Performance enhancement of an ultrafast graphene photodetector via simultaneous two-mode absorption in a hybrid plasmonic waveguide.

An ultrafast, compact, zero-biased, and complementary metal-oxide semiconductor-compatible graphene photodetector (PD) based on a silicon-on-insulator hybrid plasmonic waveguide (HPWG) is proposed. Lumerical MODE solver is employed to investigate the modal characteristics of TM-polarized modes in the HPWG composing the PD. It is shown that the input light can be completely coupled into the photonic-like and plasmonic-like fundamental TM modes at the PD section. These two modes are exploited together in the photodetection process to enhance the PD performance. A rigorous analysis is performed in order to extract the optoelectronic characteristics of the single-layer graphene (SLG) used in the proposed structure. Lumerical 3D-FDTD solver is then employed to quantify the light interaction of the two aforementioned optical modes with the SLG. With a proper design at a wavelength of 1550 nm, the PD voltage responsivity reaches 2.8 V/W, and the photocurrent responsivity is obtained as 18.5 mA/W, while the corresponding absorption length is kept below 8µm and the noise equivalent power is limited to 3.7pW/Hz. Moreover, as the PD operates under zero bias, its photoresponse is predominated by the photothermoelectric mechanism, exhibiting a bandwidth that exceeds 180 GHz while avoiding the dark current.

Efficiency analysis of cellular/LiFi traffic offloading.

Data offloading is a promising low-cost and power-efficient solution for the expected high demands for high-speed connectivity in the near future. We investigate offloading efficiency in a cellular/light fidelity (LiFi) network. This offloading efficiency is a measure of the ratio of traffic carried by the LiFi network to the total traffic carried by both LiFi and cellular networks. We consider the two scenarios of opportunistic and delayed offloading. Effects of user density, user mobility, LiFi-signal blocking, and channel characteristics are investigated. We use Zemax to simulate LiFi channels in the proposed model. Based on our results, delayed offloading can achieve up to 60% offloading efficiency while opportunistic offloading achieves up to 18% offloading efficiency.

Optical wireless communication performance enhancement using Hamming coding and an efficient adaptive equalizer with a deep-learning-based quality assessment.

Optical wireless communication (OWC) technology is one of several alternative technologies for addressing the radio frequency limitations for applications in both indoor and outdoor architectures. Indoor optical wireless systems suffer from noise and intersymbol interference (ISI). These degradations are produced by the wireless channel multipath effect, which causes data rate limitation and hence overall system performance degradation. On the other hand, outdoor OWC suffers from several physical impairments that affect transmission quality. Channel coding can play a vital role in the performance enhancement of OWC systems to ensure that data transmission is robust against channel impairments. In this paper, an efficient framework for OWC in developing African countries is introduced. It is suitable for OWC in both indoor and outdoor environments. The outdoor scenario will be suitable to wild areas in Africa. A detailed study of the system stages is presented to guarantee the suitable modulation, coding, equalization, and quality assessment scenarios for the OWC process, especially for tasks such as image and video communication. Hamming and low-density parity check coding techniques are utilized with an asymmetrically clipped DC-offset optical orthogonal frequency-division multiplexing (ADO-OFDM) scenario. The performance versus the complexity of both utilized techniques for channel coding is studied, and both coding techniques are compared at different coding rates. Another task studied in this paper is how to perform efficient adaptive channel estimation and hence equalization on the OWC systems to combat the effect of ISI. The proposed schemes for this task are based on the adaptive recursive least-squares (RLS) and the adaptive least mean squares (LMS) algorithms with activity detection guidance and tap decoupling techniques at the receiver side. These adaptive channel estimators are compared with the adaptive estimators based on the standard LMS and RLS algorithms. Moreover, this paper presents a new scenario for quality assessment of optical communication systems based on the regular transmission of images over the system and quality evaluation of these images at the receiver based on a trained convolutional neural network. The proposed OWC framework is very useful for developing countries in Africa due to its simplicity of implementation with high performance.

Underwater localization system based on visible-light communications using neural networks.

