Polarization Demultiplexing Research Articles

Design of Low-Power DSP-Free Coherent Receivers for Data Center Links

Coherent detection offers high spectral efficiency and receiver sensitivity, but digital signal processing (DSP)-based coherent receivers may be prohibitively power hungry for data centers even when optimized for short-reach applications, where fiber propagation impairments are less severe. We propose and evaluate low-power DSP-free homodyne coherent receiver architectures for dual-polarization quadrature phase shift keying (DP-QPSK) for inter- and intradata center links. We propose a novel optical polarization demultiplexing technique, for DP-QPSK and higher-order modulation formats, with three cascaded phase shifters driven by marker tone detection circuitry. We consider carrier recovery based on either optical or electrical phase-locked loops (PLLs). We propose a novel multiplier-free phase detector based on XOR gates, which exhibits less than 0.5 dB power penalty relative to a conventional Costas loop phase detector. We also study the relative performance of homodyne DP-differential QPSK, for which carrier phase recovery is unnecessary. Our proposed DSP-free architectures exhibit ∼1 dB power penalty at small chromatic dispersion compared to their DSP-based counterparts. We estimate conservatively that the high-speed analog electronics of an electrical PLL-based coherent receiver consume nearly 4 W for 200 Gbit/s DP-QPSK, assuming a 90-nm complementary metal-oxide semiconductor process.

Enhanced frequency-domain fractionally spaced equalization for coherent optical transmission system with colored noise

An enhanced minimum mean-square-error (MMSE) frequency-domain (FD) fractionally spaced equalizer (FSE) for polarization demultiplexing is proposed in the presence of colored noise. The 2-norm of the estimated channel matrix and polynomial fitting of equalization coefficients are introduced to mitigate the colored noise without requiring knowledge of the optical and electrical filtering. Simulation results show that ∼3-dB improvement of equalization performance is achieved compared to conventional MMSE FD-FSE in an 8×112-Gbit/s wavelength-division multiplexing polarization-division multiplexing-quadrature phase shift keying transmission system over a 1200-km single mode fiber link.

Tracking channel alignment in blind polarization de-multiplexing

Blind de-multiplexing methods such as the constant modulus algorithm (CMA) or independent component analysis (ICA) are an effective means of de-multiplexing dual polarization modulated optical signals mixed by the evolution of the polarization state on propagation through the transmission medium. However, the recovery matrix estimated by these methods has an inherent permutation ambiguity in the assignment of polarization channels to ports. As a consequence, frequent instances of channel alignment permutation can occur resulting in loss of data during system recovery. This paper describes a method of maintaining the initial alignment by setting the diversity combiner weight matrix to a member of the equivalence class of the estimated recovery matrix that is the nearest to the previous weight matrix. The method is verified by computer simulation of de-multiplexing by ICA of dual polarization differential quadrature phase shift keyed coherent optical transmission subject to aggressive changes in polarization state as well as experimental investigation of de-multiplexed QPSK modulation by CMA. Perfect maintenance of channel alignment and accurate recovery of the constellations is achieved.

Training Symbol-Based Equalization for Quadrature Duobinary PDM-FTN Systems

A training symbol-based equalization algorithm is proposed for polarization de-multiplexing in quadrature duobinary (QDB) modulated polarization division multiplexedfaster-than-Nyquist (FTN) coherent optical systems. The proposed algorithm is based on the least mean square algorithm, and multiple location candidates of a symbol are considered in order to make use of the training symbols with QDB modulation.Results show that an excellent convergence performance is obtained using the proposed algorithm under different polarization alignment scenarios. The optical signal-to-noise ratio required to attain a bit error rate of 2*10-2 is reduced by 1.7 and 1.8 dB using the proposed algorithm, compared to systems using the constant modulus algorithm with differential coding for 4-ary quadrature amplitude modulation(4-QAM) and 16-QAM systems with symbol-by-symbol detection, respectively.Furthermore, comparisons with the Tomlinson-Harashima precoding-based FTN systems illustrate that QDB is preferable when 4-QAM is utilized.

