  We introduce a technique to determine photon correlations of optical light
fields in real time. The method is based on ultrafast phase-randomized homodyne
detection and allows us to follow the temporal evolution of the second-order
correlation function $g^{(2)}(0)$ of a light field. We demonstrate the
capabilities of our approach by applying it to a laser diode operated in the
threshold region. In particular, we are able to monitor the emission dynamics
of the diode switching back and forth between lasing and spontaneous emission
with a $g^{(2)}(0)$-sampling rate of 100 kHz.
