In this paper, dual-wavelength laser emission of an erbium-doped fiber laser (EDFL) with a tunable distinct wavelength selection of the simultaneously produced laser lines was achieved by applying a parallel comb filter configuration based on the optical Vernier effect. The intracavity inserted proposed comb filter consists of two parallel branches to generate the Vernier effect. Each branch is an in-line Mach–Zehnder interferometer (MZI) filter, which is composed of a polarization-maintaining fiber fusion spliced between single-mode fibers with sphere shapes at both ends. The tunability of the selected laser wavelength was realized by submerging the proposed filter in different sodium chloride/water mixtures. The proposed comb filter-based Vernier effect was used to independently achieve the selection of the dual-wavelength EDFL lines and for refractive index (RI) sensing applications. The in-line MZI1 and MZI2 structures show a wavelength shift sensitivity to RI variations of −88 and 79 nm/RIU, respectively. Our proposed MZI structure presents a reliable, straightforward, and low-cost spectral comb filter for separate tunable dual-wavelength laser generation in the c-band region. Furthermore, the proposed filter structure-based Vernier effect presents a new perspective and method in the RI sensing application.

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In this paper, a fiber optic Fabry–Perot (FP) for relative humidity (RH) sensing is presented. The proposed FP cavity is constructed by splicing a 50-mm length of no-core fiber (NCF) in a single mode fiber. Then, the end side of the NCF is coated with a polyvinyl alcohol (PVA) thin film membrane with different thicknesses (1, 2, 3, 4 µm respectively) to work as a mirror. The fringes pattern of the FP undergoes a spectral shift owing to the alteration in the PVA refractive index with the ambient RH alternative. The highest obtainable sensitivity was observed at thickness of 3 µm, which is about 0.866 nm/RH%. After that, the diameter of the NCF is tuned from 125 to 65 µm using hydrofluoric acid (HF40%) to maximize the evanescent field and thus improved sensitivity to about 0.908 nm/RH% at the diameter of 95 µm for 30% to 90% RH range. The sensor shows good stability, and easy fabricated.

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We propose and demonstrate the implementation and application of a new configuration of an all-fiber comb filter based on the Vernier effect produced by parallelconnected two in-line Mach-Zehnder interferometers (MZIs). Each in-line MZI was fabricated by fusion splicing a section of panda-type polarization maintaining fiber (PMF) with peanut-shaped tapers between two single-mode fibers. These two in-line MZIs, respectively, form reference and sensing interferometers, which are parallelconnected by two 3-dB optical fiber couplers to realize the Vernier effect. By incorporating the proposed comb-filter into the erbium-doped fiber laser cavity, two output channels at 1533.2 and 1558.2 nm have been achieved. Further, the combination of the parallel-connected two in-line MZIs and polarization controllers (PC1 and PC2) promotes the lasing in a switchable and selective way. To the best of the authors' knowledge, this is the first demonstration of a comb filter that employs paralleled two peanut-shaped MZIs in PMF. The experimental results indicate that the proposed filter has the potential to be used in communication systems.