DP-16QAM Signals Research Articles

InP Photonic Integrated All-Optical Wavelength Conversion of 128 GBaud DP-16QAM Signals Using XGM in SOAs

InP Photonic Integrated All-Optical Wavelength Conversion of 128 GBaud DP-16QAM Signals Using XGM in SOAs

S-Band WDM Transmission Using PPLN-Based Wavelength Converters and 400-Gb/s C-Band Real-Time Transceivers

Multi-band WDM transmission beyond the C+L-band is a promising technology for achieving larger capacity transmission by a limited number of installed fibers. In addition to the C- and L-band, we can expect use the S-band as the next band. Although the development of optical components for new bands, particularly transceivers, entails resource dispersion, which is one of the barriers to the realization of multi-band systems, wavelength conversion by transparent all-optical signal processing enables new wavelength band transmission using existing components. Therefore, we proposed a transmission system including a new wavelength band such as the S-band and made it possible to use a transceiver for the existing band by performing the whole-band wavelength conversion without using a transceiver for the new band. As a preliminary verification to demonstrate multi-band WDM transmission including S-band, we investigated the application of a novel wavelength converter between C-band and S-band, which consists of periodically poled lithium niobate waveguide, to the proposed system. We first characterized the conversion efficiency and noise figure of the wavelength converter and estimated the transmission performance of the system through the wavelength converter. Using the evaluated wavelength converters and test signals of 64 channels arranged in the C-band at 75-GHz intervals, we constructed an experimental setup for S-band transmission through an 80-km standard single-mode fiber. We then demonstrated error-free transmission of real-time 400-Gb/s DP-16QAM signals after forward error correction decoding. From the experimental results, it was clarified that thewavelength converter which realizes the uniformlossless conversion covering the whole C-band effectively achieves the S-band WDM transmission, and it was verified that the capacity improvement of the multi-band WDM system including the S-band can be expected by applying it in combination with the C+L-band WDM system.

Single-pixel optical modulation analyzer based on phase retrieval for dual-polarization IQ modulators.

In-service monitoring and adaptive digital compensation of analog imperfections in optical transponders are vital in the next-generation optical coherent transmission systems employing extremely high-order, high-speed modulation formats. A notable example of such analog impairments is the imbalance of amplitude, phase, and/or timing between the in-phase (I) and quadrature (Q) tributaries in an optical IQ modulator, namely the IQ imbalance. Recently, an IQ-imbalance estimation technique based on phase retrieval without using a coherent receiver, the so-called single-pixel optical modulation analyzer (SP-OMA), has been proposed as an affordable in-service monitoring solution for the frequency-dependent IQ imbalance in a (single-polarization) IQ modulator. In this work, we extend the concept of the SP-OMA to dual-polarization IQ modulators. A novel phase retrieval algorithm with an alternating minimization procedure is proposed for identifying the frequency-dependent IQ imbalances on both polarization channels simultaneously from a single photodetector output. The validity and feasibility of the proposed SP-OMA for a dual-polarization IQ modulator are demonstrated numerically and experimentally with a 63.25-Gbaud DP-16QAM signal.

Ultra-wideband discrete Raman amplifier optimization for single-span S-C-L-band coherent transmission systems.

We experimentally compare the performance of two key ultra-wideband discrete Raman amplifier structures, a cascaded dual-stage structure and an in-parallel dual-band structure, in fully loaded S-C-L band coherent transmission systems over 70 km of single-mode fiber. Our results show that dual-band discrete Raman amplifier with minimized backreflections can effectively avoid unstable random distributed feedback lasing, reduce the noise figure, and therefore improve the transmission performance for signals at shorter wavelengths, versus the cascaded dual-stage structure. The average noise figure for S-band signals is 6.8 dB and 7.2 dB for the dual-band structure and cascaded dual-stage structure, respectively, while the average S-band Q2 factor is similarly improved by 0.6 dB. Moreover, the cascaded dual-stage discrete Raman amplifier requires guard bands around the 1485-nm and 1508-nm pumps as the signal and pump wavelengths overlap, which results in a bandwidth loss of ∼10 nm and reduces the potential net data throughput to 28.6 Tb/s for 30-GBaud DP-16QAM signals. However, the dual-band structure can utilize the bandwidth more effectively, which leads to a higher estimated net data throughput of 31.2 Tb/s.

