Mid-link Optical Phase Conjugation Research Articles

PMD tolerant transmission using multiple optical phase conjugators in the discretely amplified systems

Abstract In this paper, the impact of polarization mode dispersion (PMD) on system performance in coherent optical transmission assisted by optical phase conjugation (OPC) technique is numerically investigated for a 9-channel PM-4QAM system at 128Gbit/s. The OPC-aided transmission, amplified with lumped erbium doped fiber amplifier (EDFA), is a variation of the conventional dispersion-managed link, which is also called dispersion-inverted link. We demonstrate that introducing more OPCs along the link can partly suppress the PMD-induced impairments and thus improve the system performance significantly. Results of 960-km dispersion-managed transmission show that PMD effect will cause a performance penalty of 1.8 dB when using mid-link OPC (i.e., only 1-OPC), while this penalty will decrease to about 0.4 dB when employing 6-OPCs along the link. Comparing with conventional digital back-propagation (DBP) technique, a performance improvement of about 3.1 dB is observed with multi-OPCs when fiber PMD is equal to 0.1ps/ k m $\sqrt{km}$ .

Bandwidth Extension in a Mid-Link Optical Phase Conjugation

In this paper, we investigate various designs of distributed Raman amplifier (DRA) to extend amplification bandwidth in mid-link optical phase conjugation (OPC) systems and compare bands 191–197 THz and 192–198 THz giving a total bandwidth of 6 THz using a single wavelength pump. We demonstrate the use of highly reflective fiber Bragg grating (FBG) to minimize gain variation across a WDM grid by optimizing forward and backward pump powers as well as the wavelength of FBGs for original and conjugated channels. Finally, we also simulate OSNR and Kerr nonlinear reduction as a product of signals asymmetry and nonlinear phase shift (NPS) for all channels.

Kernel adaptive filtering-based phase noise compensation for pilot-free optical phase conjugated coherent systems.

We propose a kernel-based adaptive filtering method to suppress the phase noise (PN) arising from small deviations from ideal counter-phasing in the dual-pump fibre-based optical phase conjugation (OPC) of pilot-free quadrature-amplitude modulation (QAM) signals. We demonstrate experimentally and numerically that the proposed scheme achieves signal-to-noise ratio improvement over conventional PN compensation under optimised pump dithering settings in the OPC device and features no performance penalty across a range of pump-phase mismatch values, when it is used with a 16-QAM signal in an optical back-to-back configuration. We also illustrate the applicability of the method to the 64-QAM modulation format, and evaluate its performance in a transmission setup with mid-link OPC by means of numerical simulations.

Fiber Nonlinearity Compensation: DBP Versus Link-Optimized OPC in Coherent Transmission

In this letter, we numerically compare the system performance of nonlinearity-compensated transmission based on digital back-propagation (DBP) and optical phase conjugation (OPC) technique in coherent transmission with lumped erbium doped fiber amplifiers (EDFAs). The tested transmission for DBP is a conventional dispersion-managed link, while it is an optimized design, the so-called dispersion inversion link for OPC-based system. Both of them have the same amount and type of fibers, and thus the same transmission reach. The numerical results show that DBP-aided systems have better performance for a shorter transmission reach while OPC technique presents its performance advantage when the transmission reach is increased. For a given transmission distance, the performance of OPC systems can further be improved by introducing additional OPCs. Results of 3840-km (<inline-formula> <tex-math notation="LaTeX">$48\times 80$ </tex-math></inline-formula>km) dispersion-managed transmission show that only mid-link OPC is enough to get similar performance with full-field DBP. By employing 6-OPCs in the links, the SNR of OPC-based system outperforms DBP by up to 1.9dB, while this SNR advantage will increase to 3dB after 5760-km (<inline-formula> <tex-math notation="LaTeX">$72\times 80$ </tex-math></inline-formula>km) transmission.

Distributed Raman Amplification for Fiber Nonlinearity Compensation in a Mid-Link Optical Phase Conjugation System.

