ALBERT

Description: In this study, we consider a relay-assisted free-space optical communication scheme over strong atmospheric turbulence channels with misalignment-induced pointing errors. The links from the source to the destination are assumed to be all-optical links. Assuming a variable gain relay with amplify-and-forward protocol, the electrical signal at the source is forwarded to the destination with the help of this relay through all-optical links. More specifically, we first present a cumulative density function (CDF) analysis for the end-to-end signal-to-noise ratio. Based on this CDF, the outage probability, bit-error rate, and average capacity of our proposed system are derived. Results show that the system diversity order is related to the minimum value of the channel parameters.

Description: A flexible tube-lattice fiber is introduced into a terahertz time-domain spectroscopy system as a probe in an endoscope geometry and used to guide radiation to and from a sample. This allows hyperspectral imaging to be performed by scanning the endface of the probe, rather than the sample or components of the terahertz system itself. Hyperspectral imaging is demonstrated through the identification of various substances in the image and visualized through the implementation of a false-color system. The resolution of such an imaging system is discussed and quantified, finding excellent agreement with the experimental measurements.

Description: In this paper, we describe a novel update algorithm for the filter coefficients of the adaptive digital equalizer in coherent receivers, which is based on error signals evaluated in Stokes space and is insensitive to both phase-noise and frequency-offset. We also introduce an optimized decision rule in the Stokes space, which takes into account the exact statistics of noise and yields a performance improvement with respect to the minimum distance decision criterion. We compare the performance of the new algorithm to the standard constant-modulus algorithm (CMA) for polarization-multiplexed 16QAM modulation, achieving similar performance in the absence of phase noise, with comparable complexity. Differently from CMA, the proposed Stokes-space algorithm allows us to remove the phase offset between polarizations, thus enabling the use of a joint carrier-phase estimation algorithm on both polarizations, which in turns yields a nearly doubled phase noise tolerance.

Description: Given the increased interest in aerial communications, free-space optics (FSO) has gained considerable attention for its ability to deliver wider bandwidth, license-free, and secure communication. FSO was initially developed for fixed platforms. As such, the current push to introduce mobility is one of the greatest challenges for FSO developers. This paper presents the first open-loop alignment/stability analysis of hovering multirotors proven to maintain an FSO link despite inherent instability. Communication distance, wavelength, and platform deviation are among the many parameters evaluated in our model. We characterize fiber-bundle transceivers, as an example of optical arrays and their applicability to aerial FSO communication. Our simulation is based on static lab measurements combined with theoretical analysis and optical geometrical intersection models. Analyses indicated that rotational deviation has a much higher impact on performance than translational deviation. Current commercial multirotor platforms proved adequate for future FSO communication, with 16% to 30% expected throughput when employing suitable optical arrays. We conducted a parametric sweep test to determine optimal and marginal receiver parameters based on platform characteristics and performance metrics. This analysis was conducted and reported as one example of the many applications that our developed mathematical models support.

Description: In this paper, a packaged integrated coherent receiver and optical front-end for soft decision based on 25 Gbaud/s quadrature phase-shift keying (QPSK) is investigated for on-chip applications. The front-end consists of a 90° hybrid, balanced photodetectors, and unbalanced couplers monolithically integrated in a CMOS compatible silicon-on-insulator technology. The silicon photonic device is packaged onto a ceramic substrate with RF and dc connectors. The proposed front-end is an example of integration of system on chip. The integrated solution is able to provide 6.2 dB performance improvements over uncoded system without error correction. The front-end outperforms optical system with hard-decision forward error correction by 2.2 dB at the BER of 10 −7 .

Description: A high transmission efficiency multichannel wavelength division multiplexer (WDM) based on metal–insulator–metal (MIM) plasmonic waveguides with ring resonators is proposed and investigated numerically. The two-dimensional finite-difference time-domain method was used to simulate the proposed structure. The transmission efficiency of each channel is higher than 86.74%. The simulation results show that the proposed MIM waveguide with ring resonators could really function as a WDM. It would be a potential key component in the applications of the all-optical signal procession and communication systems.

Description: We numerically and experimentally investigate an on-chip solution to reduce the thermal crosstalk in indium phosphide-based photonic integrated circuits. We introduce deep trenches, fabricated through wet etch, between active and passive components. The current injected in active components and the geometry of the trenches are the parameters considered in our analysis. The trenches thermally isolate the passive components from the heat generated by active components. The thermal crosstalk is quantified by measuring the effects on the electro-optical response of an MZ modulator considered as a test structure. The heat sources are represented by semiconductor optical amplifiers placed at different distances with respect to the position of the MZ. Our experiments show how both the geometry and the position of the trenches, play a role in the reduction of the thermal crosstalk.

Description: Two-dimension hexagonal photonic crystals (PhCs) with ordered and partially disordered cavities arrays, such as missing holes, were studied using numerical modeling by the 3-D finite-difference time domain method. First, we examined an obvious case of PhCs with the ordered array of 19 coupled cavities which produce a highly directive and 19 times enhanced emission, while the single peak radiation mode and the peak wavelength width remain about the same as those for the PhC with a single cavity. The modified PhC with a partially disordered cavities array is found to emit a three times broader peak than that of the single-cavity PhC, saving the integral peak intensity and highly directive emission. These are the results of the constructive cavities-coupling radiation interference. The found peak-broadening effect may lead to efficient broadband Si-based light emitters fabrication. This is interesting for numerous sensing applications, including fiber Bragg grating interrogation.

Description: The electrically tunable liquid crystal (LC) optical channel waveguide with strong polarization-dependent mode tunability is demonstrated for the first time. When the applied voltage increases, the LC molecules encapsulated in the semicircular groove on the fused silica substrate gradually tilt upward and twist toward two sides. For TE polarization, the applied voltage slightly decreases the effective index and has a small effect on the field distribution of the guided mode. As to TM polarization, electrical tuning greatly enhances the effective index as larger as $2.234,times,10^{-1}$ and causes an obvious variation of the mode size. The TM polarization has the lowest propagation loss 0.94 dB/cm up to date. It is attributed to high-optical quality substrate and smooth groove surface produced by chemical etching with the composite mask. The LC reorientation distribution and the full-vectorial guided mode characteristics at various voltages are calculated to explain the experimental results. The proposed LC-core channel waveguide owns the unique feature of strong polarization-dependent mode tunability and can be used as promising devices, such as ultrashort phase modulator, pulse-delayed device, and polarization controlling device.

Description: We present a novel adaptive stratified-sampling (SS) algorithm to efficiently assess the polarization-dependent loss (PDL) induced outage probability (OP) in presence of fiber nonlinearity and polarization mode dispersion. We show that, at standard OP values, the SS algorithm more than quadruples the efficiency in simulation time with respect to the standard Monte Carlo method at equal accuracy. We applied the SS algorithm to polarization-division multiplexing quadrature phase-shift keying transmissions in both dispersion-managed and dispersion-unmanaged links, showing a non-negligible impact of PDL using typical ITU-T recommendations. Finally, we show that estimating the OP by neglecting the interaction of nonlinearity and PDL can be largely optimistic.

Description: This paper addresses the problem of spectrum-aware survivable strategies with failure probability constraints under static traffic in flexible bandwidth optical networks. The joint failure probability between primary and backup paths must be below the maximum tolerable joint failure probability for each traffic demand. We develop integer linear program (ILP) models for dedicated-path protection and shared-path protection in order to minimize the total number of frequency slots consumed, and we also propose a spectrum-aware dedicated protection (SADP) algorithm and a spectrum-aware shared protection (SASP) algorithm. Simulation results show that the ILP model solutions consume minimum number of frequency slots, but lead to higher average joint failure probability compared to the SADP and SASP algorithms. Moreover, both the SADP and SASP algorithms achieve a better performance in terms of total number of frequency slots consumed as compared to a conventional dedicated-path protection algorithm and a conventional shared-path protection algorithm, respectively, but lead to higher average joint failure probability.

Description: We propose and demonstrate novel temporal zone plate designs offering a higher energy efficiency and an improved background noise performance than previously proposed approaches using similar experimental configurations. Two kinds of novel temporal zone plates, namely, temporal amplitude zone plates and temporal phase Fresnel lenses, are introduced and studied. The proposed designs are demonstrated for optical linear pulse compression. In the reported experiments, temporal amplitude zone plates are shown to provide a 96% improvement in energy efficiency (or light-collecting efficiency) and significantly reduce noise background as compared with equivalent temporal intensity zone plates. On the other hand, a temporal phase Fresnel lens is shown to offer an ideal light-collecting efficiency of 100%, greatly exceeding the efficiency of an equivalent temporal phase zone plate. Performance in a practical setup is however limited by the achievable modulation bandwidth and a 37% efficiency improvement over previous designs is experimentally demonstrated.

Description: Coherent detection allows for a more effective compensation of transmission impairments in the electrical domain. However, in order to be effective, a detection strategy should be based on an accurate channel model capable of providing sufficiently accurate signal statistics. While in the linear regime such a model is available and linear impairments such as chromatic dispersion and polarization-mode dispersion can be almost fully compensated by adaptive equalizers, this is not the case for nonlinear impairments, whose mitigation is essentially based on heuristic strategies. One of the most considered strategies is the back-propagation (BP) technique, based on channel inversion. It is shown that BP is most effective only in dispersion-unmanaged links, while a low-complexity Viterbi detector with proper metrics can achieve better results in the case of dispersion-managed links. It is also shown that, in the cases where it is effective, BP is far from approaching optimal performance. Indeed, proper processing after BP can significantly increase performance.

Description: In this paper, a novel sensing technique has been designed and investigated for the direct, in situ detection of steel corrosion distributed in reinforced concrete structures. At present, structural health monitoring in reinforced concrete structures is generally focused on monitoring the corrosion risk of the reinforcing steel. It is of significant importance, however, to inform industry of both the onset of corrosion and the corrosion rate as these are key contributors to structural degradation and thus evaluating the service life of the structures. This paper aims to address the above challenges by describing a novel corrosion sensor design using birefringent photonic crystal fibres. The technique exploits fully both the birefringence of the fibres for force/pressure measurement and their very low temperature sensitivity to detect the onset of corrosion. This new type of sensor not only determines the onset of corrosion but also allows for better monitoring along the length of a reinforcement bar.

