Polarization Tributaries Research Articles

Fault-tolerant four-dimensional constellation for coherent optical transmission systems.

We propose a 4-dimensional 2-ary amplitude ring-switched modulation format with 64 symbols, which is denoted as 4D-2A-RS64 encoded over two polarization tributaries to improve the transmission performance over long-haul optical fibers in the presence of the non-linear Kerr effect. At a spectral efficiency of 6 bits per 4D, simulation results show that this format outperforms the polarization division multiplexed (PDM) 8QAM-star modulation as well as the 4D-2A-8PSK over links without inline dispersion management. We evaluate the performance for a WDM transmission of 11 × 90~Gbaud channels over a multi-span SSMF link. For an achievable information rate of 4.8bit/s/Hz, the maximum transmission distance is improved by 10.6% (400 km) and 4% (160 km) compared to PDM-8QAM-star and 4D-2A-8PSK respectively. The achieved gains are composed of a linear part and a non-linear part, respectively from the improved Euclidean-distance distribution and the constant power property of the 4D modulation. The geometric shaping of the proposed scheme is easy to implement and is robust to Mach-Zehnder modulator (MZM) imbalances and quantization errors stemming from the finite digital-to-analog converter (DAC) resolution. This robustness is compared to the one of other geometric-shaped non-linearity tolerant 4D schemes such as the 4D-2A-8PSK and the 4D-64PRS that can be both outperformed by our scheme in severe conditions.

The Impact of Kerr Nonlinearity on the SNR Variability Induced by Polarization-Dependent Loss

We provide through numerical simulations new insights on the polarization-dependent loss (PDL)-induced variability of the signal-to-noise ratio (SNR) in dual-polarization coherent systems when Kerr nonlinearity is significant. We show that the interplay between PDL and nonlinearities provides an SNR distribution broader than the one induced by PDL in the only presence of amplified spontaneous emission noise. Such a broadening is missed by handy state-of-art models as the reverse channel method (RCM) that do not account for fiber nonlinearity, thus resulting inaccurate to estimate the SNR variability at practical launch powers. We show that such an inaccuracy persists even by considering in the RCM the nonlinear noise contribution. We show that the SNR distribution broadening in the presence of fiber nonlinearity is the result of a change in dynamics between the performance of those that can be identified for each PDL draw as best- and worst-performing polarization tributaries. By numerically decoupling the main contributions to fiber nonlinearity we also identify the nonlinear effects responsible for such a change of dynamics. Finally, we qualitatively validate our findings by mean of an experimental result.

Crossing-free on-chip 2 × 2 polarization-transparent switch with signals regrouping function.

We propose and demonstrate an on-chip 2×2 polarization-transparent switch for simultaneously handling two-group polarization-division multiplexing (PDM) signals. By introducing the polarization-transparent power splitter/combiner (PPS/PPC), waveguide crossings such as those in conventional PDM switches can be avoided and, thus, the insertion losses and complexity of the device can be reduced. Furthermore, each input polarization tributary can be independently switched and routed thanks to the output selectivity of the PPS/PPC. Thus, not only a basic switch between the dual-group PDM signals, but also a precise four-channel-signal regrouping can be achieved, enabling a complete and non-redundant switching functionality. A polarization extinction ratio larger than 15dB with reasonable insertion losses is experimentally observed. Clear and open eye diagrams can be obtained with less than 1dB power penalties for all the measured paths.

Training Symbol Assisted in-Band OSNR Monitoring Technique for PDM-CO-OFDM System

We proposed and demonstrated an in-band optical signal-to-noise ratio (OSNR) monitoring method for a polarization-division-multiplexed coherent optical orthogonal frequency-division-multiplexing (PDM-CO-OFDM) system. OSNR of the PDM signals on both polarization tributaries can be simultaneously monitored by calculating the electrical SNR with low complexity by taking advantage of the uncorrelation property between the noise and training symbol, which is used for conventional frame synchronization. This OSNR monitoring technique is verified in 103.57-Gb/s 16 quadrature amplitude modulation-based PDM CO-OFDM transmission system with 240—1440-km standard single-mode fiber (SSMF) links. Experimental results show that after calibration, the OSNR estimation error is less than 0.6 dB in a dynamic range of 12—27 dB, the estimation remains accurate even after 1440-km SSMF transmission, regardless of the chromatic dispersion and polarization mode dispersion in the fiber. To further investigate the robustness of the proposed OSNR monitoring method, additional simulation is implemented on VPI and MATLAB platform. It turned out to be insensitive to nonlinear effect within reasonable range as our monitoring technique shows good performance with launching power below −2 dBm after 500-km SSMF transmission. The proposed technique could still accurately monitor the individual OSNR of each polarization tributary of the PDM signal even in the presence of 5-dB polarization-dependent loss.

