Efficient blue light generation by frequency doubling of a broad-area diode laser in a compact external cavity

An antireflection-coated broad-area laser diode is frequency stabilized in a compact unstable external cavity that was designed off-axis for transversal mode selection. The external cavity laser yielded up to 1 W near-diffraction-limited and tunable light centered around a wavelength of 970 nm. This light was frequency doubled by the use of periodically poled magnesium doped lithium niobate in a single-pass setup. With a 4-cm-long bulk crystal 57.5 mW of blue light could be generated out of 850 mW of infrared light resulting in a conversion efficiency of 6.7%. By using a 1.2-cm waveguide crystal it was possible to couple up to 64% of the infrared light into a 3.5 mu mx5 mu m channel and 142 mW of blue light at 488 nm could be obtained. An internal conversion efficiency of 44.1% and a wall-plug efficiency of 5.9% were achieved. Furthermore, the blue light was tunable over more than 5 nm.