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# Journal of Lightwave Technology - new TOC

## TOC Alert for Publication# 50

Sept.15, 2017

Optical Heterodyne Microvibration Detection Based on All-Fiber Acousto-Optic Superlattice Modulation

Sept.15, 15 2017

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

Performance Degradation of Typical 1550 nm Optical Intersatellite Communication Systems in Space Ionizing Radiation Environment

Sept.15, 15 2017

Mixed Channel Traffic Grooming for IP Over EON with SBPP-Based Cross-Layer Protection

Sept.15, 15 2017

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

Multiple Particles 3-D Trap Based on All-Fiber Bessel Optical Probe

Sept.15, 15 2017

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

Design of a Wavelength-Tunable Optical Tweezer Using a Graded-Index Multimode Optical Fiber

Sept.15, 15 2017

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

Unique Word DMT Schemes for Optical Systems With Intensity Modulation and Direct Detection

Sept.15, 15 2017

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

Sensitivity Prediction of Multiturn Fiber Coil-Based Fiber-Optic Flexural Disk Seismometer via Finite Element Method Analysis

Sept.15, 15 2017

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

Design and Implementation of an Optical Receiver for Angle-of-Arrival-Based Positioning

Sept.15, 15 2017

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

Sept.15, 15 2017

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

Low-Profile High-Power Optically Addressed Phased Array Antenna

Sept.15, 15 2017

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

Photonic Generation of High Fidelity RF Sources for Mobile Communications

Sept.15, 15 2017

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

Tomlinson–Harashima Precoding For Dispersion Uncompensated PAM-4 Transmission With Direct-Detection

Sept.15, 15 2017

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

Bulk-Silicon-Based Waveguides and Bends Fabricated Using Silicon Wet Etching: Properties and Limits

Sept.15, 15 2017

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

Triaxial Fiber Optic Magnetic Field Sensor for Magnetic Resonance Imaging

Sept.15, 15 2017

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

Control of Widely Tunable Lasers With High-Q Resonator as an Integral Part of the Cavity

Sept.15, 15 2017

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

Relative Humidity Sensing Using a PMMA Doped Agarose Gel Microfiber

Sept.15, 15 2017

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

Non-Saturation Delay Analysis of Medium Transparent MAC Protocol for 60 GHz Fiber-Wireless Towards 5G mmWave Networks

Sept.15, 15 2017

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

Accurate and Fast Demodulation Algorithm for Multipeak FBG Reflection Spectra Using a Combination of Cross Correlation and Hilbert Transformation

Sept.15, 15 2017

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

Nonlinear Mitigation of a 400G Frequency-Hybrid Superchannel for the 62.5-GHz Slot

Sept.15, 15 2017

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

SRI-Immune Highly Sensitive Temperature Sensor of Long-Period Fiber Gratings in Ge–Sb–Se Chalcogenide Fibers

Sept.15, 15 2017

This work exhibits design and characteristic study of highly sensitive temperature sensors of long-period fiber gratings (LPFGs) in Ge–Sb–Se fibers. Temperature characterization of the designed LPFGs with different working modes, operating wavelengths, and surrounding refractive indices (SRI) were studied. Results showed that temperature sensitivity of the proposed Ge–Sb–Se LPFGs reached 0.5787 nm/ at 1.55 μm at the lowest LP02 cladding mode, which is approximately 12 times higher than silica LPFGs and 1.2 times higher than that of As–Se LPFGs. When grating period of the designed Ge–Sb–Se LPFG was selected at its dispersion-turning-point, temperature sensitivity reached a maximum absolute value of 24.715 nm/°C. Influences of operating wavelength and temperature sensitivity of these LPFGs on SRI are extremely weak due to relatively high refractive index of Ge–Sb–Se fiber, indicating that the designed Ge–Sb–Se LPFG temperature sensor possesses extremely strong sensing stability on SRI.

Virtual Network Embedding for Collaborative Edge Computing in Optical-Wireless Networks

Sept.15, 15 2017

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

Low-Complexity Soft-Decision Concatenated LDGM-Staircase FEC for High-Bit-Rate Fiber-Optic Communication

Sept.15, 15 2017

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

SBS Threshold Dependence on Pulse Duration in a 2053 nm Single-Mode Fiber Amplifier

Sept.15, 15 2017

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

Achievable Rates of Space-Division Multiplexed Submarine Links Subject to Nonlinearities and Power Feed Constraints

Sept.15, 15 2017

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

Semi-Analytical Modelling of Linear Mode Coupling in Few-Mode Fibers

Sept.15, 15 2017

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

Wideband Photonic Microwave SSB Up-Converter and I/Q Modulator

Sept.15, 15 2017

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

Asymmetric Optical Bus Coupler for Interruption-Free Short-Range Connections on Board and Module Level

Sept.15, 15 2017

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

Dual-Core Optical Fiber as Beam Splitter With Arbitrary, Tunable Polarization-Dependent Transfer Function

Sept.15, 15 2017

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

Benchmarking Si, SiGe, and III–V/Si Hybrid SIS Optical Modulators for Datacenter Applications

Sept.15, 15 2017

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

Electrooptical Comparator: From Formula to Implementation

Sept.15, 15 2017

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

High-Sensitivity Gas-Pressure Sensor Based on Fiber-Tip PVC Diaphragm Fabry–Pérot Interferometer

Sept.15, 15 2017

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

Nonlinear Compensation Assessment in Few-Mode Fibers via Phase-Conjugated Twin Waves

Sept.15, 15 2017

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

Sept.15, 15 2017

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

A Humidity Sensor Based on a Singlemode-Side Polished Multimode–Singlemode Optical Fibre Structure Coated with Gelatin

Sept.15, 15 2017

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