Coherent-Optical In-Line Add/Drop of PDM Tributaries of Subcarrier Multiplexed Signals

We propose an extended coherent optical add/drop multiplexing scheme, in which an individual polarization tributary in a polarization-division multiplexed orthogonal frequency-division multiplexed signal is erased and added by frequency conversion in fiber, which is sequentially inserted in a transmission line. In the scheme, a polarization tributary can be inserted and replaced in a dual polarization spectrally overlapped ultra-dense multiplexed signal. In a proof-of-concept demonstration, we investigate the performance of the proposed optical add/drop multiplexing scheme. A 25-Gb/s quadrature phase-shift keying tributary within a dual-polarization three subcarrier orthogonal frequency division multiplexed signal is erased and added with negligible distortion of the orthogonal polarization tributary and the adjacent subcarriers.

Volterra-Based Reconfigurable Nonlinear Equalizer for Coherent OFDM

A reconfigurable nonlinear equalizer (RNLE) based on inverse Volterra series transfer function is proposed for dual-polarization (DP) and multiband coherent optical orthogonal frequency-division multiplexing (OFDM) signals. It is shown that the RNLE outperforms by 2 dB the linear equalization in a 260-Gb/s DP-OFDM system at 1500 km. The RNLE improves the tolerance to inter/intraband nonlinearities, being independent on polarization tributaries, modulation format, signal bit rate, subcarrier number, and distance.

Modified Constant Modulus Algorithm With Polarization Demultiplexing in Stokes Space in Optical Coherent Receiver

We propose a modified method of constant modulus algorithm (CMA) based on polarization demultiplexing in Stokes space for polarization demultiplexing (SS-PDM) in optical coherent receivers. SS-PDM can tolerate chromatic dispersion (CD) and polarization-dependent loss (PDL), but is sensitive to PMD. On the other hand, the CMA with high-order FIR filters could compensate for polarization-mode-dispersion (PMD) effectively, but suffers from the singularity problem which results from PDL. Therefore, we use the polarization rotation matrix estimated by SS-PDM to set the initial center taps of the CMA. The singularity problem of the CMA is avoided effectively in a much wider range of PDL and its convergence speed is also improved. We demonstrate this modified CMA in a simulation of 100-Gbit/s PDM-QPSK with PDL of 1, 3, and 5 dB, respectively. PDL is emulated by attenuating one polarization tributary before polarization multiplexing and the interaction between PDL and PMD is ignored. The singularity problem disappears as expected. Because the tolerance of PDL is affected by DGD, we further investigate the PDL tolerance under different DGD. Finally, we conduct the simulation of 100-Gbit/s PDM-QPSK with PDL of 3 dB over 3000 km SSMF and 4000 km SSMF separately. The convergence speed accelerates indeed compared with that of the conventional CMA.

Simultaneous OSNR Monitoring for Two Polarization Tributaries of a PDM Signal Using a Polarization-Diversity Nonlinear Loop Mirror Based on FWM

We investigate and experimentally demonstrate an optical signal-to-noise ratio (OSNR) monitoring technique for polarization-division-multiplexed return-to-zero ON-OFF keying (PDM-RZ-OOK) and differential-phase-shift-keying (PDM-RZ-DPSK) signals. Both polarization tributaries of the PDM channel can be simultaneously monitored based on cascaded four-wave mixing in a polarization-diversity nonlinear loop mirror. The interchannel crosstalk induced by nonlinear polarization scattering could be mitigated using the polarization diversity loop structure. As OSNR varies between 5 and 20 dB, the output power of the monitor has a dynamic range of 4.2 and 4.6 dB for the OOK and DPSK signals, respectively. We also demonstrate that the performances of the monitor are the same for both time-interleaved and synchronized PDM-RZ signals.

Towards full band colorless reception with coherent balanced receivers

In addition to linear compensation of fiber channel impairments, coherent receivers also provide colorless selection of any desired data channel within multitude of incident wavelengths, without the need of a channel selecting filter. In this paper, we investigate the design requirements for colorless reception using a coherent balanced receiver, considering both the optical front end (OFE) and the transimpedance amplifier (TIA). We develop analytical models to predict the system performance as a function of receiver design parameters and show good agreement against numerical simulations. At low input signal power, an optimum local oscillator (LO) power is shown to exist where the thermal noise is balanced with the residual LO-RIN beat noise. At high input signal power, we show the dominant noise effect is the residual self-beat noise from the out of band (OOB) channels, which scales not only with the number of OOB channels and the common mode rejection ratio (CMRR) of the OFE, but also depends on the link residual chromatic dispersion (CD) and the orientation of the polarization tributaries relative to the receiver. This residual self-beat noise from OOB channels sets the lower bound for the LO power. We also investigate the limitations imposed by overload in the TIA, showing analytically that the DC current scales only with the number of OOB channels, while the differential AC current scales only with the link residual CD, which induces high peak-to-average power ratio (PAPR). Both DC and AC currents at the input to the TIA set the upper bounds for the LO power. Considering both the OFE noise limit and the TIA overload limit, we show that the receiver operating range is notably narrowed for dispersion unmanaged links, as compared to dispersion managed links.

