Vernier effect based on cascading two Mach -Zehnder interferometers for selectable comb filter and saturable absorber applications in erbium-doped fiber laser

We present the applications of a Vernier effect based on cascading two Mach - Zehnder interferometers (MZIs) as a selectable comb filter and saturable absorber (SA) in an erbium -doped fiber laser (EDFL). Each MZI is composed of bi-tapered polarization -maintaining fibers (PMFs) constructed by fusion splicing between two single -mode fibers (SMFs), which are then connected in series to produce the Vernier effect. The first tapered MZI (called MZI-1) acted as a comb filter to provide a larger free spectral range (FSR). The second MZI (MZI-2) based comb filter has a smaller FSR. The combination of these two filters forms an envelope structure on the comb -like spectrum. In this work, the first implementation of the comb filter based on the Vernier effect was used as a wavelength -selectable filter. Through the adjustment of the polarization controller (PC1) and (PC2) inside the cavity, the FSR of the envelope can be varied, thereby switchable and adjustable multiwavelength was achieved. The second implementation uses a graphene nanoparticle (G-Nps)/polymer composite deposited onto the MZI-1 comb filter to act as an SA. At the 80 mW pump threshold, stable Q -switching EDFL operation was successfully activated at 1560.8 nm. The shortest pulse width of 2.89 mu s at a 85.91 kHz repetition rate was achieved at 330 mA (180 mW) pump power. The proposed Vernier effect based on cascading two tapered MZIs configuration can find potential applications in the fields of advanced multiwavelength fiber lasers and short pulse generation.