ALBERT

Description: Microradian tracking and pointing errors significantly affect the link performance and the bit error probability in free-space optical communication. This paper proposes the measurement method for point-ahead angle and coalignment error, which can significantly mitigate the tracking and pointing error, thus reduces the power of the free-space optical communication system. By using technologies of corner cube reflector, off-axis reflecting telescope, vacuum long-light path and anti-interference support structure design, microangles can be measured accurately, especially the coalignment error angle which is formed by two wide, incoherent beams nearly antiparallel to each other. This paper introduces the measuring equipment's structure, together with the measurement method and theoretical measurement model. The measuring accuracy analysis indicated that the measurement uncertainty of both point-ahead angle and coalignment error was superior to 0.2 μrad and the validation experimental results proved that measuring accuracies of point-ahead angle and coalignment error are all smaller than 1 μrad.

Description: We consider mixed channel traffic grooming in a shared backup path protected IP over elastic optical network (EON) and develop an auxiliary graph-based heuristic algorithm to share common optical channels between working and protection IP traffic flows. We compare integrated protection (i.e., cross-layer protection capacity sharing) with nonintegrated protection (i.e., overlay protection) to show how cross-layer sharing improves capacity utilization. Using simulations, network performance is evaluated in terms of the maximum number of frequency slots (FSs) used, the spare capacity redundancy, and the number of transponders required for a static traffic demand. For a dynamic traffic demand, the bandwidth blocking probability (BBP) is also obtained. It is found that the proposed mixed channel traffic grooming scheme can reduce both the BBP and the number of transponders required while improving the spectral efficiency compared to a system using dedicated channel traffic grooming. Integrated protection is also found to be more efficient than overlay protection under comparable conditions. We also observe that only a limited number of transponders per node and a limited number of FSs per transponder are sufficient to achieve good performance for an IP over EON with mixed channel traffic grooming and cross-layer spare capacity sharing.

Description: We propose and demonstrate an all-fiber Bessel optical tweezers for multiple microparticles (yeast cells) three-dimensional (3-D) trap. To the best knowledge of us, it is the first time to achieve the 3-D stable noncontact multiple microparticles optical traps with long distance intervals by using a single all-fiber probe. The Bessel beam is produced by splicing coaxially a single-mode fiber and a step index multimode fiber. The convergence of the output Bessel beam is performed by molding the tip of the multimode fiber into a special semiellipsoid shape. The effective trapping range of the all-fiber probe is 0 to 60 μm, which is much longer than normal single fiber optical tweezers probes. The all-fiber Bessel optical probe is convenient to integrate and suitable for the lab on the chip. The structure of this fiber probe is simple, high precision, low cost, and small size, which provides new development for biological cells experiment and operation.

Description: This paper presents a method for predicting strain and sensitivity of a fiber-optic flexural disk seismometer (FODS) based on multiturn fiber coils (MTFC) via finite element method (FEM) analysis. FEM can be used to evaluate sensitivity by taking into account the strain distribution in MTFC. A sample MTFC-based FODS was fabricated according to simulation parameters; the experimental results of strain distribution and sensitivity were determined by Brillouin optical time-domain analysis and vibrator, respectively, which in turn can be used to confirm the correctness of the prediction model. The tested strain distribution of MTFC has the same characteristic with its simulation model; the predicted sensitivity of MTFC-based FODS is 6448.0 rad/g. Three sample sensors are fabricated and tested, which have a fabrication error of 7%. All FODS simulations were carried out in the COMSOL Multiphysics environment, which has significant potential for application as a tool of predicting the FODS sensitivity when designing or fabrication an MTFC-based FODS.

Description: Combating the destructive effects of power-fading in fiber-optic systems using direct-detection is the key for increasing the transmission rates and/or reach. Some of the solutions used in the past include additional components, e.g., a dispersion-compensating fiber or an IQ-modulator, which increase the costs of the entire system. Another solution, the compensation of the channel distortions using decision-feedback equalization, is limited to its error-propagation effect. In this paper, we introduce Tomlinson–Harashima precoding for the compensation of power-fading, which allows us to use the efficiency of feedback equalization for channels with spectral zeros without the disadvantage of error propagation. In an experimental investigation with different scenarios, including transmission over 25 km up to 100 km of standard single-mode fiber with gross data rates ranging from 56 to 560 Gb/s, we show that Tomlinson–Harashima precoding significantly outperforms decision-feedback equalization in all cases. Using Tomlinson–Harashima precoding, power-fading can be efficiently compensated for using digital signal processing only, keeping the system flexible and at the same time cost-effective.

Description: Optical wireless communications have been well studied as a potential technique to achieve data rates exceeding several gigabits/second transmission. However, a simple and effective multiuser access framework for such systems remains elusive to date. In this paper, the time-slot coding scheme is employed with 16-QAM modulation format to simultaneously provide high-speed wireless communications to multiple users. With the use of unique time slotted code for each user, we demonstrate an effective multiuser access framework and evaluate its performance. The code alignment tolerance is investigated and experimentally demonstrated as 27.6%, 28.1%, 28.4%, and 28.8% at a received optical power of –18, –16, –14, and –12 dBm, respectively. Furthermore, to provide flexible data rates and to improve the coverage in multiuser scenarios, the time-slot coding scheme is incorporated with the adaptive loading function allowing different users to deploy different modulation methods. Compared to conventional time-slot coding scheme, experimental results show that satisfactory coverage (BER ≤ 3.8 × 10 −3 ) at 1 GBaud/s can be extended by up to 61.2%.

Description: We study the achievable rates of submarine fiber systems in the high-dimensional design space of variables including span length, launch power, number of spatial channels, and power feed current. We identify the regimes in which nonlinearities or power feed equipment constraints become dominant, and demonstrate that optimized system design evolves toward the linear regime as the system scales to a high number of spatial channels. We calculate the bit rate achievable by uniform and probabilistically shaped M-ary Quadrature Amplitude Modulation constellations to identify potential capacity-achieving implementations.

Description: This paper reviews and extends a method for the semi-analytical solution of the coupled linear differential equations that describe the linear mode coupling arising in few-mode fibers due to waveguide imperfections. The semi-analytical solutions obtained proved to be accurate when compared to numerical solution methods. These solutions were integrated into a multisection model with split-steps for mode dispersion and mode coupling. Simulations using this model matched the analytical predictions for the statistics of group-delays in few-mode fiber links, considering different coupling regimes with and without mode delay management.

Description: In this paper, we further explore the concept of phase-conjugated twin waves (PCTW) for nonlinear cancellation in space-division multiplexed (SDM) systems. Previously, we demonstrated that the PCTW technique can successfully provide nonlinear cancellation in SDM systems. In this paper, we investigate the cases where two and four spatial modes are copropagating in a multimode fiber, considering three link lengths (1000, 3200, and 8000 km). Weak- and strong-coupling regimes are also evaluated. Our numerical simulation results show an average performance improvement $>$ 10 dB after a 1000 km transmission link.

Description: We propose a configuration of optical heterodyne microvibration detection based on an all-fiber acousto-optic superlattice modulation structure that acts as both frequency shifter and reflector, simultaneously. The vibration information within the frequency range between 1 Hz and 150 kHz of a piezoelectric mirror has been experimentally measured by using this all-fiber optical heterodyne detection configuration. The minimal measurable vibration amplitude and the resolution are around 0.013 nm and 10 pm in the region of tens to hundreds of kilohertz, respectively. The configuration not only has advantages of compact size, easy alignment, and noncontact measurement, but also gains high accuracy, which provides a promising alternative and could be applied in the compact and portable instruments based on optical heterodyne detection.

Description: A wavelength-tunable optical fiber tweezer based on a graded index multimode fiber with a flat endface is proposed. The design offers a noncontact optical fiber tweezer generating a stable optical trap that can be tuned precisely over a large range using a common wavelength-tunable laser. This property comes from the wavelength dependence of the graded index multimode fiber design parameters, such as the numerical aperture. Using an optical fiber tweezer with a flat endface is also more desirable than the tapered one because of the easier fabrication process. Our analysis also shows that the setup can form a stable three-dimensional optical trap in the Rayleigh regime in the absence of any microfluidic flow force.

Description: A novel technique for generating RF sources for mobile communications based on dual Mach–Zehnder modulators and high-power charge-compensated modified unitraveling carrier (CC-MUTC) photodiodes is presented. A system model is developed and used to establish and mitigate nonlinearities attributed to its constituent components, i.e., waveform generators, amplifiers, modulators, and photodiodes. In particular, digital predistortion is used to improve source linearity. Extensive experiments are conducted to characterize the proposed system, using a commercial 5 MHz local thermal equilibrium signal. Optimal results are achieved with a CC-MUTC photodiode to attain an RF power greater than 12.5 dBm and an adjacent channel leakage ratio lower than −63.5 dBc.

Description: We present a fiber optic magnetic field sensor conceived for magnetic resonance imaging (MRI) applications. The sensor is based on the integration of fiber optic strain sensors (fiber Bragg gratings—FBGs) with a sensing material (Terfenol-D). The response of an FBG integrated with a block of Terfenol-D was preliminarily investigated by taking into account the dependence of the Terfenol-D magnetostrictive response on both the longitudinal and transversal magnetic fields, with different preloads. Based on the performed characterizations, a triaxial magnetic field sensor was designed, characterized, and fabricated. An algorithm enabling the demodulation of the magnetic field from the readout of the three FBGs was also implemented, by taking into account the interdependence among the different sensor responses. Experimental results demonstrate the ability of the triaxial sensor to measure the magnetic field. Performance assessment and critical analysis are reported as well, elucidating both the abilities and limitations of the implemented sensing configuration. Finally, as proof of principle, a sensing system constituted of 20 triaxial sensors has been fabricated and used to map the magnetic field strength distribution in an MRI diagnostic centre.

Description: Humidity sensors rely on humidity-induced refractive index change in the sensing material despite the sensor configuration. Polymer-based microwires can absorb water vapor molecules and detect humidity changes without the need of further coating. However, the sensitivity-simplicity trade-off is still a challenge. Sophisticated coating methods, complex resonating structures, and nanostructured films are reported as methods to enhance the device sensitivity. A simple technique, to build a high sensitivity RH sensor based on an agarose-doped Poly Methyl Methacrylate (PMMA) sensor head, is demonstrated. The waist diameter and uniform length of the PMMA doped agarose gel microfiber were measured to be 6 μm and 10 mm, respectively. The sensor can achieve power variation of up to 2.9 μW in a wide relative humidity range (50–80%), and display linear response with a correlation coefficient of 98.29%, sensitivity of 0.421 dB/%RH, and resolution of 0.431%RH. This agarose-based optical sensor provides a beneficial complement to the existing electrical ones, and will promote the employment of agarose in chemical sensing techniques.

