Optical Signal-to-noise Ratio Research Articles

Addressing nonlinear İmpairments in fiber optic communication system utilizing novel modulation schemes

Abstract Nonlinear long haul optical transmission systems (NLOTSs) are a highlighted research area in the telecommunication sector. The importance of NLOTSs is increased, by reason of system volume, number of channels and transmission span demands. To achieve the current goals in the telecommunication industry, the system faces remedies called refractive index related nonlinearities (RIrNs). These nonlinearities cause performance discreditation of NLOTSs. In this paper, RIrNs are estimated and treated for NLOTS. This is achieved by applying modulation format indicators and amplitude deviation analysis. Channel spacing, input power, system capacity, fiber length, and quantity of channels parameters are used with reference to optical signal-to-noise ratio (OSNR) and bit error rate (BER) to simulate the preferred NLOTS model.

Multi-Functional Optical Spectrum Analysis Using Multi-Task Cascaded Neural Networks

In the optical communication systems, the optical spectrum (OS) provides useful informations for optical performance monitoring and optical link diagnosis. In this paper, we investigate the neural-network based OS analysis (OSA) techniques able to simultaneously recognize the OS and estimate the optical signal to noise ratio (OSNR), cascaded filtering distortion (CFD) and carrier wavelength drift (CWD). We demonstrate that, compared with the multi-task artificial neural network (MT-ANN) and convolutional neural network (MT-CNN), the proposed multi-task cascaded ANNs (CANN) and cascaded CNNs (CCNN) can greatly improve the OSA performance and accelerate the training process by exploiting specific features and loss functions for different tasks. The CCNN able to deal with the high-resolution OS (HOS) can further improve the OSA performance compared with the CANN. The averaged OS recognition accuracy and OSNR, CFD and CWD estimation errors obtained with the CCNN (CANN) for the 15 kinds of optical signals are 99.32% (97.07%), 0.36 (0.55) dB, 0.24 (0.32) and 0.04 (0.06) GHz, respectively, even when the various OS distortions are present. The proposed CANN and CCNN with a better versatility, higher OSA accuracy and faster convergence speed are promising enabling techniques for the future intelligent OSA systems.

Performance Investigation of Back-Compatible Integrated TWDM/GPON System Using MDM And Pulse Shapes

Optical access networks are the prominent and promising cutting edge technology to provide high speed, and cost effective operation for meeting the requirements of ever-increasing demands of users. Multiple access techniques such as Wavelength and time division in passive optical networks are important innovations to provide high capacity. A mode division multiplexed back compatible integrated next generation PON2 (TWDM PON) and GPON is demonstrated with triple play supportability in this work. Proposed system offer pay as you grow feature and also provide enhanced performance by suppressing the inter-channel interference in TWDM-PON2. System successfully works on same optical distribution network unit for both PON standards without extra arrangements and has potential to support 62.5 Gbps in full duplex mode (Symmetrical downlink/uplink). Moreover, demonstrated back compatible system is analyzed by incorporating diverse modulations such as NRZ-DPSK, RZ-DPSK, RZ-DQPSK, NRZ-DPSK and linecodings NRZ, RZ. Received power, Optical signal to noise ratio (OSNR), BER and Q factor are noted at diverse link lengths and it is noteworthy that LP modes with NRZ in TWDM PON provide outstanding results.

High Performance Fiber Laser Resonator for Dual Band (C and L) Sensing

This work presents an experimental analysis and comparison of the performance of quasi-randomly distributed reflectors inscribed into a single-mode fiber as a sensing mirror both in C- and L-band. Single-wavelength emission has been obtained in either band when using these artificially controlled backscattering fiber reflectors in a ring-cavity fiber laser. Single-longitudinal mode operation with an optical signal to noise ratio (OSNR) of 47 dB and an output power instability as low as 0.04 dB have been measured when employing a C-band optical amplifier. When replaced by an L-band optical amplifier, a single-longitudinal mode behavior has also been obtained, showing an OSNR of 44 dB and an output power instability of 0.09 dB. Regarding their performance as fiber-laser sensing systems, very similar temperature and strain sensitivities have been obtained in both bands, comparable to fiber Bragg grating sensors in the case of temperature and one order of magnitude higher in the case of strain variations.

