Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s intensity- and phase-modulated formats, and highlight their resilience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity

Advanced Optical Modulation Formats

Publisher Summary
This chapter provides a taxonomy of optical modulation formats along with experimental techniques for realizing them. High-speed optical transmission systems are increasingly building on digital signal processing and digital communication techniques that are well established in radio- frequency (RF) communication systems, including efficient forward error connection (FEC), advanced modulation, line coding, polarization multiplexing, digital signal processing at both transmitter and receiver, and coherent detection. Nevertheless, an optically routed network exhibits important fundamental as well as technological differences from an RF system, and not all techniques that have proven successful in RF communication should be expected to work well for optical networks. The chapter makes clear the substantial distinctions between design conditions for optical and RF applications, namely, the presence of nonlinear distortions in the optical fiber communication channel. As a consequence, and in stark contrast to classical RF systems, the performance of an optical communication link exhibits a maximum at a certain signal power level, which represents the optimum trade-off between optical amplifier noise and fiber Kerr nonlinearity. Demodulation concepts for coherent and delay demodulation are also covered analytically. The conversion process of an (optical) bandpass signal to an (electrical) baseband signal is called demodulation and relies on photodetectors as its key component. Photodetectors used in optical fiber communications are implemented as reverse-biased semiconductor pin diodes. Advanced modulation formats that are robust to transmission impairments or able to improve spectral efficiency are being considered for next-generation lightwave systems.

2 – Advanced optical modulation formats

We first compare different polarization mode dispersion (PMD) models for evaluating PMD impairments. We show that, compared with the all-order PMD model, most of the known first- and second-order PMD models overestimate the PMD induced distortions. Then we discuss various PMD mitigation techniques. We show that modulation formats have a big impact on choosing the most effective PMD compensation techniques. Not only do the benefits of using PMD compensators (PMDCs) depend on modulation formats, but the designing of a PMDC is largely affected by modulation formats as well. Some multi-channel PMD mitigation techniques for wavelength division multiplexing (WDM) systems are also discussed.

Evaluation and mitigation of polarization mode dispersion impairments in high-speed transmission systems

We investigate the impact of the transmitter bandwidth on the nonlinear tolerance in a 112-Gb/s polarization-division-multiplexed quaternary phase-shift keying system with coherent detection over a 1200-km dispersion-managed link using standard single-mode fiber. We show that a narrower transmitter bandwidth degrades not only the noise tolerance, but also exacerbates signal degradations due to nonlinearities both in single channel and in wavelength-division-multiplexed configuration. A nonlinear threshold difference of approximately 1 dB is observed.

Impact of Transmitter Bandwidth on Transmission Performance of Coherent 112-Gb/s PDM-QPSK Systems

Fiber-optic communication systems form the high-capacity transport infrastructure that enables global broadband data services and advanced Internet applications. The desire for higher per-fiber transport capacities and, at the same time, the drive for lower costs per end-to-end transmitted information bit has led to optically routed networks with high spectral efficiencies. Among other enabling technologies, advanced optical modulation formats have become key to the design of modern wavelength division multiplexed (WDM) fiber systems. In this paper, we review optical modulation formats in the broader context of optically routed WDM networks. We discuss the generation and detection of multigigabit/s inten- sity- and phase-modulated formats, and highlight their resil- ience to key impairments found in optical networking, such as optical amplifier noise, multipath interference, chromatic dispersion, polarization-mode dispersion, WDM crosstalk, concatenated optical filtering, and fiber nonlinearity.

high-capacity optically routed wavelength division multiplexed fiber networks.

We study the influences of transmitter characteristics on polarization mode dispersion (PMD) impairments in optical transmission systems with both experiments and simulations. We show that transmitter imperfection, such as timing and amplitude jitters and pulse shape distortion, can significantly increase PMD impairments, especially when the differential group delay in the systems is large. Our simulation further shows that the increased PMD impairments due to transmitter imperfection will considerably reduce the effectiveness of using large system optical signal-to-noise ratio margin to combat PMD.

http://ieeexplore.ieee.org/iel5/68/26684/01190221.pdf

Shuai Shen

Papers

Effects of transmitter imperfection on polarization mode dispersion impairments