  An efficient method to tune the spatial coherence of a degenerate laser over
a broad range with minimum variation in the total output power is presented. It
is based on varying the diameter of a spatial filter inside the laser cavity.
The number of lasing modes supported by the degenerate laser can be controlled
from 1 to 320,000, with less than a 50% change in the total output power. We
show that a degenerate laser designed for low spatial coherence can be used as
an illumination source for speckle-free microscopy that is 9 orders of
magnitude brighter than conventional thermal light.