Underwater localization using visible-light communications is proposed based on neural networks (NNs) estimation of received signal strength (RSS). Our proposed work compromises two steps: data collection and NN training. First, data are collected with the aid of Zemax OpticStudio Monte Carlo ray tracing software, where we configure 40,000 receivers in a $100\;{\rm m} \times 100\;{\rm m}$ area in order to measure the channel gain for each detector in seawater. The channel gains represent the input data set to the NN, while the output of the NN is the coordinates of each detector based on the RSS intensity technique. Next, an NN system is built and trained with the aid of Orange data mining software. Several trials for NN implementations are performed, and the best training algorithms, activation functions, and number of neurons are determined. In addition, several performance measures are considered in order to evaluate the robustness of the proposed network. Specifically, we evaluate the following parameters: classification accuracy (CA), area under the curve (AUC), training time, testing time, F1, precision, recall, logloss, and specificity. The corresponding measures are as follows: 99.1% for AUC and 98.7% for CA, F1, precision, and recall. Further, the performance results of logloss and specificity are 7.3% and 99.3% respectively.

Free-space 16-ary orbital angular momentum coded optical communication system based on chaotic interleaving and convolutional neural networks.

Recently, orbital angular momentum (OAM) rays passing through free space have attracted the attention of researchers in the field of free-space optical communication systems. Throughout free space, the OAM states are subject to atmospheric turbulence (AT) distortion leading to crosstalk and power discrepancies between states. In this paper, a novel chaotic interleaver is used with low-density parity-check coded OAM-shift keying through an AT channel. Moreover, a convolutional neural network (CNN) is used as an adaptive demodulator to enhance the performance of the wireless optical communication system. The detection process with the conjugate light field method in the presence of chaotic interleaving has a better performance compared to that without chaotic interleaving for different values of propagation distance. Also, the viability of the proposed system is verified by conveying a digital image in the presence of distinctive turbulence conditions with different error correction codes. The impacts of turbulence strength, transmission distance, signal-to-noise ratio (SNR), and CNN parameters and hyperparameters are investigated and taken into consideration. The proposed CNN is chosen with the optimal parameter and hyperparameter values that yield the highest accuracy, utmost mean average precision (MAP), and the largest value of area under curve (AUC) for the different optimizers. The simulation results affirm that the proposed system can achieve better peak SNR values and lower mean square error values in the presence of different AT conditions. By computing accuracy, MAP, and AUC of the proposed system, we realize that the stochastic gradient descent with momentum and the adaptive moment estimation optimizers have better performance compared to the root mean square propagation optimizer.

Modulation format identification of optical signals: an approach based on singular value decomposition of Stokes space projections.

In this paper, two Stokes space (SS) analysis schemes for modulation format identification (MFI) are proposed. These schemes are based on singular value decomposition (SVD) and Radon transform (RT) for feature extraction. The singular values (SVs) are extracted from the SS projections for different modulation formats to discriminate between them. The SS projections are obtained at different optical signal-to-noise ratios (OSNRs) ranging from 11 to 30 dB for seven dual-polarized modulation formats. The first scheme depends on the SVDs of the SS projections on three planes, while the second scheme depends on the SVDs of the RTs of the SS projections. Different classifiers including support vector machine (SVM), decision tree (DT), and K-nearest neighbor (KNN) for MFI based on the obtained features are used. Both simulation and experimental setups are arranged and tested for proof of concept of the proposed schemes for the MFI task. Complexity reduction is studied for the SVD scheme by applying the decimation of the projections by two and four to achieve an acceptable classification rate, while reducing the computation time. Also, the effect of the variation of phase noise (PN) and state of polarization (SoP) on the accuracy of the MFI is considered at all OSNRs. The two proposed schemes are capable of identifying the polarization multiplexed modulation formats blindly with high accuracy levels up to 98%, even at low OSNR values of 12 dB, high PN levels up to 10 MHz, and SoP up to 45°.

On the performance of adaptive hybrid MQAM-MPPM scheme over Nakagami and log-normal dynamic visible light communication channels.

