High-order Modulation Formats Research Articles

Phase-locking-free all-optical matching system for 100 Gbaud QPSK optical signal

Photonic firewall is a monitoring protection device that will directly detect and locate optical network attacks at the optical layer, which can effectively ensure the security of optical networks. An all-optical matching system is the core part of photonic firewall, which determines the performance of a photonic firewall, so it is of great significance to research and develop all-optical matching system for high-speed and high-order modulation formats signals. At present, an all-optical matching system for binary modulation formats is relatively mature, but the all-optical matching system for high-order phase modulation format signals is still limited by how to solve the problem of phase synchronization. In this paper, a new all-optical matching system based on self-interference matching is proposed for quadrature phase shift keying (QPSK) optical signal, which avoids the introduction of local target sequence optical signals and phase-locking circuits. The theoretical analysis and simulation verification are conducted for the designed system. The results demonstrate that the system can accurately identify and locate 4-symbol or 8-symbol target sequences in the input QPSK optical signals with 16-symbol and 32-symbol data sequences at a data rate of 100 Gbaud.

On the multipath interference mitigation for high-speed IM-DD transmissions with Nyquist subcarrier modulation.

The performance of high-speed intensity modulation direct detection (IM-DD) transmissions is severely degraded due to the occurrence of multipath interference (MPI), especially when a higher-order modulation format is utilized. Here, we propose and demonstrate, for the first time to the best of our knowledge, that a Nyquist subcarrier modulation (Nyquist-SCM) format inherently exhibits resistance to the MPI. We experimentally evaluate the MPI tolerance by transmitting 56 Gbit/s PAM-4 signals and Nyquist-SCM 16QAM signals over the 2 km standard single-mode fiber (SSMF) when the C-band semiconductor laser with a linewidth of 1.7 MHz is utilized. In comparison with the PAM-4 format, the Nyquist-SCM 16QAM format can lead to an enhanced MPI tolerance of 4 dB at the KP4-FEC threshold of BER = 2 × 10-4. In addition, even with the help of MPI mitigation for the PAM-4 signals based on two newly reported methods, the utilization of Nyquist-SCM 16QAM signal can still guarantee an improved MPI tolerance of 1 dB.

High-Capacity Coherent WDM Networks Empowered by Probabilistic Shaping and End-to-End Deep Learning

To optimize the functionality of coherent optical fiber communication (OFC) systems and enhance their capacity related to long-haul transmissions, wavelength-division multiplexing (WDM) and probabilistic constellation shaping (PCS) techniques have been used. This paper develops an end-to-end (E2E) deep learning (DL)-based PCS algorithm, i.e., autoencoder (AE) for a high-order modulation format WDM system that minimizes nonlinear effects while ensuring high capacity and considers system parameters, in particular those related to the OFC channel. Only the AE of the central channel is trained to meet the specified performance objective, as the system design employs identical AEs in each WDM channel. The simulation results show that the architecture should consist of two hidden layers, with thirty two nodes per hidden layer and a ”32×modulation order” batch size to obtain optimal system performance, when designing AE using a dense layer neural network. The behavior of the AE is examined to determine the optimum launch-power ranges that enhance the system's performance. The developed AE-based PCS-WDM provides a 0.4 shaping gain and outperforms conventional solutions.

Noise equalization scheme based on probabilistic shaping and complex-valued ANN for dual-polarization continuous spectrum NFDM system with high-order modulation formats

