  We study the degree of second-order coherence of the emission of a high-power
multi-quantum well superluminescent diode with a lateral tapered amplifier
section with and without feedback. When operated in an external cavity, the
degree of second-order coherence changed from the almost thermal case of
g$^{(2)}$(0)$\approx$1.9 towards the mostly coherent case of
g$^{(2)}$(0)$\approx$1.2 when the injection current at the tapered section was
increased. We found good agreement with semi-classical laser theory near and
below threshold while above laser threshold a slightly higher g$^{(2)}$(0) was
observed. As a free running device, the superluminescent diode yielded more
than 400 mW of optical output power with good spatial beam quality of
$M^2_{slow} < 1.6$. In this case, the DSOC dropped only slightly from 1.9 at
low powers to 1.6 at the maximum output power. To our knowledge, this is the
first investigation of a high-power tapered superluminescent diode concerning
the degree of second-order coherence. Such a device might be useful for
real-world applications probing the second order coherence function, such as
ghost imaging.