Description: We propose a 400G frequency-hybrid superchannel solution based on three carriers, two edge PM-16QAM, and a central PM-64QAM carrier, compatible with the 62.5-GHz grid slot (spectral efficiency of 6.4 b/s/Hz). The proposed superchannel is experimentally assessed in long-haul transmission by copropagation with other eight similar superchannels. The optimum power ratio between superchannel carriers is analytically determined in linear and nonlinear operation regimes using the enhanced Gaussian noise model and validated by experimental and simulation results. The 400G superchannel performance is evaluated in terms of maximum reach determining the optimum launch power and considering three distinct forward-error correction (FEC) paradigms: superchannel FEC (SC-FEC) where a single FEC is applied to the entire superchannel, independent carrier FEC (IC-FEC) where an independent FEC with fixed overhead is applied to each superchannel carrier, and independent carrier flexible FEC (Flex-FEC) where optimized FEC overheads are applied independently to each superchannel carrier with the constraint of a given total overhead. When compared to the IC-FEC approach, the SC-FEC or Flex-FEC approaches enable to extend the maximum transmission distance by more than 60%, while reducing the optimum power ratio by $\sim$ 6 dB, at the cost of 2 dB higher launched power. The system performance is also analyzed for the case of nonlinear compensation via digital backpropagation (DBP) techniques, assessing the improvement in reach and evaluating their impact on the optimum power ratio and launch power. For the proposed frequency-hybrid superchannel, we demonstrate that the application of DBP can be restricted to the carrier with higher QAM cardinality, thereby significantly reducing the overall computational effort, with a maximum reach reduction of- only $\sim$ 2% over the application of DBP to all three carriers individually.

Description: Space ionizing radiation impact on the on-orbit performance of 1550 nm on off keying, differential phase shift keying, and homodyne binary phase shift keying (BPSK) based optical intersatellite communication systems is studied in this paper. Essential optoelectronic devices involved in optical communication systems were irradiated by Co 60 gamma ray to a total dose of 30 krad and the related ionizing radiation damage factors are obtained according to theoretical analyses. On this basis, ionizing radiation induced communication terminal degradation and system performance degradation are simulated. Subsequently, the ionizing radiation environment of four typical orbits is analyzed and the on-orbit performances of different intersatellite communication links are further obtained and discussed. The result shows the necessity of enhancing radiation protection of EDFAs, optical transmitters, and terminals located on GEO and HEO satellites, and it also shows the superiority of homodyne BPSK scheme against space ionizing radiation environment.

Description: A concatenated soft-decision forward error correction (FEC) scheme consisting of an inner low-density generator-matrix (LDGM) code and an outer staircase code is proposed. The soft-decision LDGM code is used for error reduction, while the majority of bit errors are corrected by the low-complexity hard-decision staircase code. Decoding complexity of the concatenated code is quantified by a score based on the number of edges in the LDGM code Tanner graph, the number of decoding iterations, and the number of staircase code decoding operations. The inner LDGM ensemble is designed by solving an optimization problem, which minimizes the product of the average node degree and an estimate of the required number of decoding iterations. A search procedure is used to find the inner and outer code pair with lowest complexity. The design procedure results in a Pareto-frontier characterization of the tradeoff between net coding gain and complexity for the concatenated code. Simulations of code designs at $\text{20}\%$ overhead showed that the proposed scheme achieves net coding gains equivalent to existing soft-decision FEC solutions, with up to $\text{57}\%$ reduction in complexity.

Description: We develop a process of fabricating silicon waveguides and devices using a bulk silicon substrate. The fabrication process mainly consists of one silicon dry etching and one silicon wet etching. The use of silicon wet etching makes the process simple and inexpensive. Because of the anisotropic nature of silicon wet etching, the bulk-silicon-based (BSB) waveguide made by the process consists of an inverted-trapezoidal core on a rectangular pedestal and a trapezoidal base beneath the pedestal. In addition, geometrically smooth BSB waveguide bends can be achieved when the radii of curvature of the bends are sufficiently large. The propagation loss of the BSB waveguide depends on wet etching conditions and it is 4.0 or 0.79 dB/cm for transverse-magnetic polarization. It is confirmed that the minimum radius of curvature of the BSB waveguide bend is 500 μm. The BSB waveguides and bends are expected to be used to implement low-cost sensors with simple geometry.

Description: As an open integrated environment deployed with wired and wireless infrastructures, the smart city heavily relies on the wireless-optical broadband access network. Smart home data are usually sent to neighbor optical network units (ONUs) through front-end wireless mesh networks (WMNs) and finally reach the optical line terminal (OLT) for decision making via the passive optical network (PON) backhaul. To reduce backhaul bandwidth saturated by this conventional approach, smart edge devices (EDs) should be deployed at sensors and ONUs so that collaborative edge computing can be performed in front-end WMNs. Moreover, the cooperation of EDs at different ONUs is also promising for computing tasks that cannot be handled within front-end WMNs due to the local bottleneck, leading to collaborative edge computing in the PON backhaul. In this paper, network virtualization is utilized to support the coordination of computing and network resources. We also describe the relationship between virtual networks and requirements of computing tasks for substrate resources. First, a graph-cutting algorithm is employed to embed as many virtual networks as possible onto the common network infrastructure in front-end WMNs, aiming at minimizing the total transmitting power. Next, we transform impossibly embedded virtual networks into new ones that must be processed through the PON backhaul where the wavelength consumption will be optimized. Simulations results demonstrate that 1) the total transmitting power assigned for nodes is effectively reduced using the graph-cutting algorithm if all computing tasks can be solved by front-end WMNs; 2) otherwise, our method accepts more virtual networks with the improvement ratio of 77%, through the PON backhaul. In addition, there is a good match between the algorithm result and the optimal number of consumed wavelengths per optical fiber cable.

Description: Stimulated Brillouin scattering (SBS) is the first nonlinear effect that limits power scaling of narrow linewidth fiber lasers. Nonlinearities typically have a reduced impact when operating at longer wavelengths. However, the SBS gain is considered wavelength independent. To investigate this further, a pulsed 2053 nm source with MHz-linewidth is amplified to >100 W peak powers in single-mode, thulium-doped fiber. The SBS thresholds were measured while varying the pulse duration. Analyzing the SBS threshold measurements suggests that the peak Brillouin gain coefficient is ∼12.2 pm/W with a spontaneous Brillouin bandwidth of ∼17.5 MHz in the passive single-mode fiber at 2053 nm. While the peak Brillouin gain coefficient is comparable to those reported at shorter wavelengths, the spontaneous Brillouin bandwidth is significantly narrower. This indicates that long wavelength sources can inhibit the onset of SBS more readily than short wavelength sources.

Description: A low-profile optically fed ultra-wideband-connected array (CA) antenna with increased operational power is presented. Introduction of a miniature optical prism into the fiber optic feed enables an efficient 90° coupling to a high-power charge-compensated-modified uni-travelling carrier photodiode connected directly at the feed point of an integrated CA antenna element. This technique significantly reduces the array profile and, herein, is implemented into a 1-D CA antenna array consisting of eight photodiode-coupled active dipole elements. The experimentally verified array achieves effective beamforming and beam steering over a 3-dB bandwidth of 6–17 GHz, as well as a peak effective isotropic radiated power of 27.5 dBm at 13 GHz.

Description: In this paper, we present a bidirectional interruption-free multimode waveguide coupler for optical bus systems on board and module level. The principle is based on directional core–core coupling and allows for adjustable coupling powers by tuning the overlap area. By adding a bending to one of the coupling partners, it is possible to obtain specific asymmetric coupling rates depending on the coupling direction (module to bus or vice versa). The proposed approach is extensively analyzed by optical simulation (beam propagation method) and measurements including experiments on the attenuation, the coupling rate, and the bit rate performance.

Description: A novel relative humidity sensor based on a singlemode-side polished multimode-singlemode fibre structure coated with gelatin material is reported. The sensing principle and fabrication method of the proposed sensor are presented. The experimental method is demonstrated to provide the optimum thickness of coating layers in order to achieve the highest sensitivity of 0.14 dB/%RH and a fast response time of 1000 ms for a given relative humidity sensing range. The developed humidity fibre optic sensor based on a gelatin coating shows great potential for many applications such as industrial production, food processing, and environmental monitoring.

Description: A wideband photonic microwave single sideband (SSB) up-converter and an in-phase/quadrature (I/Q) modulator are proposed and experimentally demonstrated. The intermediate frequency (IF) and local oscillator (LO) signals are applied to the submodulators of a dual-parallel Mach–Zehnder modulator (DPMZM) for frequency up-conversion, and the phase of the generated radio frequency signal can be arbitrarily tuned through the working point of the DPMZM. Using a polarization division multiplexing DPMZM, a wideband photonic microwave I/Q frequency up-converter can be constructed for SSB up-conversion or I/Q modulation. In the experiment, SSB signals with well-suppressed carrier and undesired sideband are generated over a wide frequency coverage of the LO (16–40 GHz) and IF (2–8 GHz) signals. In the experimental demonstration of I/Q modulation, 16- and 64-quadrature amplitude modulation signals with a symbol rate of 100 MSym/s and a carrier frequency from 10 to 40 GHz are successfully generated with low error vector magnitudes.

Description: Surface enhanced Raman scattering has recently been proposed as a label free sensing method for diagnostic applications. Raman scattering is an excellent analysis tool because a wealth of information can be obtained using a single measurement, however the weak signal has made it unsuitable for detecting low concentrations of analytes. Using plasmonic nanostructures to create SERS substrates, the Raman signal can be amplified by several orders of magnitude, but SERS substrates have been complicated to fabricate. Here we report low-cost silicon substrates based on simple fabrication method of silver nanoparticles and silicon nanowires decorated with these nanoparticles for use as a convenient practical platform for SERS-active substrates. In addition, the placement of silver nanoparticles on silicon nanowires allowed the autoaligning of the hot spots such that low cost Raman systems with normal incident laser can be used. These substrates have the ability to detect wide range of concentrations of pyridine, as low as 10 –11  M. An enhancement factor of around 6 to 8 × 10 5 was observed for silver nanoparticles alone. By depositing the same nanoparticles on silicon nanowires, the enhancement factor jumped by orders of magnitude to 10 11 .