In dual-polarization continuous spectrum nonlinear frequency division multiplexing (DP-CS NFDM) systems, the presence of noise destroys the independence between subcarriers in the nonlinear spectral domain (NSD). Additionally, as the modulation order and the number of subcarriers increase, the signal power increases and the effect of noise on the performance of the system becomes more severe. In this paper, we propose a noise equalization scheme based on probabilistic shaping (PS) and complex-valued neural network (c-ANN) for dual-polarization transmission CS NFDM system with high-power signals. The scheme utilizes PS to reduce the distribution probability of high-power constellation points of the signal with high-order modulation format, to decrease the peak-to-average power ratio of the continuous spectrum subcarriers and the average power of the signal, which can achieve the suppression and equalization of the amplifier spontaneous emission (ASE) noise and the processing noise. In addition, the scheme uses the c-ANN to effectively reduce the correlation between subcarriers caused by the noise and improve the system performance. The effectiveness of the proposed scheme is verified in the DP-CS NFDM system, and the simulation results show that the Q-factor gain is enhanced by roughly 1.1 dB and 0.8 dB for the number of subcarriers of 128 and 256, respectively, compared to the classic nonlinear Fourier transform (NFT) scheme. Among them, with different noise figure (NF) and satisfying the 7% forward error correction (FEC) threshold, the 512QAM signal with an entropy of 8 after equalization increase the transmission distance by 80 km longer than that of uniform 256QAM signals for the number of subcarriers of 128. Compared with the joint scheme based on PS and real-valued neural network (r-ANN), this scheme has better equalization performance at the same complexity, and the complexity can be reduced by 50% under the same performance.

Experimental demonstration of optical waveform transmission with multiple-eigenvalue 16-APSK modulation.

The multi-eigenvalue multiplexing-based discrete spectrum-modulated nonlinear frequency-division multiplexing (DS-NFDM) system with higher-order modulation format has been demonstrated experimentally. After designing the coefficients of the eigenvalue set and the constellation point distribution of 16-amplitude phase shift keying (16-APSK), the realizations of 14-, 30-, and 46-eigenvalue multiplexed DS-NFDM signals have been implemented. The results show that 46-eigenvalue and 30-eigenvalue multiplexed DS-NFDM signals can transmit 50 km and 400 km over a nonzero dispersion-shifted fiber (NZDSF) under soft-decision forward error correction (SD-FEC) threshold of 2.4E-2, respectively. This demonstration shows for the first time, to the best of our knowledge, the record for multiplexed eigenvalue number and data rate of the multiple-eigenvalue-based DS-NFDM system.

Improved carrier phase recovery for high-capacity optical communication systems with high-order modulation formats

A modified Viterbi-Viterbi (MVV) carrier phase recovery (CPR) algorithm is comprehensively studied based on the quadrature phase-shift-keying (QPSK) partition scheme. By improving the QPSK partition scheme for the 16-ary quadrature amplitude modulation (16QAM), MVV algorithm is developed for the optical fiber communication systems with the modulation formats of dual-polarization 64-QAM (DP-64QAM) and DP-256QAM. Numerical simulations have been carried out in a 9-channel Nyquist-spaced 32-Gbaud optical transmission system for different modulation formats and the performance of MVV CPR with respect to pilot-aided (PA) CPR has been examined. Results show that MVV CPR can significantly mitigate the laser phase noise in the system using high-order modulation formats. Compared to PA CPR, MVV CPR performs better with the increase of the transmission distance, where the impact of equalization enhanced phase noise has to be taken into account. Furthermore, MVV has been compared with other CPR approaches in terms of the performance and the computational complexity.

Reconfigurable all-optical pattern recognition for PSK and QAM signals in high-speed optoelectronic firewalls.

In the fifth generation fixed networks (F5G) era, full-fiber-connected optical networks support emerging bandwidth-hungry services. However, optical networks are vulnerable to attack by tapping or other methods, which has been paid more and more attention in modern optical infrastructure. Therefore, high-speed optoelectronic firewalls appear as one of the promising technologies to guarantee security. The most significant and challenging component of a high-speed optoelectronic firewall is all-optical pattern recognition, especially for more advanced high-order modulation formats such as phase shift keying (PSK) or quadrature amplitude modulation (QAM) to satisfy efficient enhanced fixed broadband in F5G. In this paper, what we believe to be a novel reconfigurable all-optical pattern recognition system for PSK and QAM signals is proposed with two implementation architectures. The proposed system mainly consists of a generalized XNOR (GXNOR) and a recirculating loop. The two implementation architectures are precisely two realization methods of the GXNOR part. One employs two cascaded IQ Mach-Zehnder modulators and the other is implemented by the four-wave mixing. The numerical simulation results demonstrate that the two implementation architectures can both achieve all-optical pattern recognition for the reconfigurable high-order modulation formats of QPSK, 8PSK, and 16QAM with the recorded baud rate of 260GBaud.