Description: Optical network-on-chip (ONoC) is a promising alternative to be served as the fundamental architecture for future many-core system. However, several problems of ONoC, such as power consumption, arbitration overhead, and device cost, pose many limitations to the architecture design. In this paper, a novel hierarchical ONoC structure named CWNoC is proposed, which is a 256-core architecture composed of multiple central-controlled subnets. It reduces the network complexity by dividing the whole network into several subnets and lowers the arbitration overhead by adopting centralized arbitration logic in each subnet. An efficient wavelength assignment method, making full use of broadband mcroring resonators, is also employed in CWNoC, which facilitates simplifying the optical layer and reducing the possibility of contention. The simulation results show that CWNoC has a better latency and power consumption performance. For example, when low and medium load is applied, the latency reduction can be as much as 40 ns compared with WANoC, while the total power consumption is reduced by 70%.

Description: We present a comprehensive design, fabrication, and characterization analysis of compact silicon-on-insulator bandpass filters with widely tunable bandwidth. The filter architecture is based on an unbalanced Mach–Zehnder interferometer loaded with a pair of ring resonators. A wide bandwidth tunability (from 10% to 90% FSR) can be achieved by controlling the resonant frequency of the rings while preserving a good filter off-band rejection. Design rules are provided that take into account fabrication tolerances as well as losses. Furthermore, the use of tunable couplers allows a more flexible shaping of the spectral response of the filter. The sensitivity with respect to nonlinear effects is carefully investigated. Operation over a wavelength spectrum of $hbox{20} {rm n}{rm m}$ is demonstrated, making the device suitable for channel subset selection in WDM systems, reconfigurable filters for gridless networking and adaptive filtering of signals.

Description: We present the first demonstration of combined wavelength-division multiplexed (WDM) and mode-division multiplexed (MDM) optical transmission in a hollow-core photonic bandgap fiber (HC-PBGF). For this purpose a novel low loss, broadband 310 m long HC-PBGF with a 37 cell (37c) core geometry is used. The modal properties of the HC-PBGF are characterized in detail, showing an absence of surface modes and low modal crosstalk, which enable WDM and MDM transmission with record high capacity (73.7 Tb/s) for a HC-PBGF. Several modulation formats have been tested, showing very good and stable performance. The transmission properties are assessed by looking into both single-mode transmission and MDM transmission, showing good agreement with the modal characterization of the 37c HC-PGBF.

Description: The performance of pixelated multiple-input mul-tiple-output optical wireless communication systems can be impaired by vignetting, which is the gradual fall-off in illumination at the edges of a received image. This paper investigates the effect of vignetting for a pixelated system using spatial orthogonal frequency division multiplexing (OFDM). Our analysis shows that vignetting causes attenuation and intercarrier interference (ICI) in the spatial frequency domain. MATLAB simulations indicate that for a given constellation size, spatial asymmetrically clipped optical OFDM (SACO-OFDM) is more robust to vignetting than spatial dc biased optical OFDM (SDCO-OFDM). Moreover, for the case of SDCO-OFDM, the very large zeroth subcarrier causes severe ICI in its neighbourhood causing flattening of the bit error rate (BER) curves. We show that this BER floor can be eliminated by leaving some of the lower spatial frequency subcarriers unused. The BER performance can also be improved by applying a vignetting estimation and equalization scheme. Finally, it is shown that equalized SACO-OFDM with 16-QAM has the same overall data rate as equalized SDCO-OFDM using 4-QAM, but requires less optical power.

Description: We present a simple and accurate method for obtaining modal characteristics of highly lossy waveguides. The method requires locating the positions of Lorentzian peaks for both real and imaginary parts of the propagation constants. As compared to the method calculating half width at half maximum (HWHM) of Lorentzian peaks, the present method gives much more accurate results and works even in the cases where the former fails. The method is simple to implement and converges very fast. The applicability of the method is demonstrated by taking examples of surface plasmon resonance based planar waveguide structures.

Description: We describe a model and present simulation results for the optimization of an extended amplification bandwidth hybrid Raman-Optical Parametric amplifier (HROPA) in Tandem configuration. In this configuration, the Raman and Parametric processes are separated and each one takes place in a separate span of fiber, allowing for optimization of amplification gain (e.g., > 20 dB), gain bandwidth (e.g., 170 nm) and gain ripple (e.g., 〈 4 dB). We also focus on the potential signal degradation performance due to the generation of idlers within the operational bandwidth. To overcome this limitation, we propose and model a modified HROPA design, which allows for the management and the suppression of idlers in the amplifier. The idler suppression is achieved through partitioning the channels in two sub-bands and using a wavelength-division multiplexed DEMUM/MUX pair to limit/suppress the idlers, as well as crosstalk terms generated in the amplifier. Our study shows that with proper selection of pump wavelength and power and engineering the HROPA with correct filter transfer function, we can achieve error free performance even in the case when significant misalignment of the transmitter wavelength from the center of the coarse WDM channel band exists.

Description: We report a carrier-depletion silicon Mach–Zehnder optical modulator with large optical bandwidth by adopting two symmetric arms. The fiber-to-fiber loss of the device is about 7.2–8 dB in the wavelength range from 1525 to 1565 nm. We have used the truncation method to accurately measure the loss of the optical splitter and combiner, and the on-chip loss is about 3.8 dB. The dynamic extinction ratios at the speed of 40 Gb/s are 4.9–6.4 dB in the wavelength range from 1529 to 1565 nm. By analyzing the dependence of the optical bandwidth on the optical path difference between the two arms, we find that there is an unexpected optical path difference of around 3.3 μm between the two arms, which is considered to originate from the nonuniform morphologies of the waveguide and the nonuniform doping profiles along the two arms and is responsible for the slight wavelength dependence of the static and dynamic response of the silicon Mach–Zehnder optical modulator.

Description: Organic host-guest structures with a disordered polymer as the host material have various applications in the field of organic light emitting diodes such as polymeric phosphorescent LEDs or polymeric fluorescent white LEDs. Effective mobility is a crucial parameter in the simulation of such host-guest OLEDs with conventional drift-diffusion method. In the present paper, the master equation approach is applied to calculate the effective host mobility in a host-guest structure and investigate the effect of energy disorder, guest density, charge carrier density and electric field on this parameter. By employing a parameterization scheme, an accurate model for the effective host mobility is also derived that can be employed for more precise and realistic simulation of OLEDs incorporating a host-guest layer with polymeric host.

Description: Optical/Photonic networks-on-chip (NoC) have been considered as a promising and viable paradigm to interconnect a large number of processing cores at chip level. Hybrid optoelectronic NoCs provide a more practical solution by using the electronic network for local communication while the optical network for global communication. This paper explores how to efficiently combine optical and electronic networks to build a hybrid NoC. A butterfly fat tree-based hybrid optoelectronic NoC architecture is proposed using the generic wavelength-routed optical router. Simulation results demonstrate that, compared with electronic mesh- and CMesh-based NoCs, the proposed hybrid NoC achieves the best power efficiency with comparable throughput and significantly reduces the latency under localized traffic.

Description: In engineering practice, the closed-loop optical voltage sensor (OVS) based on Pockels effect cannot reach the required precision level mainly due to the various disturbance and noise of system, so the application of OVS for low voltage measurement is restricted. Considering the cross coupling of the main and second closed-loops, the model of disturbance and noise of system that adopts the four-state modulation method is analyzed in the main closed-loop of OVS. Based on the established noise-perturbed stochastic model of OVS, we design a closed-loop detection algorithm for the OVS system to guarantee the mean-square exponential stability with a prescribed $H_infty$ performance in order to optimize the detection precision of OVS. The experimental results show that the detection precision of OVS is 0.144 V while the relative measurement error of the scale factor is within ± 0.15% in measuring low AC voltages from 140 to 500 V, which verifies the effectiveness of the proposed detection scheme.

Description: Elastic optical networks (EONs) enable network operators to have agile bandwidth management in the optical layer. In this paper, we investigate dynamic and adaptive bandwidth defragmentation (DF) in EONs with time-varying traffic using connection reconfigurations. We consider how to design DF procedure in a systematic way, and study the problems that have not been fully explored so far. Basically, we divide the procedure design into four subproblems: “How to reconfigure?,” “ How to migrate traffic?,” “When to reconfigure?,” and “What to reconfigure?,” and solve them sequentially. For “How to reconfigure?,” we investigate the combination of routing and spectrum assignment (RSA) algorithms for DF, i.e., the RSA algorithm that the connections are originally served with and the algorithm that they are re-optimized with. For “ How to migrate traffic?,” we propose to construct a dependency graph to represent the relations among the selected connections and to use it to assist the best-effort traffic migration. A move-to-vacancy method is also proposed to further reduce the traffic disruptions. For “When to reconfigure?” and “ What to reconfigure?,” we propose intelligent timing selection and adaptive DF ratio selection methods to tackle the tradeoff between the bandwidth blocking probability (BBP) performance and operational complexity. Simulation results show that the algorithm with both methods implemented (DF-AT-AR) achieves better tradeoff between BBP performance and operational complexity, when compared with existing algorithms.

Description: We present a lowpass design algorithm targeted specifically for photonic implementation. Allpass filter based systems are ideal for photonic realizations because they can be naturally realized using a variety of nanoscale dielectric components. The allpass filters based lowpass system is a particularly noteworthy example of advanced usages of allpass sub-structures. While the structure is excellent for implementation using photonic components, practical considerations about its suboptimal performance are yet to be considered. Realistic optical filters are subject to the imperfections of photonic materials and limitations from current fabrication capability. The waveguide power loss is an unavoidable source of performance degradation that arises when realizing photonic filters. In this paper, we consider the derivation of allpass filters based lowpass structures that can minimize the loss effect. The proposed algorithm will enable the utilization of low cost photonic devices for stringent design requirements, and also improve the performance of high end photonic devices.

