Side-Coupling Scheme for a High-Power Laser Diode Array With Grating Couplers: Thermal and Geometrical Issues

Grating couplers for side coupling of the light from a high-power laser diode array (LDA) into the inner cladding of a double-clad fiber were studied theoretically and implemented experimentally. In the experiments, two types of grating couplers were designed and fabricated: a gold-coated surface relief grating coupler and a gold-embedded silica grating coupler. A suitable design for heat dissipation for each grating coupler was employed to minimize thermal expansion due to the heat accumulated from the light absorption by the metal part of the grating coupler. The experimental results show that the gold-embedded silica grating coupler is superior to the surface relief gold grating coupler, because of its higher resistance to thermal expansion and better heat removal capability. In addition, the grating pitch and groove width were optimized for the highest overall coupling efficiency by taking account the backward diffraction loss and the groove wall nonverticality due to fabrication distortion. The experimental results show that the gold-embedded silica grating coupler is capable of coupling light power up to 21 W from a 976-nm continuous-wave LDA into the inner cladding of a 400-
$\mu$
m-diameter double-clad fiber with an overall coupling efficiency of 50%.