Reflection-type delayed self-homodyne/heterodyne method for optical linewidth measurements

The reflection-type delayed self-homodyne (RDSH) method presently described is based on the same principle as the delayed self-homodyne/heterodyne (DSH) method but no beam splitter or fiber coupler is required. The delayed beam is produced by utilizing a round-trip propagation passing through a solitary long optical fiber delay line. Therefore, the optical configuration is very simple, and the beat signal has a resolution twice that of the DSH method. However, the RDSH method´s drawback is that a large intensity difference between the two beams reduces the level of the beat signal. This can be improved by increasing the reflectivities of the fiber ends by dielectric coating. In this case, multiple reflection at the fiber ends should be taken into account