It is shown that a mode-locked laser can be used as a local oscillator (LO) in heterodyne ranging to bandwidth compress the received signal. The amount of bandwidth compression depends on the difference in the intermode frequencies between the LO laser and the transmitter laser, which can also be a mode-locked laser. Since the heterodyning occurs on the detector, it is only necessary to use a narrow bandwidth receiver that passes the compressed bandwidth to obtain ranging equivalent to that obtained by a wide bandwidth receiver that passes the full transmitter bandwidth. Bandwidth compression of over 500x and 1 ft range resolution was experimentally demonstrated when a 1.5 GHz He-Ne laser mode-locked transmitter signal was reduced to less than 3 MHz at detection.
Carl C. Aleksoff,
L. M. Peterson,
"Laser Heterodyne Ranging Using A Mode-Locked Local Oscillator For Bandwidth Compression," Optical Engineering 25(3), 253490 (1 March 1986). https://doi.org/10.1117/12.7973847