Dual-polarization Quadrature Amplitude Modulation Research Articles

Optimized adaptation algorithms for low-complexity adaptive equalizers.

In order to address the high-power consummation issue of conventional multi-input and multi-output (MIMO) adaptive equalizer (AEQ) for short-reach coherent transmissions, several state-of-the-art low-complexity AEQs have been proposed. In our work, optimized adaptation algorithms for low-complexity real-valued (RV) AEQs with different structures are analyzed. Moreover, an approach to avoid introducing additional computational complexity due to the optimized adaptation process is presented here. The advantages of proposed optimized adaptation algorithms are experimentally demonstrated in a 25 Gbaud dual-polarization 16-quadrature-amplitude-modulation (DP-16QAM) back-to-back (BtB) intradyne system with an overall bandwidth of 14 GHz. Experimental results show that a similar performance as the conventional AEQ could be achieved by using proposed adaptation algorithms and reducing the number of multiplications with up to ∼65%.

Low-Complexity Conditional Generative Adversarial Network (c-GAN) Based Nonlinear Equalizer for Coherent Data-Center Interconnections

Nonlinear impairments emerging from both fiber optical links and optoelectrical devices are the major bottleneck for enhancing the throughput of photonic data-center interconnection (DCI). Here, we propose a data-driven nonlinear equalizer based on a conditional generative adversarial network (c-GAN) for high-speed and large-capacity photonic DCI. Its performance is experimentally evaluated by transmitting C-band 40-channel 60-GBaud dual polarization-16 quadrature amplitude modulation (DP-16QAM) dense wavelength division multiplexing (DWDM) signals over 20-km standard single-mode fiber (SSMF). Our experimental results verify that, the proposed c-GAN equalizer outperforms both traditional nonlinear equalizers and other data-driven counterparts, in terms of both the bit-error ratio (BER) and computational complexity. Only the proposed c-GAN equalizer can guarantee the BER performance of all 40 channels to reach the threshold of 20% soft decision forward error correction (SD-FEC) at BER=0.027, leading to a net transmission capacity of 16 Tb/s. Meanwhile, the computational complexity of the proposed c-GAN equalizer can be significantly reduced by 31.8%, 80.7%, and 98.8%, respectively, in comparison with Volterra filter equalizer (VFE), multilayer perceptron (MLP), and long short-term memory neural network (LSTM-NN) schemes.

Auto Bias Control Technique for Silicon IQ Modulators Using the Dual-Arms Power Dithers

In high-capacity coherent transmission applications such as 400 Gb/s and beyond, silicon inphase and quadrature modulators (SIQMs) are widely used as a coherent transmitter assembly. The bias points of the SIQM deviate the optimum points because of environmental change. Consequently, auto bias control (ABC) is an essential technique to stabilize the bias points of the SIQM. In this paper, we described a novel method to achieve the bias point high-precision control for SIQMs. First, a pair of the differential power dithers are applied to the dual-arms of the channels I and Q, which can compose a push-pull bias locking mode. By this way, the bias error of the channel P induced by the channels I and Q can be eliminated compared with the conventional single-arm ABC method. Then an orthogonal integration operation is used to extract the dither frequency component of the output signal of the SIQM, and the intersections of the orthogonal-integral curves corresponding to the bias points of the channels I, Q, and P are the optimum bias thermal-power. The simulation and experimental results show that our method exhibits a good performance for locking the optimum thermal-power of the SIQM, and the optical signal-to-noise ratio (OSNR) penalty caused by the proposed method for a 512-Gb/s optical signal transmission with a dual-polarization 16-quadrature amplitude modulation format can be neglected in our experiment.

Nonlinear SNR estimation based on the data augmentation-assisted DNN with a small-scale dataset.