Description: A line-based fiber optic sensor employing time delay estimation to locate time-varying disturbance is proposed. The sensor consists of two line-based interferometers, multiplexed with a broadband light source, interfering unit and sensing fiber by wavelength division multiplexer, and hence detected signals with different wavelengths generated by the same disturbance are obtained. After a unique data processing algorithm, two signals with fixed time delay are achieved and the time delay is determined by the position of the disturbance. By the signal correlation method to extract the time delay, the position of the disturbance can be measured. The performance of the sensor is examined and compared with the conventional sensor, based on the null frequency of the spectrum of the phase difference generated by the disturbance, through an experimental setup with the total length of sensing fiber up to 70 km. Experimental results show that the sensor is advantageous for easy installation, low location error and flexibility, especially in long-distance perimeter security application.

Description: We report on the design of photonic crystal nanobeam cavity fully encapsulated in silica. The proposed design, based on the principle of gentle confinement of the electromagnetic field, is mostly analytical and emphasizes on the most realistic options for fabricating nanocavities, in particular in III–V semiconductor materials. After determining the field decay inside the photonic bandgap of a nanobeam photonic crystal, we engineer the envelope of the cavity mode into a Gaussian shape by shifting only progressively the lattice constant. We discuss the various implementations of such shifts and give a simple algorithm to position each hole. The resonant wavelengths are found to depend linearly on the central lattice constant and on the radius of the holes. High Q factors above 10 $^{6}$ and modal volume V close to ( λ/n) $^{3}$ are obtained. In particular, Q factors remain high for a wide range of values of the central lattice constant and of holes radii, hence showing exceptional tunability properties as well as robustness with respect to common fabrication defects.

Description: The linearity of stimulated Brillouin scattering (SBS) is studied in detail for the first time. A closed-form expression which measures the linearity of an SBS system is derived. This expression allows one to design the region of linear operation, and the amount of Brillouin gain of an SBS system, by altering some basic system parameters. The theory presented is experimentally verified using an SBS-based microwave photonic filter structure. The effect of the length of the Brillouin medium on the linearity of the filter is investigated, and a 9.7 dB extension in the dynamic range is demonstrated by reducing the length of the fibre used as the Brillouin medium, from 25 to 0.57 km, while the filter stopband rejection level remains 30 dB.

Description: We present tube-leaky fiber with microstructured glass supports as a hollow core microstructured optical fiber. Attenuation constants of tube-leaky fibers are derived by using a ray optic method and designed the wall thickness of the fibers for Er:YAG laser which is often used for medical applications. From the loss measurements of Er:YAG laser, the tube-leaky fibers supported by microstructured glass made of borosilicate-glass can deliver laser with 0.85 dB/m despite of high material absorption around laser wavelength. The high power energy transmission results of the fabricated fibers are at the order of 80–120 mJ. These energies are enough to ablate biological tissues in surgical operations.

Description: We use a nonlinear analytic approach to predict the main oscillation mode power and spurious levels in ultrapure microwave optoelectronic oscillators (OEOs). This approach takes into account N simultaneously mode falling inside the RF filter bandwidth and calculates all the dominant InterModulation Products (IMPs) that fall in the fundamental zone. We show that nonlinear microwave photonic links exhibit the capture effect. By considering this effect, we derive analytical expressions that govern the behavior of the OEOs in the steady state. We find that when the small-signal open-loop gain is increased beyond a critical value, OEOs start a multimode operation from which the spurious levels grow rapidly. Our analytical predictions are verified by numerical simulations and experimental data.

Description: We propose and discuss the extension of software-defined networking (SDN) and OpenFlow principles to optical access/aggregation networks for dynamic flex-grid wavelength circuit creation. The first experimental demonstration of an OpenFlow1.0-based flex-grid $lambda $ -flow architecture for dynamic 150 Mb/s per-cell 4 G Orthogonal Frequency Division Multiple Access (OFDMA) mobile backhaul (MBH) overlays onto 10 Gb/s passive optical networks (PON) without optical network unit (ONU)-side optical filtering, amplification, or coherent detection, over 20 km standard single mode fiber (SSMF) with a 1:64 passive split is also detailed. The proposed approach can be attractive for monetizing optical access/aggregation networks via on-demand support for high-speed, low latency, high quality of service (QoS) applications over legacy fiber infrastructure.

Description: Providing network access for multiple users in a visible light communication (VLC) system that utilizes white light emitting diodes (LED) as sources requires new networking techniques adapted to the lighting features. In this paper we introduce two multiple access techniques using expurgated PPM (EPPM) that can be implemented using LEDs and support lighting features such as dimming. Multilevel symbols are used to provide $M$ -ary signaling for multiple users using multilevel EPPM. In the first technique, the $M$ -ary data of each user is first encoded using an optical orthogonal code assigned to the user, and the result is fed into a EPPM encoder to generate a multilevel signal. The second multiple access method uses subsets of the EPPM constellation to apply MEPPM to the data of each user. While the first approach has a larger Hamming distance between the symbols of each user, the latter can provide higher bit-rates for users in VLC systems using bandwidth-limited LEDs. Both techniques are able to support up to 15 simultaneous users transmitting 200 Mb/s with a bit error rate of $hbox{10}^{-3}$ under normal lighting conditions.

Description: The intensity and wavelength dependent thermal characteristics of a minimized Er/Yb-codoped-fiber Mach–Zehnder interferometer are demonstrated theoretically and experimentally. It can be used as a high sensitivity temperature sensor if no pump is introduced. However, once a pump is launched into the interferometer, its thermal sensitivity is reduced. Under pump power of 59.4 mW, 3.1 pm/°C and 10.3 pm/°C reductions of its thermal sensitivities are achieved for the resonance dips at short and longer wavelength ranges, respectively. This suggests that its thermal sensitivity can be tuned by introducing into a pump, and its thermal stability can be improved with the pump.

Description: Visible light communications (VLC) in indoor environments suffer from the limited bandwidth of LEDs as well as from the inter-symbol interference (ISI) imposed by multipath. In this work, transmission schemes to improve the performance of indoor optical wireless communication (OWC) systems are introduced. Expurgated pulse-position modulation (EPPM) is proposed for this application since it can provide a wide range of peak to average power ratios (PAPR) needed for dimming of the indoor illumination. A correlation decoder used at the receiver is shown to be optimal for indoor VLC systems, which are shot noise and background-light limited. Interleaving applied on EPPM in order to decrease the ISI effect in dispersive VLC channels can significantly decrease the error probability. The proposed interleaving technique makes EPPM a better modulation option compared to PPM for VLC systems or any other dispersive OWC system. An overlapped EPPM pulse technique is proposed to increase the transmission rate when bandwidth-limited white LEDs are used as sources.

Description: In coherent optical systems laser phase noise can interact with digital equalization to cause equalization-enhanced impairments, which are a major obstacle for applying higher order modulation formats in coherent optical systems with digital chromatic dispersion compensation. In this paper a code-aided expectation maximization method to track phase noise in such systems is presented. A common measure of laser phase noise is the linewidth. It is shown that with $sim 11%$ redundancy, the laser linewidth tolerance for 975 km transmission distance can be increased by $50%$ , or the system reach for a laser linewidth of 5 MHz can be doubled. A phase-noise-robust 16-point 4–4–4–4 ring constellation was found to have better performance compared to 16QAM and a 2–6–8 ring constellations. Performance can be further improved with a lower code rate and fewer pilot symbols. It is also shown that algorithmic complexity can be reduced without significant reduction in the performance by reducing iterations and using low complexity codes.

Description: Directly bonded, oxide-free, InP-based epitaxial layer bonding onto nanopatterned silicon-on-insulator structures was performed and result in waveguides with an embedded effective medium. Such a medium ensures flexible form of optical confinement and could also assist heat and electric current transfer optimally to the silicon layer since there is a remarkable absence of oxide, thanks to careful surface preparation processes. The fabricated waveguides, which embed buried 1-D (trenches) or 2-D (holes forming a photonic crystal) nanopatterns have been measured by two techniques. Either the classical end-fire technique, or the “internal light source” technique used for III–V-based photonic crystal waveguides, with a layer that contains several quantum wells. Propagation losses due to scattering in the nanopatterned area are retrieved by both methods and consistently point toward a ∼20 cm $^{-1}$ loss level. A critical assessment is made of this result. Other local probe techniques allowed by the internal probe methods are reported that could help qualifying thermal transfer at these oxide-free interfaces.

Description: The search for schemes that minimize the energy associated with the transmission of information is a longstanding fundamental issue in communication theory. In this paper we extend fundamental limits to the energy consumption per unit of information, as given by Shannon’s theory, to the quantum domain. Unlike previous studies, we address a scenario where the signal may be manipulated in an arbitrary way while propagating from the transmitter to the receiver. This situation characterizes many realistic scenarios, such as multi-span fiber-optic communication systems. We obtain the ultimate quantum limit on the energy consumption in this scenario and propose a simple binary energy modulation scheme that approaches this limit within one order of magnitude for practically relevant values of spectral efficiency. Under the same conditions, the quantum energy consumption limit of the standard optically amplified coherent communication scheme is three orders of magnitude above the ultimate identified limit. Throughout the paper we consider transmission of classical information over a quantum channel.

Description: A technique that saved phosphor use by 150% was applied to warm white light-emitting diodes containing composite colloidal photonic crystals (c-CPhCs) to develop an electrical source of candlelight that exhibits a high luminous flux (52 lm) and a high color-rendering index (CRI; 94). This physiologically friendly light source, which was designed to mimic the color temperature of candlelight, provides at least eight-times more luminous flux than traditional candles do. Furthermore, a UV-based adhesive curing method for enhancing the adhesion properties of c-CPhCs was also developed. A novel, inexpensive technology was implemented to produce high-CRI candlelight LEDs for use as a residential light source.

