A laser wavelength locking control loop apparatus comprising a wavelength selective
device and a photodetector, coupled to a feedback controller circuit. Electromagnetic
radiation emitted by the laser device is provided to the wavelength selective device
which provides a first output signal at a first wavelength and a second output
signal at a second wavelength. The first and second output signals are directed
to the photodetector so as to coincide when incident thereupon. A beat signal results
from the coincidence of the first and second output signals and this is used to
measure wavelength drift.