An improved CMA for dispersion compensation in 100 Gbps DP-QPSK optical signal transmission system

An improved constant modulus algorithm (CMA) for dispersion compensation in 100Gbps Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) optical signal transmission system is proposed to achieve better reliability receiving performances. In the receiver, we combine variable step size and Mean-Square Error (MSE) to improve the conventional CMA. Compared to conventional CMA, the proposed algorithm has a better performance in adapting the time varying channel and the strong noise. Meanwhile, the proposed algorithm has a faster convergence rate than the previous one. Simulation results show that the improved CMA can achieve the dispersion and polarization demultiplexing of 100Gbps DP-QPSK optical signal transmission system efficiently.

Modulation format independent blind polarization demultiplexing algorithms for elastic optical networks

We demonstrate a novel modulation format independent algorithm for adaptive blind polarization demultiplexing for elastic optical networks (EONs). We compare the proposed algorithm with traditional constant modulus algorithm (CMA) and radius-directed algorithm (RDA), in terms of performance in PM-QPSK, PM-16QAM and PM-64QAM coherent system, by simulating in back-to-back (BTB) and transmission sceneries. The simulation result shows that the modulation format independent algorithm can achieve universal adaptive blind polarization demultiplexing for PM-mQAM signals and gain slightly better performance in condition of lower optical signal noise ratio (OSNR). Furthermore, we also carry out experiments to investigate the performance of the proposed algorithm in BTB and 800 km transmission scenarios for 16 GBaud PM-QPSK and PM-16QAM. The experimental results demonstrate the conclusion of the numerical simulations.

Joint tracking and equalization scheme for multi-polarization effects in coherent optical communication systems.

We propose a joint multi-polarization-effect tracking and equalization method based on two extended Kalman filters, which can cope with state of polarization (SOP) tracing, polarization demultiplexing, equalization for polarization dependent loss (PDL) and polarization mode dispersion (PMD) in PDM-M-QAM coherent optical communication system. The mathematical model of the proposed method is given and analyzed in detail. Through simulation, the proposed method is proved to be very effective in a 28 Gbaud/s PDM-16QAM system. With the proposed method, SOP tracing speed is up to 110 Mrad/s for azimuth angle and 1200 krad/s for phase angle, respectively, and PDL and PMD can be equalized simultaneously in the values of 10 dB and more than half of the symbol period.

Simplified polarization demultiplexing based on Stokes vector analysis for intensity-modulation direct-detection systems

A simple and effective polarization demultiplexing method is proposed based on the improved Stokes vector analysis and digital signal processor algorithm for the intensity-modulation direct-detection optical communication systems. Such a scheme could significantly simplify optical receivers with low system cost. The experimental results demonstrate the feasibility of our proposed method and show that only 1- and 1.7-dB power penalties are measured for 10- and 25-km transmissions compared to back-to-back case.

Mode Division Multiplexing Using Orbital Angular Momentum Modes Over 1.4-km Ring Core Fiber

Mode division multiplexing (MDM) systems using orbital angular momentum (OAM) modes can recover the data in D different modes without recourse to full (2 D × 2 D ) multiple input multiple output (MIMO) processing. One of the biggest challenges in OAM-MDM systems is the mode instability following fiber propagation. Previously, MIMO-free OAM-MDM data transmission with two modes over 1.1 km of vortex fiber was demonstrated, where optical polarization demultiplexing was employed in the setup. We demonstrate MDM data transmission using two OAM modes over 1.4 km of a specially designed ring core fiber without using full MIMO processing or optical polarization demultiplexing. We demonstrate reception with electrical polarization demultiplexing, i.e., minimal 2 × 2 MIMO, showing the compatibility of OAM-MDM with current polarization demultiplexing receivers.

Demonstration of 2 × ONU 80 Gbps direct detection colorless polarization division multiplexing frequency division multiplexing passive optical network uplink transmission

In this study, we propose and experimentally demonstrate a simple direct detection passive optical network (PON) uplink transmission scheme based on frequency division multiplexing and polarization division multiplexing. Two optical network units (ONUs) are assigned to two different frequency bands at two different orthogonal polarization directions. At the optical line terminal, both ONU signals can be simultaneously detected by a single photodiode without utilizing any polarization control, polarization selection, or complicated polarization demultiplexing algorithms. As a proof-of-concept, the 2×ONU 80 Gbps 32-ary quadrature amplitude modulation Nyquist single carrier signals are successfully transmitted over 2 km standard single mode fiber or 20 km large effective area fiber with the assistance of frequency domain equalization and decision-directed least-mean-square. The measured bit error rate can be below the 7% pre-forward error correction threshold of 3.8×10−3. Meanwhile, this scheme is compatible with the widely used wavelength-division multiplexed PON, which shows the promising potential and feasibility of this proposal.