Performance enhancement of 112 Gbps UWOC link by mitigating the air bubbles induced turbulence with coherent detection MIMO DP-16QAM and advanced digital signal processing

Underwater applications like real-time marine observations, video streaming, communication between underwater sensor nodes require spectrally efficient, high-speed (>100 Gbps) and reliable wireless links. In this paper, a coherently detected dual-polarization (DP)-16 quadrature amplitude modulation (QAM) based system is proposed for turbulent underwater wireless optical communication (UWOC) which can transmit a data rate of 112 Gbps. The integration of digital signal processing (DSP) with a coherent system enables error-free transmission at longer distances. The proposed single-input single-output (SISO) system achieves reliable link ranges of 150 m, 55 m, 24 m, 10 m, and 5.5 m under the pure sea, clear ocean, coastal ocean, harbor I, and harbor II water conditions. Furthermore, spatial diversity with multiple-input multiple-output (MIMO) techniques is utilized to mitigate turbulence-induced losses and path losses. At the receiver, the multiple received MIMO branches are combined with the equal gain combining (EGC) method. The transmission range improvements have been observed with 2 × 2 MIMO and 3 × 3 MIMO techniques over the SISO UWOC system. The power budget enhancement of 6 dBm is observed by utilizing a 3 × 3 MIMO configuration over 1 × 1 SISO configuration, and 3 dBm power budget enhancement has been observed in the case of a 2 × 2 MIMO configuration over 1 × 1 SISO configuration. The proposed system can further be utilized in real ocean scenarios to transmit time-sensitive information among diverse underwater sensor nodes.

Fast and blind chromatic dispersion estimation with one sample per symbol.

We propose a fast and blind chromatic dispersion (CD) estimation method by one sample per symbol after coherent detection. The CD estimation process is non-data aided, without the iterative scanning to obtain the CD values. Moreover, we identify that the proposed CD estimation method is transparent to the used modulation format and robust to the transmission impairments, including amplified spontaneous emission (ASE) noise and fiber nonlinearity. When the 35-GBaud DP-16QAM signal with a roll-off factor of 0.1 is transmitted over standard single mode fiber (SSMF) with a range from 320-km to 560-km, the error of CD estimation is less than 150-ps/nm under the condition of 8192 symbols used.

Complex-Valued Neural Network Design for Mitigation of Signal Distortions in Optical Links

Nonlinearity compensation is considered as a key enabler to increase channel transmission rates in the installed optical communication systems. Recently, data-driven approaches - motivated by modern machine learning techniques - have been proposed for optical communications in place of traditional model-based counterparts. In particular, the application of neural networks (NN) allows improving the performance of complex modern fiber-optic systems without relying on any a priori knowledge of their specific parameters. In this work, we introduce a novel design of complex-valued NN for optical systems and examine its performance in standard single mode fiber (SSMF) and large effective-area fiber (LEAF) links operating in relatively high nonlinear regime. First, we present a methodology to design a new type of NN based on the assumption that the channel model is more accurate in the nonlinear regime. Second, we implement a Bayesian optimizer to jointly adapt the size of the NN and its number of input taps depending on the different fiber properties and total length. Finally, the proposed NN is numerically and experimentally validated showing an improvement of 1.7 dB in the linear regime, 2.04 dB at the optimal optical power and 2.61 at the max available power on Q-factor when transmitting a WDM 30 × 200G DP-16QAM signal over a 612 km SSMF legacy link. The results highlight that the NN is able to mitigate not only part of the nonlinear impairments caused by optical fiber propagation but also imperfections resulting from using low-cost legacy transceiver components, such as digital-to-analog converter (DAC) and Mach-Zehnder modulator.

