Vernier effect based on hybrid fiber interferometers: a new tool for wavelength switchability and adjustable free spectral range fiber lasing

In optical communications systems, the used filter and/or demultiplexer needs to have a broad free spectral range (FSR) in order to accommodate more channels and have acceptable interchannel crosstalk. The Vernier effect applied to fiber filters is a recent effective tool to enlarge the FSR. Here, by harnessing the Vernier effect of a hybrid interferometer consisting of a Mach-Zehnder interferometer (MZI) and Sagnac interferometer (SI), we proposed and experimentally demonstrated a new kind of comb filter for a switchable and interval adjustable multi-wavelength C-band erbium-doped fiber laser (EDFL) application. In the designed comb filter, the MZI is composed of bi-tapered polarization-maintaining fibers (PMFs) fabricated by fusion splicing and has the function of achieving the switchability of the proposed dualwavelength EDFL. The SI configured by nesting tapered PMF is employed as a switchable and wavelength-spacing tuning component of triple-wavelength EDFL. In this experiment, the FSR of the MZI and the SI is designed to be close but not equal, which could be achieved by properly adjusting the length of the employed PMF, so the Vernier effect can exist and a comb spectrum with an obvious envelope is obtained. Through the adjustment of the polarization controller (PC1) and (PC2) inside the cavity, a switchable and interval-adjustable multi-wavelength EDFL was achieved. To the best of the authors' knowledge, this is the first time that an all-fiber hybrid filter based on the Vernier effect has been used to manipulate the spectral output characteristic of an EDFL and achieve a switchable multi-wavelength fiber laser.