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

Abstract

Barry L. StannWilliam C. RuffZoltan G. SztankayDepartment of the ArmyArmy Research LaboratoryAMSRL-SE-EP2800 Powder Mill RoadAdelphi, Maryland 20783-1145E-mail: stann@arl.milAbstract. This paper treats a practical adaptation of frequency modula-tion (FM) radar ranging principles to an incoherent laser radar (ladar). Inthe simplest sense, the ladar’s laser transmitter output is amplitude-modulated with a radio-frequency subcarrier, which itself is linearlyfrequency-modulated. The subcarrier signal may have a start frequencyin the tens to low hundreds of megahertz and a stop frequency in thehundreds of megahertz to low gigahertz. The difference between thestart and stop frequency, DF, is chosen to establish the desired rangeresolution DR according to the usual equation from FM radar theory,DR5c/2 DF, where cis the velocity of light. The target-reflected light isincoherently detected with a photodiode and converted into a voltagewaveform. This waveform is then mixed with an undelayed sample of theoriginal modulation waveform. The output of the mixer is processed toremove ‘‘self-clutter’’ that is commonly generated in FM ranging systemsand obscures the true target signals. The clutter-free waveform is thenprocessed coherently using the discrete Fourier transform to recovertarget amplitude and range. A breadboard of the ladar architecture wasdeveloped around a 100-mW GaAlAs diode laser operating at 817 nm.Imagery and range responses obtained show that the theoretical rangeresolution of 0.25 m was attained for a DFof 600 MHz. Embodiments ofthis ladar are likely to be practical and economical for both military andcommercial applications because low-cost continuous wave (cw) laserdiodes are used, coherent optical mixing is not required, and the post-mixing processor bandwidth is low.