At low forward currents, gain in the active region of the laser
diode is low and spontaneous emission is observed. As current
increases beyond a critical “threshold current,” round-trip
gain in the laser cavity exceeds losses and lasing action begins.
Beyond the threshold current, the light emitted by the laser
diode increases rapidly with increasing forward current, as
shown in Fig. 0. Many laser diode packages incorporate an
internal monitor photodiode which may be used in a feedback
loop to maintain constant optical output power from the laser
under varying temperature conditions and as its performance
slowly degrades over time. If the monitor photodiode is properly
biased, its current is proportional to laser light output
power.