Recirculating Fiber Loops
A recirculating fiber loop  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.
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  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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