Description: We investigate the surface plasmonic lattice solitons (PLSs) in semi-infinite graphene sheet arrays. The surface soliton is formed as the surface plasmon polaritons (SPPs) tunneling is inhibited by the graphene nonlinearity, and meanwhile the incident power should be above a threshold value. Thanks to the strong confinement of SPPs on graphene, the effective width of surface PLSs can be squeezed into deep-subwavelength scale of ∼0.002 λ . The influence of the graphene loss on the surface PLSs is also discussed. Based on the stable propagation of surface PLSs, we find that the light propagation can be switched from the array boundary to the inner graphene sheets by reducing the incident power or increasing the chemical potential of graphene. The study may find promising application in optical switches on deep-subwavelength scale.

Description: Stable octave-spanning supercontinuum (SC) in the mid-infrared (MIR) region finds extensive applications in spectroscopy, metrology, biochemistry, etc. The absorption of conventional silicon- or silicon oxide-dominated nonlinear media makes SC generation in MIR region technically challenging. In this paper, we propose ultrabroadband MIR-SC generation using a suspended germanium-membrane ridge waveguide. We theoretically showed that when pump pulses centered at 4.8 μm with pulse width at 180 fs and peak power at 800 W are injected into a 4-mm-long proposed ridge waveguide, the SC generated ranges from 1.96–12 μm (about 2.6 octaves), extending deep into the “fingerprint” region. The first-order coherence is calculated to confirm the stability of the generated SC. The performance of the SC-based frequency comb is also investigated by assuming a 100-pulses pump source at a repetition rate of 100 kHz.

Description: In this paper, a horizontal slot Si–VO 2 –Si optical waveguide is proposed and its optical properties are investigated. Numerical simulation results show that the effective index and the propagation loss of the proposed waveguide undergo substantial changes upon the VO 2 transition from insulating to metallic phase. The effective index and the propagation loss variations of the proposed waveguide are then maximized by optimizing waveguide dimensions. It is shown that 0.226 change in the effective index (Δ n eff = 0.226) and 30 dB/ μ m change in the propagation loss (Δ l dB = 30 dB/μm) are achievable using the optimum dimensions. These extraordinary variations in waveguide properties recommend the proposed waveguide as an excellent candidate for optical active device realization. To investigate these applications, performance parameters of the proposed waveguide are further studied in terms of the transition speed and the power consumption. In these studies, the VO 2 phase transition is assumed to be actuated by applying an electric field. Two examples of optical active devices based on the proposed waveguide are then presented: an electro-absorption modulator and a 1 × 2 directional coupler optical switch. Finite-difference time-domain simulation of the proposed devices shows very high extinction ratio of 21 dB along the ultrasmall propagation length of 1 μ m, for the proposed electro-absorption modulator, and high extinction ratios of ∼18.5 dB and ∼8.6 dB in off - and on -state of the proposed 1 × 2 switch, which has very small length of ∼6 μ m. Further simulations also show interesting properties of the proposed devices in terms of the power consumption, insertion loss, and bandwidth.

Description: A comprehensive experimental study on the performance of a packaged flexible-grid compliant 16 × 1 packaged MUX/DEMUX device is presented. The device relies on a bandwidth and wavelength selective filtering element array integrated on an SOI platform, equipped with on-chip polarization multiplexing functionality. Multilateral operating credentials are demonstrated through the evaluation of the device in 2 × 1 MUX, 1 × 2 DEMUX and PolMUX configurations scenarios under realistic data traffic conditions, thus confirming its suitability for next-generation flexible-grid optical networks.

Description: The finite element method (FEM) has attracted a considerable interest in the past few decades for the analysis of a wide range of dielectric waveguides. This method can handle isotropic and anisotropic material properties and arbitrary-shaped complex dielectric discontinuities more efficiently and accurately than any other methods. A modified H-field based full-vectorial finite element method is used for a rigorous analysis of a composite plasmonic waveguide as an efficient ethanol vapor sensor where a porous ZnO (P-ZnO) layer is used as low index material in between high index silicon and silver metal layer. Enhanced field confined into low index slot is utilized for ethanol vapor sensing which has many potential applications in chemical industries. It is reported here that a high waveguide sensitivity over 0.7 per RIU could be realized with our proposed design depending on the porosity of the ZnO layer. For accurate detection of refractometric changes, a compact Mach–Zehnder interferometer is designed where maximum phase sensitivities of 0.30, 0.34, 0.38, and 0.40 are shown to be achieved for $ \sim $ 50% volume fraction of ethanol into porous ZnO layer with porosity, P = 30%, 40%, 50%, and 60%, respectively. The complete investigation has been carried out at the well-known telecommunication wavelength 1550 nm and with our in-house, accurate full-vectorial FEM code.

Description: This paper considers a long-haul optical fiber cable, connecting two points on the Earth's surface that passes through earthquake-prone or other sensitive areas. Different segments of the cable are characterized by different protection levels, where a higher level through shielding represents a more costly and more resilient segment. This leads to a multi-objective optimization problem, where the two objectives are 1) the total cost of the cable and 2) the total number of potential repairs along the cable likely to be caused by earthquakes. As a measure of seismic risk, we use the concept of cable repair rate used in the civil engineering community. In our models, ground motion intensity data are used to estimate the cable repair rate, and a graph of a triangulated irregular network is used to represent the Earth's surface. We formulate this optimization problem as a multi-objective shortest path problem and solve it by a variant of the label setting algorithm. Two approximate algorithms, an interval-partition-based label-setting algorithm and an evolutionary algorithm, are also presented as methods of computational cost reduction for large-scale cases, and their results are compared. The solution leads to a Pareto front or an approximate Pareto front that enables us to choose the path and protection of the cable to either minimize cost for a given risk level or minimize risk for a given budget.

Description: We investigated a hypersensitive and tunable terahertz (THz) wave switch based on liquid-crystal-filled non-Bragg structures. Non-Bragg structures, which consist of periodically corrugated metallic tube walls, provide spectra with very sharp rising edges, making them usable for sensitive switching. Tunability can be achieved by dynamically shifting the rising edge of a THz spectrum by using an externally applied magnetic field to change the orientations of the nematic liquid crystal (E7) molecules. The simulated results revealed that the switch effects are hypersensitive and tunable in the THz frequency range and that such switches could be applicable in future THz systems.

Description: This paper provides detailed guidelines for the optimal design of contactless integrated photonic probes suitable to track and control the local optical power in photonic circuits. With reference to current technology platforms, this paper provides a guide to extract the electrical parameters of the probe and to highlight their role in defining the achievable resolution. Crucial technological and geometrical choices are discussed, together with layout and interconnection solutions oriented to a highly dense integration of the probes. Finally, the criteria for the optimal coupling of the probes to the most suitable readout electronics providing the maximization of the SNR are presented. With these guidelines in mind, transparent in-line local power monitors featuring –35 dBm sensitivity, 40 dB of dynamic range, broadband response from 1.3 to 1.6 μm, a speed down to tens of microsecond and a minimum size of tens of micrometer can be effectively designed for high performance reconfiguration and closed-loop control of complex photonic circuits.

Description: In this paper, we demonstrate an analytical model for computing the end-to-end packet delay of a converged optical/wireless 60 GHz Radio-over-Fiber (RoF) network operating under the medium-transparent MAC (MT-MAC) protocol. For the calculation of the cycle times, this model considers the protocol time consumed for contention and data exchange over both optical and wireless media, a feature of the MT-MAC that effectively enables it to provide direct and seamless interaction between the RoF Central Office and the end users. This new analytical model enables us to conduct an extensive delay performance analysis of the various performance aspects of hybrid RoF networks operating under the fixed service paradigm, such as various optical capacity availability scenarios, varying load conditions, optical network ranges, transmission window lengths, and data packet sizes. The derived theoretical results present an excellent match with the respective simulation findings, providing sub millisecond latency values for a plethora of network conditions, confirming that the MT-MAC scheme can effectively be incorporated into the upcoming mm-wave 5G era.

Description: This work exhibits design and characteristic study of highly sensitive temperature sensors of long-period fiber gratings (LPFGs) in Ge–Sb–Se fibers. Temperature characterization of the designed LPFGs with different working modes, operating wavelengths, and surrounding refractive indices (SRI) were studied. Results showed that temperature sensitivity of the proposed Ge–Sb–Se LPFGs reached 0.5787 nm/ at 1.55 μm at the lowest LP 02 cladding mode, which is approximately 12 times higher than silica LPFGs and 1.2 times higher than that of As–Se LPFGs. When grating period of the designed Ge–Sb–Se LPFG was selected at its dispersion-turning-point, temperature sensitivity reached a maximum absolute value of 24.715 nm/°C. Influences of operating wavelength and temperature sensitivity of these LPFGs on SRI are extremely weak due to relatively high refractive index of Ge–Sb–Se fiber, indicating that the designed Ge–Sb–Se LPFG temperature sensor possesses extremely strong sensing stability on SRI.

Description: Discrete multitone transmission (DMT) schemes, such as asymmetrically clipped DMT and DC-biased DMT, are popular multicarrier transmission schemes for dispersive optical channels employing intensity modulation and direct detection. These transmission schemes utilize a cyclic prefix (CP) to have a simple frequency domain equalization at the receiver. In this paper, we propose two novel DMT signaling schemes for such optical systems, where the guard interval is built of a unique word (UW), i.e., a deterministic data-independent sequence. This approach becomes possible since redundancy is added in the frequency domain and the UW is a part of the discrete Fourier transform interval. The results show that the proposed UW-DMT schemes outperform the conventional CP-DMT schemes in dispersive optical channels. Furthermore, the UW can be additionally used for other purposes, such as synchronization and channel estimation.

Description: This paper is concerned with a process of transforming the mathematical representation of a multithreshold comparator to electrooptical implementations. The basic comparator converts an analog input into one of two levels at its (binary) output. If this functionality can be made periodic (as a function of the analog input), efficient implementation of analog-to-digital conversion can be realized. This paper begins by modeling such functionality as a square-wave via its representation as a Fourier series. Some mathematical manipulations are then performed aimed at tailoring the obtained representation for implementation by optical components such as waveguides and microring resonators. Thus, three basic integrated-optical devices are proposed for realizing a comparator with well-defined binary output levels and a steep transition slope between these levels.

Description: We demonstrate a novel polyvinyl chloride (PVC) diaphragm-based fiber-tip Fabry–Perot interferometer for gas-pressure measurements with ultrahigh sensitivity. The PVC diaphragm has been coated to the end facet of a well-cut standard single-mode fiber by use of a plastic welder. An ultrahigh-pressure sensitivity of ∼65.5 nm/MPa at 1565 nm and a low-temperature cross sensitivity of ∼–5.5 kPa/°C have been experimentally demonstrated. The proposed sensor has advantages of high pressure sensitivity, miniature size, low cost, and easy fabrication.

