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请教清华大学杨昌喜教授几个“简单”问题

作者：侍子文

问题内容：

1. 在“构建”(fabricated，引杨昌喜教授语)你的Compact first-order 
polarization mode dispersion compensator based on birefringent 
crystals一文时，你和你的合作者是否知道GP公司的Fast digitally variable 
differential group delay module using polarization switching一文？

2. 如果上面问题回答“是”，是什么原因使你决定把GP公司的相关文章排除
在参考文献(References)之外？你认为GP公司的文章“创新性”(引GP公司语)
如何？

3. 请杨昌喜教授解释一下PMD补偿器(PMD compensator)和DGD模块的关系是
什么？DGD技术是不是实现PMD补偿(PMD compensation)的重要组成部分？
(注：对杨教授“PMD与DGD是两个不同的概念” 的准确论断，没有疑问。)

问题背景：

在阅读杨昌喜教授这篇有争议的文章时，发现一些有趣的现象：

尽管文章的技术细节部分(第四段至结尾)几乎全部在讨论DGD，但在文章开始的
综述部分(前3个段落)，对DGD似乎惜墨如金。文章开始的综述部分(前3个段落)，
颇有些科普文献的风范，也有顾左右而言它之嫌。

在文章的第一段(附)，杨教授对研究PMD补偿器的业界同人们似乎洋洋洒洒，
但把GP公司曾经的同事们全部“遗忘”了，在参考文献(References) 里也找
不到他们的影子。

附：杨昌喜教授一文第一段：

As the bit rate and distance of optical fibre transmission systems 
continue 
to increase, polarization mode dispersion (PMD) is becoming a major 
system 
impairment.[1_3] PMD compensation has been attracting great research 
interests. Various techniques for the first-order PMD compensation have 
been 
reported.[4_8] Heisman et al. reported a first-order PMD compensator 
based 
on differential polarization delay line technique.[4;5] Unfortunately, 
the 
mechanical movement of delay line hinders the response time of the 
compensator. Moreover, the interferometer configuration is sensitive to 
environment vibrations. Sobiski et al. reported a first-order PMD 
compensator based on polarization-maintaining fibres (PMFs) [6] with 
response time of 200 ms. Lee et al. realized a first-order PMD 
compensator 
based on the similar technique [7] with the insertion loss of 16 dB and 
the 
response time of 300 ms. The PMF has relatively small birefringence. 
The 
compensator based on the PMF thus needs a long length of PMFs to 
generate a 
certain amount of differential group delay (DGD). On the other hand, a 
long 
length of PMF may cause harmful nonlinear optical effects for the 
transmission systems. Willner et al. proposed a first-order PMD 
compensator 
based on high-birefringence nonlinearly-chirped fibre Bragg grating.[8] 
Circulators are needed for coupling the light out, which causes 
additional 
insertion loss and complexity of the compensator. Furthermore, the 
fibre 
gratings generate additional chromatic dispersion in the transmission 
system.

(XYS20050406)

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