An automatic gain control circuit in the feedback path for a laser
wavelength control circuit is described herein. This gain control circuit
automatically adjusts the amplification of the analog signals output from
a photodetector array, where the array detects a fringe pattern created
by a laser beam. Another feature of the preferred embodiment feedback
circuit is the automatic setting of a DC offset voltage that compensates
for errors in the feedback path and enables an accurate determination of
a dark level signal in the fringe pattern signal. This dark level signal
provides a reference for measuring the magnitude of the fringe pattern
signal. Varying photodetector outputs may now be more accurately
measured. The preferred embodiment feedback circuit also employs a very
fast amplifier anti-saturation circuit using LED's connected in a clamp
circuit.