Compensation of Nonlinear Impairments Using Inverse Perturbation Theory With Reduced Complexity

We propose a modification of the conventional perturbation-based approach of fiber nonlinearity compensation that enables straight-forward implementation at the receiver and meets feasible complexity requirements. We have developed a model based on perturbation analysis of an inverse Manakov problem, where we use the received signal as the initial condition and solve Manakov equations in the reversed direction, effectively implementing a perturbative digital backward propagation enhanced by machine learning techniques. To determine model coefficients we employ machine learning methods using a training set of transmitted symbols. The proposed approach allowed us to achieve 0.5 dB and 0.2 dB $Q^2$ -factor improvement for 2000 km transmission of 11 × 256 Gbit/s DP-16QAM signal compared to chromatic dispersion equalization and one step per span two samples per symbol digital back-propagation technique, respectively. We quantify the trade-off between performance and complexity.

Research on Multi-System Ultra-High Speed Optical Signal Access System

In view of the small transmission capacity and single signal modulation format of the existing optical transmission system, this paper proposes an ultra-high-speed optical signal access scheme based on NEL0670, which can realize the transmission of 100 G DP-QPSK, 200 G DP-16QAM and 400 G DP-16QAM signals, and realize flexible and intelligent reception of multi-system optical signals.

Enabling Technologies for Fiber Nonlinearity Mitigation in High Capacity Transmission Systems

Fiber nonlinearity has become a major limiting transmission impairment factor. In this paper, we discuss various nonlinearity mitigation techniques in electrical and optical domains. In electrical domain, multiple reduced complexity digital back-propagation algorithms were developed, including perturbation back-propagation. One drawback is that they can compensate for intra-channel self-phase modulation (SPM) effects only. The inter-channel fiber nonlinearity mitigation can be done in optical domain. For example, digital subcarrier multiplexing (SCM) can effectively mitigate both SPM and cross-phase modulation (XPM) effects. It can become even more beneficial for higher symbol rate systems (e.g., 64 Gbaud and 128 Gbaud) and has a capability to deliver longer reach than single carrier 32 Gbaud DP-16QAM signals and only slightly shorter reach (by up to 7%) compared to 32 Gbaud SCM systems. Another technique, the total intensity directed phase modulation, can provide per-span SPM and XPM compensation. However, this technique is limited to compensation of a few channels only. On the other hand, the proposed enhanced pre-dispersed spectral inversion can effectively compensate for SPM/XPM effects of multiple channels and become more practical by removing limitation to link symmetry even in nonuniform transmission links. Finally, subcarrier power pre-emphasis in optical superchannels can equalize performance of all subcarriers by mitigating intersubcarrier XPM and extend transmission reach by 20%.

Cost-effective 400-Gbps micro-intradyne coherent receiver using optical butt-coupling and FPCB wirings.

We present a cost-effective and bandwidth-enhanced 64-Gbaud micro-intradyne coherent receiver based on hybrid integration of InP waveguide-photodetector (WG-PD) and silica planar lightwave circuit (PLC). InP waveguide-photodetector (WG-PD) arrays are simply chip-to-chip bonded and optically butt-coupled to a silica-based dual-polarization optical hybrid chip. Multiple flexible printed circuit boards are adapted for electrical RF and DC wirings, which provide low-cost integration and good RF performance of the receiver. A 3-dB bandwidth of the fabricated coherent receiver is extended to ~36 GHz by optimization of bondwire inductance between the WG-PD array and the transimpedance amplifier (TIA), even when commercial TIAs with a typical bandwidth of ~29 GHz are used. Through optimization of the silica hybrid integrated coherent receiver, 64-Gbaud DP-16QAM signal transmission over 1050-km standard single-mode fiber is successfully demonstrated below a bit error rate of 2 × 10-3. This is the threshold for a soft decision-based forward error correction, at the optical signal to noise ratio of 23.8 dB.