Description: In this paper, we investigate a nonlinear compensation technique with two different architectures using direct modulation (DM) and external modulation (EM) techniques, termed as DM based frequency dithering (DMFD) and EM based frequency dithering (EMFD). We show that DMFD and EMFD methods operate substantially different in radio-over-fiber (RoF) system by optimizing the dithering technique relative to the LTE technology. The proposed techniques is only applicable if the condition of { $f_{L} 〈 f_{d} 〈 f_{rm RF}$ } is met, where $f_{L}$ represents the dithering boundary limit of 14 MHz, $f_{d}$ is DMFD signal frequency and $f_{rm RF}$ is the RoF carrier frequency. Analysis of the optical launch power for DMFD and EMFD methods reveal that the stimulated Brillouin scattering threshold is above ∼6 dBm for the LTE-RoF system. In addition, we also unveil that DMFD and EMFD methods do not introduce additional distortion for the linear and optimum optical launch power regions, which are frequency chirp driven regions. If the given condition is met, the proposed method improves the LTE-RoF system without any shortcoming. Finally, at 10 dBm launch power, DMFD and EMFD methods exhibits an average signal-to-noise ratio gain of ∼5.95 and ∼7.71 dB, respectively.

Description: This paper demonstrates the merit and de-merit of a low complexity, multiple transmitter generalised space shift keying (GSSK) signalling technique for short range indoor visible light communications. To illustrate the high spectral efficiency credential of GSSK, an array of $N_{{rm t}}$ white light emitting diodes (LEDs) is used to demonstrate $N_{{rm t}}$ bits/symbol transmission by exploiting the natural differences in the channel gains of the spatially separated LED array. With two low power white LEDs each having a $hbox{3}$ dB modulation bandwidth of $hbox{8}$ MHz, data transmission of up to $hbox{40}$ Mbits/s ( $hbox{20}$ Mbaud) is demonstrated. For an average received electrical signal power of ${-12.5}$ dBm and $2times 10^{4}$ data symbols, the measured symbol error rate at $hbox{20}$ Mbaud is $7 times 10^{-2}$ and $5 times 10^{-5}$ at $hbox{10}$ Mbaud ( $hbox{20}$ Mbits/s) without any error control coding. The GSSK technique is experimentally shown to be most suitable for a fixed configuration as its performance is quite sensitive to receiver mobility.

Description: Cloud computing services have driven a new design of data center networks. Hybrid switching architecture is one of the promising solutions since it makes better tradeoff between the network performance and technical feasibility. However, as the existing hybrid networks only deploy one-hop optical circuit switching (OCS) in the top layer, the flexibility and scalability is limited. To address this problem, a distributed OCS model is proposed. To reduce the high blocking ratio, the WDM and SDM technologies are introduced to increase the connectivity of the optical network. Moreover a multi-wavelength optical switch based on microring resonators is designed to enable the fast switching. Based on this model, the multi-rooted tree based hybrid architecture with deep integration of optical connection is constructed. A new way to solve the mixed traffic scheduling problem is also provided by delivering the small flows and large flows through the different networks. The simulation results indicate that the multi-rooted tree based hybrid architecture achieves better performance under various traffic patterns. It also introduces less control overhead compared with the existing traffic scheduling schemes.

Description: A novel design of a submicron power splitter is presented using plasmonics with the metal insulator metal (MIM) configuration. A simple analytical model is utilized to obtain good initial design. The final optimized design is obtained using finite-difference time domain based optimization. The proposed approach provides a simple and efficient methodology for designing 1xN power splitters using MIM configuration. The proposed power splitters are ultra wide band(∼ 2 μm) with negligible imbalance. The approach is utilized to design up to 1 × 4, 1 × 8, and 1 × 16 integrated power splitters with submicron footprint, where all the optimized designs are validated.

Description: Suppression of time-delay (TD) signature in a vertical-cavity surface-emitting laser subject to double-cavity polarization-rotated optical feedback has been verified experimentally. The TD signature, represented by the autocorrelation coefficient at the delay time in the external cavity, is measured in the two orthogonal polarizations of the laser. The results show that TD signatures with double-cavity feedback can be 22% to 35% weaker than those with single cavity feedback for the same experimental parameters. For appropriate feedback polarization, TD signatures become invisible. This is in good agreement with the theoretical prediction. Our results also show that higher feedback strength results in greater suppression in TD signatures in a wide range of polarization angle.

Description: In this paper, we present analytical expressions for the performance of dual-hop urban optical wireless communication system under the combined influence of atmospheric turbulence induced-fading and misalignment-fading (pointing errors). The system employs an amplify and forward relay with the assumption that channel state information is available at transmitting as well as receiving terminals. The atmospheric turbulence channels are modeled by independent but not necessarily identically distributed (i.n.i.d) Gamma–Gamma fading statistics with pointing error impairments. Using a tight upper bound for the end-to-end signal to noise ratio of the system, we derive the novel closed form expressions for the moment generating function and probability density function all in terms of Meijer’s G-function. Based on the aforementioned statistical results, accurate closed-form expressions for average symbol error rate for M-ary phase shift keying modulation schemes, outage probability and the ergodic capacity under four adaptive transmission policies namely optimal rate adaptation, optimal power and rate adaptation, channel inversion with fixed rate and truncated channel inversion with fixed rate are derived in terms of Meijer’s G -function function. Finally, the proposed mathematical analysis is accompanied with various numerical examples to demonstrate the effect of different system parameters on the system performance.

Description: Vacuum-propagation optical communication with high photon efficiency (many bits/photon) and high spectral efficiency (many bits/s $cdot$ Hz) requires operation in the near-field power transfer regime with a large number of spatial modes. For terrestrial propagation paths, however, the effects of atmospheric turbulence must be factored into the photon and spectral efficiency assessments. In Part I of this study [N. Chandrasekaran and J. H. Shapiro, “Photon Information Efficient Communication Through Atmospheric Turbulence—Part I: Channel Model and Propagation Statistics,” J. Lightw. Technol., vol. 32, no. 6, pp. 1075–1087, Mar. 2014], modal-transmissivity statistics were derived for the turbulent channel that depend solely on the mutual coherence function of the atmospheric Green’s function, and these bounds were evaluated for $sim$ 200 spatial-mode systems whose transmitters used either focused-beam (FB), Hermite-Gaussian (HG), or Laguerre–Gaussian (LG) modes and whose receivers either did or did not employ adaptive optics. This Part II paper derives upper and lower bounds for the ergodic Holevo capacities of classical and private information transmission over the multiple spatial-mode turbulent channel that can be evaluated from Part I’s transmissivity statistics. Also included are bounds on the ergodic capacity for on–off keying encoding and direct detection. It is shown that: 1) adaptive optics are not necessary to realize high photon information efficiency and high spectral efficiency simultaneously; 2) an FB-mode system with perfect adaptive optics outperforms its HG-mode and LG-mode counterparts; and 3) the converse is true when adaptive optics are not employed.

Description: We report a new method for constructing a single fiber optical tweezers, which can realize multi-dimensional manipulation of trapped yeast cells by using a LP $_{bf 11}$ mode beam excited in a normal communication single core optical fiber. This allows a simple and convenient orientation control on the trapped yeast cells. The LP $_{bf 11}$ mode beam, both for generating trap and orientation manipulation, has been modulated by using the tension and twisting loaded on the fiber. We present experimental results of controllable deflection and orientation manipulation of the yeast cells. To the best of our knowledge, it is the first report about the trapped yeast cells being driven by the normal single fiber optical tweezers in multi dimensions, and it constitutes a new development for single fiber optical trapping and makes possible of more practical applications in the biomedical research fields.

Description: Optical communication with high photon-efficiency (many bits/photon) and high spectral efficiency (SE) (many bits/s-Hz) cannot be achieved unless multiple spatial modes are employed. For vacuum propagation, it is known that achieving 10 bits/photon and 5 bits/s-Hz requires 189 low-loss spatial modes at the ultimate Holevo limit and 4500 such modes at the Shannon limit for on–off keying with direct detection. For terrestrial propagation paths, however, atmospheric turbulence corrupts multiple spatial-mode operation. This paper derives power-transmissivity bounds and average intermodal crosstalks for the turbulent channel that depend solely on the mutual coherence function of the atmospheric Green’s function. These statistics are then evaluated for $sim$ 200 spatial-mode systems whose transmitters use either focused-beam, Hermite–Gaussian (HG), or Laguerre–Gaussian (LG) modes and whose receivers either do or do not employ adaptive optics. It is shown that: (1) adaptive optics are not necessary for achieving both high photon information efficiency (PIE) and high SE; (2) systems employing HG or LG modes achieve the same capacities through turbulence; and (3) the orbital angular momentum carried by LG modes does not provide turbulence immunity. In the companion paper [N. Chandrasekaran, J. H. Shapiro, and L. Wang, “Photon Information Efficient Communication Through Atmospheric Turbulence—Part II: Bounds on Ergodic Classical and Private Capacities,” J. Lightw. Technol., vol. 32, no. 6, pp. 1088–1097, Mar. 2014], the transmissivity bounds are used to quantify the turbulence-induced loss in PIE versus SE performance for these mode sets.

Description: We demonstrate an all-solid Nd $^{3+}$ -doped silicate photonic crystal fiber with 13.1 W output power at 1061 nm and 64.5% slope efficiency. We also theoretically investigate and prove the effects of confinement loss and overlap fraction on a double-clad photonic crystal fiber laser. An innovative design is suggested to improve the laser beam quality.

