FREQUENCY DETECTOR FOR DISCRIMINATING MULTI-LONGITUDINAL MODE LASER OPERATION

摘要

A photodetector produces an output signal in response to irradiation by an optical beam from a pulsed laser. A delay unit delays the output signal for a period corresponding to one-half of one cycle of a beat frequency generated by interaction of two frequency components in the optical beam corresponding to two predetermined longitudinal modes of operation of the laser. A differential mixer subtractively combines the output of the photodetector with the output of the delay unit, to produce an output signal having the main profile of the laser pulses suppressed, and the beat frequency component, if present, amplified. A resonant circuit and amplifier may be provided to further amplify the beat frequency component. The output of the amplifier is integrated to produce a DC signal having a magnitude which corresponds to the amplitude of the beat frequency component. The DC signal is compared with a threshold value to discriminate between single and multimode operation of the laser. An optical shutter may be closed in response to sensed multi- longitudinal mode operation to block the laser beam and protect equipment which would otherwise be damaged thereby.