通信工程文献翻译——介绍

相干光纤通信在20世纪80年代被广泛研究，主要是因为相干接收机的高灵敏度可以延长无中继传输距离; 然而，自1990年以来，由于高容量波分复用（WDM）系统的快速进展，它们的研究和开发被打断，这些系统采用传统的强度调制和直接检测（IMDD）方案以及铒掺杂光纤放大器（EDFA）补偿链路的跨度损耗。

在2005年，相干接收机中数字载波相位估计的演示重新刺激了相干光通信的广泛兴趣。 原因是数字相干接收机使我们能够采用诸如M相移键控（PSK）和正交幅度调制（QAM）的各种频谱效率调制格式。 此外，由于在检测之后保持相位信息，因此可以均衡由于数字域中的光纤的群速度色散（GVD）和偏振模色散（PMD）引起的线性传输损伤。度调制，并且通过光纤传输的光信号的强度由光电二极管直接检测。 发射机和接收机的这种组合被称为IMDD方案，其迄今为止已经经常用于光通信系统中。 IMDD方案具有很大的优点，即接收机灵敏度独立于在实际系统中随机波动的输入信号的载波相位和极化状态（SOP）。

数字相干接收机的工作原理：相干光检测

  图1示出了称为相位分集零差接收机或同相接收机的相干光接收机的配置。

让从发射机进入的光信号

 重生相干接收机的这些优点具有创新现有光通信系统的巨大潜力。 目前正在全球努力开发基于正交PSK（QPSK）调制，极化复用和数字相干检测的以25G符号/ s的符号率的100-Gbit / s传输系统。

光纤通信系统的研究和开发始于20世纪70年代上半叶。 这种系统使用半导体激光器的强

      E s (t) = A s (t)exp(jt),                     (1)

其中A s（t）是复振幅并且是角频率。 类似地，在接收机处准备的LO的电场可以写为

          (t) = exp(jt),                       (2)

其中是LO的常数复振幅是LO的角频率。复振幅A s和与信号功率相关......

Introduction

Coherent optical fiber communications were studied extensively in the 1980s mainly because of high sensitivity of coherent receivers that could elongate the unrepeated transmission distance; however, since 1990, their research and development had been interrupted behind the rapid progress in highcapacity wavelength-division multiplexed (WDM) systems, which employed the conventional intensity-modulation and direct-detection (IMDD) scheme as well as the erbium-doped fiber amplifier (EDFA) to compensate for span loss of the link.

In 2005, the demonstration of digital carrier-phase estimation in coherent receivers re-stimulated a widespread interest in coherent optical communications. The reason is that the digital coherent receiver enables us to employ a variety of spectrally-efficient modulation formats such as M-ary phase-shift keying (PSK) and quadrature-amplitude modulation (QAM). In addition, since the phase information is preserved after detection, we can equalize linear transmission impairments due to group-velocity dispersion (GVD) and polarization-mode dispersion (PMD) of fibers in the digital domain. These advantages of the born-again coherent receiver have enormous potential for innovating existing optical communication systems. Worldwide efforts are now underway to develop 100-Gbit/s transmission systems based on quadrature PSK (QPSK) modulation, polarization multiplexing, and digital coherent detection at the symbol rate of 25Gsymbol/s.

The research and development of optical fiber communication systems started ......