Description: Optical wireless (OW) technology has attracted significant interest for indoor positioning in the past decade. An emerging form of this technology makes use of angle-of-arrival (AOA) measurements to carry out positioning via triangulation off of an optical beacon grid. Such AOA-based OW positioning systems can yield accurate position estimates—but only given sufficient attention to the optical receiver. The design, operation, and implementation of such a receiver are presented in this work. The optical receiver is designed to have a sufficiently small AOA error, being σ AOA = 1°, over a wide angular field-of-view (FOV), being 100°. The design allows the optical receiver to carry out positioning based off a 3 × 3 grid of optical beacons, where each optical beacon is uniquely identified using multiple frequency and color channels. The optical beacons are widely spaced to fully utilize the optical receiver's wide angular FOV. The overall AOA-based OW positioning system exhibits a position error of 1.7 cm, which is comparable to those obtained by more complex positioning systems. Thus, the presented AOA-based technologies can play a role in emerging indoor positioning systems.

Description: We present the design of a microstructured dual-core optical fiber with integrated electrodes and filled with liquid crystals. The dual-core structure acts as a directional coupler whose properties depend on the liquid crystal alignment. We show that with four electrodes and two separate driving voltages below 30 V on the electrodes, the beam-splitting properties of the fiber can be controlled independently and continuously for the two polarization components, thus allowing for the realization of any arbitrary 2 × 2 transfer function, such as tunable polarizers, polarization-dependent attenuators, or polarization-independent beam splitting.

Description: Recently, Si-photonics received a growing interest and started to move from laboratories to industrial product development, mainly for the applications inside data-centers. One of the weaknesses of Si is its relatively low plasma dispersion efficiency, making the size of phase modulator large. This efficiency can be improved by using the heterogeneous integration of material such as InP, InGaAsP, or SiGe to fabricate hybrid semiconductor–insulator–semiconductor (SIS) optical phase modulators. At the same time, the standard figure of merit for modulator benchmarking $V_{\pi }L_{\pi }$ does not consider the dynamic behavior of the SIS devices, nor is making the link with the system level specifications such as optical modulation amplitude (OMA), widely used in 100G to 400G parallel single mode or coarse wavelength division multiplexing applications. In this paper, we propose to simply link the modulator performance to the OMA, to derive a compact model for SIS devices and to compare hybrid device performances for various materials.

Description: We have designed and fabricated widely tunable semiconductor laser with a high- Q resonator as an integral part of the laser cavity. Wide tuning is realized by utilizing the Vernier effect of two rings with slightly different circumferences. A third ring with considerably larger circumference, and, consequently, higher Q is introduced inside the laser cavity. We study the control of such a laser and show that it is straightforward provided that the integrated laser has on-chip monitor photodiodes. This further shows the benefits of full integration as inclusion of additional monitor photodetectors is straightforward with no extra processing steps. As the complexity of photonic-integrated chips increases, the inclusion of more monitor photodetectors for control is necessary.

Description: We demonstrate a combined cross correlation and Hilbert transform-based demodulation algorithm for tracking the wavelength shifts of fiber Bragg gratings (FBGs) having a multiple peak reflection spectrum. We show how the Hilbert transform can be employed to convert the task of locating the maximum of the wavelength profile to the one of finding the zero crossing. We observed higher accuracy and fast response compared to other well-known demodulation algorithms such as the centroid detection algorithm and the cross-correlation algorithm. In addition, we show that the multipeak reflection spectrum that occurs in multimode fibers does not greatly affect the algorithm results. Finally, we experimentally recover axial strain measurements using a multipeak reflection spectrum of an FBG inscribed in a multimode gradient index CYTOP fiber using the newly developed algorithm.

Description: A broadband and nonvolatile liquid controlled silicon photonics switch is designed and fabricated. The switch consists of an adiabatic coupler where the oxide above one of the waveguides is removed. Switching is realized by exposing this waveguide to liquids with different refractive indices. The switch design is based on self-consistent orthogonal coupled-mode theory. The measured crosstalk of a 1.4-mm-long switch is less than $- {\text{38}}$  dB and $-{\text{11}}$  dB over a 100-nm wavelength range for bar and cross state, respectively. The insertion loss is less than 1 dB. Also the influence of the silicon waveguide thickness and the difference in liquid refractive indices on the switch performance is studied. Furthermore, an improved performance in cross state is demonstrated when a switch is actuated by a gas and a liquid. With a gas–liquid system a very large difference in cladding refractive index can be obtained.

Description: A magneto-modulating polarization converter (PC) based on a new type dual-core photonic crystal fiber (DCPCF) is proposed and analyzed. In DCPCF, the two cores are surrounded by a central circular air hole and two lateral elliptical air holes in the horizontal axis. The central air hole is filled with magnetic fluids (MF). Without external magnetic field, DCPCF acts as a simple polarization splitter. When magnetic field applied, due to the refractive index of MF varying with the magnetic field magnitude, the intensities of the two polarization modes would interchange alternately. At a certain magnetic field, the two polarization modes would rotate 90° at the output ports. In this case, a magneto-modulating PC is realized. The performance of the PC is analyzed by the finite element method. The simulation results show that the length of the PC is 1007 μm. The polarization mode conversion can be realized at a magnetic field magnitude of 40.5 mT with a room temperature 21 °C. A high extinction ratio greater than 70 dB is obtained for mode-conversion around 1550 nm wavelength. In addition, the PC works well in the temperature range from 0 °C to 60 °C by regulating the magnetic field magnitude. The optical fiber-based PC eliminates the silicon-on-insulator circuit chip, which can be integrated with optical system easily and conveniently.

Description: In this paper, we study the channel capacity and region for both the single-input-single-output (SISO) channel and broadcast channel (BC) in visible light communication (VLC) systems, under the peak optical power, average optical power, and electrical power constraints. Under the condition that the input signal is continuous, we develop a closed-form lower bound (termed ABG lower bound) and an upper bound for SISO channel using the entropy power inequality and Lagrangian function method. Moreover, a closed-form achievable rate region (termed ABG region) is derived for the VLC BC. Furthermore, for a multi-light-emitting diode and multiuser VLC system, we propose an achievable rate expression for each user, and then investigate a VLC BC beamforming design problem by utilizing the obtained closed-form expression. The beamforming design problem is shown to be NP-hard, and we transform this problem into a convex semidefinite program by using the semidefinite relaxation technique. Finally, numerical results are presented to evaluate the performance of the proposed ABG lower bound/region and the beamforming design.

Description: We derive the closed-form information capacity of optically amplified polarization modulation with direct detection and show that it exceeds a tight lower bound for the capacity of three parallel intensity modulated direct detection systems.

Description: A time division multiplexing-passive optical network (TDM-PON) has been attracting attention with a view to realizing the cost-effective mobile fronthaul (MFH) of a mobile base station (BS). A cost-effective MFH is needed because a lot of BSs will be densely deployed in fifth generation mobile networks (5G). A TDM-PON is capable of reducing the network cost as the number of subscribers increases. However, the number of remote radio heads that can be accommodated in a TDM-PON is limited since a 5G MFH has a throughput of more than 1 Gbps. The accommodation of not only the MFH but also multiple services will enable us to construct a more cost-effective network that will in turn achieve cost-effective 5G MFH. Therefore, we propose the accommodation of the fronthaul and secondary services in a TDM-PON. Our proposed TDM-PON allows us to increase the number of accommodated ONUs. We use a traffic monitor to capture the MFH signal and then estimate the unallocated interval in the PON domain by using the characteristics of the MFH signal. The signals of the secondary services are inserted in the unallocated interval of the MFH signal. We estimate the unallocated interval by using this characteristic. In this paper, we report experimental and the theoretical results for our proposed technique obtained with a 10 Gigabit-Ethernet PON prototype. Moreover, we also discuss the variation in throughput when we increase the number of ONUs.

Description: Future generation mobile communications running on mm-wave frequencies will require great robustness against frequency selective channels. In this paper, we evaluate the transmission performance of 4.9 Gb/s wavelet-coded orthogonal frequency division multiplexing (OFDM) signals on a 10 km fiber plus 58 m wireless radio-over-fiber link using a mm-wave radio frequency carrier. The results show that a 2 × 128 wavelet-coded OFDM system achieves a bit-error rate of 1e-4 with nearly 2.5 dB less signal-to-noise ratio than a convolutional coded OFDM system with equivalent spectral efficiency for 8 GHz-wide signals with 512 subcarriers on a carrier frequency of 86 GHz. Our findings confirm the Tzannes’ theory that wavelet coding enables high diversity gains with a low complexity receiver and, most notably, without compromising the system's spectral efficiency.

Description: We present a thorough description of the dominating intramodal and intermodal four-wave mixing interactions occurring in a highly nonlinear few-mode fiber and describe their phase matching conditions. Those interactions that result in few-mode parametric amplification using a single-frequency pump are of particular interest. Thus, based on the phase matching conditions of such interactions, we outline the dispersion properties that a fiber should possess in order to achieve few-mode parametric amplification, while having minimal modal crosstalk. Accordingly, we design and optimize two fibers such that they meet the dispersion requirements for parametric amplification of two and four spatial modes, respectively. The two-mode fiber provides a maximum differential modal gain (DMG) of 0.21 dB across the C-band with a minimum gain of 9.5 dB per mode, while the four-mode fiber provides a maximum DMG of 1.51 dB with a minimum gain of 6.5 dB per mode over a 19 nm bandwidth in the C-band. The designed fibers are highly nonlinear dispersion-shifted few-mode fibers that provide both high nonlinearity and low dispersion for several modes in the C-band, which have not been demonstrated simultaneously to date. We also take practical fabrication issues into account and analyze and compare the tolerances of the structural parameters of both fibers to small deviations from their optimal values.

