Wavelength stabilization of HPDL array - Fast-axis collimation optic with integrated VHG

Volume holographic gratings (VHG) provide the capability of narrowing and stabilizing the wavelength of semiconductor lasers by forming an external cavity laser (ECL). The standard configuration of these ECL's is to use a collimating lens followed by the VHG to provide feedback to the resonator and lock the wavelength. In this configuration both elements have to be carefully aligned with tolerances in the sub-μm and mrad range. The present paper presents a fast-axis collimation lens (FAC) with integrated VHG for locking a laser diode bar. Besides the advantage of having only a single element, the integrated element is also less sensitive to alignment tolerances with respect to the locking due to the large divergence angle of the uncollimated array compared to a collimated array. Using a standard AR coated array with 19 emitters an output power of 67.4 W was achieved. The spectral bandwidth was within 1 nm over the whole power range. Due to high stability requirements in this application, glass was chosen as the VHG material. Though the refractive index is low compared to standard FAC lenses, the design and manufacturing process of the lens still guarantees a diffraction limited collimated beam.