Gradient-descent noise whitening techniques for short-reach IM-DD optical interconnects with severe bandwidth limitation.

Bandwidth limitation in optoelectrical components and the chromatic dispersion-induced power fading phenomenon cause severe inter-symbol interference (ISI) in high-speed intensity modulation and direct detection (IM-DD) optical interconnects. While the equalizer implemented in the receiver's digital signal processing procedure can mitigate ISI, it also inevitably enhances the noise located in the decayed frequency region, known as equalization-enhanced colored noise (EECN). Additionally, the nonlinear impairments of the modulator and photodetector also deteriorate the performance of the IM-DD system, especially for high-order modulation formats. In this work, we propose a gradient-descent noise whitening (GD-NW) algorithm to address EECN and extend it by introducing nonlinear kernels to simultaneously mitigate EECN and nonlinear impairments. The proposed algorithms are compared with conventional counterparts in terms of the achievable baud rate and the receiver optical power sensitivity. As a proof-of-concept experiment, we validate the principles of the proposed algorithms by successfully transmitting 360-GBd on-off-keying (OOK) and 180-GBd 4-level pulse-amplitude-modulation (PAM-4) signals in the back-to-back case under a 62-GHz brick-wall bandwidth limitation. 280-GBd OOK and 150-GBd PAM-4 transmissions are also demonstrated over 1-km standard single-mode fiber with a bit error rate below 7% hard-decision forward error correction aided by the proposed approach.

ISRS impact-reduced routing, modulation, band, and spectrum allocation algorithm in C + L-bands elastic optical networks

C + L-bands elastic optical networks (EONs) cope with growing traffic demands by leveraging the C and L bands of deployed optical fibers. In this scenario, the inter-band stimulated Raman scattering (ISRS) becomes more serious and degrades the quality of transmission (QoT) of services, and should be considered to guarantee the lightpaths establishment for services in the routing, modulation, band, and spectrum allocation (RMBSA). In this paper, we propose an ISRS impact-reduced RMBSA algorithm that reduces the ISRS impact on services at the RMBSA process. In the routing phase, selecting the route with more available contiguous aligned frequency slots (FSs) and shorter transmission distance, the less ISRS impact of requests can be suffered and the spectrum resources of each link can be used as much as possible. In the modulation phase, the lower-order modulation format which can achieve the least required FSs and has higher OSNR tolerance is selected. In the band and spectrum allocation phase, allocation of higher-order modulation format requests to the C-band to reduce the ISRS impact from C-band to L-band. Meanwhile, centralized assignment of more required FSs requests to reduce the ISRS impact of these requests on other requests. Simulation results indicate that the proposed RMBSA algorithm can effectively decrease the blocking probability, improve the resource utilization and request average optical signal-to-noise ratio.

Adaptive feedback-driven probabilistic shaping for high-order modulation formats in a fiber–THz seamless integration system at 320 GHz

In this Letter, we propose a novel, to the best of our knowledge, adaptive feedback-driven probabilistic constellation-shaping (FBD-PCS) method based on the robustness evaluation criteria and employ variational autoencoder (VAE)-based equalizers to implement polarization demultiplexing and nonlinear equalization for the recovery of high-order PCS-QAM signals. We experimentally demonstrate the fiber–THz 2 times 2 MIMO system with a net rate of 366.4 Gbit/s using dual-polarization 40 Gbaud PCS-64QAM signal over a 20 km SSMF and 6 m wireless link. Specifically, the feedback mechanism drives the fiber–THz system to solve optimization problems, adaptively matching the optimized distribution of transmitted symbols that maximizes normalized generalized mutual information (NGMI). We also examine six scenarios to explore nonlinear resistances of FBD-PCS symbols and the robustness of VAE-based equalizers. The results demonstrate the superiority of FBD-PCS over the Maxwell–Boltzmann (M–B) distributions in practical nonlinear-dominant systems. Additionally, the FBD-PCS signals can break limitations for ultrahigh rate transmission with the help of advanced equalizers.