Description: Flexible grid optical networks (FONs) can accommodate diverse kinds of service connections with higher networking flexibility and spectrum-utilization efficiency by employing a finer resource allocation granularity (known as frequency slot) and a more sophisticated path-routing and resource-allocation algorithm (known as routing and spectrum assignment algorithm), when compared to the traditional wavelength division multiplexing optical networks. However, the continuity and the contiguity constraints in spectrum allocation may possibly introduce spectrum fragments, which cannot be utilized by the subsequent service connections and thus reduce the amount of available resources as well as the networking flexibility in FONs. Therefore, many algorithms have been proposed to decrease the amount of spectrum fragments by re-optimizing the fragmented resources. These algorithms are known as spectrum defragmentation algorithms, which always induce traffic disruption or require extra components. In order to avoid traffic disruption and the requirement of extra components, some grouping RSA algorithms have been proposed to suppress spectrum fragments from their generation by pre-dividing the spectrum resources into several either fixed or variable groups. However, since the spectrum resources in one group can only be assigned to the service connections of one specific kind, the flexibility of these grouping algorithms is always limited by the kinds of service connections. In this paper, we introduce hybrid grouping mechanism into spectrum assignment and propose a hybrid-group-based RSA algorithm, most-used-first hybrid grouping (MUFHG) RSA algorithm, to suppress the spectrum fragments generated in FONs. By employing hybrid grouping mechanism, the proposed MUFHG algorithm sorts the spectrum resources into several flexible groups with specified allocation starting FSs. And each spectrum group can accommodate several kinds of service connections if the bandwidth requirements of - hese service connections have multiple relations, which guarantees that the remained or the released spectrum resources in each group can always be re-used. Therefore, spectrum fragments are only generated in the spectrum bands between two adjacent flexible groups. In this way, the proposed hybrid-group-based algorithm can significantly suppress spectrum fragments from their generation. Besides, the proposed MUFHG algorithm helps improve the blocking performance of the network by employing the most-used-first strategy to maximize the number of less frequently used spectrum resources for subsequent service connections. The simulation results indicate that the proposed MUFHG algorithm gains notable reductions in both spectrum fragments and bandwidth blocking probability with neither traffic disruption nor extra components.

Description: A derivation of a general formula for the bit error rate of 2-D time-spreading wavelength-hopping optical code division multiple access systems under the impact of four-wave mixing (FWM) is presented. The effect of many system parameters such as the transmitted power per chip, code properties, wavelength spacing, and transmission fiber length is investigated. The analysis reveals that the generated FWM light contributes not only as an additional crosstalk component, but also produces as extra three noise terms, namely, signal-FWM, interferer-FWM, and FWM–FWM noise. Theoretical results show the detrimental effect of FWM on the system performance at high chip power levels; resulting in a 4-dB penalty at a power of 15 dBm. Furthermore, the paper presents many aspects useful in the system design.

Description: We developed a wavefront matching (WFM) method based on the full-vector finite-element beam propagation method (BPM). The WFM method is one of the optimization algorithms based on BPM, and it can deterministically design optimum waveguide structures from desired characteristics. The developed WFM method can be applied to optical waveguides with high refractive-index contrast and polarization control devices, which require full-vector analysis. In this paper, the principle and design procedure of the full-vector WFM method are described. Its validity and effectiveness are demonstrated by designing polarization control devices, such as a polarization converter based on a vertical asymmetric taper waveguide and a polarization splitter-rotator based on taper and Y-branch waveguides.

Description: We experimentally demonstrate a novel ultrabroadband and wide-angle unidirectional coupling scheme by combining the coupling effect of single nanoslit and chirped grating effect. The operation principle is numerically and experimentally clarified through analysis and compare. Numerical analysis shows that the fabricated chirped-nanoslits grating could unidirectionally couple surface plasmon polaritons from free-space light over an ultrabroadband (1.3  $\mu$ m) and wide-angle ( ${\text{30}}^{\circ }$ ) ranges. Excellent unidirectional coupling performance with extinction ratio ER $>10$  dB (a peak value ER = 22 dB) over an ultrabroad operating bandwidth of 300 nm and wide angular range of ${\text{26}}^{\circ }$ is experimentally demonstrated.

Description: Optical code division multiple access (OCDMA) systems can support multiple classes of service by differentiating code parameters, power level, and diversity order. In this paper, we analyze bit error rate (BER) performance of a multiclass 1-D/2-D OCDMA system and propose a new approximation method that can be used to generate accurate estimation of system BER using a simple mathematical form. The proposed approximation provides insight into proper system level analysis, system level design and sensitivity of system performance to the factors such as code parameters, power level, and diversity order. Considering code design, code cardinality, and system performance constraints, two design problems are defined and their optimal solutions are provided. We, then, propose an adaptive OCDMA passive optical networks (OCDMA-PON) that adaptively shares unused resources of inactive users among active ones to improve upstream system performance. Using the approximated BER expression and defined design problems, two adaptive code allocation algorithms for the adaptive OCDMA-PON are presented and their performances are evaluated by simulation. Simulation results show that the adaptive code allocation algorithms can increase average transmission rate or decrease average transmitted optical power of optical network units for dynamic traffic patterns. According to the simulation results, for an adaptive OCDMA-PON with BER value of $10^{-7}$ and user activity probability of 0.5, transmission rate (transmitted optical power) can be increased (decreased) by a factor of 2.53 (0.25) compared to fixed code assignment.

Description: A full-vector beam propagation method based on a finite-element scheme for a helicoidal system is developed. The permittivity and permeability tensors of a straight waveguide are replaced with equivalent ones for a helicoidal system, obtained by transformation optics. A cylindrical, perfectly matched layer is implemented for the absorbing boundary condition. To treat wide-angle beam propagation, a second-order differentiation term with respect to the propagation direction is directly discretized without using a conventional Padé approximation. The transmission spectra of twisted photonic crystal fibers are thoroughly investigated, and it is found that the diameters of the air holes greatly affect the spectra. The calculated results are in good agreement with the recently reported measured results, showing the validity and usefulness of the method developed here.

Description: High finesse hollow-core photonic bandgap fiber (HC-PBF) resonating Fabry–Perot gas cells are presented. These gas cells are made with a piece of HC-PBF sandwiched by two single mode fibers with mirrored ends. A HC-PBF cavity made with 6.75-cm-long HC-1550-06 fiber achieved a cavity finesse of 128, corresponding to an effective optical path length of ∼5.5 m. Experiment with a 9.4-cm-long Fabry–Perot gas cell with a finesse of 68 demonstrated a detection limit of 7 ppm acetylene. Compared with a single-path nonresonating HC-PBF, the use of a high finesse resonating HC-PBF cavity can reduce significantly the effect of modal interference on gas detection and improve the detection sensitivity. The cavity-enhanced HC-PBF gas cells enable stronger light–gas interaction and can be used to develop all-fiber gas sensors with high sensitivity and fast response.

Description: This paper presents a simple solution to simultaneous transcendental equations of propagation constants for leaky and degenerate modes. Using a block version of the Sakurai–Sugiura method (SSM) and a criterion for distinguishing physical solutions from spurious ones, we solved a nonlinear matrix equation formulated by the multipole method in a two-dimensional cylindrical coordinate system. Numerical examples showed that we can obtain accurate propagation constants and extract field distributions of leaky and degenerate modes of holey fibers without imposing symmetry. In addition, the condition numbers of the eigenvalues are efficient criteria for distinguishing solutions computed by the block version of the SSM.

Description: We propose and demonstrate a sensing system with fiber Bragg grating sensors designed as discrete prolate spheroidal sequences (DPSS). In this way, we can encode all of the sensing devices in the network making sure that each sensor is completely distinguishable from each other even under overlapping conditions. Since these devices are complex structures involving unique magnitude and phase response, the demodulation method should be able to recover the associated magnitude and phase response in order to identify each device in the sensing network. To do so, we use a single side band modulated optical source sweeping over the operational wavelength range of the sensors, the reflected signal from the sensors is sent to a vector network analyzer in which it is obtained the magnitude and phase ratio of the network in the microwave domain. Experimental demonstration has been carried out including the manufacturing of DPSS structures. Feasibility of the interrogation technique for devices involving magnitude and phase distinction has been validated, not only to identify encoded sensors in a measurement network but also to allow overlapping between them, which allow to increase the number of sensors allocated in the sensing network.

Description: We propose and design the silicon phoxonic crystal waveguides to achieve forward stimulated Brillouin scattering (FSBS). The waveguides with honeycomb-lattice structure are able to support the photonic and phononic band gaps simultaneously. By the optimized design, the corresponding optical and acoustic defect modes are tightly confined in the line defect to enhance acousto-optic coupling, resulting in the intramode and intermode FSBS. The three-dimensional (3D) full-vectorial theoretical formulation is applied to analyze the influences of structural parameters on FSBS gain. By the coupled-mode equations, we explore the FSBS process taking into account the slow light enhanced factor, two-photon and free-carrier absorptions in the slow-light phoxonic crystal waveguide. The results indicate that 18 dB Stokes amplification is obtained with 100 mW pump power in a short waveguide length of 200 μm. Such approach enables the realization of on-chip FSBS with CMOS technologies.

Description: In this paper, a high-sensitivity ammonia gas sensor is proposed based on a silica-gel-coated microfiber coupler (MFC). The MFC structure is formed by the two tapered fibers with 3 μm waist diameter each, which were fabricated by using a customized microheater brushing technique. Silica gel coating was prepared by a sol-gel technique and applied on the surface of the MFC as a thin layer. The spectral characteristics of the proposed sensor were studied under various ammonia gas concentrations. The experimental results show that the coating thickness strongly affected the sensitivity of the MFC-based sensor to ammonia gas concentration. For the sensor with a 90 nm silica gel coating thickness, the highest measurement sensitivity is 2.23 nm/ppm for ammonia gas concentration, and the resolution is as good as 5 ppb, while the measured response and recovery times are ∼50 and 35 s, respectively. Finally, it is demonstrated that the proposed sensor offers good repeatability and selectivity to ammonia gas.

Description: Offset quadratic-amplitude modulation (QAM) based filterbank multicarrier (FBMC/OQAM) is an attractive candidate to improve the spectral containment of optical fiber communication systems, especially when considering a sufficiently high number of subcarriers. As for other multicarrier modulations, the chromatic dispersion (CD) compensation is simplified in FBMC/OQAM systems since it is performed in the frequency domain. Unfortunately, FBMC/OQAM systems are sensitive to the laser phase noise (PN). The PN becomes difficult to mitigate when the number of subcarriers increases due to the increased symbol period. It results in intercarrier interference and intersymbol interference due to the loss of OQAM orthogonality. In this paper, we consider the use of moderate numbers of subcarriers to allow for simpler PN tracking. Consequently, more advanced CD compensation methods are required and a trade-off between CD and PN compensations needs to be studied. In this paper, the frequency sampling equalizer is used for the CD compensation, whereas an innovative adaptive maximum likelihood estimator is used for the PN compensation. A methodology is then presented to analyze this performance trade-off between CD and PN compensations, and design the desirable system parameters such as the number of subcarriers and the equalizer length. This is illustrated in the case of a terrestrial long-haul FBMC/OQAM transmission system, with 400-kHz laser linewidth and a 1000-km optical link.

