Soft aperture spatial filtering: 1.5W in a single spatial mode from a highly multi-mode laser diode in an external cavity

TL;DR

This paper introduces a soft aperture spatial filtering technique in an external cavity to achieve high-power, single-mode operation from a highly multi-mode broad-area laser diode, enhancing spatial coherence without intra-cavity slit filters.

Contribution

The study presents a novel mode-selective external cavity design that uses the diode aperture as its own spatial filter, enabling high-power single-mode emission from broad-area laser diodes.

Findings

Achieved 1.5W CW power at 1064 nm in a single spatial mode.

Demonstrated high beam quality with broad-area diodes of 200 and 300 μm widths.

Compared efficiency of soft filtering to traditional hard-filtering methods.

Abstract

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 semiconductor wave-guide is inherently spatially multi-mode (in the slow axis). We demonstrate a method to majorly improve the spatial coherence of a high-power broad-area diode laser 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 $1.5\rm{W}$ CW power at $1064 \rm{nm}$ with high beam quality.