High-Capacity Multi-Stage Operation of Polarization-Diversity Silicon Photonics 8 × 8 Optical Switch

For practical and wide use of optical switches, cascadability and high throughput are of great significance. We demonstrate up to 8-stage cascaded operation of a polarization-independent silicon photonics 8 × 8 optical switch where 11-channel 32/44-Gbaud DP-16QAM signals are loaded up on all the paths in a cyclic way. The results verify that the 8 × 8 switch is capable of offering a throughput of 48.2 Tb/s with a wall-plug energy efficiency of 0.37 pJ/bit. Using an additional optical module with the characteristics equivalent to the 8 × 8 switch, 9-stage cascaded operation is also emulated. The successful performance observed in these experiments promises the realization of optical switch fabrics with higher port counts by arranging the 8 × 8 switches in a multi-folded Clos network.

On the Cascadability of All-Optical Wavelength Converter for High-Order QAM Formats

With the evolution to ever more complex modulation formats for optical signals, all-optical wavelength converters offer great benefits to optical networks, thanks to the possibility of translating the wavelength of an optical signal regardless of its data rate or modulation format. To deliver its full advantages over conventional optical-electrical-optical conversion, an all-optical wavelength converter has to possess important features in terms of transparency, bandwidth, and cascadability for practical applications in real systems. In this paper, we review a recent development of an all-optical wavelength converter, which exhibits seamless conversion in the C-band with a low noise figure of 6.2 dB for dual-polarization (DP) phase-modulated signals. We then carry out an investigation by both experiment and calculation of the cascaded operation of the developed wavelength converter for high-order quadrature amplitude modulation (QAM) formats. A cascadability comparable to an erbium-doped fiber amplifier is achieved for DP-QPSK, DP-16QAM, and DP-64QAM signals, thanks to the exemplary characteristics of the wavelength converter.

Fractional Fourier Transformation-Based Blind Chromatic Dispersion Estimation for Coherent Optical Communications

In this paper, we propose and demonstrate a blind chromatic dispersion (CD) estimation method based on fractional Fourier transformation (FrFT). Through numerical simulations, the proposed CD estimation method is shown to be robust against the amplified spontaneous emission noise and nonlinear interference. Only 2048 samples are required for reliable CD estimation for single-carrier 28 GBd DP-QPSK or 32 GBd DP-16QAM signals, and the standard deviation can be as low as 98.9 and 103.6 ps/nm, respectively. The feasibility of the proposed CD estimation method has been experimentally verified using 28 GBd DP-QPSK and 14 GBd DP-16QAM signals over various transmission distances with CD ranges of 46 332–77 220 ps/nm and 5148–51 480 ps/nm, respectively. Compared with some other CD estimation methods, the method based on FrFT has advantages in the aspects of less computation complexity and robustness to transmission impairments.

Laser Rate Equation-Based Filtering for Carrier Recovery in Characterization and Communication

We formulate a semiconductor laser rate equation-based approach to carrier recovery in a Bayesian filtering framework. Filter stability and the effect of model inaccuracies (unknown or unuseable rate equation coefficients) are discussed. Two potential application areas are explored: Laser characterization and carrier recovery in coherent communication. Two rate equation-based Bayesian filters, the particle filter and extended Kalman filter, are used in conjunction with a coherent receiver to measure frequency noise spectrum of a photonic crystal cavity laser with less than 20 nW of fiber-coupled output power. The extended Kalman filter is also used to recover a 28-GBd DP-16 QAM signal where a decision-directed phase-locked loop fails.

Fast and robust chromatic dispersion estimation based on temporal auto-correlation after digital spectrum superposition.