In this paper, we introduce the idea of using adaptive hybrid modulation techniques to overcome channel fading effects on visible light communication (VLC) systems. A hybrid $ M $M-ary quadrature-amplitude modulation ($ M{\rm QAM} $MQAM) and multipulse pulse-position modulation (MPPM) technique is considered due to its ability to make gradual changes in spectral efficiency to cope with channel effects. First, the Zemax optics studio simulator is used to simulate dynamic VLC channels. The results of Zemax show that Nakagami and log-normal distributions give the best fitting for simulation results. The performance of $ M{\rm QAM} $MQAM-MPPM is analytically investigated for both Nakagami and log-normal channels, where we obtain closed-form expressions for the average bit-error rate (BER). The optimization problem of evaluating the hybrid modulation technique settings that lead to the highest spectral efficiency under a specific channel status and constraint of outage probability is formulated and solved using an exhaustive search. Our results reveal that the adaptive hybrid scheme improves system spectral efficiency compared to ordinary QAM and MPPM schemes. Our results reveal that the adaptive hybrid scheme improves system spectral efficiency compared to ordinary QAM and MPPM schemes. Specifically, at low average transmitted power, $ - 32\;{\rm dBm} $-32dBm, the adaptive hybrid scheme shows 280% improvement in spectral efficiency compared to adaptive versions of ordinary schemes. At higher power, $ - 20\;{\rm dBm} $-20dBm, 6.5% and 725% improvement are obtained compared to ordinary QAM and ordinary MPPM, respectively. Also, the adaptive hybrid scheme shows great improvement in average BER and outage probability compared to ordinary schemes. The hybrid scheme shows 28%, 34%, and 38% improvement, respectively, for $ m = 1,2,3 $m=1,2,3 for Nakagami channels at $ {\rm BER}{ = 10^{ - 3}} $BER=10-3. Also, the outage probability of hybrid schemes of $ {\rm BER}{ = 10^{ - 3}} $BER=10-3 shows 30% and 14% better performance than ordinary $ M{\rm QAM} $MQAM and MPPM schemes, respectively.

Hybrid two-level MPPM-MDPSK modulation for high-speed optical communication networks.

A hybrid optical modulation approach is described, which layers a continuous wave $M$M-ary differential phase-shift keying ($M{\rm DPSK}$MDPSK) and a two-level ($2L$2L) multipulse pulse-position modulation (MPPM) intensity-modulated signal for improved spectral efficiency. These $2L$2L techniques are a generalization of earlier hybrid MPPM-$M{\rm DPSK}$MDPSK techniques and have the added advantage of reducing transmitter and detector complexities over previous hybrid modulation approaches. The spectral and power efficiencies for the proposed $2L$2L-MPPM-$M{\rm DPSK}$MDPSK modulation techniques are formulated and shown to have the highest spectral efficiency in comparison to other hybrid techniques with lower implementation complexity. The performance of the proposed $2L$2L hybrid techniques is quantified over free-space optical (FSO) networks as well as fiber networks and verified using Monte Carlo simulation. For FSO channels, the proposed $2L$2L-MPPM-$M{\rm DPSK}$MDPSK technique outperforms the traditional MPPM-$M{\rm DPSK}$MDPSK scheme by approximately 2 dB at a bit-error rate (BER) of ${10^{-4}}$10-4 and a spectral efficiency of 2.5 bit/s/Hz. Similarly, in optical fiber, the proposed scheme relaxes the impact of nonlinearity in comparison to traditional MPPM-$M{\rm DPSK}$MDPSK. Specifically, at a ${\rm BER}{=10^{-3}}$BER=10-3, the $2L$2L-MPPM-$M{\rm DPSK}$MDPSK technique outreaches the MPPM-$M{\rm DPSK}$MDPSK by 2000 km at a spectral efficiency of 2.5 bit/s/Hz and an average transmit power of $-{3}\,\,{\rm dBm}$-3dBm.

NLoS underwater VLC system performance: static and dynamic channel modeling.

