High-power dense wavelength division multiplexer (HP-DWDM) for diode lasers using volume Bragg gratings (VBG)

The power scaling of narrow-band and near-diffraction-limited solid state laser radiation by spectral beam combining has been demonstrated during the last years with high efficiencies > 90 % [1,2] in large multiplexing modules (l ≈ 1 m). We present a diode laser prototype based on a compact dense wavelength division multiplexer (DWDM) for diode laser bars with low average beam quality (M² = 45). The prototype with a radiance of 70 GW/(m²sr) (without polarization coupling, at an injection current of I = 120 A) consists of five spectrally stabilized and passively cooled diode laser bars with the center wavelengths 973 nm, 974.5 nm, 976 nm, 977.5 nm and 979 nm. To achieve a good stabilization performance ELOD (Extreme LOw Divergence) diode laser bars have been chosen [3,4]. The spectral width defined by 95 % power inclusion is < 0.6 nm for each beam source across the entire operating range from 30 A to 120 A. Due to the spectral stabilization the output power of each bar changes in the range of ± 5 %. Near the threshold current and at high injection currents (100 A to 120 A) the output power of the stabilized diode laser bar slightly increases compared to the free running diode laser. A beam transformation with a pair of step mirrors adapts the beam quality in both axes before the radiation enters the multiplexer.