ALBERT

Description: Time–frequency packing (TFP) transmission provides the highest achievable spectral efficiency with a constrained symbol alphabet and detector complexity. In this paper, the application of the TFP technique to fiber-optic systems is investigated and experimentally demonstrated. The main theoretical aspects, design guidelines, and implementation issues are discussed, focusing on those aspects which are peculiar to TFP systems. In particular, adaptive compensation of propagation impairments, matched filtering, and maximum a posteriori probability detection are obtained by a combination of a two-dimensional equalizer and four eight-state parallel Bahl–Cocke–Jelinek–Raviv (BCJR) detectors. A novel algorithm that ensures adaptive equalization, channel estimation, and a proper distribution of tasks between the equalizer and BCJR detectors is proposed. A set of irregular low-density parity-check codes with different rates is designed to operate at low error rates and approach the spectral efficiency limit achievable by TFP at different signal-to-noise ratios. An experimental demonstration of the designed system is finally provided with five dual-polarization QPSK-modulated optical carriers, densely packed in a 100-GHz bandwidth, employing a recirculating loop to test the performance of the system at different transmission distances.

Description: We investigated nonlinear optical characteristics of Tungsten disulfide (WS 2 ) films and experimentally demonstrated their high potential for application as nonlinear saturable absorbers in passively mode-locked fiber lasers. Side polished fiber (SPF) was fabricated and WS 2 film was overlaid to provide an efficient evanescent field interaction. The WS 2 film was prepared using two methods: liquid phase exfoliation to form few-layer nano-sheets, and chemical vapor deposition (CVD) to grow uniform multilayer WS 2 on a SiO 2 substrate. Two SPF saturable absorbers were prepared by either spin coating WS 2 solution or lifting off the multilayer CVD WS 2 on SPF. An all-fiber ring cavity was built and the WS 2 film overlaid on SPF was employed as a mode locker along with Er-doped fiber as a gain medium. Using the spin-coated WS 2 SPF, stable soliton-like pulses were generated with a spectral width of 5.6 nm and 467 fs pulse duration. The fiber laser cavity containing CVD WS2 SPF generated a transform-limited soliton pulse train with a spectral width of 8.23 nm and a pulse duration of 332 fs. Our study confirmed a high potential of WS 2 film as a novel 2-D nonlinear optical material for laser applications.

Description: We propose and experimentally demonstrate an on-chip all-optical differential-equation solver capable of solving second-order ordinary differential equations (ODEs) characterizing continuous-time linear time-invariant systems. The photonic device is implemented by a self-coupled microresonator on a silicon-on-insulator platform with mutual coupling between the cavity modes. Owing to the mutual mode coupling within the same resonant cavity, the resonance wavelengths induced by different cavity modes are self-aligned, thus avoiding precise wavelength alignment and unequal thermal wavelength drifts as in the case of cascaded resonators. By changing the mutual mode coupling strength, the proposed device can be used to solve second-order ODEs with tunable coefficients. System demonstration using the fabricated device is carried out for 10-Gb/s optical Gaussian and super-Gaussian input pulses. The experimental results are in good agreement with theoretical predictions of the solutions, which verify the feasibility of the fabricated device as a tunable second-order photonic ODE solver.

Description: We demonstrate a 100 Gb/s short reach system using a multicarrier transmitter based on a gain switched monolithically integrated laser. An optical comb source with 12.5-GHz free spectral range is achieved by gain-switching an integrated passive feedback laser. The 100 Gb/s wavelength division multiplexed, single sideband, direct detection, orthogonal frequency division multiplexed (WDM-SSB-DD-OFDM) system operates over 25 km standard single mode fiber exhibiting a spectral efficiency of 1.8 b/s/Hz. Receiver sensitivity of –14.2 dBm is achieved after 25 km transmission. Performance optimization with phase and amplitude precompensation is employed to improve the SSB OFDM modulation thereby reducing the interchannel interference and overcoming the power fading induced by the optical filter. We also present a theoretical analysis of the SSB-OFDM modulation.

Description: In this paper, narrow-band emission lines are generated by means of two random distributed feedback fiber laser schemes. Spectral line-widths as narrow as 3.2 pm have been measured, which significantly improves previous reported results. The laser is analyzed with the aim of obtaining a spectral line-width as narrow as possible. Additionally a variation of this setup for multi-wavelength operation is also validated. Both schemes present a simple topology that use a combination of phase-shifted fiber Bragg gratings and regular fiber Bragg gratings as filtering elements.

Description: We report on the fabrication and characterization of 19-cell hypocycloid-shape Kagome fibers with core size larger than 100 μm. These inhibited coupling fibers present low propagation loss (100 dB/km) over broad transmission range with low chromatic dispersion combined with ultra-low power overlap with silica surround, making them an efficient solution for ultra-high power laser handling, ultra-fast laser delivery, and plasma photonics applications.