Blind nonlinearity equalization by machine-learning-based clustering for QAM-based quantum noise stream cipher transmission

In quantum noise stream cipher (QNSC) systems, it is difficult to compensate fiber nonlinearity by digital signal processing (DSP) due to interactions between chromatic dispersion (CD), amplified spontaneous emission (ASE) noise from erbium-doped fiber amplifier (EDFA) and Kerr nonlinearity. Nonlinearity equalizer (NLE) based on machine learning (ML) algorithms have been extensively studied. However, most NLE based on supervised ML algorithms have high training overhead and computation complexity. In addition, the performance of these algorithms have a lot of randomness. This paper proposes two clustering algorithms based on Fuzzy-logic C-Means Clustering (FLC) to compensate the fiber nonlinearity in quadrature amplitude modulation (QAM)-based QNSC system, including FLC based on subtractive clustering (SC) and annealing evolution (AE) algorithm. The performance of FLC-SC and FLC-AE are evaluated through simulation and experiment. The proposed algorithms can promptly obtain suitable initial centroids and choose optimal initial centroids of the clusters to achieve the global optimal initial centroids especially for high order modulation scheme. In the simulation, different parameter configurations are considered, including fiber length, optical signal-to-noise ratio (OSNR), clipping ratio and resolution of digital to analog converter (DAC). Furthermore, we measure the Q-factor of transmission signal with different launched powers, DAC resolution and laser linewidth in the optical back-to-back (BTB) experiment with 80-km single mode fiber. Both simulation and experimental results show that the proposed techniques can greatly mitigate the signal impairments.

Monolithically Integrated 8 × 8 Transmitter-Router Based on Tunable V-Cavity Laser Array and Cyclic Arrayed Waveguide Grating Router

A monolithic 8 × 8 transmitter-router chip fabricated on InGaAlAs-InP multi-quantum well wafer for distributed wavelength routing network in the O-band with a channel spacing of 400 GHz is experimentally demonstrated. A tunable V-cavity laser (VCL) array, a cyclic-arrayed waveguide grating router (AWGR) and a semiconductor amplifier (SOA) array are integrated using the quantum-well intermixing (QWI) technique for its fabrication simplicity and cost effectiveness. A 200 GHz-spaced compact-size VCL is demonstrated with a 27 nm tuning range and a side-mode suppression ratio (SMSR) around 40 dB using a single-electrode controlled tuning. The chip output power is 24.6 mW. Measurement results of the transmitter-router chip show that all 64 input-output combinations have cyclic response spectra. After passing through the SOA with 80 mA bias current, an output power of about 9.6 mW and optical signal-to-noise ratio (OSNR) up to 39 dB have been measured. The demonstrated transmitter-router chip is fabricated on the same quantum well structure without requiring multiple epitaxial growth and complex grating fabrication. The characteristics of multi-wavelength and multi-port transmissions enable the distributed optical routing networks with flexible bandwidth allocation, which can find wide application in datacenters and high-performance computers.

MDG and SNR Estimation in SDM Transmission Based on Artificial Neural Networks

The increase in capacity provided by coupled SDM systems is fundamentally limited by MDG and ASE noise. Therefore, monitoring MDG and optical SNR is essential for accurate performance evaluation and troubleshooting. Recent works show that the conventional MDG estimation method based on the transfer matrix of MIMO equalizers optimizing the MMSE underestimates the actual value at low SNR. Besides, estimating the optical SNR itself is not a trivial task in SDM systems, as MDG strongly influences the electrical SNR after the equalizer. In a recent work we propose an MDG and SNR estimation method using ANN. The proposed ANN-based method processes features extracted at the receiver after DSP. In this paper, we discuss the ANN-based method in detail, and validate it in an experimental 73-km 3-mode transmission link with controlled MDG and SNR. After validation, we apply the method in a case study consisting of an experimental long-haul 6-mode link. The results show that the ANN estimates both MDG and SNR with high accuracy, outperforming conventional methods.