In this paper, we review different designs of distributed Raman amplifiers which have been proposed to minimize the signal power profile asymmetry in mid-link optical phase conjugation systems. We demonstrate how the symmetrical signal power profiles along the fiber can be achieved using various distributed Raman amplification techniques in the single-span and more realistic multi-span circumstances. In addition, we show the theoretically predicted results of the Kerr nonlinear product reduction with different Raman techniques in mid-link optical phase conjugator systems, and then in-line/long-haul transmission performance using numerical simulations.

Performance Evaluation of Highly Nonlinear Fiber (HNLF) Based Optical Phase Conjugation (OPC) in Long Haul Transmission of 640 Gbps 16-QAM CO-OFDM

This paper presents the quantitative measurement through an experimental test of 640 Gbps 16-QAM coherent-optical orthogonal frequency-division multiplexing (CO-OFDM) over 800 km optical fiber with mid-link optical phase conjugation (OPC) using highly nonlinear fiber (HNLF). The first focus is the OPC parameter optimization, including the optimization of HNLF length and signal/pump power that inputs into OPC. Four different HNLFs, as the illustrative examples, are investigated. The second focus is to investigate the effects of fiber dispersion, nonlinearity, and amplified spontaneous emission (ASE) noise on the long-haul transmission of 16-QAM CO-OFDM signal, and the OPC compensation efficiency. The performance evaluation focuses on the conversion efficiency (CE), received signal constellation, Q-factor improvement, and bit error rate (BER) at the receiver end. Such end-to-end performance evaluation is important because the 16-QAM CO-OFDM signal status is heterogeneous and the mitigation of transmission impairments to the signal is still unclear. The OPC parametric optimization is achieved experimentally using commercially available HNLFs with different scenarios and the numerical results are interpreted in conjunction with simulations.

Probabilistic Shaping for the Optical Phase Conjugation Channel

Probabilistic constellation shaping is studied and developed for optical phase conjugation (OPC)-based nonlinearity compensation of Kerr nonlinearities in optical fiber links. The mid-link OPC scenario is considered for dispersion compensated systems. It is demonstrated in simulations and experimentally that transmission strategies optimal for classical additive white Gaussian noise (AWGN) channels can be sub-optimal for these systems without nonlinearity compensation. On the contrary, when nonlinearity compensation is applied with mid-link OPC, the channel noise is demonstrated to be Gaussian and AWGN-like transmission strategies thus remain effective. A channel-agnostic probability mass function (PMF) optimization algorithm is proposed for the input constellation in order to further improve the shaping gains in both scenarios. Operating arbitrary PMFs on arbitrary channels is enabled by a channel-agnostic digital signal processing (DSP) chain. After ≈2000 km of transmission, mid-link OPC is demonstrated to provide ≈1 dB of gain in effective SNR, which translates to ≈0.4 bits/QAM symbol of gain in achievable information rate. The gain is then increased by an extra 0.2 bits/QAM symbol by applying probabilistic shaping.

Enhancing the Signal Power Symmetry for Optical Phase Conjugation Using Erbium-Doped-Fibre-Assisted Raman Amplification

We propose a novel dual-order backward-pumped distributed Raman amplification scheme assisted erbium-doped-fibre (EDF) providing nearly perfect signal power symmetry (>93% symmetry level) over two 50.4 km single mode fibre spans. Compared with conventional dual-order Raman schemes, this scheme only requires an additional short (25 cm) erbium-doped fibre to compensate the loss from the passive components between spans, significantly improving the overall link symmetry. Unlike a conventional hybrid Raman/EDFA approach with separate amplifier modules, the proposed scheme offers cost savings by utilizing the Raman pumps to activate the erbium-doped fibre, avoiding the need for an EDF-designated pump. In an optical transmission system with four 50.4 km fibre spans, our novel Raman scheme presented in this paper enables mid-link optical phase conjugation (OPC) to compensate up to 37 dB of nonlinear Kerr inter-signal interference. This represents a 12 dB advantage in compensation over conventional dual-order Raman amplification. Our experimental and simulated results also demonstrate that the proposed configuration provides 7 dB nonlinear threshold enhancement in a 200 Gb/s DP-16QAM 200 km inline transmission system using a mid-link OPC, exceeding the enhancement observed with the conventional dual-order Raman scheme. Our simulation results also show that the optimum Q2 factor using the proposed scheme outperforms the conventional schemes at 2000 km.

