The waveguide saturable absorber fabricated in the bulk of YAG:Cr4+ crystal

In this paper we present a novel waveguide design for a saturable absorber based on the YAG:Cr⁴⁺ crystal. Such a waveguide is designed to be compatible with conventional fibers. Hence, such Waveguide Saturable Absorber (WSA) features low-loss butt-coupling fiber lasers. We fabricated 25 mm long waveguide with a depressed cladding in YAG:Cr⁴⁺ under a crystal surface at 150 mum depth. Femtosecond laser beam was used for waveguide microfabrication. Transmittance of WSA upon power of CW linear polarized laser beam entering into the waveguide. Polarization is along crystal axis. To characterize the WSA we coupled a 1.064 mum CW Gaussian laser beam by a lens, the beam waist radius was 20 mum at a focal plane of the lens. Field distribution at the output end of WSA was close to Gaussian one with radius of 20 mum. The CW laser beam was chopped at 10 Hz frequency and duty cycle factor of 1/20 in order to keep average power lower than 0.5 W in order to limit heating effects. The experimental curve was theoretically treated taking into account the well known model of tetrahedrally coordinated Cr⁴⁺.