Jingchi Cheng

Relative phase noise induced impairment in M-ary phase- shift-keying coherent optical communication system using distributed fiber Raman amplifier

We show for the first time, to the best of our knowledge, that, in a coherent communication system that employs a phase-shift-keying signal and Raman amplification, besides the pump relative intensity noise (RIN) transfer to the amplitude, the signals phase will also be affected by pump RIN through the pump-signal cross-phase modulation. Although the average pump power induced linear phase change can be compensated for by the phase-correction algorithm, a relative phase noise (RPN) parameter has been found to characterize pump RIN induced stochastic phase noise. This extra phase noise brings non-negligible system impairments in terms of the Q-factor penalty. The calculation shows that copumping leads to much more stringent requirements to pump RIN, and relatively larger fiber dispersion helps to suppress the RPN induced impairment. A higher-order phase-shift keying (PSK) signal is less tolerant to noise than a lower-order PSK.

Relative phase noise estimation and mitigation in Raman amplified coherent optical communication system.

The interplay between the stochastic intensity fluctuation of Raman pump laser and cross-phase modulation (XPM) effect in transmission optical fiber leads to additional phase noise, namely, relative phase noise (RPN) of signal in multi-level modulated coherent optical communication system. Both theoretical analysis and quantitative simulation have been performed to investigate the characteristics and impact of RPN. Being low-pass in nature, RPN is different from XPM induced phase noise in PSK/OOK hybrid system, and has not been considered yet. The noise power of RPN can accumulate incoherently along transmission links. With a proper signal model, we study the impact of RPN to the coherent optical communication system through Monte Carlo simulation. RPN will cause more cycle slips in Viterbi-and-Viterbi (V-V) phase estimation (PE), and the quantitative analysis of cycle slip probability is carried out. When using sliding window V-V without any optimization, the Q factor penalty of RPN on DQPSK signal can be as large as around 5 dB in strong RPN condition. However, it can be reduced by over 3 dB when using an optimal block size or optimal averaging weights.

Relative Phase Noise-Induced Phase Error and System Impairment in Pump Depletion/Nondepletion Regime

Although the pump relative intensity noise transfer phenomenon in traditional Raman amplified amplitude-shift keying fiber communication systems has been investigated intensively, the stochastic intensity fluctuation of Raman pump laser will interplay with fiber nonlinearity and leads to additional phase noise of signal in the multilevel modulated coherent optical communication system. Such phase noise is defined as relative phase noise (RPN). In this paper, we present comprehensive analysis regarding the characteristics and impairments of RPN through both theoretical derivation and Monte Carlo simulation. We derive the analytical expressions of phase error variance for M-ary PSK and 16QAM modulation formats in consideration of RPN using decision-aided maximum-likelihood phase estimation algorithm. The analysis can be used to predict RPN induced impairment theoretically. Bit error rate (BER) performance is evaluated and compared for QPSK, 8PSK, 16PSK, and 16QAM modulation formats, and the result shows that in the context of RPN 16QAM signal outperforms 16PSK which has the same spectral efficiency. Finally, we extend the analytical result obtained in pump nondepletion regime to depletion regime via numerical analysis. It is observed by comparison that Q-factor penalty will increase in depletion regime.

Carrier Phase Estimation Through the Rotation Algorithm for 64-QAM Optical Systems

A novel low-complexity two-stage digital feedforward carrier phase estimation algorithm based on the rotation of constellation points to remove phase modulation for a 64-ary quadrature amplitude modulation (QAM) system is proposed and analyzed both experimentally and through numerical simulations. The first stage is composed of a Viterbi and Viterbi (V&V) block, based on either the standard quadrature phase shift keying (QPSK) partitioning algorithm using only Class-1 symbols or a modified QPSK partitioning scheme utilizing both Class-1 and outer most triangle-edge (TE) symbols. The second stage applies the V&V algorithm after the removal of phase modulation through rotation of constellation points. Comparison of the proposed scheme with constellation transformation, blind phase search (BPS) and BPS+MLE (maximum likelihood estimation) algorithm is also shown. For an OSNR penalty of 1 dB at bit error rate of 10-2, the proposed scheme can tolerate a linewidth times symbol duration product (Δν · Ts) equal to 3.7 × 10-5, making it possible to operate 32-GBd optical 64-QAM systems with current commercial tunable lasers.

Pump RIN-induced impairments in unrepeatered transmission systems using distributed Raman amplifier.

