High-efficiency wavelength and polarization selective grating-waveguide structures for Yb:YAG thin-disk lasers

Abstract

We report on Grating Waveguide Structures (GWS) with a high diffraction efficiency used in Littrow configuration to select (and tune) the wavelength of an Yb:YAG thin-disk laser. The structures are composed of a multilayer HR coating, on which an additional low index layer (SiO 2 ) and high index layer (Ta 2 O 5 ) was deposited. A binary grating with a period of 580 nm is etched on top of the structure with a groove depth of 87 nm for GWS 1 and 72 nm for GWS 2. The simulation results show that the diffraction efficiency in the -1 st order can reach a value of 99.99% for TE polarization, whereas it is only about 20% for TM polarization at 1030 nm. The grating was fabricated by standard interference lithography followed by a dry etching process to the desired groove depth. The spectroscopic measurement exhibited a diffraction efficiency of 99.6% for GWS 1 at 1030 nm and 99.7% for GWS 2 at 1048 nm. The devices were placed as end-mirror into the resonator of a Yb:YAG thin-disk laser. An output power of up to 110 W could be obtained from the laser in fundamental-mode operation (M 2 ≈ 1.2) with GWS 1, corresponding to an optical efficiency of η oo = 36.2%. In multi-mode operation (M 2 ≈ 6) a power of 325 W with η oo = 53.2% could be extracted. The spectral bandwidth of the emitted beam was measured using an Optical Spectrum Analyzer (OSA) to be less than 20 pm in fundamental-mode. We also showed a continuous wavelength tuning range of 46 nm for GWS 1 and of 38 nm for GWS 2. With a commercially available Stokes polarimeter the degree of linear polarization (DOLP) was measured to be higher than 98.6% over the whole power and wavelength tuning range.