Description: The iterative demodulation and decoding algorithm introduced in 2005 by Colavolpe, Barbieri, and Caire to cope with channels affected by phase noise needs pilot symbols to bootstrap. However, pilot symbols reduce the spectral efficiency of the system and, consequently, system's throughput. The aim of this paper is to show that trellis-based demodulation can be used to bootstrap the iterative process without the need of pilot symbols. Also, the complexity issue of trellis-based demodulation is addressed in this paper. The result is that the performance of iterative demodulation and decoding after the iterations is virtually unaffected by complexity reduction, provided that the reduced-complexity demodulator guarantees cycle-slip-free operation. From the numerical results presented in this paper, we show that cycle-slip-free operation can be achieved with substantial complexity reduction also for phase noise associated with linewidths of practical interest.

Description: The optical pulse evolution in a highly nonlinear normal dispersion-increasing fiber has been considered, both experimentally and theoretically. It was found that large spectral broadening in tapered waveguides could occur without temporal instabilities and impose the linear frequency modulation, i.e., chirp, required for high-quality pulse compression. The pedestal-free pulses have been demonstrated after dechirping in a standard single-mode fiber.

Description: This work proposes a novel localized surface plasmon resonance (LSPR) biochemical sensor featuring high sensitivity and a high resolution. The sensor was divided into two subcomponents according to their distinct functions; namely, single-mode fiber and metal array. Single-mode fibers located on the left and right sides of the sensors function as the input and output for optical fiber signals. A metal array comprising an arrangement of cylindrical nanometal particles served as the detection area of the sensor. To effectively reduce the memory capacity and calculation time, two innovative techniques (i.e., object meshing and boundary meshing) were integrated with the finite element method. With the area of the triangular elements used as a basis, the object boundary, small object, medium object, and large objects were meshed at a ratio of 1:8:160:1600. The improved numerical simulation methods and six design procedures were adopted to develop and analyze the proposed LSPR biochemical sensor. The results show that the novel LSPR biochemical sensor outperformed two current high-performance biochemical sensors and provided additional advantages such as short length (approximately 430 μm), high resolution (approximately –120 dB), and high sensitivity (approximately 127 604 nm/RIU).

Description: We propose an ultra-broadband super light absorber by integrating different-sized tapered hyperbolic metamaterial (HMM) waveguides, each of which has a different and wide absorption band due to broadband slow-light response, into a unit cell. We numerically demonstrate that such an absorber is superior to a single-sized HMM absorber in terms of absorption bandwidth, while maintaining a comparable absorption efficiency. A three different-sized HMM absorber presents the capability of working with an ultra-wide frequency band ranging from 1 to 30 THz, which is much larger than previously proposed absorbers working in the same spectral region. Such a design shows great promise for a broad range of applications such as thermal emitters, photovoltaics, optical-chemical energy harvesting, and stealth technology, where ultra-wideband absorption is in very high demand.

Description: This paper proposes a ring-based integrated wireless optical network architecture and an associated protocol that involves Ethernet passive optical network (EPON) and long term evolution (LTE) wireless network. The architecture along with the proposed protocol is instrumental toward the reduction of handover delay. The proposed ring-based EPON architecture facilitates the implementation of the X2 interface for LTE network by enabling the optical network units of the EPON backhaul to directly communicate with each other. The work further discusses about an open access network architecture where a single EPON can be used by multiple mobile service providers without compromising information security. Extensive simulations have been carried out to evaluate the performance of the proposed network. An analytical model has been introduced to calculate the queuing delay experienced by the X2 interface. The model has been validated with the simulations results.

Description: High spectrum efficiency and fast restoration speed are highly desired for survivable elastic optical networks (EONs). In this paper, we take the advantages of failure-independent path-protecting preconfigured cycles (FIPP p -cycles) and investigate how to realize spectrum efficient resilience design with them. We first study the problem of offline service provisioning with FIPP p -cycles. We formulate an integer linear programming model and prove that the problem is $mathcal{NP}$ hard. Then, several time-efficient heuristics are designed for FIPP p -cycle formulation and related routing, modulation format, and spectrum assignment. Extensive simulations on offline provisioning verify that the heuristics can obtain near-optimal solutions. Next, we consider online service provisioning with FIPP p -cycles in dynamic EONs. In order to overcome the decrease of protection efficiency during dynamic network operation, we propose a p -cycle reconfiguration scheme to reoptimize protection structures on-the-fly. Simulation results demonstrate that the proposed algorithms can improve spectrum efficiency and reduce bandwidth blocking probability simultaneously.

Description: Photo-thermo-refractive (PTR) glasses are the class of polyfunctional materials that combine the properties of several monofunctional materials such as photo-refractive, laser, luminescent, and plasmonic ones. Based on PTR glasses, various diffractive holographic elements and photonic devices were developed in both the volume and fiber versions. In this paper, the fabrication of optical planar waveguides on PTR glass by low-temperature ion exchange is reported for the first time. Planar waveguides were fabricated through substituting the sodium ions in glass by silver, potassium, rubidium, and cesium ones from the nitrate melts. The silver waveguides were shown to have the largest depth (27 μm) and reveal no birefringence. For the silver waveguides, an increase in the refractive index is caused by differences in the polarizabilities of cations exchanged. The maximum increment of the refractive index was observed in the cesium waveguides (0.0512). An increase in the refractive index and also appearing the birefringence in potassium, rubidium, and cesium waveguides are found to be due to the compressive mechanical stresses and their relaxation. The potentialities of the ion exchange technology for fabricating, in PTR glasses, planar gradient waveguides with low losses (0.5 dB/cm) are demonstrated, the potentialities extending the application field of PTR glasses in photonics.

