What is “cyclic fiber ring”? How much do you know about it?

Definition: An optical fiber ring through which light can cycle many times

A cyclic fiber ring is a fiber optic device in which light can cycle back and forth many times. It is mainly used in long distance optical fiber communication system. Even with a finite length of optical fiber, signal light can be transmitted over very long distances by winding many times. This helps to study the harmful effects and optical nonlinearity that affect the light quality of the signal.

In laser technology, cyclic fiber loops can be used to measure the linewidth of a laser, especially when the linewidth is very small (<1kHz). This is an extension of the self-heterodyne linewidth measurement method, which does not require an additional reference laser to obtain a reference signal from itself, which requires the use of long single-mode fibers. The problem with self-heterodyne detection technology is that the required time delay is of the same order as the reciprocal of the line width, so that the line width is only a few kHz, and even less than 1kHz requires very large fiber lengths.

Figure 1: Schematic diagram of a cyclic fiber ring.

The basic reason for using fiber loops is that a medium length fiber can provide a long time delay because light travels many turns in the fiber. In order to separate the light transmitted in different loops, an acousto-optic modulator can be used in the loop to produce a certain frequency shift (for example, 100MHz). Because this frequency shift is much larger than the line width, light that has traveled a different number of turns in the loop can be separated in the frequency domain. In the photodetector, the original laser light and the beat of the light after the frequency shift can be used to measure the line width.

If there is no amplifying device in the loop, the loss in the acousto-optic modulator and fiber is very large, and the light intensity will decay seriously after several loops. This severely limits the number of loops when the linewidth is measured. Fiber amplifiers can be added to the loop to eliminate this limitation.

However, this creates a new problem: although the light passing through different turns is completely separate, the beat signal comes from different pairs of photons, which changes the beat spectrum as a whole. The optical fiber ring can be reasonably designed to effectively inhibit these effects. Finally, the sensitivity of the cyclic fiber loop is limited by the noise of the fiber amplifier. It is also necessary to consider the nonlinearity of the fiber and the non-Lorentz lines in the data processing