Format-independent polarization-demultiplexing technique for dual-polarization intensity modulated signals.

We show through simulations how polarization demultiplexing of dual-polarization, intensity modulated signals of arbitrary format can be performed by only using the information in Stokes space. The technique would be applicable for short-range communications within data centers.

Low-Complexity and Adaptive Nonlinearity Estimation Module Based on Godard's Error

A low-complexity and adaptive nonlinearity estimation module is proposed and experimentally demonstrated based on Godard's error and low-pass filter (LPF). The computational complexity of the proposed adaptive estimation module is significantly reduced by avoiding utilizing the polarization demultiplexing, frequency offset compensation, and carrier phase recovery in the process of finding the optimal value $\gamma\xi_{\mathrm{opt}}$ compared with the previous approach based on phase noise variance. The performance of the proposed module is experimentally verified in a 40-Gb/s polarization-division-multiplexing-quadrature-amplitude modulation (PDM-QPSK) coherent optical communication system over 720-km single-mode fiber (SMF). Experimental results show that the computational complexity of the proposed nonlinearity estimation module is only ∼7.35% of the previous approach and ∼27.8% of that with an LPF inserted.

Decision-free radius-directed Kalman filter for universal polarization demultiplexing of square M-QAM and hybrid QAM signals

We propose and experimentally demonstrate a universal and blind polarization-state tracking scheme for M-quadrature amplitude modulation (QAM) signals based on a decision-free radius-directed linear Kalman filter (RD-LKF). The polarization tracking performance is investigated through simulation and experiments for both quadrature phase-shift keying (QPSK) and 16-QAM signals. The influence of the filter parameters on the static polarization demultiplexing performance and dynamic tracking capability are discussed via simulation. The optimization strategy of the filter parameters in modulation-format-independent scenarios is proposed and simulations are carried out to evaluate the polarization demultiplexing penalty for QPSK, 16QAM, and hybrid QPSK/16QAM signals. Finally, the proposed decision-free RD-LKF is experimentally compared to the respective algorithms of constant modulus algorithm and multimodulus algorithm for QPSK and 16-QAM and the advantage of ultrafast polarization tracking ability is confirmed.

Polarization Demultiplexing for Stokes Vector Direct Detection

The conventional intensity modulation with direct detection faces the capacity bottleneck beyond 100-Gb/s per wavelength, due to its 1-dimension (1-D) modulation and detection. To increase the spectral efficiency, there emerge a variety of direct detection subsystems based on the Stokes vector receiver (SV-R). The Stokes space contains the intensity information of three independent polarizations. Therefore, the SV-R enlarges the detection dimensions at receiver to 3. The SV-R can recover any 2-D or 3-D signal modulated in Stokes space. One of the most significant features of SV-R is the DSP-enabled polarization demultiplexing. Instead of a 2 × 2 Jones matrix, in Stokes space, a 3 × 3 rotation matrix (RM) characterizes the polarization variation during fiber transmission. Previously, we have demonstrated a training-assisted polarization demultiplexing algorithm, which uses three basis vectors in Stokes space to estimate the RM. In this paper, we propose a blind polarization demultiplexing using the signal distribution in Stokes space. Combined with the blind adaptive algorithm, the new method converts the two-stage polarization demultiplexing (RM estimation and channel equalization) to a single-stage blind 3 × 2 real-value multiple-input multiple-output equalization, which significantly simplify the DSP procedures. We experimentally demonstrate the SV-R based on either training or blind demultiplexing algorithms, and compare the performance between them.

A Highly Flexible Polarization Demultiplexing Scheme for Short-Reach Transmission

We propose and experimentally demonstrate an effective and highly flexible polarization demultiplexing scheme based on a Stokes analyzer, which could be also applied to the nonorthogonal polarization division multiplexing (NPDM) schemes. Compared with the conventional direct detection, the proposed method has better tolerance to the polarization-dependent loss. Furthermore, such an approach can significantly enhance system flexibility without increasing cost and complexity since the transmitted signals are not required to maintain strict orthogonality, and no additional polarization-tracking feedback circuit is needed. Experimental results show that any polarization multiplexing degree that is larger than 23° can be adaptively demultiplexed after 10-km transmission using optical orthogonality monitoring and digital-signal-processing (DSP) algorithms.