Description: A multi-sensor addressing method for fiber Bragg grating aided fiber loop ringdown (FBG-FLRD) sensor array is proposed. It is capable of simultaneous measurement of temperature and force. Light from a wavelength-swept fiber laser (WSFL) is modulated into pulsed light to illuminate FBG-FLRD sensor array. Based on the time sequence of pulsed lights reflected by fiber Bragg grating array, each FBG-FLRD sensor can be distinguished. The time interval between the trigger signal of the WSFL and the pulsed light reflected by FBG offers temperature variation information. By measuring the ringdown time of each FBG-FLRD, the loss of fiber ring induced by temperature and force can be obtained. And then simultaneous measurement can be implemented in real time. To prove the validity of the proposed system, a six elements array is experimentally validated with interrogation frequency of 27.5 Hz.

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Description: We report the results of a study of an optical sensor based on a channel-drop technique with two cascaded cavities in photonic-crystal slabs. Quality factors and intensities of the resonant modes of the sensor were analyzed with three-dimensional simulations. With the introduction of a reflector in the bus-channel and by control of the coupling between the two cavities and the drop-channel, the drop efficiency can be remarkably increased. In addition to the simulation, the two cavity sensor is fabricated and tested for optical response to water and oil infiltration. Both direct visual imaging and quantitative analysis were applied in experiment. A difference of refractive index $Delta n = 0.12$ between water and oil samples results in a wavelength shift of 18.3 nm, which greatly matches the simulation result of 20 nm and indicates a sensitivity of 153 nm RIU −1 . Both resonant peaks for water and oil infiltration have good selectivity in their transmission spectrum. The contrast between the broadband output of the bus-channel and the highly wavelength-selective outputs of the drop-channel opens opportunities for the two cascaded-cavity system as a fundamental building block for a multiplex drop-channel array for all-optical sensing, which can be widely used for bio/chemical detection and environmental monitoring.

Description: Nonlinearity-induced phase noise has become a major obstacle in long-haul coherent fiber-optic communication systems. Such phase noise has been shown to be signal dependent, and correlated over time. We propose a code-aided expectation-maximization algorithm to mitigate such nonlinear phase noise, iteratively utilizing both the time correlation of the nonlinearity-induced impairments and a soft-decision error-control code. Simulation and experimental results show that on a dual-polarization wavelength-division-multiplexed 16 QAM system, launch-power tolerance can be increased by ${1.5}$ dB, and the optical signal-to-noise ratio requirement can be relaxed by ${0.3}$ dB to achieve the same ${Q^2}$ -factor.

Description: Microring resonators have been the fundamental building blocks of integrated photonic networks. They play key roles in a number of on-chip photonic elements, such as filters, modulators, switches, and sensors. However, the applications of microring resonators in these components are hindered by the evanescent coupling, which requires very precise position control and is hard to be realized in standard photolithography. Here, we demonstrate a novel, general, and robust mechanism to achieve microdisk-based end-fire injection and collection devices. Instead of coupling light via evanescent waves, here the light is injected into the cavity by connecting a waveguide and a deformed microdisk directly. The light on resonance can be reflected back and detected at the same waveguide, making the devices to be “easy come easy go.” The reflectance is as high as 95% and can be periodically obtained from the long-lived resonances with free spectral range around 18 nm. The proposed mechanism is found to be robust to cavity shape, refractive index, and waveguide width and all the parameters can be larger than 500 nm, making the designed devices to be easier realized by standard photolithography. We believe this research will boost the development of low-cost silicon photonic devices.

Description: We proposed theoretically analyzed and experimentally demonstrated novel 1 × 4 channel flatband optical multiplexer/demultiplexer (MUX/DeMUX) that is based on Si-nanowire microring-assisted multiple delaylines. The proposed optical MUX/DeMUX scheme, which includes all-pass microring-type phase controllers and a Banyan-type 4 × 4 coupler, exhibits superior spectral flatness to any Si-nanowire optical DeMUXs that have previously been experimentally demonstrated.

Description: Analysis of the experimental data on microscopic, absorptive, fluorescent, and Raman-scattering properties of Bismuth (Bi)-doped yttria-alumino-silicate glass (Y-Al-SiO 2 :Bi)-based nanoengineered optical fibers, exhibiting broadband near-infrared fluorescence, is presented. Among the other well-established characteristics, inherent to Bi-doped silica fibers codoped with Aluminum (Al), a trend of spatial distributions of Bi atoms and Bi-related fluorescence-active centers is determined, being their concentrating in ring-like areas around the core's center, at approximately a half-distance to core/cladding interface. At the same time, the formation in this region of nanosized Bi clusters, supposedly weakly or nonfluorescing, is revealed for the fibers. These phenomena are argued to underlie worsening of the pump-to-signal overlap factor, which deteriorates efficiency of lasers and amplifiers based on such or similar Bi-doped alumino-silicate fibers.

