光电子产生高频微波信号简介及现状

号等。

相对于注入式锁频只能获得倍频而言，锁相环更加灵活，可以产生几乎任意的频率，当然也更加复杂一些。例如以图2中的经过光电调制器位相调制的，或者脉冲激光产生的光频梳作为光源，利用光锁相环将两个激光器有选择性的锁频到其中两条光梳齿信号，再通过差频产生高频信号，如图4。f1和f2分别是两个锁相环的参考信号频率,通过两激光之间的差频便可以以产生N*frep+f1+f2的微波信号。

图4. 利用光频梳和光锁相环产生任意频率示意图。

3. 使用锁模脉冲激光器，通过光电探测器把光脉冲信号转化为微波信号[18-20]。

此方法的主要优点是可以获得频率稳定性非常好，位相噪声极低的信号。通过将激光的频率锁频到非常稳定的原子分子跃迁光谱，或者是是极其稳定的光腔，以及利用自倍频消除系统频移等等技术，可以获得重复频率非常稳的光脉冲信号，从而获得位相噪声超低的微波信号[21-23]。如图5.

图5. 不同信号源相对位相噪声比较。摘自参考文献[21]

但是因为脉冲的重复频率是跟激光器的腔长成反比的，而传统的锁模激光器体积较大，因此难以直接获得高频的微波信号。另外传统的脉冲激光体积，重量和能耗，以及对于环境的苛刻要求都限制了它们主要是在实验室应用。为了克服这些困难，最进美国和德国兴起研究利用非线性效应在很小的高品质的啾啁模光腔内产生频率稳定的光频梳，进而产生高频的低噪声微波信号[24-29]。

4. 光电耦合振荡器(opto electronic oscillator) [30-33]，如图6。 

图6. 光电耦合振荡器示意图

传统的产生微波或者激光的方法之一是使用一个自反馈的闭环回路，只要闭环中的增益大于损耗，自激振荡就能产生微波或者激光。该闭环的品质因素Q越高，产生的信号位相或者频率噪声越小。为了增加回路的品质因素，直接的办法是增加回路长度并且尽量降低传播损耗。但是较长的回路通常能够会支持产生多个振荡模式，如果加入一个窄带宽的滤波器，就可以获得单频低噪声的微波振荡信号。光电耦合振荡器就是一种基于此想法产生的微波信号源，它充分利用了光纤的低传播损耗的特性，使用较长的光纤提高回路的Q值，可以产生位相噪声极低的微波信号。自从九十年代该方法提出，该类型的振荡器获得了广泛的研究和长足的发展,目前已有商业化的光电耦合振荡器[34]。最近更有发展出频率可大范围调节的光电振荡器[35, 36]。基于这种架构的微波信号源主要的问题是回路较长，在其自由普(FSR)及其倍频频率噪声会显著增高。另外所用的光电元件较多，成本高，体积难以缩小，而且较长的光纤对于环境的扰动较敏感。

以上初略介绍了几种光电子产生微波信号的方法，以及各自的优缺点。最后提一句，利用光电子产生微波还有另外一个好处是，可以将光信号通过光纤以极低的损耗以分布式的，远距离的传播到各个使用终端再转换为微波信号，而且抗电磁干扰的能力比传统电子元件有显著提高。

本文作者：livingwater 发表于微波射频社区
http://club.mwrf.net/thread-44927-1-1.html

本文的写作主要有参考[2, 3]，以及结合笔者自己在该领域的研究经验和体会，有不准确不全面之处请谅解。 

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