Fiber nonlinearity is one of the major impairments for long-haul transmission systems. Therefore, estimating the nonlinear signal-to-noise ratio (SNRNL) is indispensable to guarantee the quality of transmission and manage the operation of optical networks. The deep neural network (DNN) has been successfully applied for the SNRNL estimation. However, the performance substantially degrades, when the mega dataset is inaccessible. Here, we demonstrate an accurate SNRNL estimation based on the data augmentation (DA)-assisted DNN with a small-scale dataset in the frequency domain. When the 95-GBaud dual-polarization 16 quadrature amplitude modulation (DP-16QAM) signal with variable optical launch powers from -2 to 4-dBm is numerically transmitted from 80-km to 1520-km standard single-mode fiber (SSMF), we identify that, in comparison with traditional DNN scheme, the DA allows the reduction of the training dataset size by about 60% while keeping the same mean absolute error (MAE) as 0.2-dB of SNRNL estimation. Meanwhile, the DA-assisted DNN scheme can reduce the MAE by about 0.14-dB compared with the traditional DNN scheme, when both SNRNL estimation schemes use 100 raw datasets which contain 700 symbols. Due to these observations, the DA-assisted DNN scheme is promising for field-trial nonlinear SNR estimation, especially when the collection of mega datasets is inconvenient.

System impact and calibration method of power imbalances for a dual-polarization in-phase/quadrature optical transmitter.

For the latest 400-Gb/s or upcoming 1-Tb/s single-carrier optical fiber communications systems, dual-polarization quadrature amplitude modulation (DP-QAM) based on in-phase (I) and quadrature (Q) optical transmitter is the only possible solution. In a coherent DP-IQ transmitter, the power difference between the I and Q branches or orthogonally polarized (X and Y) channels are known as the IQ or XY power imbalance, respectively. Uncompensated IQ and XY power imbalances are always troublesome and can significantly limit the performance in long-haul transmission. In this work, we first experimentally investigate the impact of IQ and XY power imbalances on various QAM formats and baud rates in high-speed transmission. A DP-16-QAM up to 86 GBd is investigated, aiming for beyond 400 Gb/s applications. Then, a convenient pre-calibration method based on optical interference is proposed and experimentally demonstrated to detect and compensate both the IQ and XY power imbalances. This method can use the existing modulator structure and low-speed photodiode in the DP-IQ transmitter to obtain the optical interference power by injecting a specific coded electrical signal. After adjusting the time skew of each channel, the power imbalance between two channels can be calibrated by minimizing the optical interference power. The results fit well with the given theoretical analysis, and the proposed technique can be adopted to the DP-IQ transmitter with different QAM formats.

Implementation of Soft Decoding Mechanism for Addressing Nonlinearities in Long-Distance Optical Transmission System

A soft decoding technique is discussed in this paper to improve the performance of long-distance optical networks (LDOTNs). LDOTNs are affected by phase noise and nonlinearities generated inside the fiber. The investigations of the proposed LDOTN were carried out by dual-polarization 16-quadrature amplitude modulation (DP-16QAM), DP-64QAM over single-mode fiber (SMF) and digital signal processing (DSP) methodologies. The improved performance of the presented mechanism is discussed over SMF based on constellation shaping (CS). The CS of the presented LDOTN is then compared to the standard 16-QAM and 64-QAM using international telecommunication union-telecommunication (ITU-T) standard G-652.D and G-657.A1 SMF. The soft detecting procedure enables the LDOTNs to attain significant outcomes.

Performance analysis of nyquist superchannel with multicast overlay system

We demonstrate the performance of Nyquist superchannel system along with multicast overlay system which employs Nyquist dual polarization quadrature amplitude modulation (DP-16QAM) and polarization shift keying (POLSK) modulation to transmit 640 GB/s as unicast and differential phase shift keying (DPSK) modulation for 40Gb/s as multicast. The performance of multicast overlay system transmits total 5.1Tb/s data rate using 8*640 GB/s unicast signal and 40Gb/s multicast signals. From the simulation results, best BER, input power, and Q-factor have been analyzed for multicast overlay system. In addition, the proposed design Nyquist DP-16QAM modulation offers efficient utilization of bandwidth for transmission of each channel at the high data rate.

