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M.I.T. Lincoln Laboratory is actively developing laser and detector technologies that make it possible to build a threedimensional laser radar (3-D ladar) with several attractive features, including capture of an entire 3-D image on a single laser pulse, tens ofthousands ofpixels, few-centimeter range resolution, and small size, weight, and power requirements. The laser technology is based on diode-pumped solid-state microchip lasers that are passively Q-switched. The detector technology is based on Lincoln-built arrays of avalanche photodiodes (APD s) operating in the Geiger mode, with integrated timing circuitry for each pixel. The advantage ofthese technologies is that they offer the potential for small, compact, rugged, high-performance, systems which are critical for many applications.

We investigated the fundamental limits to the performance of a laser vibrometer that is mounted on a moving ground vehicle. The noise floor of a moving laser vibrometer consists of speckle noise, shot noise, and platform vibrations. We showed that speckle noise can be reduced by increasing the laser spot size and that the noise floor is dominated by shot noise at high frequencies (typically greater than a few kilohertz for our system). We built a five-channel, vehicle-mounted, 1.55 μm wavelength laser vibrometer to measure its noise floor at 10 m horizontal range while driving on dirt roads. The measured noise floor agreed with our theoretical estimates. We showed that, by subtracting the response of an accelerometer and an optical reference channel, we could reduce the excess noise (in units of micrometers per second per Hz(1/2)) from vehicle vibrations by a factor of up to 33, to obtain nearly speckle-and-shot-noise-limited performance from 0.3 to 47 kHz.

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