Experimental demonstration of 480 Gbit/s coherent transmission using a nanosecond switching tuneable laser

Fast-switching tuneable lasers with a wide wavelength coverage and with noise and linewidth levels suitable for high-order modulation formats can facilitate the implementation of highly flexible and reconfigurable optical metro, access and inter/intra data center networks. In this work, we show the characterization of a tuneable laser capable of covering a wavelength range of 35 nm in the C-band with nanosecond switching time and low linewidth and use it to demonstrate 480 Gbit/s 16QAM transmission over 25 km of single-mode fiber for a wavelength range of 19 nm.

23.1-Gb/s 135-GHz Wireless Transmission Over 4.6-Km and Effect of Rain Attenuation

In this article, we have experimentally demonstrated a fiber-wireless-integration <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D$</tex-math> </inline-formula> -band (110–170 GHz) transmission system. To extend the wireless transmission distance while maintaining the wireless bit rate as high as possible, we designed high-gain lens horn antenna modules consisting of a pair of dielectric plano-convex lenses and horn antennas for long-haul wireless links. In addition, we employ the Volterra nonlinear equalization (VNE) algorithm based on the multiple-input–multiple-output (MIMO) structure to precisely compensate for the in-phase/quadrature (I/Q) imbalance and nonlinear impairment of quadrature amplitude modulation (QAM) signals mainly caused by photoelectric (O-E) conversion and electric-photo (E-O) conversion. Meanwhile, one of the methods adopted in our experiment to decrease the influence of nonlinearity on high-order QAM signals is coupling the high-order modulation format with the probabilistic shaping (PS) technology. As a result of outdoor field experimental verification, the demonstration at 135 GHz carrier with a net rate of 23.1 Gb/s over 10 km optic-fiber and 4.6 km wireless transmission distance has been successfully carried out. It is, as we know, the first time that a record-breaking product of net bit rate and wireless transmission distance, i.e., 23.1 Gb/s <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> 4.6 km <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$=$</tex-math> </inline-formula> 106.3 Gb/s <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\cdot$</tex-math> </inline-formula> km, has been accomplished based on a fiber-wireless-integration system. Also, for the first time, we have measured the effect of rain attenuation for the 135 GHz millimeter-wave (mm-wave) wireless link and established more accurate rainfall models for the Shanghai region of China by comparing them with multiple rainfall models. This work can be used as a complement to the International Telecommunication Union-Radiocommunication sector (ITU-R) recommendations.

Clone-comb-enabled high-capacity digital-analogue fronthaul with high-order modulation formats

Clone-comb-enabled high-capacity digital-analogue fronthaul with high-order modulation formats

Joint-Transceiver Equalization Technique over a 1.4 km Multi-Mode Fiber Using Optical MIMO Technique in IM/DD Systems

In optical fiber communication, recent advances in multiple-input and multiple-output (MIMO) systems using space-division multiplexing have helped achieve higher spectral efficiency and data rates. Propagating higher-order modulation formats over MIMO systems further strengthens the capacity of the transmission link. In the optical-MIMO system, the dispersion impairments originating from a 1.4 km long multi-mode fiber (MMF) are mitigated using the proposed joint-transceiver equalization technique. A numerical convex optimization algorithm is used to compute and optimize the pre- and post-equalization (PPE) coefficients jointly restricted by cost and power budgets. The potential of the proposed joint-PPE technique is tested on an MMF link, which is severely degraded by dispersion compared to a single-mode fiber channel. From the experimental results, the average optical received power gain necessary to reach 10−4 bit-error rate is improved by nearly 2.5 dB using the joint-PPE compared to the post-equalization only based on the minimum mean-squared error principle. When the efficiency of the conventional zero-forcing (ZF) principle-based PPE and the joint-PPE is compared, the joint-PPE scheme outperforms the ZF-PPE by approximately 1.5 dB. The enhancement in the transmission quality is observed with experimentally measured eye diagrams using the joint-PPE scheme. Under the analyzed scenarios, computer simulation also confirms the hypothesis, which establishes the effectiveness of the proposed joint-transceiver equalization over the conventional ZF-PPE scheme. Moreover, the simulated performance benefits of the joint-PPE are evaluated using the singular value decomposition (SVD) technique. Improvement of ≈3.86 dB in the average optical received power gain required to reach 10−4 bit-error rate is witnessed with the PAM-4 format. Overall, the joint-transceiver equalization technique is proven to be beneficial in optical MIMO systems.