OSNR-Aware Modeling and Optimization of Multi-Core Fiber-Based Spectrally–Spatially Flexible Optical Networks

We focus on the optical signal-to-noise ratio (OSNR)-aware optical path (lightpath) planning problem in spectrally and spatially flexible optical networks (SS-FONs) connected using weakly coupled multi-core fibers (MCFs) in which distance-adaptive and super-channel transmission is realized using multiple modulation formats. In the SS-FON considered, the quality of transmission (QoT) of optical signals is degraded due to the inter-core crosstalk (XT) effect occurring in MCFs. To account for the XT impairment when planning lightpath connections, we make use of a reliable QoT model based on the OSNR estimation, in which the XT impairment is integrated with other physical-layer impairments. To model the lightpath planning optimization problem, we develop a novel mixed-integer programming (MIP) formulation that incorporates the OSNR model. In addition, we propose an efficient heuristic method that is capable of solving larger instances of the optimization problem considered. The results of numerical experiments indicate the low scalability of the MIP method due to the presence of XT and high effectiveness of the heuristic method. The analysis of three different network topologies and two types of MCFs shows a high impact of XT on network performance and limited performance gains from the presence of the central core in a 7-core MCF.

Real-time transmission of geometrically-shaped signals using a software-defined GPU-based optical receiver.

A software-defined optical receiver is implemented on an off-the-shelf commercial graphics processing unit (GPU). The receiver provides real-time signal processing functionality to process 1 GBaud minimum phase (MP) 4-, 8-, 16-, 32-, 64-, 128-ary quadrature amplitude modulation (QAM) as well as geometrically shaped (GS) 8- and 128-QAM signals using Kramers-Kronig (KK) coherent detection. Experimental validation of this receiver over a 91 km field-deployed optical fiber link between two Tokyo locations is shown with detailed optical signal-to-noise ratio (OSNR) investigations. A net data rate of 5 Gbps using 64-QAM is demonstrated.

Optical performance monitoring using lifelong learning with confrontational knowledge distillation in 7-core fiber for elastic optical networks

We propose a novel optical performance monitoring (OPM) scheme, including modulation format recognition (MFR) and optical signal-to-noise ratio (OSNR) estimation, for 7-core fiber in elastic optical networks (EONs) by using the specific Stokes sectional images of the received signals. Meanwhile, MFR and OSNR estimation in all channels can be utilized by using a lightweight neural network via lifelong learning. In addition, the proposed scheme saves the computational resources for real implementation through confrontational knowledge distillation, making it easy to deploy the proposed neural network in the receiving end and intermediate node. Five modulation formats, including BPSK, QPSK, 8PSK, 8QAM, and 16QAM, were recognized by the proposed scheme within the OSNR of 10-30 dB over 2 km weakly coupled 7-core fiber. Experimental results show that 100% recognition accuracy of all these five modulation formats can be achieved while the RMSE of the estimation is below 0.1 dB. Compared with conventional neural network architectures, the proposed neural network achieves better performance, whose runtime is merely 20.2 ms, saving the computational resource of the optical network.

Demonstration of turbulence mitigation in a 200-Gbit/s orbital-angular-momentum multiplexed free-space optical link using simple power measurements for determining the modal crosstalk matrix.