Description: The paper develops a statistical model for the signals received in phase-sensitive optical time domain reflectometry (OTDR) probed by highly coherent sources. The backscattering process is modelled by a set of discrete scatterers with properly chosen parameters. Explicit equations for calculating the amplitude and the phase of the backscattered signal are obtained. The developed model predicts spectral and autocorrelation characteristics of the amplitude signals that are validated by experimental results. Characteristics of the phase signals, practicable for studying the sensing applications of the OTDR system, are presented and studied as well, demonstrating good correspondence with experiment. A more detailed modelling of distributed vibration sensing systems and their response to disturbances along an optical fiber will be possible as an extension of the developed formalism.

Description: A five channel step index plastic optical fiber proposal for a multiplexer/demultiplexer having insertion losses ( IL ) of 2.9–4 dB, pass bandwidths at −3 dB > 30 nm, crosstalk attenuation >30 dB and size of ∼65 mm × 55 mm, is demonstrated. It is based on a reflective diffraction grating with blazed profile and an aspheric lens. The theoretical analysis presented is used to further reduce the system size to ∼37 mm × 30 mm and to increase the number of channels to 8 keeping ILs 〈 4.5 dB. Experimental results have good agreement with theoretical expectations.

Description: The design and development of a plastic optical fiber macrobend temperature sensor is presented. The sensor can operate in a temperature range from −55 to 70 °C and has a linear response versus temperature with a sensitivity of 8.95·10 −4 °C −1 . The sensor system uses the ratio of transmittance at two wavelengths to implement a self-referencing technique in order to avoid undesirable power fluctuations influence. The transmittance ratio precision is 0.1%. An analysis has been developed to find the two wavelengths which ratio offers the highest linearity and sensitivity response. Experimental results are successfully compared with theoretical approaches.

Description: We review recent high capacity ultralong haul demonstrations using amplification schemes with extended C+L optical bandwidth and optimized nonlinear performance. These demonstrations achieve 49.3 Tb/s capacity over 9100 km with the use of C+L EDFAs and 54 Tb/s capacity over 9150 km using hybrid-Raman EDFAs. Using the same hybrid-Raman EDFAs, a capacity of 52.2 Tb/s over 10 230 km (534 Pb/s*km) was also demonstrated. Different combinations of three 16QAM-based coded modulation schemes with spectral efficiencies SE = 4.86, 5.4/5.45, 6.14 bit/s/Hz were used in order to maximize capacity for each amplification scheme and transmission distance. We also compare the merits of optimized transmission performance of hybrid Raman-EDFA and C+L EDFA amplification schemes based on our loop experiments. At the nonlinear limit, the use of hybrid Raman-EDFA provides a modest 9.5% increase in capacity at a cost of ∼2× higher electrical power.

Description: Flexible grid optical networks allow a better exploitation of fiber capacity, by enabling a denser frequency allocation. A tighter channel spacing, however, requires narrower filters, which increase linear intersymbol interference (ISI), and may dramatically reduce system reach. Commercial coherent receivers are based on symbol by symbol detectors, which are quite sensitive to ISI. In this context, Nyquist spacing is considered as the ultimate limit to wavelength-division multiplexing (WDM) packing. In this paper, we show that by employing a limited-complexity trellis processing at the receiver, either the reach of Nyquist WDM flexi-grid networks can be significantly extended, or a denser-than-Nyquist channel packing [i.e., a higher spectral efficiency (SE)] is possible at equal reach. By adopting well-known information-theoretic techniques, we design a limited-complexity trellis processing and quantify its SE gain in flexi-grid architectures where wavelength selective switches over a frequency grid of 12.5 GHz are employed.

Description: We analyze the shortest path lengths between node pairs of real optical transport networks. From the analysis, we find that Johnson ${rm S}_{B}$ distribution is suitable for the shortest path length modeling. The validity of the distributions is evaluated in terms of the Kolmogorov–Smirnov (KS) statistic. Johnson ${rm S}_{B}$ distribution provides an average KS statistic of 0.0423, which indicates its good accuracy. We also show that the key parameters of the shortest path lengths, such as the mean, the median, and the standard deviation, can be estimated from the convex area of the network. We develop the proposed Johnson ${rm S}_{B}$ distribution model for the shortest path lengths using the basic information of the networks. The developed model is able to estimate path-length dependent system parameters, such as the appropriate modulation formats in transparent optical networks with an average error of only $6.4{%}$ . It is noteworthy that these estimations can be made without full knowledge of the network. Only the node locations are required.