Description: Application of multi-level coded modulation (MLCM) for 16-ary constellations in coherent systems is studied. An MLCM system with Reed-Solomon component codes and multi-stage decoder is considered. A systematic numerical method for finding set-partitioning and optimal code rates is presented. The method only requires the probability density function of received samples and can be applied to any constellation regardless of irregularity or lack of symmetry. Performance of the designed MLCM system is verified in presence of nonlinear phase noise and normal phase noise. For nonlinear phase noise limited system, it is shown that the block error rate (BLER) of the system can be improved using our approach for set-partitioning. For phase noise limited system, both BLER and bit error rate (BER) are studied for a phase noise optimized and square 16-quadrature amplitude modulation (16QAM). Post forward-error correction (FEC) BER performance of the optimized constellation over square 16QAM is studied for different levels of phase noise. It is shown that the optimized constellation along with MLCM system decreases the required signal-to-noise ratio by several dB at high phase noise regime and low post FEC BER.

Description: Metamaterial-based perfect absorbers utilize the intrinsic loss, with the aid of appropriate structural design (completely suppress transmission and reflection), to achieve near unity absorption at a certain frequency. The frequency of the reported absorbers is usually fixed and operates over a limited bandwidth, which greatly hampers their practical applications. Active or dynamic control over their resonance frequency is urgently necessary. Herein, we propose a novel approach for efficient tuning of the frequency of the absorber by shifting the movable part of the composite structure composed of the fixed and movable parts. The concept is rather general and applicable to various absorbers as long as the sandwich structure design is valid. The demonstrated continuous tuning of metamaterial absorber can find practical applications in detection, imaging, spectroscopy and selective thermal emitters.

Description: Nanophotonic waveguides can be engineered in order to exhibit slow mode propagation thereby enhancing the nonlinear responses. In such waveguides, loss and nonlinear coefficients are strongly wavelength dependent, a property that must be considered when the signal to pump detuning is large. Exact formulas for the parametric gain and conversion efficiency, accounting for the dispersion of losses and nonlinearity, are derived here. They can be applied to any waveguide presenting such features; in particular they have been calculated for a III–V semiconductor photonic crystal waveguide, where narrow- and broad-band amplification are predicted. The asymmetry of losses causes major asymmetries in the gain and conversion efficiency, which are no longer simply related as in the case of waveguides in which loss do not depend on the wavelength.

Description: We investigate dynamic pre-configured-cycle (p-cycle) protection design for spectrum-sliced elastic optical networks (EONs). Several novel algorithms are proposed to use the spectrum planning of working and backup resources. We first combine the protected working capacity envelope (PWCE) p-cycle design with spectrum planning, and design an algorithm that can achieve dynamic p-cycle design in EONs. Then, to resolve the coverage issue of the PWCE-p-cycles, we consider dynamic p -cycle design with Hamiltonian cycles and propose to use topology partition for reducing the lengths of backup paths. Simulation results show that our proposed algorithms achieve lower blocking probability than the shared path-protection (SPP) algorithm, while the average length of backup paths per request can be controlled well. To the best of our knowledge, this is the first attempt to address dynamic p-cycle protection design with spectrum planning for EONs.

Description: Generating radio-frequency signals over the entire spectrum, from 1 to 100 GHz, typically requires the use of multiple oscillators designed only for specific narrow bands of operation within that spectrum. An optoelectronic system based on modulation side-band-injection-locked lasers is capable of generating RF frequencies tunable over a span of more than seven octaves with low phase noise . We present the results of our efforts to develop a heterogeneously integrated version of this system based on a silicon-photonic integrated circuit coupled to III–V semiconductor gain chips. Towards that end, we have successfully demonstrated an integrated laser module and achieved tunable RF generation with a $sim$ 1 Hz linewidth.

Description: This paper investigates the use of a pilot signal in reducing the electrical peak-to-average power ratio (PAPR) of an orthogonal frequency division multiplexing (OFDM) intensity-modulated optical wireless communication system. The phase of the pilot signal is chosen based on the selected mapping (SLM) algorithm while the maximum likelihood criterion is used to estimate the pilot signal at the receiver. Bit error rate (BER) performance of the pilot-assisted optical OFDM system is identical to that of the basic optical OFDM (with no pilot and no PAPR reduction technique implemented) at the desired BER of less than $hbox{10}^{-3}$ needed to establish a reliable communication link. The pilot-assisted PAPR reduction technique results in higher reduction in PAPR for high order $(M>4)$ constellations than the classical SLM. With respect to a basic OFDM system, with no pilot and no PAPR reduction technique implemented, a pilot-assisted $M$ -QAM optical OFDM system is capable of reducing the electrical PAPR by over about $hbox{2.5}$ dB at a modest complementary cumulative distribution function (CCDF) point of $hbox{10}^{-4}$ for $M = hbox{64}$ . Greater reductions in PAPR are possible at lower values of CCDF with no degradation to the system’s error performance. Clipping the time domain signal at both ends mildly (at $hbox{25}$ times the signal variance level) results in a PAPR reduction of about $hbox{6.3}$ dB at the same CCDF of $hbox{10}^{-4}$ but with an error floor of about $hbox{3} times hbox{10}^{-5}$ . Although it is possible to attain any desired level of electrical PAPR reduction with signal clipping, this will be at a cost of deterioration in the systems’s bit error performance.

Description: We report on numerical and experimental results on macro bending losses in single-cell Kagome-lattice hollow-core photonic crystal fibers. Fibers with different cladding layer numbers have been investigated. Results show that a cladding with at least two rings produces a reduction in bend sensitivity by more than one order of magnitude relative to a single layer cladding. This result is an original way to confirm the main role played by the inhibiting coupling mechanism in such microstructured fibers.

Description: A time multiplexed rectangular Zernike modal wavefront sensor based on a nematic phase-only liquid crystal spatial light modulator and specially designed for a high power two-electrode tapered laser diode which is a compact and novel free space optical communication source is used in an adaptive beam steering free space optical communication system, enabling the system to have 1.25 GHz modulation bandwidth, 4.6° angular coverage and the capability of sensing aberrations within the system and caused by atmosphere turbulence up to absolute value of 0.15 waves amplitude and correcting them in one correction cycle. Closed-loop aberration correction algorithm can be implemented to provide convergence for larger and time varying aberrations. Improvement of the system signal-to-noise-ratio performance is achieved by aberration correction. To our knowledge, it is first time to use rectangular orthonormal Zernike polynomials to represent balanced aberrations for high power rectangular laser beam in practice.

Description: In the hybrid optical code division multiple access (OCDMA) architectures using different types of optical encoder/decoder (E/Ds) between an optical line terminal and optical network units, a hybrid 40G-OCDMA-PON with a single multi-port and super-structured fiber Bragg grating (SSFBG) E/Ds is studied as a candidate system beyond next-generation PON stage 2 (NG-PON2). In this paper, we have developed uniformed and sampled SSFBG E/Ds with different refractive index profiles and have compared their code performances in a hybrid 40G-OCDMA-PON system. The experimental results show that the sampled profile improves the code dependent performances presented in many OCDMA systems. We have also demonstrated a full-duplex 4-user  $times$  40 Gb/s hybrid OCDMA-PON system by applying the sampled SSFBG E/Ds. An asynchronous full-duplex 50 km transmission with a total capacity of 160 Gb/s (4-user  $times$  40 Gb/s) has been achieved for the first time. Furthermore, we focus on major requirements in NG-PON2 and discuss several issues to introduce the hybrid 40G-OCDMA-PON for optical access networks.

Description: Using the power flow equation, the state of mode coupling in plastic-clad silica fiber and organic glass-clad PMMA fiber is investigated in this article. Coupling coefficients are determined by extending our previously reported calculation method to the narrowing beams that are launched with distributions wider than the corresponding steady state distributions. It was found that the organic glass-clad PMMA fiber showed stronger mode coupling than the plastic-clad silica fiber. Consequently, the fiber length for achieving the steady state distribution was shorter for organic glass-clad PMMA fibers than for plastic-clad silica fibers.

Description: Mode-dependent loss (MDL) is known to be a major issue in space-division multiplexed (SMD) systems. Its effect on performance is complex as it affects both the data carrying signal and the accumulated amplification noise. In this paper we propose a procedure for characterizing the MDL of SDM systems by means of standard measurements that are routinely performed on SDM setups. The figure of merit that we present for quantifying MDL incorporates the effect on the transmitted signal and the noise and is directly related to the spectral efficiency reduction.

Description: We analyze, both theoretically and experimentally, the phase noise of the radio frequency (RF) beatnote generated by optical mixing of two orthogonally polarized modes in an optically pumped dual-frequency vertical external cavity surface emitting laser (VECSEL). The characteristics of the RF phase noise within the frequency range of 10 kHz–50 MHz are investigated for three different nonlinear coupling strengths between the two lasing modes. In the theoretical model, we consider two different physical mechanisms responsible for the RF phase noise. In the low frequency domain (typically below 500 kHz), the dominant contribution to the RF phase noise is shown to come from the thermal fluctuations of the semiconductor active medium induced by pump intensity fluctuations. However, in the higher frequency domain (typically above 500 kHz), the main source of RF phase noise is shown to be the pump intensity fluctuations which are transferred to the intensity noises of the two lasing modes and then to the phase noise via the large Henry factor of the semiconductor gain medium. For this latter mechanism, the nonlinear coupling strength between the two lasing modes is shown to play an important role in the value of the RF phase noise. All experimental results are shown to be in good agreement with theory.

Description: In this paper we introduce a simple and effective method for substantially reducing the beam spot wandering and scattering effects in the free space optical (FSO) communications using a spherical concave mirror (SCM). The advantages of employing SCMs for focusing the light onto a small area photodetector are improved efficiency in collecting income scattered light beam in a turbulence channel and the detachment between the position of the SCM focal point and fluctuations of the refractive index of the channel. The proposed method is experimentally evaluated in an indoor controlled turbulence environment over a propagation span of up to 104 m. The results testify that SCM can effectively compensate the optical spot scattering and wandering effect, thus leading to improved performance of the FSO system.

Description: We investigate the fiber parameters which meet the design conditions for effectively three-spatial-mode transmission and low bending loss in a ring-core fiber for mode-division multiplexing transmission. We also clarify the effective area and the difference of the effective index while satisfying the design conditions.