We experimentally demonstrate turbulence mitigation in a 200-Gbit/s quadrature phase-shift keying (QPSK) orbital-angular-momentum (OAM) mode-multiplexed system using simple power measurements for determining the modal coupling matrix. To probe and mitigate turbulence, we perform the following: (i) sequentially transmit multiple probe beams at 1550-nm wavelength each with a different combination of Laguerre-Gaussian (LG) modes; (ii) detect the power coupling of each probe beam to LG0,0 for determining the complex modal coupling matrix; (iii) calculate the conjugate phase of turbulence-induced spatial phase distortion; (iv) apply this conjugate phase to a spatial light modulator (SLM) at the receiver to mitigate the turbulence distortion for the 1552-nm mode-multiplexed data-carrying beams. The probe wavelength is close enough to the data wavelength such that it experiences similar turbulence, but is far enough away such that the probe beams do not affect the data beams and can all operate simultaneously. Our experimental results show that with our turbulence mitigation approach the following occur: (a) the inter-channel crosstalk is reduced by ∼25 and ∼21 dB for OAM +1 and -2 channels, respectively; (b) the optical signal-to-noise ratio (OSNR) penalty is <1 dB for both OAM channels for a bit error rate (BER) at the 7% forward error correction (FEC) limit, compared with the no turbulence case.

Blind and low-complexity modulation format identification based on signal envelope flatness for autonomous digital coherent receivers.

Modulation format identification (MFI) is a critical technology for autonomous digital coherent receivers in next-generation elastic optical networks. A novel and simple MFI scheme, to the best of our knowledge, based on signal envelope flatness is proposed without requiring any training or other prior information. After amplitude normalization and partition, the incoming polarization division multiplexed (PDM) signals can be classified into quadrature phase shift keying (QPSK), 8 quadrature amplitude modulation (QAM), 16QAM, and 64QAM signals according to envelope flatnesses R1, R2, and R3 of signals in different amplitude ranges. The feasibility of the proposed MFI scheme is first verified via numerical simulations with 28 GBaud PDM-QPSK/-8QAM/-16QAM/-64QAM signals. Only by using 4000 symbols can the proposed MFI scheme achieve a 100% correct identification rate for the four modulation formats over a wide optical signal-to-noise ratio (OSNR) range. Proof-of-concept experiments among 28 GBaud PDM-QPSK/-8QAM/-16QAM systems under back-to-back and long-haul fiber transmission links are implemented to further demonstrate the effectiveness of the proposed MFI scheme. The experimental results show that the proposed MFI scheme can obtain a 100% correct identification rate when the OSNR value of each modulation format is higher than the threshold corresponding to 7% FEC and is resilient towards fiber nonlinearities. More importantly, the proposed MFI scheme can significantly reduce computational complexity.

Multi-wavelength-tunable thulium-doped fiber laser generation based on dual-pass Mach–Zehnder and Sagnac loop hybrid filters

A wavelength-tunable thulium-doped fiber laser (TDFL) based on a Mach–Zehnder (MZ) filter and Sagnac loop was proposed in this study. The dual-pass MZ fiber filter, and Sagnac loop were fabricated to operate as reflectors in a linear cavity. The laser operating threshold was set to 138 mW. A single-wavelength laser output with an optical signal-to-noise ratio (OSNR) of up to 38 dB was realized with a tuning range of 1854–1882 nm. The tuning range was obtained with a tuning interval of 6 nm by tuning the polarization controller (PC), which yielded a power shift of less than 0.42 dB. Stability of the single-wavelength center wavelength power fluctuation was less than 0.39 dB within 20 min. When the laser was operated in the dual-wavelength condition, the OSNR was more than 30.5 dB, and the power fluctuation was less than 0.89 dB within 20 min at room temperature. The tunable triple-wavelength laser emission could be realized experimentally by adjusting the PC, which yielded an OSNR of more than 30.8 dB. The proposed TDFL is suitable for fiber-sensing system applications.

