Key technologies for high-speed digital coherent optical communications

In this paper, based on the proposed simulation model, the impact of dispersion on the systems performance in 160Gbit/s optical communication systems is detailedly discussed. As chromatic dispersion (CD) and polarization mode dispersion (PMD) are caused by different physical effect, the effects of CD and PMD on systems performance are also analyzed separately in the paper. At first, the CD and CD slope tolerance in 160Gbit/s systems are carefully simulated and analyzed, at the same time, self-phase modulation (SPM) effect is also discussed in this part, and the result shows that for 1dB eye-opening penalty, CD tolerance would be about ±3.1ps/nm and CD slope and SPM also all seriously limit the transmission distance of systems. Furthermore, the impact of PMD on systems property are discussed by virtue of statistical method, and result shows that PMD tolerance of 160Gbit/s is 0.7ps with the outage probability of 10^-5 of 1dB eye-opening penalty and it will increase to 1.8ps for the first-order PMD compensated systems. These results show that CD, CD slope, PMD and SPM all are key challenges in 160Gbit/s systems.

Chapter 3 - Optical Communication Fundamentals

Summary form only given. In high-speed fiber optic transmission systems, fiber group-velocity dispersion is periodically compensated to reduce the nonlinear waveform distortion in a dispersion managed system. Polarization mode dispersion (PMD) is another limiting factor for 10 Gb/s optical transmission system. However, chromatic dispersion management could be different when the waveform distortion caused by PMD is considered. We experimentally study the optimization of a 10 Gb/s optical transmission system considering the interaction between nonlinear waveform distortion and PMD waveform distortion as well as chromatic dispersion variance of a single mode (SM) fiber. A general guideline is derived for a system designer based on the experimental results.

Polarization-multiplexing quadrature phase shift keying (PM-QPSK) has become a key ingredient in the design of 100 and 200-Gb/s dense wavelength-division multiplexed (DWDM) networks. The performance of the system using these formats varies according to the shape of the pulses employed by the optical carrier: non-return to zero (NRZ), return to zero (RZ) or carrier-suppressed return to zero (CSRZ). In this paper, we analyse the tolerance of PM-QPSK to linear and nonlinear optical impairments, including amplified spontaneous emission (ASE) noise, crosstalk, distortion by optical filtering, chromatic dispersion (CD), polarization mode dispersion (PMD) and fibre Kerr nonlinearities. The simulation results show that RZ formats with a low-duty cycle have a higher tolerance to CD, PMD and intrachannel nonlinearities due to the reduction of pulse-to-pulse interaction. Conversely, the PM–NRZ-QPSK format has a higher spectral efficiency (SE) than other options and shows better tolerance to filtering distortion and CD in filterless networks.

Polarization mode dispersion (PMD), polarization dependent loss (PDL) and chromatic dispersion (CD) of fiber optic components and fiber transmission links are key issues in fiber optical communication. Of interest are the respective static values as well as their variations in time which might be due to environmental changes or due to rerouting. Basic aspects will be briefly addressed, a variety of standard and state-of-the-art characterization and monitoring techniques will be reviewed and recent experimental results will be presented.

Optical performance monitoring (OPM) becomes an inviting topic in high speed optical communication networks. In this paper, a novel technique of OPM based on a new elaborated computation approach of singular spectrum analysis (SSA) for time series prediction is presented. Indeed, various optical impairments among chromatic dispersion (CD), polarization mode dispersion (PMD) and amplified spontaneous emission (ASE) noise are a major factors limiting quality of transmission data in the systems with data rates lager than 40 Gbit/s. This technique proposed an independent and simultaneous multi-impairments monitoring, where we used SSA of time series analysis and forecasting. It has proven their usefulness in the temporal analysis of short and noisy time series in several fields, that it is based on the singular value decomposition (SVD). Also, advanced optical modulation formats (100 Gbit/s non-return-to zero dual-polarization quadrature phase shift keying (NRZ-DP-QPSK) and 160 Gbit/s DP-16 quadrature amplitude modulation (DP-16QAM)) offering high spectral efficiencies have been successfully employed by analyzing their asynchronously sampled amplitude. The simulated results proved that our method is efficient on CD, first-order PMD, Q-factor and OSNR monitoring, which enabled large monitoring ranges, the CD in the range of 170-1700 ps/nm.Km and 170-1110 ps/nm.Km for 100 Gbit/s NRZ-DP-QPSK and 160 Gbit/s DP-16QAM respectively, and also the DGD up to 20 ps is monitored. We could accurately monitor the OSNR in the range of 10-40 dB with monitoring error remains less than 1 dB in the presence of large accumulated CD.

High-speed transmission limitations due to Polarization Mode Dispersion

Polarization mode dispersion (PMD) is one of the major obstacles in high-speed (above 100 Gbits rate) and long-haul optical communication system. In order to meet the requirements of high-sped optical communication quality, some PMD compensation and mitigation schemes had been developed. Forward error correction (FEC) is useful to increasing the system margin. The combination of distributed fast polarization scrambler (D-FPS) and FEC is an effective method to improving the optical system’s PMD tolerance. The fundamental reason is that D-FPS can accelerate the redistribution of the link PMD to enable FEC to be more effective during the periods that would otherwise have PMD outages. Another advantage of this scheme is that it can mitigate the influence of PMD in multichannel system without feedback control and compensation based on each channel. The principle and performance of scrambling is introduced in this paper and the fundamental idea of improving PMD mitigation by using D-FPS combined with FEC is proposed. Some key factors such as scrambling speed and the number of scrambler which affecting the performance of this scheme are also introduced. Finally, proposing an assumption that the impact for system performance caused by the style of FPSs is distributed along the fiber link, and the mathematical estimation model is established. The combination of D-FPS and FEC would be a promising approach for performance improvement in ultra-high-speed optical communication system.