Description: In indoor environments, visible light communications paradigm is emerging as a viable promising solution complementary to well-known radio frequency technology. At the same time, the information about user's location is useful for accessing the medium via space-division multiplexing, handling over or providing access to location-based contents. In this paper, we present two localization mechanisms based on the wavelength domain by assuming that each anchor point uses a spectrally dedicated signature for the user to readily identify it. The first approach, i.e., wavelength-based localization, assumes a simultaneous transmission of three different pulse streams emitted by the red–green–blue (RGB) light emitting diodes (LEDs). The second method, i.e., color-based localization, considers the subsequent transmission of RGB pulses. Localization is then computed through traditional received signal strength and time difference of arrival approaches. Moreover, we resort to the properties of metamerism so that the red, green, and blue components used by LEDs provide the white light sensation to the human eye. The performances of the two proposed schemes are close to theoretical bounds. Even in the worst cases, the estimation error variance is in the order of $10^{-4}$  m $^2$ . Finally, the signaling request for estimating user position is less than others in the literature and is independent from the number of anchor points.

Description: We propose a hybrid Tamm-plasmon-polariton (TPP) mode based sensing scheme for accurate determination of temperature. The sensing architecture is comprised of dual “metal-distributed-Bragg-reflector (DBR)” based structure, which sandwiches a common layer acting as a temperature sensing medium. Hybrid-TPP modes formed as a consequence of individual TPP mode-coupling is characterized by two distinct sharp reflectivity minima within the photonic bandgap of DBR for normal incidence of broadband source. The symmetric hybrid mode exhibits discernible variation in terms of change in reflectivity and resonance wavelength as the temperature of the sensing layer alters. On the other hand, the antisymmetric mode remains unchanged that gives rise to self-referenced temperature sensing scheme. We obtained a spectral sensitivity of 50 pm/°C by optimally choosing the thickness of sensing layer. In addition, the sharp TPP resonances gives rise to improved detection accuracy of such a sensing configuration as compared to competing optical sensing technologies.

Description: The figure of merit is proposed for all-dielectric waveguides for absorption overtone spectroscopy as the measure of probing efficiency of molecular overtones. It is defined as the power in the evanescent tail over the total power carried by the guided mode. The figure of merit was calculated for the proposed waveguide structures and then compared. We address each waveguide structure in probing overtones. We show that the figure of merit can be substantially increased due to the downscaling of the physical dimensions of the waveguide and microfiber. Such a configuration enables integration possibilities for ultrasensitive devices harnessing evanescent excitation of molecular overtones on miniature and portable chips.

Description: The longitudinal offset technique permits to improve the accuracy of the coupling coefficients of integrated directional couplers and provides designs that can be easily implemented with current fabrication tolerances. In this paper, we address the additional degree of freedom offered by this technology in order to tailor the differential group delay in coupled-resonator optical filters. We present the characterization of several devices exploiting this feature and we discuss their potential applications.

Description: All-optical arbitrary-order temporal differentiators are demonstrated with phase-modulated fiber Bragg gratings (PM-FBGs) in transmission for the first time. The transmissive PM-FBGs are designed by employing a novel two-step nonlinear optimization method, which consists of unconstrained nonlinear optimization method step and constrained nonlinear optimization method step. Specifically, the first unconstrained nonlinear step is used to get appropriate parameters as input to the second step, while the second constrained nonlinear step is employed to generate more accurate result based on the output of the first step. The proposed method does not impose much restriction on the initial input parameters and also improves the result accuracy compared with previous one-step nonlinear optimization method. Examples of 0.5th-order, first-order, and second-order differentiators are designed and numerically simulated. The numerical results show that the designed PM-FBG differentiators are very accurate with a bandwidth up to 500 GHz. Moreover, the proposed method can be applied in designing arbitrary-order differentiators with the differentiation order n  > 0.

Description: The advent of many-core processors with hundreds of processing cores collocated on single silicon dies requires scalable and efficient on-chip interconnects. Research suggests conventional electrical interconnects will be inadequate to support large-scale many-core processors. Thus, emerging technologies such as on-chip silicon nanophotonics may help meet the demands of future many-core processors by providing high-bandwidth and low-power communication over extended distances. Nanophotonic interconnects require a large external laser source which is often wasted during periods of low network utilization. We propose laser pooling techniques, where we share laser power among all the nodes and effectively share the link bandwidth and power. Furthermore, we design both static and dynamic techniques to scale laser power with network load. Evaluation and simulations of the proposed architecture estimate the proposed architecture, powering 16 optical channels with 0.5 dB/cm waveguide cladding losses, would require only 5.1 W of laser power versus 12.3 W of laser power for the baseline architecture, almost 58.5% laser power savings, while incurring only a 22% saturation throughput penalty.

Description: In this paper, we experimentally demonstrate the transmission of 112 Gb/s four-level pulse amplitude modulation over 100-m OM4 multimode fiber employing a multimode 850-nm vertical-cavity surface-emitting laser (VCSEL) at the transmitter side and equalization techniques at the receiver's digital signal processing (DSP). The penalties imposed by the strong bandwidth limitations due to the optical components as well as the low modal bandwidth of the fiber are compensated by three variant DSP schemes at the receiver, i.e., 1) a finite-impulse response (FIR) filter, 2) a maximum likelihood sequence estimation equalizer (MLSE), and 3) an FIR filter followed by an MLSE equalizer (FIR/MLSE) in a cascaded form. We evaluate all three aforementioned equalization schemes under two different transmitter implementations, i.e., employing a 30-GHz arbitrary waveform generator and a lower bandwidth 15-GHz commercially available digital-to-analog converter and we infer about the applicability of each DSP scheme under these implementations. We show that the hybrid implementation of an FIR followed by a 16-state MLSE can enable the 100-m transmission below the 7% hard decision (HD) forward error correction (FEC) threshold limit and outperforms its other two counterparts for the back-to-back case as well as after 100-m transmission for the high-bandwidth transmitter implementation. On the other hand, lower bandwidth DAC implementations, i.e., 15 GHz, require an increased state MLSE without the need for a preceding FIR filter to bring the bit error rate (BER) below the HD-FEC limit after 100-m OM4 fiber transmission. DSP complexity versus BER performance is assessed for all the aforementioned scenarios evaluating the impact of the transmitter's bandwidth on the overall system's performance. Our proposed solutions show that 112 Gb/s 100-m OM4 multimode links based on VCSELs - nd standard OM4 fiber can enable next generation 100 and 400 Gb/s wavelength division multiplexed optical interconnects.

Description: A systematic experimental analysis of the stationary and dynamic performance of 1.5-μm InP-based vertical-cavity surface-emitting lasers with two dielectric distributed Bragg reflectors is presented. The electrical, thermal, and optical characteristics are studied as a function of the cavity length, strain in the active region and mesa capacitance.

Description: In this paper, compact optical subassemblies are demonstrated based on a novel silicon interposer, which is designed and fabricated in a wafer scale process. The interposer includes the design of optical and electrical connections. A low-cost fabrication method, wet etching, is used to define light inputs and outputs as well as create the required recesses in the interposer to embed the chips into the silicon wafer at the same time. Impedance matched traces, for the high speed signals of the CMOS and opto–electronic ICs, are designed using advanced design system software and transferred onto the interposer by photolithography and electro-plating, which are accomplished on the deeply etched topology. The whole process flow of the silicon interposer patterning is designed and demonstrated, and the challenges are discussed. After the process, the optoelectronic dies and their complimentary CMOS parts are flipped and bonded on the interposer in close proximity, and a mechanical optical interface (MOI) is mounted for light coupling. Both transmitter and receiver subassemblies provide 12 parallel optical interconnections, and are scaled down to an area measuring 6 by 8 mm. Signal integrity testing is performed on a probe station for 10 Gb/s data signal delivering clear eye patterns for all channels (in both Rx and Tx subassemblies). The performance is further characterized using bit error rate (BER) testing. Both transmitter and receiver assemblies outperform a reference SFP+, with receiver sensitivity of −10 dBm at a BER lower than 10 −12 after compensating for the MOI insertion loss. Finally, we also test the assemblies for crosstalk and demonstrate that the current design has a maximal additional penalty lower than 0.2 and 0.8 dB for transmitter and receiver, respectively.

Description: Thunderbolt interconnect technology has adopted copper and optical cables. Single cable can support 2 × 20 Gb/s data rate, which is driven by 4K video. Future 8K video and virtual reality will push the bandwidth requirement even higher. Key technologies developed to enable this high data rate for consumer electronics are discussed, such as robust copper and optical cables, miniature optical engine, and 2 × 25.625 Gb/s low power integrated circuits for vertical cavity surface emitting laser based optical link. Copper and optical interconnect technologies are compared on the basis of cost, power, form factor, and scalability. Same circuit with four channels (4 × 25.625 Gb/s) can be used in 100G data center optical interconnect. Total power consumption is 146 mW for each 25.625 Gb/s optical link, which gives 5.69 mW/Gb/s. Among the commercially available optical ICs we evaluated at 25 Gb/s, this work has lowest power consumption and smallest die area in industry.

Description: The increased communication bandwidth demands of high performance computing (HPC) systems calling at the same time for reduced latency and increased power efficiency have designated optical interconnects as the key technology in order to achieve the target of exascale performance. In this realm, technology advances have to be accompanied by the development of corresponding design and simulation tools that support end-to-end system modeling in order to evaluate the performance benefits offered by optical components at system scale. In this paper, we present recent advances on electro-optical printed circuit boards (EOPCB) technology development pursued within the European FP7 PhoxTroT research program and directed toward system-scale performance benefits in real HPC workload applications. We report on high-density and multilayered EOPCBs together with all necessary building blocks for enabling true optical blade technology, including multimode polymer-based single- and dual-layer EOPCBs, a board-compatible optically interfaced router chip, and passive board-level connectors. We also demonstrate a complete optical blade design and evaluation software simulation framework called OptoHPC that tailors optical blade technology development toward optimized performance at HPC system scale, allowing for its validation with synthetic workload benchmark traffic profiles and for reliable comparison with existing HPC platforms. The OptoHPC simulator is finally utilized for evaluating and comparing a 384-node HPC system relying on optically enabled blades with the state-of-the-art Cray XK7 HPC network when performing with a range of synthetic workload traffic profiles, revealing the significant throughput and delay improvements that can be released through application-oriented optical blade technology.

