Tunable actively Q-switched fiber laser based on fiber Bragg grating

We report a tunable actively Q-switched fiber laser. A Fabry-Perot cavity is formed by a tunable fiber Bragg grating and two short-wave pass dichroic mirrors, (SWP-0-R1550-T1064-0525-C) and (SWP-45-RU1550- TU1064-PW-0525-C) incident angles, with high reflectivity ( 3+ /Yb 3+ double-clad fiber was pumped by a high power diode laser (JOLD-30-FC-12-976). The diode laser has output fiber with a core diameter and numerical aperture of 200-μm and 0.22, respectively. The Er 3+ /Yb 3+ fiber has a core diameter of 7-μm and numerical aperture of 0.17. The pump was coupled into the doped fiber through the collimating aspherical lens with focal distance 18-mm, 45° dichroic mirror, focusing aspheric lens with focal distance of 8-mm, and a 7.5° cleave at one end of the doped fiber. The use of aspherical lenses allows reducing the spherical aberration caused by the large numerical aperture of the fibers and increasing the coupling efficiency for both pump and signal. An acousto-optic IR modulator with a diffraction efficiency of <60% at first order was inserted in the cavity. The pump power used in experiments was limited to a maximum power of 5-W. The laser was tunable over 5-nm in the wavelength region of 1550-nm with a wavelength selectivity of 0.8-nm from 1549 to 1544-nm. The Q-switch operation was achieved at pulse repetition rate of 120- kHz. Average output power was 1-W at 5-W of pump with a pulse duration of 530-ns.