  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.