Combating Fiber Nonlinearity Using Dual-Order Raman Amplification and OPC

We experimentally demonstrate, for the first time, a significant 34-dB nonlinear product power compensation in a mid-link optical phase conjugation (OPC) system using an optimized dual-order distributed Raman amplification (DRA) technique. The dual-order backward (BW)-pumping scheme shows a record signal power symmetry of 97% over 50.4-km single-mode fiber (SMF) spans. We also demonstrate that the required accuracy for span-to-span power alignment is within ±1 dB in order to maintain 20-dB nonlinear product compensation. For a 256-Gb/s dual-polarization-16 QAM transmission over 100.8 km ( $2\times50.4$ km) SMF with mid-link OPC, the proposed Raman amplification scheme enables the OPC to improve the nonlinear threshold by ~7 dB and the optimum signal launch power by ~5 dB compared with the system without OPC.

Experimental demonstration of 72% reach enhancement of 36Tbps optical transmission system using mid-link optical phase conjugation

We experimentally demonstrate nonlinear noise compensation in an optical phase conjugation assisted 1st order Raman amplified 30x30Gbaud DP-QPSK transmission system with a spectral efficiency of 3.6b/s/Hz. We show that by optimizing the link symmetry, even with only 1st order Raman amplification a single, mid-link, optical phase conjugation compensates for 90% of the signal-signal nonlinear interference resulting in a 2.3dB performance enhancement. We show that increasing the number of optical phase conjugations in the presence of 10% residual nonlinearity results in a reduction in the performance enhancement owing to an enhancement in the nonlinear noise generation efficiency of the system. We achieve a record 72% optical phase conjugation enabled reach enhancement of the 30x30Gbaud DP-QPSK signals.

Transmission performance improvement using random DFB laser based Raman amplification and bidirectional second-order pumping.

We demonstrate that a distributed Raman amplification scheme based on random distributed feedback (DFB) fiber laser enables bidirectional second-order Raman pumping without increasing relative intensity noise (RIN) of the signal. This extends the reach of 10 × 116 Gb/s DP-QPSK WDM transmission up to 7915 km, compared with conventional Raman amplification schemes. Moreover, this scheme gives the longest maximum transmission distance among all the Raman amplification schemes presented in this paper, whilst maintaining relatively uniform and symmetric signal power distribution, and is also adjustable in order to be highly compatible with different nonlinearity compensation techniques, including mid-link optical phase conjugation (OPC) and nonlinear Fourier transform (NFT).

Qualitative Comparison of Kerr Nonlinearity Mitigation Schemes in a Dispersion-Managed Link for 4 × 28-GBd 16-QAM Signals

We present an experimental investigation of different Kerr nonlinearity mitigation schemes in a 2-span dispersion–compensated link of 160–km length for 4 × 28–GBd single–polarization 16–QAM signal transmission. We consider optical phase conjugation (OPC) based on mid–link spectral inversion, digital backpropagation (DBP), and transmission of phase-conjugated twin-waves with either digital coherent superposition (DCS) or all–optical coherent superposition employing phase–sensitive amplification (PSA). The experiments are performed in a single–channel and a wavelength-division multiplexing (WDM) scenario. For single–channel operation, phase-conjugated twin waves with DCS provided superior performance compared to all the other mitigation schemes with a maximum Q2–factor improvement of 1.7 dB compared to signal transmission without any nonlinearity mitigation. The DBP, however, showed a higher nonlinear threshold than all the other schemes in our experiments. In the WDM scenario, DBP showed worse performance than the other mitigation schemes whereas the DCS again provided a higher Q2–factor improvement than the other techniques with a maximum Q2–factor improvement of 1.9 dB. The PSA and the mid–link OPC showed similar nonlinearity mitigation performance both in single-channel and WDM scenarios.