Description: Optical interconnect is a potential solution to attain the large bandwidth on-chip communications needed in high-performance computers in a low-power and low-cost manner. Mode-division multiplexing (MDM) is an emerging technology that scales the capacity of a single wavelength carrier by the number of modes in a multimode waveguide, and is attractive as a cost-effective means for high bandwidth density on-chip communications. Advanced modulation formats with high spectral efficiency in MDM networks can further improve the data rates of the optical link. Here, we demonstrate an intra-chip MDM communications link employing advanced modulation formats with two waveguide modes. We demonstrate a compact single wavelength carrier link that is expected to support 2 × 100 Gb/s mode multiplexed capacity. The network comprised integrated microring modulators at the transmitter, mode multiplexers, multimode waveguide interconnect, mode demultiplexers, and integrated germanium on silicon photodetectors. Each of the mode channels achieves 100 Gb/s line rate with 84 Gb/s net payload data rate at 7% overhead for hard-decision forward error correction (HD-FEC) in the OFDM/16-QAM signal transmission.

Description: We demonstrate a four-port optical switch for the fat-tree photonic network-on-chip architecture, which contains four silicon Mach–Zehnder optical switch elements tuned by the plasma dispersion effect. The optical signal-to-noise ratio of the device is over 10.0 dB in the wavelength range from 1525 to 1565 nm. The routing functionality of the device is verified and 50- × 32-Gb/s wavelength-division-multiplexing data transmission for one optical link is demonstrated. The rise and fall times (10%–90% and 90%–10%) of the optical switch element are both ∼2 ns. The average power consumption of the device is ∼56.8 mW.

Description: The functionality of novel parallel and series high-speed vertical-cavity surface-emitting laser (VCSEL) arrays, which can greatly relax the tradeoff between output power and modulation speed, is demonstrated. Both types of array structure allow improvement in the output power with no degradation in their maximum modulation speed as compared to a single reference unit. The observed invariant electrical–optical bandwidth shown for these array structures is mainly due to the effective reduction of the differential resistance and parasitic capacitance, arising from the Zn-diffusion and oxide-relief apertures fabricated in the VCSEL unit. This in turn minimizes the degradation in the RC -limited bandwidth with the VCSEL arrays. Furthermore, the dense packing of single VCSELs, with Zn-diffusion apertures for optical mode control, minimizes the coupling loss between the output from the array into a standard multimode fiber (MMF). Compared with the single VCSEL unit, the parallel VCSEL array shows a significant enhancement in transmission performance over a standard OM4 MMF, which includes a larger eye margin, a higher signal-to-noise ratio, as well as a higher error-free data rate (48 versus 44 Gbit/sec). The device modeling technique is used to perform device analysis. From the results we can conclude that the improvement with the parallel array is because the value of the internal impedance is closer to the 50-Ω signal source, which minimizes the microwave reflection induced timing jitter in the eye-patterns.

Description: The goal of this study is to establish a phase comparison method using high-frequency microwave photonic signals. The signal is generated as a beat frequency of a dual-wavelength optical signal. We propose a double differential phase comparison method suitable for a phase-locked loop. This is achieved by using an optical frequency shifter and two low-frequency photomixers (optical-to-electrical signal converters). The basic approach of the method is that the phase difference between the high-frequency dual-wavelength signals can be measured as a differential phase of the detected low-frequency microwave signals by two respective low-frequency photomixers without high-frequency photomixers. With this approach, the phase comparison of any range of frequencies of the dual-wavelength signals can be performed by two identical low-frequency photomixers. In an application of this method for separate fiber-coupled stations, a common laser signal transmission is effectively carried out with a dual-wavelength optical signal. We conducted an experiment to verify the effectiveness of the double differential phase comparison method by comparing the direct phase comparison method.

Description: Quadrichromatic light-emitting diode (QLED) cluster is a four-color solid-state apparatus suitable for simultaneous illumination and communications. Unlike traditional red/green/blue (RGB) LEDs, its extra color provides not only one new wavelength-division multiplexing data channel but also better color quality in illumination. Taking full consideration of the high quality of color rendering index (CRI) requirement with tunable color temperature (CT), this paper investigates the constellation design of color shift keying (CSK) to maximize the minimum pairwise Euclidean distance (MED) for communication performance optimization. Beyond existing works, maintaining a high-level CRI with a specified CT complicates our design optimization problem. We propose to transform the CRI requirement into a set of linear constraints on one of the LED source composition while jointly incorporating the CT constraints. Both simulation results and prototype CSK communication testbed measurements based on commercial multicolor LEDs (LUMILEDS Luxeon C) illustrate that, under the same luminous flux and CT conditions, our proposed flux independent CSK constellation for QLEDs can significantly enhance the MED, bit error rate, and illumination color qualities.

Description: Automotive optical gigabit Ethernet is a technology that is expected to realize autonomous driving cars with notable benefits: electromagnetic interference free, lighter weight, and higher data communication. Plastic optical fibers have been used in automotive optical networks since 1998. To secure a higher data rate, two issues must be considered: the need for modal power distribution (MPD) control and the lack of link margin. Here, we performed a combined study of MPD and the frequency characteristics of a 15-m optical fiber cable with multiple connections. The results reveal that the links have transmission directivity and the number of connections influences the bandwidth with different MPDs, as shown in encircled angular flux (EAF) profiles. We also show that automotive optical connections typically contain an air gap that generates higher mode radiation. To minimize this radiation, we filled the gap with a cured gel. We found that the EAF profile shifts from 13.8° to 15.4° at 50% EAF with a 1.75-dB insertion loss improvement based on the shift at z/a = 3.1 (where z indicates the axial misalignment at the connection, and a is the core radius). The approach presented here is an effective solution for facilitating the rapid realization of automotive optical gigabit communication.

Description: As a counterpart of analog radio-over-fiber technology, digital radio-over-fiber (D-RoF) system, such as common public radio interface (CPRI), is a matured and robust solution to support RF signal delivery in traditional mobile fronthaul networks. In view of recent progresses in delta-sigma modulation, data compression, and advanced error correcting coding, the efficiency of D-RoF is significantly improved, which motivates researchers to re-evaluate the role of D-RoF in future mobile fronthaul networks to support 5G and beyond wireless communications. In this paper, we demonstrate two critical technologies to improve the transmission efficiency and flexibility of D-RoF systems. A fast-statistical-estimation based data compression algorithm is proposed to reduce the number of quantization digits in a D-RoF-based mobile fronthaul with low complexity and high quality. Combined with resampling and advanced modulation formats, data-transmission efficiency of a 25-Gbit/s D-RoF testbed is improved by around five times compared with uncompressed CPRI systems. On the other hand, we also experimentally demonstrate a point-to-multi-point (PTMP) D-RoF system with multiband modulation, which exhibits higher flexibility and better compatibility with multiple services and different radio-access technologies compared to existing schemes based on time interleaving. An experiment of 13.3-Gbit/s 4-band PTMP bidirectional D-RoF MFH is demonstrated. Combined with data compression, error free delivery of 6.4-Gbit/s 1024-QAM 5G-New-Radio-like signals is realized.

Description: Orthogonal frequency division multiplexing (OFDM) is attracting increasing attention in optical communication systems, thanks to its inherent benefits such as high spectral efficiency and resistance to frequency-selective channels. In this paper, a novel energy and spectrally efficient scheme called asymmetrically clipped absolute value optical OFDM (AAO-OFDM) is proposed for intensity-modulated direct-detection systems. In AAO-OFDM, absolute value optical OFDM (AVO-OFDM) signals on the even subcarriers and asymmetrically clipped optical OFDM (ACO-OFDM) signals on the odd subcarriers are combined for simultaneous transmission, which employs all the subcarriers requiring no dc biases. For AVO-OFDM scheme, the frequency symbols are first modulated on the even subcarriers, which are then fed into an inverse fast Fourier transform block. Afterward, the absolute values of the bipolar time-domain signals are taken to guarantee non-negativity, while their signs are mapped to the complex-valued symbols and modulated on the odd subcarriers. Since there remain unused odd subcarriers, other useful symbols can be modulated on them, which leads to the conventional ACO-OFDM scheme. At the receiver, the ACO-OFDM symbols on the odd subcarriers are demodulated first, which are reconstructed and removed from the received signals. Afterward, the remaining signals are utilized to detect the AVO-OFDM symbols with the aid of the demodulated sign symbols on the odd subcarriers. Theoretical analysis and simulation results show that AAO-OFDM has lower peak-to-average power ratio than other optical OFDM schemes, which makes it less sensitive to the nonlinearity of the optical devices. Furthermore, it achieves better bit error rate performance compared to its counterparts for the same spectral efficiency.

Description: In this paper, a modified derivative-free surrogate-based trust region optimization algorithm is proposed for optimizing the dispersion properties of photonic crystal fibers (PCFs) for the first time to the best of our knowledge. The modal analysis of the PCF is made using the full vectorial finite difference method with perfect matched layer boundary condition. The numerical results are compared with another trust region algorithm and other metaheuristic techniques to show the strength of the reported technique. Further, the modified algorithm is also used to achieve a nearly ultra-flattened zero dispersion over a wide range of wavelengths from 1.45 $\mu {\rm{m}}$ to 1.6 $\mu {\rm{m}}$ using an index guiding soft glass PCF selectively infiltrated with a nematic liquid crystal. In order to show the strength of the reported algorithm, a highly negative flat dispersion compensation PCF is also designed over wavelength range from 1.4 $\mu {\rm{m}}$ to 1.6 $\mu {\rm{m}}$ . Such a design has a negative dispersion of −155 ± 0.5 ps/Km·nm over the studied wavelength range. The trust region algorithms show a strong potential as an efficient tool for the design and optimization of different photonic devices. Further, these algorithms can be used as powerful techniques for solving the inverse problems.

Description: In this paper, we report the performance of an imaging multiple-input multiple-output (MIMO) visible light communication (VLC) system. The VLC transmitter consists of a two-dimensional (2-D), individually addressable Gallium Nitride micro light-emitting diode ( μ LED) array. The receiver uses a 2-D avalanche photodiode array fabricated using complementary metal oxide semiconductor (CMOS). Using integrated CMOS-based LED drivers, a data rate greater than 1 Gb/s was obtained at a link distance of 1 m with the system field of view of 3.45° using four channels. At a reduced link distance of 0.5 m, a data rate of 7.48 Gb/s was obtained using a nine channel MIMO system. This demonstrates the feasibility of compact MIMO systems that offer substantial data rates.

Description: A complete model describing both single- and dual-loop optoelectronic oscillators (OEO) is introduced. It is compared to several experimental configurations, with excellent agreement in all cases. The physical insight into noise coupling mechanisms brought by the model further allows us for the design of ultralow noise OEO. Phase noise performances at 10 GHz with a single 1 km delay line and with a dual 1 km/100 m delay lines are reported. An optimized dual loop configuration exhibits low phase noise floor at high offset frequency (–160 dBc/Hz at 100 kHz) and low spur levels (–145 dBc/Hz), here again in close agreement with our model.

