LASER PULSE COMPRESSION RANGING SYSTEM USING DOUBLE-CHIRPED PULSES

摘要

1,249,843. Lasers. UNITED AIRCRAFT CORP. 27 Feb., 1970, No. 9661/70. Heading H1C. [Also in Division H4] Laser rangefinding apparatus has a pair of Q switched lasers A and B, Fig. 2, each controlled by the same rotating mirror 14. The mirror rotates about axis 16 so that while the resonator of laser A formed by reflectors 12, 18 and 14 is increasing in length, the resonator of laser B is decreasing in length. The resonator mirrors 12, 18, 181 and 14 are arranged so that the lasers give simultaneous output pulses which are superimposed into one beam by mirrors 24 and 26. Because one resonator shortens as the other lengthens, the frequency of the pulse from laser A falls during the pulse, while that of laser B rises, Fig. 3. The combined output is thus a so called " double chirped " pulse, and allows advantage to be taken of the laser bandwidth, giving high resolution. The lasers are preferably of the CO 2 type, and the pulse lasts for 1 Á see, allowing considerable energy to be transmitted, but since this is a relatively long time for radar applications the receiving apparatus, Fig. 5, includes a compression circuit to restore the advantages of the laser bandwidth. The laser pulse, which may be amplified before transmission, is scattered from a target and received by a square law photodetector. The resulting electrical signal is subjected to high pass filtering to give an a.m. signal which is used to modulate a 300 MHz carrier in a balanced modulator carrier suppressed circuit 58. The resulting signal, in the form of a.m. sidebands, is fed to compressor 16 which has a group delay which increases linearly with frequency. The effect of this is to amplify the high frequencies of the signal and to attenuate and delay the low frequencies, hence producing a spike output, Figs 7, 8 (not shown), which may be visually displayed.