We report on the demonstration of a switchable and adjustable spacing multiwavelength nanosecond pulsed ytterbium-doped fiber laser (YDFL) operating in a mode-locking regime by macrobending loss tuning induced in a figure-eight knot shape. The figure-eight knot-based Mach–Zehnder interferometer (MZI) was established by wrapping a section of single-mode fiber into two elliptical loops by twisting one end of the fiber onto the other without splicing. Combining the proposed MZI with a polarization controller (PC) acts as a selective comb filter and a nanosecond pulse generator. By appropriately adjusting the bending diameter of the MZI structure inside the YDFL cavity, the laser can be operated in a single- and dual-wavelength with a maximum spacing range of 18.5 nm. Furthermore, the number of lasing wavelengths was directly controlled by only altering the pump power; dual and triple wavelengths were achieved as the pump power was increased from 127 to 210 mW, respectively. The appropriate adjustment of the PC cascaded with the proposed comb filter in the laser cavity allows for the generation of up to quadruple switchable wavelengths of nanosecond mode-locking pulses with a pulse duration of 6.2 ns and repetition rate of 71.4 MHz located between 1061.17 and 1038.92 nm. The experimental results indicate that the proposed configuration of the MZI-based comb filter can provide great potential in various photonics applications.
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 parallel-connected 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 parallel-connected 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.
We report on a humidity sensor based on passband spectrum resulting due to self-imaging effect in single-mode–multimode–single-mode fiber structure experimentally. The sensor performance with varying NCF diameter was investigated to find appropriate size of maximized evanescent fields. Different tuned diameters of NCF of 100, 80, 60, and 40 μm were obtained by chemical etching process based on hydrofluoric (HF) acid immersion. The experimental results reveal that almost a threefold increase in sensitivity was obtained at the proposed optimum NCF diameter of 60 μm coated with copper oxide nanoparticles embedded in poly polyvinyl alcohol. The highest wavelength and intensity sensitivity of −0.581 nm / RH % and −0.456 dB / RH % were obtained, respectively, in the RH range of 30% to 100% with 8.11-s response time. To the best of our knowledge, this is the first demonstration of RH sensor based on NCF coated with CuO NPs thin film.
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