Phase sensitive detector and reference generator for use in third derivative locking of the frequency of a laser to a saturated absorption feature

Design criteria are discussed and circuits are presented for a phase sensitive detector and its reference signal generator to be used in third derivative locking of the frequency of a laser to the centre of a saturated absorption feature. The use of a phase locked loop enables the operating frequency to be continuously variable over the range 4.7-47 kHz so that it can be matched to a mechanical resonance of the transducer used to frequency modulate the laser.