High Spectral Efficiency Photonics-Based Polarization Multiplexing OFDM Signal Delivery at W-Band

We have implemented a large-capacity, long-haul, and high spectral efficiency photonics-based polarization multiplexing communication system at W-band. The signal delivery up to 119.1 Gb/s with spectral efficiency of 9.9 bit/s/Hz is achieved over a 10 km optic-fiber link and 4600 m wireless link. In our study, we utilize an orthogonal frequency division multiplexing (OFDM) scheme to compensate for the signal impairments caused by the dispersion effect and the wireless multipath effect. Based on a pair of antennas, the antenna polarization multiplexing technique is used to realize the polarization-multiplexed mm-wave signal delivery. Also, only one communication link is required, instead of the complex 2×2 line-of-sight multiple-input multiple-output (MIMO) link. Instead of Cassegrain antennas, the combination of plano-convex lens and horn antenna greatly extends the wireless distance. For increasing the spectral efficiency and throughput of the communication system, we employ higher-order modulation formats, probabilistic shaping (PS) techniques, and a series of equalization algorithms. Based on photonics technology, we have created a record-setting distance-rate product of 547.9 Gb/s•km (119.1 Gb/s × 4.6 km) for an independent channel at W-band.

100 Gbit/s PAM-4 Linear Burst-Mode Transimpedance Amplifier for Upstream Flexible Passive Optical Networks

Next-generation passive optical networks (PONs) with upstream rates of 50 Gbit/s and beyond will require a new class of burst-mode transimpedance amplifiers (BMTIAs) that are linear to enable (digital) equalization of channel impairments. Such linear BMTIAs also enable higher-order modulation formats like 4-level pulse amplitude modulation (PAM-4). In this paper, we demonstrate operation of a novel linear BMTIA integrated together with a commercial off-the-shelf 25G-class avalanche photodiode (APD), achieving 50 Gbit/s non-return-to-zero (NRZ) operation with a sensitivity of -23.7 dBm optical modulation amplitude (OMA) and dynamic range exceeding 21.7 dB and 100 Gbit/s PAM-4 operation with a sensitivity of -15.8 dBm OMA and dynamic range exceeding 15.4 dB, both at a bit error ratio (BER) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-2}$</tex-math></inline-formula> . In addition, fast burst-mode gain-control and balancing circuits limit loud-soft sensitivity penalties in the case of AC-coupled circuits to less than 1.3 dB. The chip was designed in a 0.13 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m SiGe:C BiCMOS technology, has an area of 1.2×1.7 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{2}$</tex-math></inline-formula> and consumes between 260 mW and 310 mW. This receiver paves the way to a next-generation class of BMTIAs, supporting the ITU-T G.9804.3 Amd 1 standard.

