Low-loss wavelength locking of a 49-element single mode diode laser bar with phase-plate beam correction

The paper reports on the use of dual-axis correction for a full-length diode laser bar from Bookham, with 49 single mode emitters and 30 W cw output at 975 nm. Fast-axis smile and lens error correction is combined with laser-cut slow axis collimation to give highly parallel beams with far-field divergence of 2.6 mrad (fast axis) and 13 mrad (slow-axis). Result shows the improvement in fast-axis divergence, in the form of far-field pattern vs. emitter number. Wavelength locking of the ultra-collimated single-mode array is investigated, using a volume Bragg grating (VBG) with 15% diffraction efficiency and 200 pm bandwidth from Ondax Inc. The single-mode emitters require better collimation than the more usual BA emitters for efficient feedback from the VBG, due to their small aperture (1 x 5 mum). The power loss associated with the VBG insertion and locking is less than 10%, lower than the nominal reflectivity of the grating.