Soft Aperture Spatial Filtering: 1.5W in a Single Spatial Mode From a Highly Multi-Mode Laser Diode in an External Cavity

Broad area laser diodes are attractive for the high optical power they can produce. Unfortunately, this high power normally comes at the cost of severely reduced spatial coherence since the wide area of the diode wave-guide is inherently spatially multi-mode along the slow axis. We demonstrate a method to significantly improve the spatial coherence of a high-power broad-area diode by placing it in an external cavity that is mode selective. We design the cavity, such that the diode aperture acts as its own spatial filter, obviating the need for an intra-cavity slit-filter, and optimally utilizing the entire gain medium. We demonstrate this soft filtering method using wide diodes of $200 \rm {\mu m}$ and $300 \rm {\mu m}$ widths and compare its power-efficiency to the standard approach of hard-filtering with a slit. We obtain high-gain operation in a pure single-mode, demonstrating up to 1.5 W CW power at 1064 nm with excellent beam quality ( $M^{2}=1.3$ at 90% power).