Description: We present a comprehensive numerical model for intrinsic small-signal modulation response of both reflective, and traveling-wave semiconductor optical amplifiers. We investigate the small-signal photon and carrier density spatial distribution, modulation response, and $-3text{dB}$ bandwidth for uniform and lossless traveling-wave modulation current model. The analysis shows that the current model does not significantly affect the modulation response as long as the modulation frequency is within the bandwidth. One of the most important results of our analysis is the discovery of the bandwidth maximum in case of a reflective semiconductor optical amplifier operating with low to moderate input optical powers and high current densities. The bandwidth can be further improved by choosing the optimal amplifier length.

Description: We study the effect of phase noise canceled polarization-insensitive all-optical wavelength conversion (AOWC) of orthogonal frequency-division multiplexing (OFDM) signals, which is based on four-wave-mixing in high-nonlinear optical fiber using dual-pump. An AOWC experiment of polarization division multiplexing OFDM (PDM-OFDM) 8/16/32-QAM signals is investigated. A DFB laser with linewidth of 10 MHz is employed as the coherent dual-pump seed. The measured results show that the received signal after wavelength conversion by utilizing coherent DFB dual-pump has the same BER performance as back-to-back. Furthermore, AOWC of 557-Gb/s superchannel discrete Fourier transform-spread PDM-OFDM with eight-QAM (DFT-S PDM-OFDM 8-QAM) signal is proposed and experimentally demonstrated based on the studied coherent dual-pump scheme. Negligible OSNR penalty (〈0.8 dB) is observed at 7% FEC limit ( ${rm BER} = 3.8 times 10^{-3}$ ) after wavelength conversion. Our demonstration shows that the phase noise transferred from the pumps can be effectively eliminated. It also shows that AOWC enables practical implementation on the polarization multiplexing dynamic optical networks at the nodes where wavelengths conflict.

Description: This article presents results from the integration of a noncontact physiological radar monitoring system (PRMS) with a type I polysomnography (PSG) system to perform sleep monitoring. The PRMS system consists of two continuous-wave Doppler radars operating at the industrial, scientific, and medical (ISM) band of 2.45 GHz. The system can acquire data, perform digital processing, and output appropriate conventional analog outputs with a latency of approximately 130 ms, which can be recorded and displayed by a gold standard sleep monitoring system along with other standard sensor measurements. Radar data was also used to successfully detect paradoxical motion that was simulated using linear movers and to categorize normal breathing, apnea, and hypopnea in sleeping subjects with obstructive sleep apnea (OSA).

Description: A quantitative and explicit validation of the performance and safety of microwave systems and devices that have electromagnetic interaction with the human body is critical in the technological development process. Although a numerical model of the system environment can be ideally simulated, it cannot reflect the realistic environment that is vulnerable to various electrical, mechanical, and environmental interferences. Hence, the presence of the human body is the best measurement environment for these systems. However, newly designed devices/ systems that rely on the human body electromagnetic field interactions require multiple tests/measurements under a controlled environment. This environment is needed to validate the performance in all the possible scenarios of operation and make sure of the safety of those devices and systems. For example, a breast imaging system needs to be evaluated by detecting tumors in multiple locations, and it is unthinkable to do that on a real patient; hence, breast phantoms are needed to obtain an optimum system design and algorithms before moving to human clinical trials. Moreover, some experiments, such as the specific absorption rate (SAR) and hyperthermia, cannot be done on human beings due to the need to monitor the variation of the power intensity and temperature inside tissues. Employing live human participants for testing devices exposes the entire test procedure to several inherent uncertainties, such as respiratory movement, cardiovascular vibration, and variable skin humidity in addition, of course, to the safety concern of the new devices. Also, the application of the devices and systems on human subjects or human-related materials is a serious ethical issue where the researchers must receive an ethical clearance from the proper authorities, and it can be difficult to reasonably estimate and investigate the level of risks from various scientific, physical, and psychological aspects beforehand. Thus, the utilization of ar- ificial tissue-emulating (ATE) phantoms is highly beneficial for testing a device or system.

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Description: We report record 10 Gb/s bi-directional data transmission over a single 10 m SI-POF, by employing blue microlight-emitting diodes (μLEDs) at a single wavelength, APD receivers, and a PAM-32 modulation scheme. The implementation of 10 Gb/s LED-POF links takes advantage of the bi-directional configuration, which doubles the overall channel capacity, and APDs, which provide an enhanced link power budget owing to their improved sensitivity compared with conventional p-i-n photodiodes. Moreover, the high spectral efficiency of the PAM-32 modulation scheme employed, together with equalization techniques, enable the full utilization of the link bandwidth and the transmission of data rates higher than those obtained with conventional on–off keying. Simulation and experimental results demonstrate the feasibility of such a bi-directional link, and simultaneous 5 Gb/s data transmission is realized in each direction, achieving an aggregate data rate of 10 Gb/s with a BER 〈 10 −3 . The crosstalk penalty between the two directions of the link is measured to be less than 0.5 dB.