Simultaneous PMD Mitigation for Two Polarization Tributaries of a PDM Signal using only One All-Optical Regenerator

A scheme of polarization-mode-dispersion (PMD) mitigation in a polarization-division-multiplexing (PDM) system is proposed and demonstrated with 2×10Gb/s return-to-zero on-off-keying (RZ-OOK) transmission. Simultaneous mitigation for two polarization tributaries of the PDM signal is achieved based on the self-phase-modulation (SPM) effect and offset filtering in a polarization nonlinear loop configuration. The improvement of eye-diagram-based signal-to-noise ratio (SNR) is 3.5 dB in the presence of a 7.2-ps differential-group delay (DGD) when the pulsewidth of the PDM signal is ∼16.6ps.

Adaptive frequency-domain equalization in digital coherent optical receivers

We propose a novel frequency-domain adaptive equalizer in digital coherent optical receivers, which can reduce computational complexity of the conventional time-domain adaptive equalizer based on finite-impulse-response (FIR) filters. The proposed equalizer can operate on the input sequence sampled by free-running analog-to-digital converters (ADCs) at the rate of two samples per symbol; therefore, the arbitrary initial sampling phase of ADCs can be adjusted so that the best symbol-spaced sequence is produced. The equalizer can also be configured in the butterfly structure, which enables demultiplexing of polarization tributaries apart from equalization of linear transmission impairments. The performance of the proposed equalization scheme is verified by 40-Gbits/s dual-polarization quadrature phase-shift keying (QPSK) transmission experiments.

On the Tolerance of 111-Gb/s POLMUX-RZ-DQPSK to Nonlinear Transmission Effects

We experimentally investigate the tolerance of 111-Gb/s polarization multiplexed-return to zero-differential quadrature phase-shift keying (POLMUX-RZ-DQPSK) to the different nonlinear transmission impairments in different links structures. We analyze the effect of dispersion maps for four different fiber types and show that with sufficiently large per-span under-compensation in the double periodic dispersion map, a dispersion managed (DM) link can nearly have a similar performance to a non-DM link. Furthermore, we confirm that the super large effective area fiber family can provide around 3 dB advantage in the tolerance to nonlinear effects compared to standard single-mode fiber. Finally, we demonstrate that interleaving the two polarization tributaries of the 111-Gb/s POLMUX-RZ-DQPSK signal provides only a negligible advantage in terms of tolerance to nonlinear effects.

Detection of a Single 40 Gb/s Polarization-Multiplexed QPSK Channel With a Real-Time Intradyne Receiver in the Presence of Multiple Coincident WDM Channels

We have characterized the performance of a single-ended intradyne receiver when multiple wavelength-division multiplexed (WDM) channels are incident. Detection of a single 40 Gb/s dual-polarization QPSK channel is achieved within a band of up to 17 WDM channels using a receiver with real-time digital signal processing and without optical demultiplexing. Measurements are presented of the performance in a back-to-back configuration as well as after transmission over 400 km of standard single-mode fiber. The dependence of the coincident channels' interference on various parameters, such as the net chromatic dispersion and the orientation of the coincident channels' polarization tributaries relative to the receiver polarizer, is explored. We also investigate using a single interfering channel with its power appropriately scaled to represent multiple interfering channels as an alternative configuration for evaluating receiver performance. The single-interferer approach has significant disadvantages including increased polarization sensitivity and different scaling of the interference term arising from the channel-channel beating compared to true multi-channel interference.

100-Gb/s Direct-Detection OFDM Transmission on Independent Polarization Tributaries

We propose a low-overhead method of polarization-multiplexed direct detection optical orthogonal frequency-division-multiplexing transmission that requires no digital multiple-input and multiple-output processing at the receiver. Optical filtering of carrier components allows a simple polarization separation. Using 16-quadrature amplitude modulation and only 31-GHz bandwidth, 100-Gb/s transmission over 80 km of standard single-mode fiber is successfully demonstrated. The crosstalk penalty was only 1.2 dB. We also investigated the laser frequency offset tolerance for the proposed 100-Gb/s transmission method.

PDM-QPSK: on the system benefits arising from temporally interleaving polarization tributaries at 100Gb/s

We experimentally study, over a dispersion-managed link relying on low chromatic dispersion fibre, the origins of the system benefits provided by temporally interleaving the polarization tributaries of 100Gb/s coherent RZ-PDM-QPSK by half a symbol period. Hence, we demonstrate that the amount of benefits provided by this technique is dependent on the configuration of the WDM transmission system.

Impact of Temporal Interleaving of Polarization Tributaries Onto 100-Gb/s Coherent Transmission Systems With RZ Pulse Carving

We report in this letter on a 100-Gb/s spectrally efficient transmission over 16 x 100 km of standard single-mode fiber employing erbium-doped fiber amplifiers. We feed it with polarization-division-multiplexed quadrature phase-shift keying signals with 50% return-to-zero carving which we recover with coherent detection. We evaluate the impact of interleaving the polarization-multiplexed tributaries onto the tolerance to nonlinearities and we discuss in particular the benefit of this interleaving when nonlinear effects interact with polarization-mode-dispersion related impairments.