Experimental investigation on transmission system of dual-polarization mode-division-multiplexing signals with few-mode erbium-doped fiber amplifiers

The transmission capacity of optical fiber communication can be effectively improved by the combination of dual-polarization (DP) high-order modulation format with mode division multiplexing (MDM) technology. We build up an experimental system for amplification and transmission of DP-MDM signals with orthogonal mode combination (OMC) and identical mode combination (IMC). The MDM transceiver units are composed of mode-selective photonic lanterns (MSPLs) and few-mode polarization controllers (FMPCs). The few-mode erbium-doped fiber amplifier (FM-EDFA) used here is constructed by homemade isolation and wavelength division multiplexers (IWDMs). The amplification and transmission experiments for 100 Gb/s dual-polarization quadrature phase shift keying (DP-QPSK) MDM signals are carried out by the commercial optical transport network (OTN) platform. An effective method of optimally receiving DP-MDM signals are presented by appropriately controlling the FMPC in the MDM receiver unit. The experimental results show that, (1) OMC DP-MDM signals have a better performance than IMC cases in differential modal gain (DMG), which provides a new means to reduce the DMG; and (2) under the optimization conditions, OMC and IMC DP-MDM systems possess a close receiver sensitivity, but the former has a larger optical power margin than the latter by 2.2 dB.

High accurate and modulation transparent frequency offset compensation scheme using data-aided feedback structure in coherent optical communication systems

The high-order modulation format has the advantages of high spectral efficiency and large system capacity. However, the traditional frequency offset estimation and compensation algorithms, such as fast Fourier transform-based frequency offset estimation (FFT-FOE) that relies on a large number of complex FFT operations and differential frequency offset estimation (Diff-FOE) that relies on finite phase difference between adjacent symbols, are difficult to meet the frequency offset estimation requirements for any high-order modulation format. In this work, a novel FOE scheme using QPSK training symbols aided feedback structure is proposed. The performance of the proposed FOE scheme is evaluated in a polarization multiplexing QPSK-aided 32/64-quadrature amplitude modulation (QAM) simulation platforms with a symbol rate of 28 GBaud. The results show that has the technical advantages of low complexity, suitable for arbitrarily high-order modulation, tolerance for laser phase noise and wide tracking frequency offset range.

Digital Non-Linear Transmitter Equalization for PAM-N-Based VCSEL Links Enabling Robust Data Transmission of 100 Gbit/s and Beyond

This paper demonstrates why VCSEL-based transmitter systems are not able to capitalize on higher-order PAM modulation formats (such as PAM-4) to push the link data rate towards 100 Gbit/s with BER of better than 10−5. First, the non-linear and biasing-dependent VCSEL behavior is analyzed in great detail to prove why state-of-the-art VCSELs with linear equalization are not able to reach these bandwidth targets. Consequently, based on an enhanced VCSEL model, a digital non-linear transmit equalizer is proposed to overcome voltage-dependent relaxation frequency shifts as well as data-dependent VCSEL bandwidth variations. The proposed digital non-linear equalizer is compared with a standard linear FFE structure using PAM-4 transmission experiments of 100 Gbit/s and beyond. This way, the proposed non-linear equalizer reveals a BER improvement of more than 30-fold compared to a linear FFE at a data rate of 80 Gbit/s. Finally, at 112 Gbit/s PAM-4 the non-linear equalizer was successfully demonstrated at a BER of 1.5·10−7.

An Analog EIC-PIC Receiver With Carrier Phase Recovery for Self-Homodyne Coherent DCIs

Coherent techniques, used for high-capacity optical links, conventionally use high-speed ADCs and digital signal processing, which make them power hungry. For short-reach coherent links such as data center interconnects, power consumption, form factor, cost, and latency have to be greatly reduced. Co-designed photonic and electronic integrated circuits, in the optical and electrical analog domains, respectively, can achieve these goals. In this brief, for the first time, we present integration of a silicon photonic integrated coherent receiver (ICR) front-end with an electronic carrier phase recovery chip, as a part of a complete coherent receiver solution. A phase shifter in the ICR (fabricated in a 220 nm silicon-on-insulator technology) receives feedback from the CPR chip, and the combination compensates for the time varying phase offset between the modulated signal and the unmodulated carrier in the closed loop configuration. In this proof-of-concept demonstration, we present experimental results obtained from the stand-alone ICR, and its system level integration with CPR chip, for QPSK signals. The technique can be extended to a higher-order modulation formats, such as 16-QAM, for capacity scaling. The proposed scheme is suitable for carrier-forwarded coherent links, such as polarization multiplexed carrier based self-homodyne links.