Description: Integrated polarization converters (PCs) with high (>99%) conversion efficiency open up many new possibilities in an InP-based photonic integrated circuit. In this paper, we describe how such a PC can be added to a circuit containing semiconductor optical amplifiers (SOAs), in order to obtain polarization independent amplification. Polarization independence is obtained by placing the PC halfway between two identical SOA sections. This approach has the advantage that no compromises in design and fabrication are needed. The polarization conversion is found to be very high, above 99.5%, when using a tolerant two-section PC. The extra insertion loss due to the converter is below 0.5 dB. The polarization-dependent gain (PDG) of the SOA reduces from 17 dB to only 0.3 dB by the inclusion of the PC. This is comparable to the best PDG values found in the literature with other techniques. The reduction is achieved over the whole C-band and for varying pump currents.

Description: We present the design, fabrication, and measurement results of low-insertion-loss and low-crosstalk broadband $2times 2$ Mach–Zehnder switches for nanosecond-scale optical data routing applications. We propose a simulation framework to calculate the spectral characteristics of switches and use it to design two switches: one based on directional couplers, the other using two-section directional couplers for broader bandwidth. We show that driving the switch in a push–pull manner enables to reduce insertion loss and optical crosstalk at the expense of the optical bandwidth. We achieve a good correlation between simulations and devices fabricated in IBM's 90-nm photonics-enabled CMOS process. We demonstrate a push–pull drive switch with insertion loss of $sim$ 1 dB and an optical crosstalk smaller than $-$ 23 dB over a 45-nm optical bandwidth in the O-band. We further achieve a transition time of $sim$ 4 ns with an average phase shifter consumption of 1 mW and a heater efficiency of $sim$ 25 mW $/pi$ .

Description: The optical spectra of composite one-dimensional photonic crystals (1-D PCs), fabricated by microstructuring of Si, are investigated. The composite PC is based on a Si-air structure and consists of two periodic 1-D PCs with various lattice constants a with $a_{1} = 2.7$ μm and $a_{2} = 4$ μm and various filling fractions f . The PCs are fabricated using photolithography followed by anisotropic chemical etching of (1 1 0) Si. Reflection and transmission spectra of this structure are measured using a Fourier transform infrared spectrometer combined with an IR microscope. The transmission spectra obtained demonstrate extended photonic stop bands (SBs), with characteristic transmission bands between the SBs. Theoretical and experimental transmission results indicate that the position of the extended SBs does not depend on the sequence of the individual PCs within the composite structure. In general, the calculated electric field distribution in the composite structure is similar to that of the individual PC components.

Description: We investigate five-channel polarization division multiplexed Nyquist WDM superchannel using offset-16QAM and 16QAM modulation format, under the condition of transmitter-side digital spectral shaping technique, respectively. The offset-16QAM scheme can greatly relax the implementation complexity, because the tap number of 35 is sufficient for the required finite impulse response (FIR) filter, compared with 81-tap FIR filter of the 16QAM scheme. About 1.3-dB back-to-back sensitivity improvement is obtained for the offset-16QAM scheme. In particular, the offset-16QAM scheme has better tolerance of DACs’ constraints, including the untracked jitter, sampling rate, and resolution. Finally, the practical implementation of offset-16QAM-based Nyquist WDM superchannel with TS-DSS is evaluated, by taking the phase difference deviation among wavelength channels and phase noise into account. No more than 1.3-dB required-OSNR penalty can be obtained, in case the phase difference is varied from 0° to 360°. When the laser with a linewidth of 100 kHz is used, there still exists about 1-dB overall performance improvement, compared with the 16QAM scheme.

Description: We investigate the optimum linewidth of the spectrum-sliced incoherent light (SSIL) source using a gain-saturated semiconductor optical amplifier (SOA) for the maximum capacity and longest transmission distance. For this purpose, we carry out experimental and simulation studies on the transmission performance of a 10-Gb/s on–off keying signal generated by using the SSIL source over a wide range of the SSIL linewidth. We find out that there are two windows of the linewidth for the high-speed operation of the SSIL source: ultra-narrow (i.e., linewidth $ll$ receiver bandwidth) and very wide (i.e., linewidth $gg$ receiver bandwidth). However, when the linewidth of the SSIL source is very wide, the 10-Gb/s signal generated by using this SSIL suffers severely from fiber chromatic dispersion and optical filtering. The simulation results are confirmed by experimental data measured by using an ultranarrow fiber Fabry–Perot filter (bandwidth = 700 MHz) and a bandwidth-tunable optical filter (bandwidth = 20 ∼ 53 GHz). Thus, we can conclude that the optimum linewidth of SSIL for capacity and transmission distance is ultranarrow. We also present a couple of drawbacks of the ultranarrow SSIL source, compared to the conventional wide-linewidth SSIL one, such as a large spectrum-slicing loss, a large SOA input power required for the suppression of excess intensity noise inherent in the incoherent light source, and the susceptibility to in-band crosstalk.

