Scannerless imaging ladar using a laser diode illuminator and FM/cw radar principles

We describe the research and development of a scannerless three- dimensional (3-D) imaging laser radar (ladar) performed at the Army Research Laboratory for reconnaissance applications. Range information is obtained by a frequency modulation/continuous wave (FM/cw) radar technique implemented by amplitude modulation of a near-IR diode laser with an rf subcarrier that is linearly frequency modulated. The diode's output is projected to floodlight the downrange image area. The returned signal is focused onto the cathode of an image intensifier tube (IIT) where it is mixed with a delayed replica of the laser modulation applied to the cathode bias to modulate the tube gain. The output image of the IIT is modulated at an intermediate frequency (IF) that is sampled in time by a conventional charge-coupled device (CCD) camera. Image frames over one period of the frequency modulation are collected and stored. A discrete Fourier transform is calculated over the IF waveform to establish the ranges to all scatterers in a pixel. This processing scheme yields a scannerless ladar possessing high range resolution with no range ambiguities. We constructed a breadboard version of this ladar and used it to collect 256 X 256 pixel images of targets at 1-km ranges with 0.375-m range resolution. We present imagery collected during field experiments and discuss the direction of future research to enhance the ladar's performance.