Improved Thermal Performances of Resonant Reflection Waveguide Grating Structure

We present the intra-cavity characterization (within an Yb:YAG thin-disk oscillator) of a single-layer resonant waveguide grating (RWG) using a crystalline material (namely Sapphire) as substrate. The operating principle of the present device is the same as in [1]. It offers high reflectivity > 99% with a narrow spectral bandwidth of typically <1 nm FWHM for a given polarization and angle of incidence. Commonly single-layer dielectric waveguide gratings are based on sub-wavelength grating integrated with a high-index waveguide (such as $\text{Ta}_{2}\mathrm{O}_{5}$ or $\text{Nb}_{2}\mathrm{O}_{5})$ coated on a fused silica substrate. Often, they suffer from heating caused by the absorption in the waveguide layer, in addition to the relatively low thermal conductivity of the substrate, thus limiting their applications in high average power lasers. In the present contribution, we report on a RWG composed of a $\text{Ta}_{2}\mathrm{O}_{5}$ waveguide layer coated on a structured sapphire substrate with better thermal performance at high average laser power (i.e., at high-power densities on the grating surface). The designed device consists of a single $\text{Ta}_{2}\mathrm{O}_{5}$ layer of 200 - 235 nm thickness deposited upon a binary grating with a period of 515 nm and a groove depth of 120 nm etched into a sapphire $(\text{Al}_{2}\mathrm{O}_{3})$ substrate [2]. The RWG was designed to operate for TE polarization at 1030 nm and an angle of incidence of $\sim 10^{\circ}$ . The suitable choice of substrate material offers a thermal conductivity that is an order of magnitude higher than fused silica. Before its implementation in the laser cavity, the fabricated sample was first characterized in a spectroscopic setup to measure its reflectivity for both TE and TM polarizations. At the central wavelength of our laser i.e. 1030 nm, the reflectivity for TE and TM polarizations were measured to be $R_{TE},=99.2\pm 0.2\% \ \mathrm{a}\ R_{TM}=8\pm 0.2\%$ , respectively.