34-GBd Linear Transimpedance Amplifier for 200-Gb/s DP-16-QAM Optical Coherent Receivers

High spectral efficiency offered by the coherent optical communication links makes them attractive for the next-generation optical communication links. Using advanced modulation schemes such as dual-polarization quadrature-amplitude modulation (DP-QAM) data rates beyond 200 Gb/s can be achieved. A key component of such links is the wide-bandwidth and high-linearity coherent optical receiver. In this paper, we present a fully differential (FD) optical receiver architecture consisting of a variable-gain transimpedance amplifier (VG-TIA) followed by a VG amplifier (VGA). The proposed optical receiver employs a dual-feedback automatic gain control (AGC) loop that controls both the front-end VG-TIA and the following VGA to achieve both low-noise and high-linearity operation. A new photodiode (PD) dc current cancellation scheme is developed and implemented for the full differential front-end TIA. A prototype dual-TIA chip is fabricated in a 0.13- $\mu \text{m}$ SiGe BiCMOS process. The presented TIA achieves 20-pA/ $\sqrt {\text {Hz}}$ input-referred noise (IRN) density, 27-GHz, 3-dB bandwidth, and 1.5% total harmonic distortion (THD) at 1-mApp input PD current and 500-mVpp output voltage swing. This enables the 34-GBd operation with the bit error rate (BER) of $10^{-10}$ and $5.4\times 10^{-4}$ using DP-QPSK and DP-16-QAM formats at optical signal-to-noise ratios (OSNRs) of 25 and 30 dB, respectively, demonstrating the 100- and 200-Gb/s single wavelength optical coherent receiver operation. The dual-TIA chip occupies an area of $1.4\,\,\text {mm}\times 1.6$ mm and consumes 313 mW per channel at 34 GBd from a 3.3-V supply.

Performance of 120 Gbps single channel coherent DP-16-QAM in terrestrial FSO link under different weather conditions

The performance of a 120 Gbps single-carrier coherent transceiver system is evaluated in a terrestrial Free Space Optical Link (FSOL) under the haze, rain, and fog weather conditions. In-phase and Quadrature (IQ) based Dual Polarization 16 Quadrature Amplitude Modulation (DP-16-QAM) is implemented in OptiSystem v.13 simulation platform with homodyne detection and Digital Signal Processing (DSP) at the receiver. The IQ based optical modulator and the DSP unit can be fabricated in the form of Integrated Circuits (ICs) help to keep the system cost low. The system performance is evaluated in terms of Optical Signal to Noise Ratio (OSNR) requirements at an acceptable Bit Error Rate (BER ≤ 2 × 10−3, i.e., below the FEC limit) under low turbulence environment. Simulation results show that the system maintains good OSNR performance under severe weather conditions. This transceiver system shall be useful in short-reach, low-cost, high-capacity, robust communication link with dynamic environmental conditions.

A full-duplex optical fiber/wireless coherent communication system with digital signal processing at the receiver

In this paper, a full-duplex, 120 Gbps optical fiber/wireless system is presented for high-speed and multicasting communication link. Both the wired and wireless systems use Dual Polarization 16 Quadrature Amplitude Modulation (DP-16-QAM) with homodyne detection. Blind Phase Search (BPS) algorithm, Constant Modulus Algorithm (CMA) and Viterbi Phase Estimation (VPE) algorithm are used at the receiver end to compensate for signal distortions such as Chromatic Dispersion (CD), Polarization Mode Dispersion (PMD), nonlinear impairments, frequency and phase mismatch. The system performance is analyzed in terms of Error Vector Magnitude (EVM), Bit Error Rate (BER ≤ 10−12) and constellation diagram based on OptiSystem V.13 simulation platform. The maximum transmission distances achieved are 275 km and 1.1 km for Standard Single Mode Fiber (SSMF) and Free Space Optical Link (FSOL), respectively.

Dual-polarization wavelength conversion of 16-QAM signals in a single silicon waveguide with a lateral p-i-n diode [Invited

A polarization-diversity loop with a silicon waveguide with a lateral p-i-n diode as a nonlinear medium is used to realize polarization insensitive four-wave mixing. Wavelength conversion of seven dual-polarization 16-quadrature amplitude modulation (QAM) signals at 16 GBd is demonstrated with an optical signal-to-noise ratio penalty below 0.7 dB. High-quality converted signals are generated thanks to the low polarization dependence (≤0.5 dB) and the high conversion efficiency (CE) achievable. The strong Kerr nonlinearity in silicon and the decrease of detrimental free-carrier absorption due to the reverse-biased p-i-n diode are key in ensuring high CE levels.