Spectral efficiency and performance improvement of coherent optical transmission system

This paper presents an orthogonal frequency division multiplexing (OFDM) for a long-haul optical transmission system with high-rate transferability to alleviate dispersion effects. In addition, we suggest combining polarization division multiplexing (PDM) with coherent OFDM (CO-OFDM) to increase spectral efficiency (SE). Based on OptiSystem (2021) version 18.0" software package, a 100 Gbps single-channel PDM-CO-OFDM transmission system is investigated using different modulation formats; bipolar phase keying (BPSK), quadrature phase shift keying (QPSK), Eight-Phase-Shift Keying modulators (8-PSK), and quadrature amplitude modulation (16-QAM). A 60 km span of standard single-mode fiber (SSMF) cable is employed in this investigation. The system's performance and spectral efficiency have been evaluated by comparing against the different modulation schemes. The outcomes that were got show that the BPSK modulation scheme has the longest transmission distance and requires a lesser level of optical signal to noise ratio (OSNR) at the receiver side. Concerning spectral efficiency, 16-QAM outperforms the others. Farther, the impact of employing ultra-low loss and large effective area fiber in reducing loss and nonlinear effects in the optical channel for 16-QAM modulation formats is examined. The result found that the system with advanced fiber has superior performance than the SSMF. The bit error rate (BER) of 0.033 (20% concatenated forward error correction (FEC) threshold) is used as a baseline.

Improved Receivers for Optical Wireless OFDM: An Information Theoretic Perspective

We consider performance enhancement of asymmetrically-clipped optical orthogonal frequency division multiplexing (ACO-OFDM) and related optical OFDM schemes, which are variations of OFDM in intensity-modulated optical wireless communications. Unlike most existing studies on specific designs of improved receivers, this paper investigates information theoretic limits of all possible receivers. For independent and identically distributed (IID) complex Gaussian inputs, we obtain an exact characterization of information rate of ACO-OFDM with improved receivers for all SNRs. It is proved that the high-SNR gain of improved receivers asymptotically achieve 1/4 bits per channel use, which is equivalent to 3 dB in electrical SNR or 1.5 dB in optical SNR; as the SNR decreases, the maximum achievable SNR gain of improved receivers decreases monotonically to a non-zero low-SNR limit, corresponding to an information rate gain of 36.3%. For practically used constellations, we derive an upper bound on the gain of improved receivers. Numerical results demonstrate that the upper bound can be approached to within 1 dB in optical SNR by combining existing improved receivers and coded modulation. We also show that our information theoretic analyses can be extended to Flip-OFDM and PAM-DMT. Our results imply that, for the considered schemes, improved receivers may reduce the gap to channel capacity significantly at low-to-moderate SNR.

Nonlinearity mitigation of self-phase modulation effect in coherent optical system

Abstract In this paper, the proficiency of nonlinearity compensation studied on the polarization division multiplexing 0.3 return-to-zero quadrature phase-shift keying (PDM-0.3RZ-QAM) coherent optical system. The considerable nonlinear effect is the self-phase modulation (SPM) in the presence of chromatic dispersion. The nonlinearity mitigation technique based on the digital backpropagation (DBP) algorithm is implemented on the receiver side. The performance of the system was investigated in different terms as the optical signal-noise ratio, bit error rate, and the maximum transmission distance. The results demonstrated that the penalties were decreased by 3 dB. The maximum achievable distance was extended to 2000 km.

Thulium-doped fiber laser with bidirectional output in a ring laser cavity

A thulium-doped fiber laser (TDFL) with bidirectional output was proposed and demonstrated herein. Clockwise (CW) and counter-clockwise (CCW) lasing output with different lasing wavelengths was realized by two uniform fiber Bragg gratings (UFBGs) and two optical circulators. The UFBGs located at 1941.238 and 2049.325 nm were incorporated in the laser cavity to act as the wavelength-selective element. Switchable operation was realized through tuning the gain and loss of the circulating light by applying curvature on a section of the single-mode fiber. When the TDFL was operated in the CW direction, single-wavelength output with a center wavelength, optical signal-to-noise ratio (OSNR), and maximal output power of 2049.335 nm, 33.175 dB, and 280.6 mW was obtained, and the fluctuation of the center wavelength and the output power were less than 0.041 nm and 1.400 dB within an observation time of 60 min. When the TDFL was operated in the CCW direction, single-wavelength output with a center wavelength, OSNR, and maximal output power of 1941.312 nm, 37.108 dB, and 486 mW could be obtained, and the maximal fluctuation of the center wavelength and the output power were 0.128 nm and 0.832 dB within an observation period of 60 min. In addition, the proposed TDFL can operate in both CC and CCW directions simultaneously with different output wavelengths. The potential application of a TDFL strain sensor with sensitivities of 1.33 and 1.22 pm/με in the CW and CCW directions was also demonstrated and the proposed TDFL may also well suited for use in medical and free-space optical communication systems.

