Intensity-modulated diode laser radar using frequency-modulation/continuous-wave ranging techniques

This paper treats a practical adaptation of frequency modulation (FM) radar ranging principles to an incoherent laser radar (ladar). In the simplest sense, the ladar's laser transmitter output is amplitude-modulated with a radio-frequency subcarrier, which itself is linearly frequency-modulated. The subcarrier signal may have a start frequency in the tens to low hundreds of megahertz and a stop frequency in the hundreds of megahertz to low gigahertz. The difference between the start and stop frequency, Delta F, is chosen to establish the desired range resolution Delta R according to the usual equation from FM radar theory, Delta R=c/2 Delta F, where c is the velocity of light. The target-reflected light is incoherently detected with a photodiode and converted into a voltage waveform. This waveform is then mixed with an undelayed sample of the original modulation waveform. The output of the mixer is processed to remove ''self-clutter'' that is commonly generated in FM ranging systems and obscures the true target signals. The clutter-free waveform is then processed coherently using the discrete Fourier transform to recover target amplitude and range. A breadboard of the ladar architecture was developed around a 100-mW GaAlAs diode laser operating at 817 nm, Imagery and range responses obtained show that the theoretical range resolution of 0.25 m was attained for a Delta F of 600 MHz. Embodiments of this ladar are likely to be practical and economical for both military and commercial applications because row-cost continuous wave (cw) laser diodes are used, coherent optical mixing is not required, and the post-mixing processor bandwidth is low. (C) 1996 Society of Photo-Optical Instrumentation Engineers.