Description: This paper reports a hybrid integrated light source fabricated on a Si platform using a spot-size converter (SSC) with a trident Si waveguide. Low-loss coupling for 1.55 μm and 1.3 μm wavelengths was achieved with merely the simple planar form of a Si waveguide with no use of complicated structures such as vertical tapers or an extra dielectric core overlaid on the waveguide. The coupling loss tolerance up to a 1 dB loss increase was larger than the accuracy of our passive alignment technology. The coupling efficiency was quite robust against manufacturing variations in the waveguide width compared with that of a conventional SSC with an inverse taper waveguide. A multi-channel light source with highly uniform output power and a high-temperature light source were fabricated with a 1.55 μm quantum well laser and a 1.3 μm quantum dot laser, respectively. The integration scheme we report can be used to fabricate light sources for high-density, multi-channel Si optical interposers.

Description: A novel design for multiple symmetric image encryption system based on a phase encoding is presented. The proposed encryptor utilizes a photonic bandgap (PBG) block in order to ensure high reflectivity over a relatively wide frequency range of interest. Also, the proposed encryptor can be utilized to encrypt two images simultaneously through the use of two nematic liquid crystal (NLC) layers across the PBG block. The whole system has been simulated numerically using the rigorous finite difference time domain method. To describe the robustness of the encryption, a root mean square of error and the signal to noise ratio are calculated. The statistical analysis of the retrieved images shows that the proposed image encryption system provides an efficient and secure way for real time image encryption and transmission. In addition, as the proposed system offers a number of advantages over existing systems such as simple design, symmetry allowing integrated encryptor/decryptor system, ultra high bandwidth and encrypting two images at the same time, it can be suitably exploited in optical imaging system applications.

Description: Next-generation optical communication systems will continue to push the ( bandwidth $cdot$ distance) product towards its physical limit. To address this enormous demand, the usage of digital signal processing together with advanced modulation formats and coherent detection has been proposed to enable data-rates as high as 400 Gb/s per channel over distances in the order of 1000 km. These technological breakthroughs have been made possible by full compensation of linear fiber impairments using digital equalization algorithms. While linear equalization techniques have already matured over the last decade, the next logical focus is to explore solutions enabling the mitigation of the Kerr effect induced nonlinear channel impairments. One of the most promising methods to compensate for fiber nonlinearities is digital back-propagation (DBP), which has recently been acknowledged as a universal compensator for fiber propagation impairments, albeit with high computational requirements. In this paper, we discuss two proposals to reduce the hardware complexity required by DBP. The first confirms and extends published results for non-dispersion managed link, while the second introduces a novel method applicable to dispersion managed links, showing complexity reductions in the order of 50% and up to 85%, respectively. The proposed techniques are validated by comparing results obtained through post-processing of simulated and experimental data, employing single channel and WDM configurations, with advanced modulation formats, such as quadrature phase shift keying (QPSK) and 16-ary quadrature amplitude modulation (16-QAM). The considered net symbol rate for all cases is 25 GSymbol/s. Our post-processing results show that we can significantly reduce the hardware complexity without affecting the system performance. Finally, a detailed analysis of the obtained reduction is presented for the ca- e of dispersion managed link in terms of number of required complex multiplications per transmitted bit.

Description: Discrete multi-tone (DMT) modulation is known to be an efficient single-transmitter technique for visible-light communication. However, the use of this technique in a multiple transmitter environment requires effective subcarrier and power allocation design in order to exploit the full potential of spatial multiple-transmitter diversity. Spatial reuse of the subcarriers in the presence of interference and power constraints increases the efficiency of multiple access (MA) DMT communication. In this paper, we propose an algorithm that manages interference-constrained subcarrier reuse between different transmitters and power redistribution between different subcarriers in a heuristic manner. The algorithm simulation shows an improvement in the average bit-rate as compared with a conventional DMT method. Furthermore, the effectiveness of the proposed MA-DMT scheme increases with the number of users.

Description: In this paper, a comparison between metal-insulator-metal (MIM) waveguides made of silver and newly emerging plasmonic materials is reported. In particular, titanium nitride (TiN) from the class of nitrides, and gallium zinc oxide (GZO) from the class of transparent conducting oxides, are proposed as alternatives to conventional metals for the more flexible exploitation of plasmonic properties. Depending on the specific application, the new choices of the plasmonic material allow for tuning of the surface plasmon resonance and may also reduce typical conductive losses. Moreover, compared to noble metals, these new plasmonic materials have the extremely important property of being compatible with the mature CMOS technology. In this paper, the specific application of MIM waveguides made of TiN and GZO for dispersion engineering (slow-wave propagation and negative effective index) is considered for the first time.

Description: We present a novel hybrid miniature dual-cavity Fabry–Perot sensor for simultaneous pressure and temperature measurements. The pressure sensing cavity is composed of an UV-molded cavity covered by a metal/polymer composite diaphragm for achieving a high pressure sensitivity while maintaining a miniature sensor size. Another intrinsic polymer/silica cavity is adopted for temperature sensing, which enables a high temperature sensitivity even with a short cavity length due to the large thermal expansion of the polymer. The sensor is fabricated by using a unique UV molding process with simple and safe procedures. The overall sensor size is around 150 μm in diameter and 343 μm in length. Experimental studies show that the sensor exhibits a good linearity over a pressure range of 6.89 to 27.58 kPa with a pressure sensitivity of 0.0122 μm/kPa at 26 °C, and a temperature range of 26.0 °C to 50.0 °C with a temperature sensitivity of 0.0029 μm/°C. An optical signal processing method is developed to retrieve the two cavity length changes, which is demonstrated to have a better resolution and a faster speed than the conventional method. The sensor is expected to benefit many fronts that require simultaneous pressure and temperature measurements with minimum intrusiveness, especially for biomedical applications.

Description: This paper presents and demonstrates a simple pattern effect suppression technique for the downstream repeater and the upstream booster semiconductor optical amplifiers (SOAs) of an SOA-based, optically amplified passive optical network (PON) system. The synchronized gain-clamping light injection technique is applied to an SOA mainly consisting of an optical-electrical-optical converter. The feasibility of the technique is confirmed experimentally by applying it to a downstream amplification repeater in a Gigabit Ethernet PON (GE-PON) and a upstream booster amplifier in a 10G-EPON with a high output power optical network unit (ONU). In both cases, the pattern effect is significantly suppressed and the system loss budget is greatly improved. For the GE-PON repeater a large access span loss of over 40 dB was achieved with a trunk span distance of 51.8 km, and for the 10G-EPON the loss budget was increased to over 40 dB by using high-output power ONU.

Description: For optical waveguides with high index-contrast and sharp corners, it is challenging to develop efficient, accurate and general full-vectorial mode solvers. The classical mode-matching method is capable of computing modes to high accuracy for waveguides with right-angle corners, but it is not convenient to implement when the waveguide has lossy components. Numerical variants of the mode-matching method, such as the Fourier modal method (as a mode solver), are simpler to implement. In this paper, a recently developed pseudospectral modal method for diffraction gratings is reformulated as a full-vectorial waveguide mode solver. As demonstrated in a number of examples, the method can be used to calculate waveguide modes to high accuracy, and it is relatively simple to implement.

Description: A complete characterization of the working parameters of a long-wavelength vertical-cavity surface-emitting laser (VCSEL) is presented. A technique is described for extracting values for the parameters of semiconductor laser rate equations based on simple expressions for the laser power and linewidth as a function of the bias current. The differential carrier lifetime at threshold is estimated by using the linear relation between the laser linewidth and the bias current that is obtained for below threshold operation. High resolution CW optical spectrum measurements are performed to apply this technique for extraction of the parameters of a 1550-nm single-transverse mode VCSEL. Intensity noise spectrum analysis is used to complete our parameter extraction procedure and to compare with parameter values obtained from laser linewidth measurements.

Description: In this paper, we propose a kind of phase conjugate fiber optic gyroscope (FOG). The natural time reversal property of the phase conjugation process can eliminate much of the nonreciprocal effects in optical gyroscope, so this can improve the FOG performance for better applications. We use the degenerate four-wave mixing (DFWM) process to generate counter-propagating phase conjugated waves with the advantage that this process is free from phase matching constrains. DFWM is also a parametric amplification process, so this can remedy the power attenuation in the interacting process and improve the signal to noise ratio. This method can improve sensitivity of the FOG as well. We also take advantage of the evanescent field outside the optical waveguide to interact with the imposed pump field. In this way, we can maximize the nonlinear interaction through choosing proper waveguide dimensions and the cladding materials.

Description: An analog joint source channel coding (JSCC) system is developed for wireless optical communications. Source symbols are mapped directly onto channel symbols using space filling curves and then a non-linear stretching function is used to reduce distortion. Different from digital systems, the proposed scheme does not require long block lengths to achieve good performance reducing the complexity of the decoder significantly. This paper focuses on intensity-modulated direct-detection (IM/DD) optical wireless systems. First, a theoretical analysis of the IM/DD wireless optical channel is presented and the prototype communication system designed to transmit data using analog JSCC is introduced. The nonlinearities of the real channel are studied and characterized. A novel technique to mitigate the channel nonlinearities is presented. The performance of the real system follows the simulations and closely approximates the theoretical limits. The proposed system is then used for image transmission by first taking samples of a set of images using compressive sensing and then encoding the measurements using analog JSCC. Both simulation and experimental results are shown.