Wavelength Translation of Dual-Polarization Phase-Modulated Nyquist OTDM at Terabit/s

Wavelength translation is one of the key functions to realize spectral defragmentation in an elastic optical add/drop network. Optically translating the wavelength of Terabit/s signals is challenging in the sense that it requires a signal processing scheme to be independent of the symbol rate and modulation format, wideband, and highly efficient simultaneously. In this paper, we present an all-optical wavelength translation for a single-carrier 1.376-Tb/s dual-polarization quadrature phase-shift keying (DP-QPSK) Nyquist optical time-division multiplexing (OTDM) signal by using an all-optical wavelength converter which fulfills all of the basic requirements. A seamless wavelength translation covering almost C-band is achieved with a boost in OSNR thanks to the improved conversion efficiency. The system performance is measured by a polarization-insensitive coherent receiver using linear sampling which implements simultaneously the demultiplexing of the time-domain tributaries, polarization demultiplexing, and carrier-phase recovery for the dual-polarization phase-modulated OTDM signal. The employed receiving scheme, therefore, resolves one of the most shortcomings of the conventional OTDM receiver, which is polarization sensitive due to the use of nonlinear sampling.

Highly Robust Dual-Polarization Doubly Differential PSK Coherent Optical Packet Receiver for Energy Efficient Reconfigurable Networks

A dual polarization doubly differential phase shift keying coherent optical packet receiver that is robust against frequency offset transients and polarization demultiplexing problems is described in detail for energy efficient reconfigurable networks. Its performance is not affected by large frequency offsets. This leads to greatly reduced waiting times compared with previous coherent optical packet receivers, which in turn can significantly increase throughput in the coherent optical packet switched networks.

Experimental Assessment of the Adaptive Stokes Space-Based Polarization Demultiplexing for Optical Metro and Access Networks

We experimentally analyze an adaptive polarization-demultiplexing (PolDemux) technique based on the representation of the state of polarization of the signal in the Stokes space. The performance and convergence speed of the Stokes PolDemux are compared with the constant modulus algorithm, for dual polarization QPSK and 16QAM modulation formats in a system with a maximum polarization rotation of 6 kHz. Moreover, the applicability of the adaptive Stokes algorithm is tested in an ultradense wavelength-division multiplexing scenario over 80 km of standard single-mode fiber. The BER results show a very small power penalty confirming the suitability of the adaptive Stokes method in systems where the accumulated chromatic and polarization-mode dispersion are small, such as high capacity optical metro and access networks scenarios.

Extended Kalman Filter vs. Geometrical Approach for Stokes Space-Based Polarization Demultiplexing

By deriving the physical model of the system, which includes definitions of both state variables and measurement equations, we apply an extended Kalman filter to Stokes space-based polarization demultiplexing for complex-modulated signals. The convergence ratio, tracking, computational complexity, and system performance of this method are investigated and compared with the geometrical approach previously proposed to adaptive computation of the best fit plane. An analysis of the tuning parameters of both methods reveals that the Kalman filtering provides a more robust and stable polarization demultiplexing of signals. Nevertheless, if properly tuned, the geometrical approach attains a similar performance, with a gain of 90% in terms of complexity reduction.

Proposal for a compact silicon microring resonator-based polarization demultiplexer

A compact silicon-based polarization demultiplexer (P-DEMUX) is proposed by using a microring resonator, where the metal-loaded strip waveguides are used as the bus channels. The modal characteristics of transverse electric (TE) modes for the hybrid plasmonic waveguides are analogous to those for the strip dielectric ones, while those of transverse magnetic (TM) modes show significant differences, leading to strong polarization-dependence. Consequently, the input TE mode can efficiently output from the drop port at the resonant wavelength, while the input TM mode directly outputs from the through port with nearly negligible coupling. The present P-DEMUX can be easily employed to construct an on-chip wavelength/polarization division multiplexing optical transmission system, further increasing the optical interconnect capacity. Results show that a P-DEMUX with a radius of 2.003 μm for the microring is achieved at the wavelength of 1550 nm, where the extinction ratio and insertion loss are, respectively, ∼22.36 (20.09) dB and ∼0.39 (0.84) dB for the TE (TM) mode. In addition, fabrication tolerances to the key structural parameters are investigated and field evolution through the P-DEMUX is also presented.