We investigate and experimentally demonstrate a fast and robust chromatic dispersion (CD) estimation method based on temporal auto-correlation after digital spectrum superposition. The estimation process is fast, because neither tentative CD scanning based on CD compensation nor specific cost function calculations are used. Meanwhile, the proposed CD estimation method is robust against polarization mode dispersion (PMD), amplified spontaneous emission (ASE) noise and fiber nonlinearity. Furthermore, the proposed CD estimation method can be used for various modulation formats and digital pulse shaping technique. Only 4096 samples are necessary for CD estimation of single carrier either 112 Gbps DP-QPSK or 224 Gbps DP-16QAM signal with various pulse shapes. 8192 samples are sufficient for the root-raised-cosine pulse with roll-off factor of 0.1. As low as 50 ps/nm standard deviation together with a worst estimation error of about 160 ps/nm is experimentally obtained for 7×112 Gbps DP-QPSK WDM signal after the transmission through 480 km to 9120 km single mode fiber (SMF) loop using different launch powers.

Heterogeneously Integrated PLC With Low-Loss Spot-Size Converter and Newly Developed Waveplate PBS for DC-DP-16QAM Receiver

Digital coherent transmission systems with multilevel and multicarrier modulation formats have been moving beyond 100-G systems. In this paper, we report a one-chip 400-G coherent receiver founded on a silica-based planar lightwave circuit (PLC) integration platform. To integrate a polarization beam splitter (PBS), two polarization rotators, four-optical hybrids, and 16 high-speed photodiodes (PDs) in one chip, we employed a 5%-Δ waveguide with a minimum bending radius of 300 μm, and the heterogeneous integration of InP-PDs. To achieve a compact receiver without any degradation in optical passive performance, we also investigated a three-branched spot size converter with a low loss and a high-fabrication tolerance and a waveplate type PBS with a high-polarization extinction ratio. Using these techniques, we fabricated a dual-carrier dual-polarization (DP) 16-level quadrature amplitude modulation (QAM) receiver PLC, and successfully demonstrated 32-Gbaud DP-16QAM signal demodulation for each channel.

Reducing the complexity of perturbation based nonlinearity pre-compensation using symmetric EDC and pulse shaping

Perturbation based nonlinearity pre-compensation has been performed for a 128 Gbit/s single-carrier dual-polarization 16-ary quadrature-amplitude-modulation (DP 16-QAM) signal. Without any performance degradation, a complexity reduction factor of 6.8 has been demonstrated for a transmission distance of 3600 km by combining symmetric electronic dispersion compensation and root-raised-cosine pulse shaping with a roll-off factor of 0.1. Transmission over 4200 km of standard single-mode fiber with EDFA amplification was achieved for the 128 Gbit/s DP 16-QAM signals with a forward error correction (FEC) threshold of 2 × 10(-2).

400 Gbit/s single-carrier and 1 Tbit/s three-carrier superchannel signals using dual polarization 16-QAM with look-up table correction and optical pulse shaping

A 448 Gbit/s single-carrier dual-polarization 16-ary quadrature-amplitude-modulation (DP 16-QAM) signal and a 1.206 Tbit/s three-carrier DP 16-QAM signal are demonstrated using look-up table (LUT) correction and optical pulse shaping. The LUT correction is used to mitigate the effects of transmitter-based pattern-dependent distortion due to the high symbol rates. A programmable optical filter is employed to narrow the modulated signal spectrum and thereby enhance the spectral efficiency and reduce the requirements on the receiver bandwidth and analog-to-digital converter sampling rate. By combining these techniques, the back-to-back required optical signal-to-noise ratios are 26.6 dB and 27.2 dB for BER = 10(-3), and transmission over 1200 and 1500 km of standard single-mode fiber with EDFA amplification was achieved for the 448 Gbit/s signal (12% forward error correction (FEC) overhead) and 1.206 Tbit/s signal (20% FEC overhead), respectively.

Experimental optical generation of DP-16QAM modulation format for high spectral efficiency optical transmission

In this paper, we present a simple experimental method to generate a DP-16QAM optical signal for high spectral efficiency optical transmission. A 16QAM optical signal is generated by passing a DP-QPSK signal through a linear polarization filter in a certain angle so that a square 16QAM can be achieved. Afterwards, we use a polarization multiplexer in order to produce a dual polarization signal. This method proved to be a straightforward and stable way to generate a DP-16QAM signal. In addition, we show that it can provide the same quality in terms of BER per OSNR demonstrated elsewhere.