A frequency offset locking system for the short-pulsed laser outdoors

hi recent years, with the rapid development of laser radar technology both in military and civil fields, the research on laser radar has come to the stage of application throughout the world. This paper studies the offset frequency locking system used for a short-pulsed laser outdoors. It is often the case that if the received IF signal bandwidth of coherent laser radar becomes more narrow in coherent detection, the signal noise ratio will be improved greatly. Therefore the technique is needed for offset locking the heterodyne frequency between two lasers. However the unstable ambient temperature will produce an effect on the behavior of a gas laser, and the offset locking technique should have such characterized features as in a frequency offset locking loop used for pulsed lasers. The heterodyne frequency between two lasers is necessitated to have a wide enough bandwidth so as to prevent itself from a loss of locking and to have a high enough discriminating frequency accuracy so as to make the radar possess a high accuracy of measuring the target speed. hi order to attack the problems mentioned above, this paper designs an offset frequency locking system with two frequency discrimination loops. One has an all-digital frequency discriminator with a bandwidth of 600MHz and accuracy of 5MHz in discriminating frequency to an optical pulse width 30 ns; the other has a coincidence gate circuit discriminator with a bandwidth of 20MHz and an accuracy of less then 500kHz in discriminating frequency to an optical pulse width 30 ns.