Narrow beam divergence of laser diodes with a Bragg grating in external cavity

Conventional high-power diode lasers present poor spatial beam quality along their slow axis, being 10 to 50 times more divergent than the limit of diffraction in that direction. To overcome this problem, the use of external cavities remains of great interest because of their compactness and simplicity compared with injection locking by a separate master laser. We demonstrate a novel external cavity technique that allows for efficient spatial mode selectivity. The key component to realize mode filtering is a thick transmission type phase volume grating that may exhibit both high diffraction efficiency and very sharp angular bandwidth around the Bragg incidence. The grating was recorded with an argon laser at 488 nm in a phenanthrenequinone-doped poly(methyl methacrylate (PQ:PMMA) sample which can be available as thick as several mm with good optical quality. The intrinsic Bragg diffraction efficiency was 85% at 970 nm and the full width at half maximum of the angular selectivity curve was 3.5 mrad. In conclusion we have successfully demonstrated a novel compact technique that allows for significant improvement in beam quality of broad area diodes as well as narrowing of spectral bandwidth. Similar experiments are being carried out with a diode laser array.