This paper deals with the effects and compensation of Polarization Mode Dispersion (PMD), which has found to be a considerable attention in high data rate systems, now-a-days. In general, a small amount of PMD is to be considered in high data rate systems, since it limits the data rate. Unlike the chromatic dispersion phenomenon, the PMD is found to be a time varying and an unstable phenomenon. Thus compensation of PMD is required since it distorts the signal and broadens the pulse in a statistical manner. In this paper we analyze the PMD effects by varying the length and birefringence of the fiber and compensate the effect of PMD using the OPTSIM simulation for 10, and 40 Giga bits per second (Gb/s) transmission systems. Here the Compensation of PMD is done by an optical compensation method called Deterministic Differential Group Delay (DDGD) and the results have been simulated and analyzed.

A simple method to monitor chromatic dispersion (CD) and polarization mode dispersion (PMD) simultaneously is proposed using two polarization-modulation pilot tones with different frequencies. It has been demonstrated that RF output power increases with group velocity delay (GVD) and differential group delay (DGD) and the power ratio of the two pilot tones increases with GVD and decreases with DGD, thus CD and PMD can be distinguished and monitored simultaneously.

At present, optical fiber communication systems are developing in the direction of large capacity, high speed, long distance, etc., making nonlinear effects and polarization mode dispersion (PMD) and other optical fiber performance defects that were negligible for low-rate systems become the main factors limiting system upgrades and transmission distances. Polarization mode dispersion is an important factor affecting the performance of optical fiber communication systems, and it has become one of the main obstacles to the development of high-speed and long-distance optical fiber communication systems. It can cause pulse widening in the digital transmission system, thereby limiting the transmission rate of the system. Therefore, the influence of polarization mode dispersion on the transmission system and the compensation method are analyzed in this paper.

In order to adapt to the ever-increasing demands of telecommunication needs, the network operators already consider 100 Gb/s per dense wavelength division multiplexing (DWDM) channel transmission. At those data rates, the performance of fiber-optic communication systems is degraded significantly due to intra- and interchannel fiber nonlinearities, polarization mode dispersion (PMD), and chromatic dispersion [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. In order to mitigate the signal distortions at ultra-high bit rates, some new technologies have been proposed and deployed in optical systems, and they represent a distinctive new trend in optical fiber communications. These new technologies include digital signal processing (DSP)-aided optical channel equalization, digital coherent receiving, multilevel modulations and optical polarization multiplexing (or optical multiple-input multiple-output technologies) [1, 2, 3, 4, 5, 6]. To deal with chromatic dispersion and PMD a number of channel equalization techniques have been proposed recently including digital-filtering approach [1] and maximum likelihood sequence detection/estimation (MLSD/E) [2]. To simultaneously suppress chromatic dispersion and PMD, orthogonal frequency division multiplexing (OFDM) has been proposed [4, 5]. On the other hand, to deal with intrachannel nonlinearities someone may use either constrained coding [6].

Optical communication transmission systems improvement based on chromatic and polarization Mode dispersion compensation simulation management

Existing on-off-keying modulation in optical communication becomes inadequate to meet exponentially growing bandwidth demand. New receiver architectures and algorithms are being sought to confront this challenge. To achieve this goal, we propose innovative adaptive algorithms to simultaneously track optical polarizations and compensate any channel impairment without the need of any optical/electronic phase-locked-loop in Phase-Shift-Keying modulation for optical coherent receivers. These algorithms are demonstrated to be capable of coping with various impairments such as chromatic dispersion and polarization mode dispersion in optical channels to allow transmission of high speed data at 40 Gbps and beyond in even low-speed-grade long-haul fibers.

We experimentally demonstrate a novel digital signal processing (DSP) structure for reduced guard-interval (RGI) OFDM coherent optical systems. The proposed concept is based on digitally slicing optical channel bandwidth into multiple spectrally disjoint sub-bands which are then processed in parallel. Each low bandwidth sub-band has a smaller delay-spread compared to a full-band signal. This enables compensation of both chromatic dispersion (CD) and polarization mode dispersion using a simple timing and one-tap-per-symbol frequency domain equalizer with a small cyclic prefix overhead. In terms of the DSP architecture, this allows for a highly efficient parallelization of DSP tasks performed over the received signal samples by deploying multiple processors running at a lower clock rate. It should be noted that this parallelization is performed in the frequency domain and it allows for flexible optical transceiver schemes. In addition, the resulting optical receiver is simplified due to the removal of the CD compensation equalizer compared to conventional RGI-OFDM systems. In this paper we experimentally demonstrate digital sub-banding of optical bandwidth. We test the system performance for different modulation formats (QPSK, 16QAM and 32QAM) over various transmission distances and optical launch powers using a 1.5% CP overhead in all scenarios. We also compare the proposed RGI-OFDM architecture performance against common single carrier modulation formats. At the same total data rate and signal bandwidth both systems have similar performance and transmission reach whereas the proposed method allows for a significant reduction of computational complexity due to removal of CD pre/post compensation equalizer.