SPM compensation for long-haul CO-OFDM systems with midlink optical phase conjugation

Midlink optical phase conjugation (M-OPC) based on four wave mixing (FWM) in semiconductor optical amplifier (SOA) can be used to compensate for the fiber nonlinearity effect in CO-OFDM systems. Simulation of 40Gb/s CO-OFDM systems without optical dispersion compensation is presented. For a single-channel system, even greater benefits are obtained and the nonlinear limits are significantly decreased: the maximum Q can be increased by 3dB for all transmission distances greater than 2400km, the nonlinear threshold (NLT) (for Q>10dB) is increased by >4dB. Due to cross phase modulation (XPM) from neighbouring WDM channels, the maximum Q and NLT are increased by about 1.1dB and 1dB for 50-GHz-spacing WDM systems separately. These results show that M-OPC can mitigate the fiber nonlinearity impairment for both single-channel and WDM systems.

Fiber nonlinearity compensation for OFDM super-channels using optical phase conjugation

We experimentally demonstrate that mid-link optical phase conjugation (OPC) effectively compensates fiber nonlinearity in coherent optical OFDM super-channels. The OPC was produced by pump × subcarrier degenerate four-wave-mixing in a 1-km highly nonlinear fiber. The nonlinear threshold for the 10 × 80-km 604.7-Gb/s 16-QAM test system was increased by 4.8 dB. The performance at the optimum power was only improved by 0.2 dB because the OPC module produces a 1.6 dB penalty for the back-to-back system. FWM theory shows that the 'noise' processes of OPC modules utilizing χ3 nonlinearities could be reduced by increasing the pump power, which will improve back-to-back performance with the OPC module.

Reduction of Parametric Amplified Noise in Nonlinear Fiber Channels by Use of Wiener Filtering

As a parametric amplified noise, nonlinear phase noise limits the channel capacity in multispan fiber-optics transmissions. In this paper, the reduction of nonlinear phase noise by use of Wiener filtering is studied. Extracting signal from time-domain concepts, parametric amplified noise covering signal spectrum in frequency domain can be suppressed considerably. Timing jitter and ghost pulses due to intrachannel nonlinearities can also be compensated. Nonparametric Wiener filtering is used at the end of six configurations of dispersion compensation (DC) transmission lines: pre- and postlumped DC, distributed DC without optical phase conjugation (OPC), dispersion inversion (DI) about midlink OPC with, respectively, one-for-multispan, single type span and alternate type span on each side of OPC. It is found that single-DI and alternate-DI schemes have higher input power capacity than one-for-multi-DI scheme. Further reductions in the nonlinear phase noise accumulation for all these DC maps are obtained with Wiener filtering, and higher absolute dispersion can decrease signal fluctuation due to nonlinear phase noise, as well.

Optical phase conjugation for ultra long-haul phase-shift-keyed transmission

Nonlinear phase noise (Gordon-Mollenauer phase noise) can limit the transmission distance for phase-shift-keyed modulation formats. In this paper, the compensation of nonlinear phase noise by a midlink optical phase conjugation (OPC) is studied. A proof-of-principle experiment is presented showing an over 4-dB improvement in Q factor when OPC is employed in a differential phase-shift-keying (DPSK) system. Also, an ultra long-haul OPC-based differential quadrature phase-shift-keying (DQPSK) transmission experiment is studied to show the impact of self-phase modulation (SPM)-induced impairments, including nonlinear phase noise, in a transmission line. OPC results in a 44% increase in transmission distance when compared to a conventional transmission system using dispersion compensating fiber (DCF) for chromatic dispersion compensation.