Mode hop free single mode fibre ring laser with frequency scanning using a fibre ring resonator filter combined with Mach-Zehnder interferometer

In order to obtain stable single longitudinal mode (SLM) operation from a fibre laser, both broad band and narrow band tunable filters are required. A fibre ring resonator filter (FRRF) simply composed of two directional couplers can be used for this purpose together with a cheap, commercially available, tunable band pass filter having a FWHM of a few nm. However, if one tries to make a FRRF by splicing two commercial directional couplers, it is difficult to increase the free spectral range (FSR) to greater than 1 GHz because of the short length of fibre to handle. This small FSR usually gives rise to laser mode jumping to FRRF modes rather than to main cavity modes, which makes the system inappropriate for frequency scanning. In this paper, we introduce a FRRF combined with a Mach-Zehnder interferometric filter (MZIF) as a simple solution to this problem.