In this paper, the impact of water channels under different communication link parameters is studied for underwater visible light communication (UVLC). The objective is to highlight the best results for non-line of sight (NLoS) communication links. In addition, NLoS links are studied under different parameters: LED colors, viewing angle, receiving angle, and data rates. The results are obtained and plotted using MATLAB simulation. The performance of the received power is first measured at different wavelengths and data rates. Then, the best results are further investigated at different viewing angles and receiving angles. The obtained results show that using cyan color provides more depth for the NLoS case, as well as a low bit error rate compared to the other colors. Most of the literature is concerned with unpractical configurations in underwater scenarios, such as an empty sea or assuming no human-object or blockage environment. We use the practical setup in Zemax Optics Studio to allow a precise description of ray tracing and high order of reflections inside a sea water environment. The channel impulse response (CIR) is obtained for static channel modeling, including a blockage environment to evaluate the best transmitters in sea water. Also, we are able to compare the average delay and the average delay spread of the source colors. The reflection characteristics of the sea water are considered as wavelength dependent. The CIR obtained by Zemax Solver and MATLAB indicates that cyan is the best source in sea water for different LED chips. Moreover, other previous studies assume perfect alignment scenarios between divers, which is not practical and not suitable for real channel gain results. Accordingly, we present a comprehensive dynamic channel modeling and characterization study for UVLC. Our study is based on Zemax programming language (ZPL) combined with Zemax Optics Studio. Using ZPL enables us to apply a mobility algorithm for divers and measure the channel gain variations due to random motion. We introduce a dynamic motion in a single-input single-output scenario and a single-input multiple-output scenario in the presence of blockage divers. Statistical analyses are studied for the appropriate distributions that can fit the data with various transmitter and receiver specifications. All dynamic scenarios are performed using cyan color in sea water, as it is proven to have satisfactory performance. The statistical results are beneficial for further analysis. As case studies, we consider various underwater scenarios, and the resulting parameters of statistical distributions can be used for future analysis in UVLC dynamic environments.

General silicon-on-insulator higher-order mode converter based on substrip dielectric waveguides.

A general silicon mode-converter waveguide that converts a fundamental mode to any higher-order mode is proposed. Specifically, dielectric substrip waveguides are inserted in the fundamental mode propagation path so that the conversion is done directly in the same propagation waveguide, without coupling the power into another waveguide as it happens in traditional mode converters. The device has a very small footprint compared to traditional converters. A mathematical model is developed to determine the design parameters of the used dielectric material and analyze the whole performance of the proposed device. Both the effective index method (EIM) and the perturbative mode-coupled theory are used in our mathematical analysis to get exact values for both the coupling coefficient and the length of the used dielectric material, so as to ensure a maximum coupled power transfer to the higher-order mode. In addition, full vectorial 3D-FDTD simulations are performed to validate our mathematical model. Our results show good agreement between the approximate EIM method and accurate full vectorial 3D-finite-difference time-domain (FDTD) simulations in characterizing the device parameters and performance. In order to validate the design model, two mode converters are simulated, fabricated, and tested for converting a fundamental TE0 mode into both first- and second-order (TE1 and TE2) modes, respectively. Good insertion losses and low crosstalks are obtained. Good agreement between simulated and fabricated results are achieved.

Photonic crystal-based compact hybrid WDM/MDM (De)multiplexer for SOI platforms.

A compact hybrid wavelength- (WDM) and mode-division (de)multiplexer (MDM) is proposed, and its performance is evaluated. The design of the device is based on 2D photonic crystals with a square lattice and Si rods. The device can multiplex two eigenmodes, TM0 and TM1, and two wavelengths, 1550 and 1300 nm. Two identical multimode interference couplers and an asymmetric directional coupler are used in implementing both the wavelength- and mode-division multiplexing functions, respectively. To avoid back-reflections, tapers are used at waveguide junctions. The structure is compact with dimensions of 29 µm×12 µm, which is suitable for on-chip integration. Simulation results reveal that the insertion losses and crosstalks are less than -1.0927 and -11.9024 dB, respectively, for all four channels.

Bidirectional mode-division multiplexers with antireflection gratings.

A bidirectional mode-division multiplexer (BMDM) with antireflection gratings is designed, and its performance in terms of S-parameters is presented. A BMDM can (de)multiplex three modes with only two waveguides and a Bragg grating. The impact of return losses on the performance of BMDMs is studied and antireflection gratings are designed to reduce their effects. A theoretical analysis of the proposed (de)multiplexer is developed based on the perturbative coupled-mode theory. Analytical expressions for the coupled-mode equations of the proposed device are obtained, taking into account the effects of return losses. Both duty cycle and teeth depths of the antireflection gratings are determined based on optimizing a cost function. In addition, FDTD simulation of the proposed device is performed, and its S-parameters are obtained and studied.