Analysis and Demonstration of Network Utilization Improvement Through Format-Agnostic Multi-Channel Wavelength Converters

The effectiveness of using all-optical wavelength converters (AO-WCs) to improve optical network utilization is evaluated by performing both network simulations and experiments. The employed AO-WCs enable format-agnostic and multi-channel operations, in which the wavelengths of multiple input channels are simultaneously shifted by the same amount. Such unique features are not available in conventional optical-electrical-optical-conversion-based wavelength converters (OEO-WCs). Network simulations show that the wavelength conversion can double the network utilization (the carried traffic), while the number of required wavelength converters can be significantly reduced by 60% by employing AO-WCs instead of OEO-WCs. In addition, simulation results confirm the importance of AO-WC cascadability for achieving low blocking probabilities. Our simulation analysis also identifies an optimum AO-WC specification value, the maximum wavelength shift, for maximizing sharing of the installed AO-WCs. In the experiment, the performance of a prototyped AO-WC is evaluated under a realistic network setup employing a wavelength-selective-switch-based optical node installing the AO-WC and fiber transmission lines. Wavelength division multiplexing signals of different modulation formats, including dual-polarization quadrature phase shift keying (DP-QPSK), dual-polarization 16 quadrature amplitude modulation (DP-16QAM), and on-off keying (OOK), spanning over a 1000 GHz bandwidth are successfully shifted simultaneously by 1400 GHz or 200 GHz without significant signal quality degradation, confirming the guard-band-less, multi-channel, and modulation-format/ line-rate-agnostic capabilities of the AO-WC.

Blind third-order dispersion estimation based on fractional Fourier transformation for coherent optical communication

In this paper, we propose a blind third-order dispersion estimation method based on fractional Fourier transformation (FrFT) in optical fiber communication system. By measuring the chromatic dispersion (CD) at different wavelengths, this method can estimation dispersion slope and further calculate the third-order dispersion. The simulation results demonstrate that the estimation error is less than 2% in 28GBaud dual polarization quadrature phase-shift keying (DP-QPSK) and 28GBaud dual polarization 16 quadrature amplitude modulation (DP-16QAM) system. Through simulations, the proposed third-order dispersion estimation method is shown to be robust against nonlinear and amplified spontaneous emission (ASE) noise. In addition, to reduce the computational complexity, searching step with coarse and fine granularity is chosen to search optimal order of FrFT. The third-order dispersion estimation method based on FrFT can be used to monitor the third-order dispersion in optical fiber system.

Experimental Evaluation of Impairments in Unrepeatered DP-16QAM Link with Distributed Raman Amplification

The transmission impairments of a Raman amplified link using dual-polarization 16-quadrature amplitude modulation (DP-16QAM) are experimentally characterized. The impact of amplitude and phase nois ...

Performance analysis of coherent optical communication system for higher order dual polarization modulation formats

The ever increasing demand of data traffic due to emerging services and applications has evolved the need of high capacity, high speed and highly spectral efficient optical systems with advanced modulation formats. We compare the performance of single channel coherent optical system for 112Gb/s with Dual polarization modulation techniques i.e. DP- QPSK(Dual Polarization Quadrature phase shift keying), DP-16QAM(Dual Polarization −16 Quadrature amplitude Modulation) and DP-32QAM(Dual Polarization −32 Quadrature amplitude Modulation) with DSP (Digital Signal Processing)module for SSMF(standard single mode fiber) in terms of tolerance to fiber nonlinearities which is investigated through the effect of power and distance on %EVM and requirement of OSNR using numerical simulations. ned shows that the coherent optical systems with higher order modulation formats are less tolerant to fiber nonlinearities i.e the performance of DP-QPSK coherent systems with DSP module is better than the DP-16QAM and DP-32QAM coherent systems as they are more tolerant to fiber nonlinearities. The results obtained can be used for the design of compensation techniques for fiber nonlinearities in cohResults obtaierent optical systems using higher order modulation formats.

A Modif ied Adaptive DBP for DP 16-QAM Coherent Optical System

A modified adaptive digital backpropagation (A-DBP) algorithm based on intensity variance is proposed in this letter. In the simulation, a 25-GBd dual-polarization 16-quadrature amplitude modulation transmission system with a transmission distance of 2000 km is established to investigate the performance of the proposed A-DBP. The simulation results show that the proposed A-DBP can dynamically estimate the optimal fiber nonlinear coefficient $\gamma _{\mathrm{ opt}}$ and achieve comparable or even better performance than the DBP with full knowledge of fiber. Moreover, the proposed A-DBP can achieve better performance compared with the previous A-DBP based on intensity variance. Moreover, the proposed A-DBP requires fewer iterations to determine $\gamma _{\mathrm{ opt}}$ compared with the previous A-DBP based on intensity or phase variance.