High spectral efficiency modulation format based unrepeatered transmission systems using distributed Raman amplifier (DRA) have attracted much attention recently. To enhance the reach and optimize system performance, careful design of DRA is required based on the analysis of various types of impairments and their balance. In this paper, we study various pump RIN induced distortions on high spectral efficiency modulation formats. The vector theory of both 1st and higher-order stimulated Raman scattering (SRS) effect using Jones-matrix formalism is presented. The pump RIN will induce three types of distortion on high spectral efficiency signals: intensity noise stemming from SRS, phase noise stemming from cross phase modulation (XPM), and polarization crosstalk stemming from cross polarization modulation (XPolM). An analytical model for the statistical property of relative phase noise (RPN) in higher order DRA without dealing with complex vector theory is derived. The impact of pump RIN induced impairments are analyzed in polarization-multiplexed (PM)-QPSK and PM-16QAM-based unrepeatered systems simulations using 1st, 2nd and 3rd-order forward pumped Raman amplifier. It is shown that at realistic RIN levels, negligible impairments will be induced to PM-QPSK signals in 1st and 2nd order DRA, while non-negligible impairments will occur in 3rd order case. PM-16QAM signals suffer more penalties compared to PM-QPSK with the same on-off gain where both 2nd and 3rd order DRA will cause non-negligible performance degradations. We also investigate the performance of digital signal processing (DSP) algorithms to mitigate such impairments.

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.

Experimental Verification of Relative Phase Noise in Raman Amplified Coherent Optical Communication System

The intensity fluctuation of a Raman pump laser source will not only transfer to the signals intensity through stimulated Raman scattering effect, which has been investigated comprehensively from both theoretical and experimental perspective, but also affect signals phase through cross-phase modulation (XPM) effect. The phase noise is characterized by relative phase noise (RPN) parameter. Although the analytical model of RPN was proposed and the simulations were performed, no experimental verification has been carried on so far. In order to verify the statistical property of RPN, we conducted an experiment to measure the RPN transfer function using a continuous-wave signal and a modulated Raman pump laser diode. The measurement result for the co-pumping configuration has achieved good agreement with the theoretical calculation, validating the proposed theory. In addition to the measurement, we found clear Raman pump induced phase modulation in the experiment using a CO-OFDM signal, demonstrating that RPN will occur in a practical transmission system. With the measured RPN transfer function and the prior knowledge of Raman pump relative intensity noise, we can predict the RPN induced system performance degradation in a theoretical manner.

Relative phase noise induced impairment in CO-OFDM optical communication system with distributed fiber Raman amplifier

In this Letter, we demonstrate that the interplay between Raman pump relative intensity noise and cross-phase modulation leads to a relative phase noise (RPN) that brings non-negligible performance degradation to coherent optical orthogonal frequency-division multiplexing (CO-OFDM) transmission systems with co-pumped Raman amplification. By theoretical analysis and numerical simulation, we proved that RPN brings more system impairment in terms of Q-factor penalty than the single carrier system, and relatively larger walk-off between pump and signal helps to suppress the RPN induced impairment. A higher-order modulated signal is less tolerant to RPN than a lower-order signal. With the same spectral efficiency, the quadrature-amplitude modulation format shows better tolerance to RPN than phase-shift keying. The reported findings will be useful for the design and optimization of Raman amplified CO-OFDM multi-carrier transmission systems.

Characterization and Optimization of Unrepeatered Coherent Transmission Systems Using DRA and ROPA

We present a detailed investigation of the singe-channel 100G polarization-multiplexed quadrature-phase-shift-keying-based unrepeatered coherent transmissions using various order distributed Raman amplifiers (DRAs). The impact of main factors on the system performance is discussed including intrachannel nonlinearity, amplified spontaneous emission, double Rayleigh backscattering, and pump relative intensity noise (RIN)-induced impairments. After optimizing the pump power configuration, it is found that compared to a first- and a third-order DRA, a second-order DRA can provide the best performance owing to relative high optical signal-to-noise ratio (OSNR) and low pump RIN-induced penalty. By using the Giles model, the optimization of a remote optically pumped amplifier is performed in terms of the Erbium-doped fiber length and its farthest position, with given input signals power and OSNR.

A Robust and Efficient Frequency Offset Correction Algorithm With Experimental Verification for Coherent Optical OFDM System

Frequency offset correction (FOC) is essential for a coherent optical orthogonal frequency-division multiplexing (CO-OFDM) system. In this paper, we proposed a robust and efficient FOC algorithm by designing training symbols in frequency domain. This algorithm has an advantage in overcoming imperfect frequency response induced by transmitter devices and physical channel implementations. To further improve the stability of the FOC algorithm against the optical signal-to-noise ratio degradation, a two-step iterative operation is introduced. The proposed algorithm has been utilized in a practical 100-km 16QAM-CO-OFDM transmission system to compensate the frequency offset dynamically. The efficiency and robustness of the algorithm have been validated by outstanding and stable experimental results.