Time domain diversity combining with delay-and-advanced operation in two layered asymmetrically clipped optical OFDM system

This paper demonstrates the advantage of application of time domain diversity combining (TDDC) at the transmitter over the time domain diversity combining receiver (TDDR) for a single layer of ACO-OFDM in terms of reduced complexity. The paper further demonstrates the implementation of time domain delay-and-advanced operation with TDDC at the transmitter for a two-layered LACO-OFDM system, where all the subcarriers are utilized and an improved BER performance is achieved. The new improved 2-LACO-OFDM system achieves 2.7 dB, 3.3 dB and 3.7dB better optical signal to noise ratio (OSNR) than the ACO-TDDC, 2-LACO-TDDC, 3-LACO-TDDC respectively.

Intelligent Optical Performance Monitoring Based on Intensity and Differential-Phase Features for Digital Coherent Receivers

An intelligent optical performance monitoring scheme for simultaneous modulation format identification (MFI) and optical signal-to-noise ratio (OSNR) monitoring is proposed and experimentally demonstrated in digital coherent receivers. The proposed scheme introduces convolutional neural network (CNN) to automatically extract and monitor modulation format and OSNR dependent features (the empirical distribution of intensity and differential-phase) which can be obtained after constant modulus algorithm (CMA) equalization. The experiment results show that 100% identification accuracies for all modulation formats (e.g. 28-GBaud PDM-QPSK/−8PSK/ −8QAM/−16QAM/−32QAM/−64QAM) are achieved at OSNR values are lower than the corresponding theoretical 20% FEC limit (BER = 2.4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> ). Furthermore, under the chromatic dispersion (CD) variation from 0-ps/nm to 1200-ps/nm, the root-mean- square error (RMSE) and mean absolute error (MAE) of OSNR monitoring for all modulation formats are less than 0.0896-dB and 0.0657-dB, respectively. Subsequently, the influence of frequency offset and fiber nonlinearity effect on the intelligent optical performance monitoring scheme is also analyzed. We believe that our proposed multi-parameter monitoring scheme has the potential to be applied in the next-generation intelligent elastic optical networks.

Performance Investigation of Error-Feedback Noise Shaping in Low-Resolution High-Speed IM/DD and Coherent Transmission Systems

In this paper, we comprehensively investigate the performance of error-feedback noise shaping (EFNS) in low-resolution high-speed intensity-modulation direct-detection (IM/DD) PAM4 and coherent DP-mQAM transmission systems by simulation and experiments. The influence of EFNS parameters and digital to analog converter (DAC) parameters on EFNS performance is first numerically investigated in a low-resolution 56-GBaud 2-km IM/DD PAM4 transmission system. The combination of clipping and EFNS is also numerically evaluated and the impact of overloading is deeply analyzed. For coherent transmission, the EFNS performance is investigated for different modulation formats including QPSK, 16QAM and 64QAM. The optical signal to noise ratio (OSNR) penalties for different modulation formats are evaluated numerically in a low-resolution 56-GBaud 80-km coherent transmission system. Finally, we experimentally demonstrate the EFNS performance in low-resolution 32-GBaud 10-km IM/DD PAM4 and 28-GBaud 80-km coherent DP-64QAM transmission experiments. Extensive numerical and experimental results show that EFNS can effectively mitigate the quantization noise of low-resolution DAC and thus reduce received optical power or OSNR penalty significantly. Although EFNS is a little inferior to DRE at the PNoB of 3, both of them have very similar performance when PNoB <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\geq$</tex-math></inline-formula> 4. Compared with DRE, EFNS has lower computational complexity and no processing latency. In addition, no channel response is required for EFNS.