On the Filter Narrowing Issues in Elastic Optical Networks

This paper describes the problematic filter narrowing effect in the context of next-generation elastic optical networks. First, three possible scenarios are introduced: the transition from an actual fixed-grid to a flexi-grid network, the generic full flexi-grid network, and a proposal for a filterless optical network. Next, we investigate different transmission techniques and evaluate the penalty introduced by the filtering effect when considering Nyquist wavelength division multiplexing, single side-band direct-detection orthogonal frequency division multiplexing, and symbol-rate variable dual polarization quadrature amplitude modulation. Also, different approaches to compensate for the filter narrowing effect are discussed. Results show that the specific needs per each scenario can be fulfilled by the aforementioned technologies and techniques or a combination of them, when balancing performance, network reach, and cost.

Detection and alignment of dual-polarization optical quadrature amplitude transmitter IQ and XY skews using reconfigurable interference.

Dual-polarization quadrature amplitude modulation (DP-QAM) is one of the feasible paths towards 100-Gb/s, 400-Gb/s and 1-Tb/s optical fiber communications systems. For DP-QAM transmitter, the time mismatch between the in-phase and quadrature (IQ) or x-polarized and y-polarized (XY) tributary channels is known as the IQ or XY skew. Large uncompensated IQ or XY skew can significantly degrade the optical fiber communications system performance. Sometimes, time-interleaved return-to-zero (RZ) DP signal is preferred with lower nonlinear polarization scattering induced penalty. In this work, detection and alignment of DP-QAM transmitter IQ and XY skews using reconfigurable interference is experimentally demonstrated. For IQ skew detection, a total dynamic range of 26.4 dB is achieved with ~1-dB power change for 0.5-ps skew from well alignment. For XY skew detection, it shows 23.2-dB dynamic range, and ~1.5-dB power change is achieved for 1-ps XY skew. Fast detection algorithm for arbitrary skew is also proposed and experimentally verified. The scheme is compatible with different modulation formats, flexible data sequences, and variable waveforms.

Linewidth-Tolerant Dual Polarization QAM Transmission System Using Twin Local Lights

A novel digital coherent transmission system based on dual polarization quadrature amplitude modulation (DP-QAM) is proposed. In this system, a DP-QAM signal and twin local lights are generated from one continuous wave (CW) light source, and transmitted simultaneously. Because optical phase noise is canceled in the proposed receiver, linewidth tolerance is improved without using a coherent light source in the receiver. The proposed scheme successfully demodulates a 3.125-GBd DP-quadrature phase shift keying (i.e., DP-4-QAM) signal. After 50-km transmission, no degradation in bit error rate was observed even at the linewidth of 45 MHz.

Fiber-wireless integration for 80 Gb/s dual-polarization-16 quadrature-amplitude-modulation signal transmission at Q-band

Abstract. We experimentally demonstrate a Q-band seamlessly integrated fiber-wireless system, where80 Gb∕s dual-polarization (DP) 16 quadrature-amplitude-modulation (DP-16QAM) is transmitted over 50-kmsingle-mode fiber-28 (SMF-28) wire link, 0.5-m 2×2 multiple-input multiple-output wireless link, and finally,another 50-km SMF-28 wire link. Conventional multimodulus-algorithm equalization is introduced at the receiverto realize polarization demultiplexing and to minimize crosstalk between the two polarizations. For the first time,we realize fiber-wireless integration signal transmission with high-order modulation (16QAM) and polarizationmultiplexing at Q-band. © 2014 Society of Photo-Optical Instrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.53.10.106104] Keywords: optical communication; fiber-wireless integration; polarization multiplexing; 16 quadrature-amplitude-modulation.Paper 140783 received May 15, 2014; revised manuscript received Sep. 16, 2014; accepted for publication Sep. 17, 2014; publishedonline Oct. 9, 2014.