Description: Photonic integration of optical packet switching modules is crucial to compete with existing electronic switching fabrics in large data center networks. The approach of coding the forwarding packet information in an in-band label enables a spectral-efficient and scalable way of building low-latency large port count modular optical packet switching architecture. We demonstrate the error-free operation of the four in-band label extraction from $hbox{160 Gb/s}$ optical data packets based on photonic integrated silicon-on-insulator ring resonators. Four low-loss cascaded ring resonators using the quasi-TM mode are used as narrowband filters to ensure the detection of four optical labels as well as the error-free forwarding of the payload at limited power penalty. Due to the low-loss and less-confined optical quasi-TM mode the resonators can be very narrowband and have low insertion loss. The effect of the bandwidth of the four ring resonators on the quality of the payload is investigated. We show that using four rings with 3dB bandwidth of $hbox{21 pm}$ and only an insertion loss of $hbox{3 dB}$ , the distortion on the payload is limited ( ${〈}1.5,{rm dB}$ power penalty), even when the resonances are placed very close to the packet’s central wavelength. We also investigate the optical power requirements for error-free detection of the label as function of their spectral position relative to the center of the payload. The successful in-band positioning of the labels makes this component very scalable in amount of labels.

Description: A microfiber-enabled Fabry–Pérot interferometer (FPI) constructed by splicing a section of microfiber between two cleaved standard single-mode fibers (SMFs) with unique relative fiber cross section relationship has been proposed and experimentally demonstrated. The opening air cavity between the two SMF ends connected by the microfiber serves as an FP cavity and also a direct sensing head. The sensing characteristics of the FPIs with different cavity lengths and microfiber diameters have been studied. A force sensitivity as high as 167.41 nm/N (∼200 pm/μϵ) and a high refractive index (RI) sensitivity of 1330.8 nm/RIU (around a RI of 1.33) have been achieved by using the microfiber-based FPI with ∼21 μm cavity length and ∼44 μm microfiber diameter. Such a device has several merits such as simple configuration, compactness and reliability in operation owing to the extremely low thermal cross-sensitivities.

Description: A plasmonic bus waveguide with a side-coupled T-shaped (TS) or a reverse T-shaped (RTS) resonator consisting of a parallel and a perpendicular cavities is proposed. The compact configuration could serve as a wavelength demultiplexing device as a forbidden band is achieved based on the symmetric distribution of resonators. By shifting one cavity away from the center of the resonator, the system exhibits electromagnetically induced transparency (EIT) like transmission at the wavelength of the former forbidden band. The electromagnetic responses of the structure could be handled with certain flexibility by changing the asymmetric behavior of the TS or RTS resonator. Similar characteristics for two proposed structures could be obtained except for the center wavelength that is determined by the two cavities in the RTS resonator or by the cavity parallel to the bus waveguide in the TS resonator.

Description: The paper introduces a novel architecture for optical access networks that simultaneously provides complete flexibility and security. At the same time, the distribution architecture is completely passive. Unlike the other architectures in literature, the proposed architecture does not possess a security-flexibility tradeoff. Complete flexibility allows to switch OFF appropriate number of active components at low network loads making this design a green technology. The discussed architecture has a long reach, which is independent of the number of users in the network.

Description: We present a theoretical and experimental investigation on refractive index (RI) sensing characteristics of single mode fiber (SMF) based modal interferometers. Theoretical analysis reveals that interference between different modes in an SMF has a quite different response to the RI variation of the external medium. The interference between the core and lower order cladding modes has negative RI sensitivity whereas that between the core and higher order modes, or between two different order cladding modes have positive sensitivity. A single-mode-multimode-single-mode (SMS) fiber Michelson interferometer with a large-core step-index multimode fiber (MMF) is employed for experimental verification. In the SMS-based Michelson interferometer, the MMF acts as a mode coupler to excite cladding modes in the SMF. The RI response of the SMS-based structures with two different lengths of MMF are respectively tested in sodium-chloride water solutions. Experimental results show excellent agreements with the theoretical analysis.

Description: The characteristics of polymer fiber Bragg gratings (FBGs) embedded in composite materials are studied in this paper and are compared with characteristics of their silica counterparts. A polymer FBG of 10 mm length which exhibits a peak reflected wavelength circa 1530 nm is fabricated and characterized for this purpose. A silica FBG with a peak reflected wavelength circa 1553 nm is also embedded in the composite material for a comparison study. The fabricated composite material sample with embedded sensors is subjected to temperature and strain changes and the corresponding effects on the embedded polymer and silica FBGs are studied. The measured temperature sensitivity of the embedded polymer FBG was close to that of the same polymer FBG in free space, while the silica FBG shows elevated temperature sensitivity after embedding. With an increase in temperature, spectral broadening was observed for the embedded polymer FBG due to the stress induced by the thermal expansion of the composite material. From the observed wavelength shift and spectral bandwidth change of the polymer FBG, temperature and thermal expansion effects in the composite material can be measured simultaneously.

Description: Fibers with a longitudinal strain distribution can be used to shape the stimulated Brillouin scattering (SBS) gain spectrum or to increase the SBS threshold power. Theoretical analysis and experimental results on the SBS gain spectrum of coiled fibers with a continuous and arbitrary distribution of longitudinal strain are presented. The concept of a critical gain factor for determining the SBS threshold power with a simple formula is reviewed for fibers with different parameters and spectral shapes. A fiber coiling machine for high coiling forces is described for realizing permanent strain distributions with a maximum strain of more than 3%. SBS spectra were broadened to a spectral width of 1.7 GHz. Measurements verify a significant SBS gain suppression by a factor of 40, which is in excellent agreement with the theoretical analysis. Finally, synthetic triangular and broadened Lorentzian SBS spectral shapes with a spectral width of 550 MHz are demonstrated. The results can be used to suppress SBS in high-power fiber lasers and amplifiers or to tailor almost any arbitrary SBS spectral shapes, which could be useful for slow-light or active optical filter applications.

Description: The direct patterning of hybrid-polymer microring resonators with minimal residual layers by UV-assisted nanoimprint lithography is reported. The proposed stamp-and-repeat technology requires no post-processing. The imprint polymer was applied by spin-coating as a 130–150 nm thin initial film for an optimized processing. The importance of the initial film thickness is discussed in detail. Aspect ratios of more than 5:1 were realized with 2 µm high ridge-waveguides and sub-400 nm coupling gaps on maximal 130 nm thin residual layers. The achieved ratio of structure height to residual layer thickness of 15.4 (2 µm versus 130 nm) was much larger than the typical values in high-resolution imprinting and superseded the removal of the residual layer completely. The resonators are thought as biosensor transducers. High quality devices with Q-factors up to 13 000 were produced with a minimal set of process steps.

Description: We propose a hollow core Bragg fiber (HC-BF) with a heterostructured cladding based on the ternary 1-D photonic crystal (T-1DPC) in this paper. The distinguishing ternary unit cell of T-1DPC is formed from a polyetherimide (PEI) layer sandwiched by two As $_{2}$ Se $_{3}$ layers of the same thickness. We demonstrate its capability for mid-infrared broadband and low-loss transmission by numerically simulating and analyzing the omnidirectional photonic bandgap (OPBG) and the modal loss characteristics. The results show that the T-1DPC-based heterostructured cladding can effectively broaden the OPBG, which is only due to the blue-shift of the lower bandgap edge wavelength. Compared with that for the binary-1DPC-based HC-BF, the transmission loss for the T-1DPC-based HC-BF can be reduced by three orders of magnitude over most of the OPBG range. The large loss contrast is essentially attributed to the enhancement of the multilayer reflection from the T-1DPC-based cladding. Most notably, for a T-1DPC-based cladding with three groups, the transmission band with loss lower than 0.01 dB/m for the HE $_{11}$ mode can cover almost the whole 3 to 5 μm range, with the exception of several loss peaks near the short-wavelength edge. Even with the cladding material absorption included, the transmission loss for the HE $_{11}$ mode in the T-1DPC-based HC-BF can be still lower than 0.1 dB/m over the whole OPBG range.

Description: This paper reports an uncooled transmitter system using a digital super-mode (DS) distributed Bragg reflector (DBR) tunable laser, which is able to act as an athermal, wavelength agnostic transmitter suitable for wavelength division multiplexed (WDM) passive optical network (PON) applications. An open-loop laser current control algorithm is designed to compensate autonomously for wavelength drift, thus allowing constant operating wavelength to be achieved regardless of ambient temperature. An improved wavelength accuracy of ±3 GHz is achieved when using low bandwidth feedback from the central office using information from a centralized shared wavelength locker. The entire laser start-up, channel selection and subsequent wavelength control is autonomous and has been implemented on micro-controllers and field programmable gate arrays. We demonstrate a three channel WDM-PON system comprising an uncooled packaged DS-DBR laser in the presence of two neighboring interfering channels. Error free transmission over 40 km single mode fiber of 10 Gb/s externally modulated NRZ data, is achieved for each of 48 C-band channels on the 100 GHz ITU grid. Successful athermal operation is demonstrated by sweeping the ambient temperature of the laser from 15 to 70 °C with a maximum wavelength deviation for any channel of no more than 0.1 nm.

Description: We proposed a novel polarization splitter based on single-polarized elliptical-hole core circular-hole holey fibers (EC-CHFs). We employed the full-vector finite element beam propagation method to demonstrate the polarization splitter which is designed by the large hole EC-CHFs (air filling fraction in core is 36.73%) and small hole EC-CHFs (air filling fraction in core is 4.08%) can completely split an arbitrarily polarized light beam into two orthogonal polarization states without any crosstalk. In addition, we also calculated the tolerance and wavelength dependence of this kind of polarization splitter.

Description: We present a multifunctional scheme to implement reconfigurable temporal Fourier transformation (TFT) and temporal imaging (TI) flexibly. This structure is composed of a time lens followed by a section of dispersive fiber with proper length. More specifically, the output waveforms presenting either the spectrum profile or the scaled waveform profile of the input signal are achieved by simply changing the length of the fiber, which is theoretically analyzed and experimentally verified. This proposal requires neither the use of an input dispersive device preceding the time lens nor a second time lens after the dispersion. Hence, it is a simple and practical alternative to the implementation of both TFT and TI.

