定义：
光可以在其中循环很多圈的光纤环

循环光纤环是一种光纤装置，光可以在其中循环往返很多圈。它主要应用于长距离光纤通信系统中。即使采用有限长度的光纤，通过绕制多圈，信号光可以传输非常长的距离。这有助于研究影响信号光质量的有害效应和光学非线性。 

在激光器技术中，循环光纤环可用于测量激光器的线宽，尤其是当线宽非常小时（<1 kHz）。这是自外差线宽测量方法的扩展，自外差方法不需要采用额外的参考激光器就可以从其自身得到参考信号，这种方法需要采用长的单模光纤。自外差探测技术的问题在于需要的时间延迟与线宽的倒数具有相同的量级，这样线宽只有几kHz，甚至小于1kHz时需要非常大的光纤长度。 

图1：循环光纤环的示意图。 
采用光纤环的基本原因就是可以采用中等长度的光纤提供长的时间延迟，因为光在光纤中传输很多圈。为了分立不同环路中传输的光，可以在环路中采用声光调制器产生一定的频移（例如100MHz）。因为这一频移远大于线宽，因此在环路中走了圈数不同的光可以在频域中被分开。在探测器中，原始的激光与其经过频移后光的拍音可以用来测量线宽。 

如果环路中没有放大装置，声光调制器和光纤中的损耗很大，光强会经过几个环路后衰减很严重。这严重限制了线宽测量时环路的数量。可以在环路中加入光纤放大器来消除这一限制。但是，这会产生一个新的问题：尽管经过不同圈数的光是完全分离的，拍频信号还是来自于不同的光子对，这会整体上改变拍频光谱。可以合理设计光纤环，来有效抑制这些效应。最终，循环光纤环的灵敏度受光纤放大器噪声的限制。另外还需要考虑的是光纤非线性以及数据处理时的非洛伦兹线型。

Definition: fiber loops in which light can circulate many times

A recirculating fiber loop [1] is a fiber-optical setup where light can do many round trips in an optical fiber. Its main use is for studying long-haul transmission in optical fiber communication systems. Even with a limited length of fiber, the propagation of signals over very long lengths can be studied by using multiple passes. This can help to investigate, e.g., the deleterious effects of amplifier noise and optical nonlinearities on the signal quality.

In laser technology, recirculating fiber loops are also used for the measurement of the linewidth of a laser, particularly in cases where this linewidth is very small (potentially below 1 kHz). The method is an extension of the self-heterodyne linewidth measurement, where the use of an extra reference laser can be avoided by deriving the reference signal from the laser output itself, using a long delay, which is provided by a long single-mode fiber. The problem with the self-heterodyne technique is that the required delay time is usually of the order of the inverse linewidth, and this leads to very large fiber lengths for linewidths of a few kilohertz only, or even below 1 kHz.

Figure 1: Schematic setup of a recirculating fiber loop.

The basic idea of using a recirculating fiber loop is that a long delay can be provided by a moderately long fiber, if the light makes multiple round trips through that fiber. In order to keep the light from different round trips well separated, an acousto-optic modulator in the loop shifts the optical frequency by a certain amount (e.g. 100 MHz) in each round trip. As that frequency shift is much larger than the linewidth, the components corresponding to different numbers of round trips are well separated in the frequency domain. At the detector, beat notes of the original laser light with different frequency-shifted components can be used to measure their linewidths.

If there is no amplifying element in the loop, the losses from the acousto-optic modulator and from the fiber are significant, and the light intensity rapidly decays during several round trips in the loop. This strongly limits the number of round trips which can be utilized for the linewidth measurement. To remove this limitation, the effective loss of the loop can be strongly reduced by inserting a fiber amplifier in the loop (as shown in Figure 1). However, this introduces a new problem: even though the light components with different numbers of round trips are clearly separated in the optical domain, the beat signal from the detector has contributions from different pairs of these components, which in general can greatly modify the resulting beat spectrum. It has been shown [3] that for a suitable design of the loop, this effect can be effectively suppressed. Ultimately, the sensitivity of recirculating fiber loop will be limited by noise from the fiber amplifier. Other aspects to be considered are the fiber nonlinearity and the data processing for non-Lorentzian line shapes.

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