Etalon-based wavelength locking apparatus and alignment method

An apparatus for producing a control signal for a laser source, comprising:

• a beam splitter that receives light from a laser source;
• an etalon configured to receive a first portion of the light from the laser source;
• a first detector that detects a forward transmission signal of the etalon to provide a first signal, the forward transmission signal being a first part of the first portion of the light from the laser source, the forward transmission signal being reflected by the beam splitter and passing through the etalon to the first detector;
• a second detector that detects a backward reflection signal of the etalon to provide a second signal, the backward reflection signal being a second part of the first portion of the light from the laser source, the backward reflection signal being output from the etalon and passing through the beam splitter to the second detector;
• a third detector that detects a second portion of the light from the laser source to provide a third signal, the second portion of the light from the laser source passing through the beam splitter to the third detector;
• and
• circuitry that provides the control signal based on the first signal, the second signal and the third signal,
• wherein the control signal is indicative of a comparison of (a) a difference between the first and second signals and (b) the third signal,
• wherein the circuitry includes:
• an operational amplifier arranged to subtract the second signal from the first signal to produce a fourth signal indicative of the difference between the first signal and the second signal;
• and
• a comparator arranged to compare the fourth signal to the third signal.