All-Fiber 2.07 μm Distributed Feedback Laser Based on π-Phase-Shifted FBG Inscribed in Heavily Doped Holmium Fiber by Femtosecond Laser Pulses

Abstract

Distributed-feedback (DFB) fiber laser is a versatile source of low-noise single-frequency radiation that finds applications in spectroscopy, optical communications and sensing devices. Depending on the active medium, different lasing wavelengths can be obtained: 1.03–1.12 μm for Yb, 1.53–1.62 μm for Er, and 1.73–1.93 μm for Tm-doped fibers. Special fiber Bragg grating (FBG) with a length of 20–70 mm and phase shift in the structure, directly written in an active medium, serves as distributed feedback cavity of this type of laser. Commonly, special UV photosensitive fibers and phase masks inscription technique are used for an FBG fabrication. Alternative femtosecond (fs) point-by-point technique [1] has a number of advantages as compared to UV-based ones: 1) wide variety of active fibers can serve as a host material for an FBG, thanks to the nonlinear mechanism of fs pulses absorption, 2) FBG period can be easily tuned by controlling fs pulses frequency and velocity of fiber translation during the inscription process, 3) each fiber grating pitch can be written independently, which means that phase shift with an exact value and position along FBG can be introduced. Despite the number of publications dealing with holmium fiber lasers [2], there no publications devoted to DFB ones. At the same time, such type of laser sources has a great potential as a master oscillator in all-fiber coherent beam combination systems [3].