Modeling, simulation, and fabrication of bi-directional mode-division multiplexing for silicon-on-insulator platform.

A strip waveguide-based bi-directional mode-division multiplexer is proposed. A mathematical model has been proposed to analyze the performance, and the results are simulated. The design concept of this device to (de)multiplex three modes simultaneously has been studied previously for slab waveguides, both mathematically using the perturbative mode-coupled theory and by simulation using 2D FDTD Solutions (FDTD, finite difference time domain). As slab waveguides are not suitable for extracting fabrication parameters for most silicon-on-insulator applications, we apply the concept to a more practical device that involves strip waveguides rather than slab waveguides. The effective index method (EIM) has been used to develop the mathematical model and to get approximate forms for both the profiles and coupling coefficients. The return loss of different modes is taken into consideration to fully characterize the device performance. Simple formulas for both insertion and return losses of all multiplexing modes have been derived. In addition, full vectorial 3D FDTD simulations are performed so as to validate our mathematical model. Different design parameters have been used to get numerical results of the proposed device. Our results reveal that the EIM has enough accuracy to characterize the performance of our device compared to that of the complex full vectorial simulation. In order to validate the used model, the device has been fabricated and tested. Good insertion losses and crosstalks for all modes have been obtained.

Performance evaluation of hybrid DPSK-MPPM techniques in long-haul optical transmission.

In this paper, we evaluate the performance of hybrid differential phase shift keying-multipulse pulse position modulation (DPSK-MPPM) techniques in long-haul nonlinear-dispersive optical fiber transmission. An expression for the nonlinear interference variance is obtained analytically using the Gaussian noise (GN) model. We derive upper-bound expressions that take into account the fiber nonlinearity impact on the DPSK-MPPM system's performance for both bit- and symbol-error rates (BER and SER). The tightness of the BER's upper bound is verified using Monte Carlo simulation. The numerical analysis is carried out based on the proposed setup supplemented by a realistic simulation scenario for the DPSK-MPPM long-haul optical transmission system. Our results reveal that while the hybrid DPSK-MPPM technique outperforms both traditional DPSK and MPPM techniques under amplified spontaneous emission (ASE) noise (linear limit), it is less robust when fiber nonlinearity is considered. However, under the impact of low nonlinearity, the performance of a hybrid technique still surpasses the traditional ones. We also discuss the effect of some wavelength-division multiplexing (WDM) parameters on optimal system performance. The nonlinear interference penalties on the maximum reachable distances by both hybrid and traditional modulation systems are then investigated at a forward-error correction (FEC) requirement (BER=10-3). In particular, at an average launch power of -8 dBm, the hybrid DQPSK-MPPM system with a total frame length of eight time slots including two signal time slots outreaches a traditional DQPSK system by 950 km.

Performance analysis of an all-optical OFDM system in presence of non-linear phase noise.

The potential for higher spectral efficiency has increased the interest in all-optical orthogonal frequency division multiplexing (OFDM) systems. However, the sensitivity of all-optical OFDM to fiber non-linearity, which causes nonlinear phase noise, is still a major concern. In this paper, an analytical model for estimating the phase noise due to self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM) in an all-optical OFDM system is presented. The phase noise versus power, distance, and number of subcarriers is evaluated by implementing the mathematical model using Matlab. In order to verify the results, an all-optical OFDM system, that uses coupler-based inverse fast Fourier transform/fast Fourier transform without any nonlinear compensation, is demonstrated by numerical simulation. The system employs 29 subcarriers; each subcarrier is modulated by a 4-QAM or 16-QAM format with a symbol rate of 25 Gsymbol/s. The results indicate that the phase variance due to FWM is dominant over those induced by either SPM or XPM. It is also shown that the minimum phase noise occurs at -3 dBm and -1 dBm for 4-QAM and 16-QAM, respectively. Finally, the error vector magnitude (EVM) versus subcarrier power and symbol rate is quantified using both simulation and the analytical model. It turns out that both EVM results are in good agreement with each other.