Description: The paper demonstrates a standardized process of vapor phase doping to fabricate large core Yb–doped preforms with longer useful length in reproducible manner. The optimization of the process led to successful achievement of Yb-doped core thickness of 4.5 mm (in 14.8 mm of preform diameter) by depositing up to 30 number of core layers with controlled amount of generated precursor vapors. The influence of the process parameters was studied rigorously to enhance the useful preform length up to 380 mm. A combination of Yb and Al in different proportions was doped into the core with uniform dopant concentration along the length by adjusting few process parameters efficiently. The Al 2 O 3 concentration up to the level of 17.8 mol% has been achieved successfully which resulted in NA of 0.31. This is the highest ever doping of Al in passive fibers by any modified chemical vapor deposition process. The Yb 2 O 3 content in the active fibers is as high as 0.47 mol%.

Description: In view of the increasing frequency and damage severity of disasters, network operators have become more concerned with providing disaster-resiliency measures for their optical network infrastructure, whereas mitigating network service interruption due to the disaster region failures in the optical physical medium merely by increasing network redundancy is deemed spatially inefficient and very costly, with recent advancements, wireless technology is a potential candidate solution for efficient medium diversification. This paper addresses the challenge of efficiently designing disaster-resilient wireless-link-augmented optical network infrastructure. We formulate this problem as an optimization model of finding the subset of links in an optical network topology whose wireless augmentation maximizes postdisaster recovery of overall network availability for a given budget constraint. To overcome the computational complexity of finding the optimal design solution, a novel greedy heuristic algorithm is proposed. Performance comparisons with an exhaustive enumeration search and simple heuristics demonstrate the efficiency and scalability of our heuristic algorithm.

Description: In order to characterize the channel capacity of a wavelength channel in a wavelength-division multiplexed (WDM) system, statistical models are needed for the transmitted signals on the other wavelengths. For example, one could assume that the transmitters for all wavelengths are configured independently of each other, that they use the same signal power, or that they use the same modulation format. In this paper, it is shown that these so-called behavioral models have a profound impact on the single-wavelength achievable information rate. This is demonstrated by establishing, for the first time, upper and lower bounds on the maximum achievable rate under various behavioral models, for a rudimentary WDM channel model.

Description: This paper demonstrates the feasibility of layered space-time coding (STC) in an outdoor image-sensor-based (IS-based) visible light communication (VLC) system. We examined that for low-resolution IS-based VLC channel where intensity-modulated signals from two different light emitting diodes (LEDs) are detected by one pixel of an IS, STC allows us to decouple them; thus, succeeding to receive them with no errors. Consequently, STC offers extended transmission distance to pixel-resolution-limited IS-based VLC links. In the layered STC presented in this paper, additional bit streams are laid on the ${2^ntimes 2^n}$  LED array for increasing the transmission rate per symbol duration for the case where the pixel resolution is improved. A prototype of a three-layered STC is built with an ${8times 8}$  LED array, where each of the LEDs is modulated at 1 kb/s and a high-speed camera with IS operating at 1000 fps. Our experimental results validate that the two additional bit streams (layer-2 and -3), aligned in the layer-1 STC matrix pair, are extracted with no errors when the receiver comes within 155 and 55 m, respectively, from the LED array, without decreasing 210 m of the transmission distance of layer-1 bit stream.

Description: We present simulation and experimental results on a silicon photonic strictly nonblocking $4times 4$ electrooptic Mach–Zehnder-based switch fabric. We propose a simulation framework based on the transfer matrix approach that enables calculating the transmission spectra of any type of multistage interconnect switch network. The model is used to analyze the spectral characteristics of the switch fabric. We also show experimental results on a fabric designed and fabricated in IBM's 90-nm photonics-enabled CMOS process. The fabric monolithically integrates the CMOS logic, the switch drivers, and all the photonics. We fully characterized all the transmittances of the switch and demonstrate onchip insertion loss between 1.5 and 3 dB and a crosstalk less than –25 dB for all the signal paths.

Description: The FEC limit paradigm is the prevalent practice for designing optical communication systems to attain a certain bit error rate (BER) without forward error correction (FEC). This practice assumes that there is an FEC code that will reduce the BER after decoding to the desired level. In this paper, we challenge this practice and show that the concept of a channel-independent FEC limit is invalid for soft-decision bit-wise decoding. It is shown that for low code rates and high-order modulation formats, the use of the soft-decision FEC limit paradigm can underestimate the spectral efficiencies by up to 20%. A better predictor for the BER after decoding is the generalized mutual information, which is shown to give consistent post-FEC BER predictions across different channel conditions and modulation formats. Extensive optical full-field simulations and experiments are carried out in both the linear and nonlinear transmission regimes to confirm the theoretical analysis.

Description: We propose an analytical time-domain model for microring and microdisk modulators, which considers both their electrical and optical properties. Theory of the dynamics of microring/microdisk is discussed, and general solutions to the transfer matrix representation are presented. Both static and dynamic predictions from the model are compared to measurement results to demonstrate the accuracy of our model. Static predictions and measurements are presented for power and phase responses, whereas dynamic predictions and measurements are presented for small-signal and large-signal operations. The model verifies that the chirping and modulation bandwidth of the modulators depend on the detuning state. Finally, the accuracy and scalability of several techniques employed in the model are discussed.