Description: A single passband microwave photonic filter (MPF) based on stimulated Brillouin scattering (SBS) is analyzed and experimentally demonstrated. The proposed MPF can be tuned over a dramatically large frequency range, which overcomes the disadvantage in previously reported schemes based on SBS that the frequency tuning range is limited within two folds of the Brillouin frequency shift. The single passband MPF is obtained by enhancing the amplitude of the microwave passband generated by SBS gain while suppressing the amplitude of the microwave passband generated by SBS loss through optimizing the key parameters of the SBS process, including the pump power, the length of high nonlinear fiber, and the polarization states of the pump and signal waves. A theoretical model is established to describe the operation principle of the SBS-based MPF and illustrate the mechanism for the single passband, and an experiment is carried out to verify the theoretical analysis. In the experiment, the central frequency of the single passband MPF can be tuned from 0 to 40 GHz, which is only limited by the bandwidth of the adopted electro-optic modulator and photodetector. The main to secondary sidelobe ratio can reach 55 dB and the full width at half-maximum bandwidth is 16 MHz. The achieved MPF is specifically suitable for applications in ultrahigh selective filtering.

Description: Potential of directly modulated vertical cavity surface emitting lasers (VCSELs), widely employed for low-cost and energy-efficient intradata center transmissions, is here studied for transmissions up to access/metro scenarios. In particular, the manuscript refers to long-wavelength VCSELs for intensity modulation/direct detection (IM/DD) on–off keying transmission over standard single-mode fiber (SSMF). The intent of this paper primarily consists of demonstrating information rate and fiber reach enhancement in potentially uncooled environments or with relaxed thermal management requirements through the use of time–frequency packing (TFP) technique in a VCSEL-based transmission. TFP has been applied here to IM/DD transmission for the first time, although not in a multiple channel system. The digital signal processing unit implementation is clearly simplified with respect to coherent systems (no frequency synchronization and no phase error recovery), where TFP was firstly introduced, as it only takes into account the photo-detected signal intensity, regardless of the phase information. On the other hand, the signal processing is here more complex than in a traditional IM/DD system. Through the use of a commercial C-band 4 G VCSEL, net information rate up to 12.5 and 11.2 Gb/s has been measured for 25 and 45 km of SSMF, respectively, in an unamplified optical link and with power budget equal to 19 and 16 dB, respectively. The transmission is intrinsically polarization independent and wavelength independent and tolerance to temperature increase up to 60 °C has been analyzed together with energy consumption.

Description: In order to enhance electro–optical system-in-package capabilities for silicon photonics, a cost effective fabrication process for optical waveguides integration on thin glass substrate interposer is demonstrated. First, a femtosecond laser ablation coupled with a hydrofluoric acid etching is developed to create microgrooves at the glass surface. Second, a dry film lamination followed by chemical mechanical planarization is achieved to define surface optical waveguides by filling the microchannels. Physical characterizations of the fabricated waveguides are performed using optical and scanning electron microscopy. Finally, optical mode profile and loss characterizations confirm the optical functionality of waveguides which prove to be multimodal at 1550 nm.

Description: A novel technique to enlarge the dynamic range of strain measurement for slope-assisted Brillouin optical time domain analysis (SA-BOTDA) is proposed by introducing a new parameter called Brillouin phase-gain ratio, which combines Brillouin phase shift and Brillouin gain. With the new technique, the dynamic range of strain measurement can be enlarged, and the pump-power-dependency problem mitigated. In the experiment, a 100-MHz frequency span of linear slope is demonstrated with a 25-ns pump pulse, which is 3.3 times of that in conventional SA-BOTDA, and a dynamic strain with a large amplitude of about 1000 $\mu \varepsilon$ is successfully measured.

Description: We present a new optimization-based approach for synthesizing photonic infinite impulse response filters formed using microresonators. Taking into account the finite internal losses in these resonators, we cast the synthesis of filter coefficients as a semidefinite programming problem designed to restrict root-magnitudes set by the internal loss, while meeting filter-mask specifications. We provide a discussion on the formulation of the algorithm, and show how it can be used to design the optical components in a second-order unit cell. In addition, we provide an example of forming a fourth-order low-pass filters by cascading two unit cells. Our simulations show that fourth-order low-pass filters can achieve band-rejection ratios of 60 dB with integrated microresonators that possess an intrinsic quality-factor Q 0 of ${\text{1.1}}\times{\text{10}}^5$ .

Description: An integrated high-order phase-shifted Bragg grating, comprising six quarter-wave sections between Bragg grating mirrors in a laterally-corrugated strip waveguide has been realized in silicon-on-insulator technology. A box-like transmission window is created within the 10-nm-wide grating reflection band, realizing a sharp bandpass optical filter with out-of-band rejection exceeding 40 dB and a steep roll-off of ∼300 dB/nm in the transition band. The sharp optical filter has been experimentally tested in microwave photonics (MWP) signal processing applications, namely spectral separation of an optical sideband comprising 1.25 Gb/s data from a 15-GHz-spaced carrier, and sideband suppression for dispersion compensation in a radio-over-fiber link. The results of the characterizations indicate negligible power penalty in terms of bit-error rate for the sideband separation and robust mitigation of dispersion-induced transmission impairment. The device has an ultrasmall footprint of ∼450 × 0.5 μ m 2 , and can be monolithically integrated with germanium photodiodes or silicon modulators as well as other passive subsystems to implement advanced on-chip MWP signal processing functionalities.

Description: Extensive explorations are undertaken of the feasibility of utilizing self-phase modulation (SPM) to mitigate the channel fading effect associated with digital-orthogonal-filtering-enabled software-reconfigurable intensity-modulation direct-detection passive optical networks (PONs). A comprehensive theoretical model is developed and subsequently verified, based on which numerical simulations are undertaken to investigate the effectiveness of the effects of both SPM and adaptive channel power loading in maximizing the signal transmission capacity of each individual channel in the aforementioned PON systems. It is shown that, for the channel experiencing the worst channel fading effect, the SPM effect can enhance its transmission capacity by a factor as large as 2, and a further 45% transmission capacity improvement is also obtainable when adaptive channel power loading is applied. The signal transmission capacity improvement enabled by the coexistence of these two effects increases almost linearly with transmission distance. The research work not only provides a new means for dynamically manipulating the signal transmission capacity of each individual channel, but also allows the utilization of low-cost optical components without comprising the overall PON system performance.

Description: Coiling size dependence of group delay spread (GDS) in coupled multicore fibers (MCFs) without intentional twisting is investigated by using a coupled-wave theory. It is shown that the GDS does not simply decrease with increasing bending radius and the optimum coiling size to decrease GDS depends on a core-to-core distance of MCFs. It is also found that a coupled MCF with a trench-assisted profile is suitable for realizing lower GDS with higher spatial density compared with a coupled MCF with a step-index profile.

Description: Unlike ultralong coherent optical systems that seriously suffer from fiber nonlinearities, short-reach noncoherent systems such as data center interconnections, which utilize small, cheap, and low-bandwidth components, are sensitive to nonlinearities that are mainly produced by devices responsible for electrical signal amplification, modulation, and demodulation. One of the most promising schemes for these applications is the four-level pulse amplitude modulation format combined with intensity modulation and direct detection; however, it can be significantly degraded by linear and nonlinear intersymbol interference. Linear and nonlinear signal degradation can efficiently be handled by different types of equalizers. In many cases, the straightforward linear equalizer cannot lower the error rate at the acceptable level. Therefore, much stronger equalizers based on nonlinear models such as the Volterra series are proposed. Volterra filter that can also be orthogonalized by the Wiener model is well described in the existing literature, and, in this paper, we investigate the most critical points related to high-speed Volterra filter design and implementation. Several experiments are carried out in order to indicate filter requirements/complexity, acquisition, and stability. We also provide a simple guidance for filter complexity reduction and useful hints for channel acquisition.

Description: In super-Nyquist wavelength division multiplexed (WDM) systems with frequency spacing smaller than the signal baudrate, the maximum-likelihood (ML) decoder in the receiver is usually introduced to compensate for intersymbol interference due to tight spectral filtering, such as polybinary shaping. After the ML decoder, symbol errors tend to propagate, causing excess continuous errors. Considering that forward error correction (FEC) is commonly introduced, the excess continuous errors degrade bit-error ratio (BER) performance after FEC, so-called post-FEC BER. In order to suppress the performance degradation, we introduce iterative decoding between the first ML decoder for polybinary shaping and the second FEC decoder in the receiver. First, we calculate BER characteristics of polybinary-shaped super-Nyquist WDM quadrature phase-shift keying (QPSK) signals. The results show that iterative decoding is effective for improving post-FEC BER performance. A lager pre-FEC BER threshold for post-FEC BER 〈 10 −5 is obtained in super-Nyquist WDM case than in the Nyquist WDM case, although a higher signal-to-noise ratio (SNR) is required. Next, we measure the BER characteristics of three-channel duobinary-shaped super-Nyquist WDM 12.5-Gbaud dual-polarization QPSK signals. The iterative decoding reduces the optical SNR penalty by 0.8 dB. A larger pre-FEC BER threshold of 3.1 × 10 −2 is obtained in the duobinary-shaped super-Nyquist WDM case, compared with the threshold of 2.2 × 10 −2 in the Nyquist WDM case.

Description: A time-varying phase shift associated with each phase matching point of homogeneous weakly coupled multicore fibers is proposed to describe the stochastic time evolution of the intercore crosstalk (ICXT) observed experimentally. This model for the stochastic ICXT time evolution represents a generalization of current ICXT theoretical models and potentiates the development of new time-adaptive ICXT mitigation techniques. The model is first proposed considering a single polarization scheme, and is then generalized to a dual polarization scheme and to account for the polarization coupling between cores. Comparison between spectrograms of the crosstalk transfer function (XTTF) amplitude evaluated using the single polarization model and measured experimentally shows excellent agreement for short (few minutes) decorrelation times. For large decorrelation times (above 1 h), some differences in the behavior of the time evolution of the XTTF amplitude are observed. Nevertheless, excellent match between the mean and variance of the XTTF amplitude evaluated from the model and from analytical expressions proposed in the literature is observed for short and large decorrelation times. It is also shown that the decorrelation time of the short-term average crosstalk remains unchanged when the dual polarization scheme is considered. Similar spectrograms of the XTTF obtained with the single and dual polarization ICXT models are also shown.