Description: In this study, we investigate the spectral distribution of the light scattered in step-index polymer optical fibers by illuminating the fibers transversely to their symmetry axis. For that purpose, we have performed wavelength-dependent scattering measurements over the spectral range 400–750 nm in three different commercial step-index polymer optical fibers. We have estimated the mean size of the most influential scattering centers in these polymer optical fibers, analyzing theoretically the experimental results obtained.

Description: We experimentally demonstrate multigigabit capacity bidirectional hybrid fiber-wireless systems with RF carrier frequencies at the W-band (75–110 GHz) that enables the seamless convergence between wireless and fiber-optic data transmission systems in access networks. In this study, we evaluate the transmission performances in two scenarios: a fiber-wireless access link that directly provide high-speed connections to wireless end users, and a fiber-wireless-fiber signal relay where a high capacity wireless link can be used to bridge two access fiber spans over physical obstacles. In both scenarios, we compare the transmission performances in terms of achievable wireless distances with and without using a high-frequency electrical power amplifier at the wireless transmitter. A downlink 16-Gbit/s QPSK signal and an uplink 1.25-Gbit/s ASK signal transmission over the two implementations are experimentally evaluated in terms of bit-error-rate (BER) performance. Results show that the use of the power amplifier can extend the wireless distance from ∼1 m to over 15 m in an indoor environment with BER performance within the 7% FEC threshold.

Description: A low-complexity model for signal quality prediction in a nonlinear fiber-optic network is developed. The model, which builds on the Gaussian noise model, takes into account the signal degradation caused by a combination of chromatic dispersion, nonlinear signal distortion, and amplifier noise. The center frequencies, bandwidths, and transmit powers can be chosen independently for each channel, which makes the model suitable for analysis and optimization of resource allocation and routing in large-scale optical networks applying flexible-grid wavelength-division multiplexing.

Description: An Er 3+ -doped ZBLAN fiber amplifier for Q-switch-ed pulses at 2.79 μm is reported. Over 24 μJ of pulse energy at an average output power of 1.0 W was achieved at a maximum available pump power of 9.4 W. The efficiency of this pulsed laser fiber amplifier is about 10%. Our simulation predicts that over 300-μJ pulses can be achieved with this fiber amplifier when a 120-W pump is used.

Description: This paper reports results obtained using fiber Bragg grating (FBG)-based sensors to investigate the displacement mode shapes of a cantilevered steel propeller blade, using FBG arrays for vibration monitoring for the first time. The experimental data obtained are cross compared with those from a finite element analysis of the same blade, undertaken using proprietary software. In the experimental configuration used, a network of gratings, forming a series of sensor arrays, was mounted on the blade under study to monitor its bending modes, while a further set was mounted perpendicular to this array to monitor torsional modes. To obtain the shape of the strain modes generated in the blade at specific frequencies, the dynamic response of the FBG arrays, as a function of time, was captured and then processed using Fourier transform algorithms to show the natural frequencies of the blade. As a result, the displacement modes shapes for the bending, torsional, and coupled modes of the first nine natural frequencies of the plate were obtained. The experimental data show very good agreement with theoretical analysis. This paper demonstrates the potential of using the lightweight, minimally invasive sensing technique described for the analysis of propeller blades and, thus, illustrating an effective method to overcome the deleterious effects of propellers seen in some commercial propeller designs.

Description: A magnetic-field-tuned photonics device based on magnetic fluid (MF) and a square tapered no-core fiber (NCF) sandwiched between two single-mode fibers (SMFs) has been demonstrated experimentally and theoretically. The enhanced evanescent field effect in the NCF is achieved by tapering the square NCF utilizing a fusion splicer. The spectral dependence of the proposed device on the applied magnetic-field intensity has been investigated. The results indicate that the multimode interference spectrum exhibits a blue-shift with the increment of the magnetic-field intensity. A maximal sensitivity of −18.7 pm/Oe is obtained for a magnetic field strength ranging from 25 to 450 Oe. The proposed tunable device has several advantages, including low cost, ease of fabrication, simple and compact structure, and high sensitivity. Therefore, the magnetic-field-tuned square tapered NCF is expected to find potential applications in the fields of optical fiber sensors, as well as fiber communications.

Description: A residual dispersion compensating octagonal photonic crystal fiber (OPCF), with an elliptical array of circular air-holes in the fiber core region, is proposed. The full-vector finite-element method with perfectly matched layer boundary is used as the analysis tool. It is demonstrated that it is possible to obtain large average negative dispersion of –562.52 ps/(nm · km) over 240 nm and –369.10 ps/(nm · km) over 630 nm wavelength bands for the fast and the slow axis, respectively. In addition to large negative dispersion, ultra-high birefringence, high nonlinearity, and zero-dispersion wavelengths with low confinement loss are also warranted. The proposed OPCFs would be a promising candidate for residual dispersion compensation, supercontinuum generation, and other applications.

Description: An all-optical delay flip flop is designed and described using two quantum dot semiconductor assisted Mach–Zehnder interferometers in this study. The flip flop has one enable input that allows for both positive and negative edge triggered operation without changing the design of the circuit. It is reconfigurable by design. Numerical simulations have been conducted to assess the performance of the circuit.

Description: Most research works emphasized on the acoustic sensor miniaturization are not optimized for applications under high surrounding pressure. For underwater acoustic sensing, measuring the small acoustic pressure from the huge hydrostatic pressure makes the design of the sensor challenging. In this paper, we attempt to solve the problem by adopting a center-embossed diaphragm as the sensitive structure. The deformation angular error is defined to evaluate the optical performance degradation and the optical sensitivity reduction under hydrostatic pressure. Simulations indicate that the central embossment is beneficial to maintain optics-related properties, although the structural sensitivity is reduced. Then, the diaphragm design guideline for underwater acoustic sensing is regulated. An optical fiber extrinsic Fabry–Pérot interferometer probe based on the diaphragm, which was micromachined by double-side etching of the silicon on insulator, was designed and assembled. Experimental results show that the interferometric fringe preserves similar shapes at any tested depth from 0 (in air) to 50 cm. The recovered signal detected by the sensor coincides well with the corresponding transmitted signal. The pressure sensitivity response is flat in frequency range from 10 to 2 kHz, of which value is about $-$ 154.6 dB re. 1/ $mu$ Pa. It agrees well with the theoretical predication. These results demonstrated that the designed sensor according to the guideline can be used as an underwater acoustic sensor. Moreover, the sensor has potential applications in smart unmanned platforms and swiftly deployable arrays.

Description: Spatially and spectrally resolved imaging has become an important tool for multimode fiber characterization, both in the realms of group index dispersion measurement and modal geometry analysis. However, limited resolution in group delay space, distributed scattering and noise sometimes make the task of identifying meaningful modal interference figures difficult. In this paper, an application of multivariate statistical analysis to S $^2$ datasets that effectively isolates mode pair interferences, both spectrally and spatially, is presented. A method to use those components to retrieve the power in each mode is also shown.

Description: A simple chromatic dispersion (CD) compensation scheme for a certain frequency is first proposed by utilizing a phase modulator in conjunction with a polarizer. Properly adjusting the input polarization states, the frequency response of the dispersive link can be controlled. Experimental results show that the dispersion-induced power fading (PF) occurred at 14 and 10.8 GHz over 40- and 65-km transmission is fully maximized or compensated. Furthermore, the spurious-free dynamic range of the proposed analog photonic link is improved to 96 dB·Hz 2/3 for the 40-km fiber transmission, which is 12 dB higher than that of the phase modulation-based link. On the other hand, an improved scheme for CD compensation is further demonstrated, where a polarization beam combiner is employed instead of a polarizer. Such an approach achieves the broadband CD compensation since it can be equivalent to the sum of two first schemes. In this case, the PF is greatly mitigated for the RF signal with a bandwidth up to 15 GHz. Meanwhile, the error vector magnitude reduces to ∼5% with the fluctuation of only 0.7%.

Description: We have proposed and experimentally demonstrated a novel approach to stabilize the photonic microwave generated by the period-one (P1) nonlinear dynamics of an optically injected semiconductor laser by introducing external subharmonic microwave modulation technique. External optical injection first drives a semiconductor laser into single sideband (SSB) P1 nonlinear dynamics whose frequency can be tuned by varying the optical injection strength, and then an external subharmonic microwave modulation is applied to the laser for stabilizing the frequency of the P1 oscillation. The experimental results show that after adopting 1/3 and 1/4 subharmonic microwave modulation, significant reduction up to more than three orders of magnitude of generated microwave linewidth can be achieved. Furthermore, we have analyzed the linewidth narrowing effects under subharmonic modulation with different powers and frequencies, and mapped the subharmonic microwave locking region in the parameter space of the power and frequency of the external subharmonic microwave.

Description: Design tools have existed for decades for standard step-index fibers, with analytical expressions for cutoff conditions as a function of core size, refractive indexes, and wavelength. We present analytical expressions for cutoff conditions for fibers with a ring-shaped propagation region. We validate our analytical expressions against numerical solutions, as well as via asymptotic analysis yielding the existing solutions for standard step-index fiber. We demonstrate the utility of our solutions for optimizing fibers supporting specific eigenmode behaviors of interest for spatial division multiplexing. In particular, we address large mode separation for orbital angular momentum modes and fibers supporting only modes with a single intensity ring.

Description: We experimentally demonstrate a sensitivity-enhanced fiber-optic pressure sensor using a simplified hollow-core photonic crystal fiber (SHC-PCF). The sensing head is fabricated by splicing a single mode fiber to a section of SHC-PCF, while the air claddings at the end of the PCF are selectively blocked by glue sealing and femtosecond laser machining. By immersing the end into water, a compressible Fabry-Pérot cavity can be formed in the fiber core of the PCF. Since the air core is opened up to the air claddings at the splicing joint, changing of environmental pressure will cost a large change of the cavity length to balance the pressure, which greatly enhances the pressure sensitivity of the sensor. A pressure sensitivity of 18.15 $mu$ m/kPa is experimentally demonstrated in the pressure range from 110 to 130 kPa. This sensor has potential applications in the area of highly sensitive pressure measurement.