Description: We experimentally investigate an improved fast orthogonal frequency division multiplexing (OFDM) scheme using intensity modulation and full-field detection. This new fast OFDM algorithm exhibits better back-to-back and transmission performance than the conventional one, and is shown to support a 40-Gb/s signal over a 480-km re-circulating loop-based single-mode fiber (SMF) with 1-dB penalty. We compare this scheme with the direct-detection (DD) system using the same algorithm, and show that the DD system cannot support 60-km SMF at 40 Gb/s. Finally, we demonstrate the proposed scheme over 124 km of BT Ireland's field-installed fiber without inline optical amplification. The results show that this scheme can be a promising solution to address the gap between direct detection and coherent detection for applications in short metro networks and long-reach Ethernet and access networks.

Description: We describe branched-fiber sensing with a novel Brillouin time-domain analysis configuration for temperature or strain sensing within a 2-D expanded area. By using a branch configuration with an appropriate number of branches and the newly proposed multiprobe pulse arrangement, the system dynamic range may surpass that of the conventional Brillouin time-domain analyzer, which employs a unicursal fiber configuration. The proposed Brillouin sensing would be useful particularly when the sensed area is 2-D, rather than when the area expands 1-D.

Description: The boundaryless beam propagation method has been reported to suffer from reflections due to frequency aliasing. We propose an alternate explanation of reflection in the method, based on eikonal analysis of the wave equation in the mapped space, and show that reflection starts much before aliasing happens. We theoretically predict the reflection coefficient profile in a simulation and introduce an internal absorbing boundary condition (ABC), where reflection becomes appreciable. The a priori knowledge of the window size of the ABC does away with the arbitrariness in the design of such boundaries. Finally, we use the boundaryless scheme with the ABC to successfully model a dielectric bend.

Description: This study presents a photonic integrated transceiver for application in a data readout unit of a sensor network. The device was realized in a generic InP-based technology. The circuit uses an asymmetric coupler and a PIN photodiode for input signal detection and a Mach–Zehnder amplitude modulator for encoding of output sensor data. Small-signal modulation bandwidth of 11.1 GHz was measured, eye-diagrams with a dynamic extinction ratio of 11 dB were recorded, and transmission of a 10-Gb/s signal over 25 km of SMF fiber with bit error rate below 10 −10 was achieved.

Description: We report a new low-noise compact fiber acoustic sensor that implements a microfabricated silicon diaphragm with a π/2 phase step combined to a single-mode fiber to form a simple interferometric sensor head. Compared to high-sensitivity diaphragm-based fiber Fabry–Perot sensors, this sensor has a similar strain resolution, it operates over a much broad range of wavelengths (∼±150 nm), its response depends very weakly on temperature and on the distance between the fiber and the diaphragm, and both microfabrication and assembly are simpler. A prototype sensor is reported with an average minimum detectable pressure as low as 5.4 μPa/√Hz between 1 and 30 kHz, in agreement with a theoretical model.

Description: Polymer waveguides with different lengths have been fabricated between two single mode fibers by using a new approach of self-writing using single photon polymerization via a xenon lamp instead of a monochromatic laser source. A photopolymerizable liquid drop deposited between two aligned fibers forms a polymer bridge between the cores of the two fibers when irradiated by light from one of the fibers. Polymer waveguides bridges 40 to 600 μm long have been fabricated between two optical fibers with single mode transmission loss 0.5 to 1.26 dB over a broad wavelength range.

Description: In perturbation-based digital fiber nonlinearity mitigation methods, such as pre-distortion and post-equalization, quantization of perturbation coefficients is employed to reduce the computational and implementation complexity. However, quantization introduces errors, which results in performance penalties. By analyzing the occurrence probability of the perturbation coefficients, an MMSE-based optimization of perturbation coefficients quantization is proposed and demonstrated in ultra-large-area-fiber transmission systems with a 36Gbd DP-16QAM modulation format. Compared to conventional uniform quantization schemes, a significant reduction in quantization errors has been realized for both the real and imaginary parts of the perturbation coefficients. Compared with the nonquantization case, computational term reduction by a factor of 50 is achieved in 2400 km ULAF transmission with 〈 0.2 dB Q degradation. For the same Q improvement, optimized quantization scheme further reduce the computational term by 50% compared to that of the uniform quantization method. We also analyze the tolerance to link dispersion.

Description: Presents the 2015 author/subject index for this publication.

Description: Most advanced wireless communication systems use spectrally efficient complex modulation to pack more users into the crowded frequency spectrum. Complex modulation simultaneous modulates a signal's amplitude and phase (polar coordinates) or, equivalently, the I and Q components (rectangular coordinates).

Description: Satellite-based systems are used countless times a day. The navigation systems in our cars and smartphones employ a minimum of seven to ten satellites to give us directions; several satellites are required when we check weather conditions and in mapping our location when we search for a local restaurant; when calling abroad, when surfing the Internet on an airplane or ocean cruise, when the Super Bowl is broadcast live worldwide-in cases like these and many more, several satellites must be used at the same time. The failure of a transmitter onboard any one of these satellites can lead to a service interruption that is eventually translated into a negative economic impact for the satellite operators. In short, reliability in space is essential.