Single etalon optical wavelength reference device

This invention relates to wavelength locking of lasers and optical test instruments. A compact wavelength monitoring and control assembly for a narrow band source is provided comprising a narrow bandpass, wavelength selective transmission filter element, of Fabry-Perot structure, through which two collimated beams from a laser source are directed onto two photodetectors. The differential output of the two photodetectors is used in a feedback loop to stabilize the source to the desired wavelength. Through the dependence of the wavelength transmission of the Fabry-Perot etalon on angle and physical parameters, a wavelength variation of the source is converted into a transmission loss, which is different for the detectors, so that the wavelength change is detected by changes in the signals output from the detectors. Through the controlled variation of the source wavelength in the vicinity of the transmission maximum of the etalon, the absolute frequency of the Fabry-Perot locking point may be confirmed by the variation of the ratio of the detector signals.