Daniel G. Fouche

Three-dimensional imaging laser radar with a photon-counting avalanche photodiode array and microchip laser

We have developed a threedimensional imaging laser radar featuring 3-cm range resolution and single-photon sensitivity. This prototype direct-detection laser radar employs compact, all-solid-state technology for the laser and detector array. The source is a Nd:YAG microchip laser that is diode pumped, passively Q-switched, and frequency doubled. The detector is a gated, passively quenched, two-dimensional array of silicon avalanche photodiodes operating in Geigermode. After describing the system in detail, we present a three-dimensional image, derive performance characteristics, and discuss our plans for future imaging three-dimensional laser radars.

Detection and false-alarm probabilities for laser radars that use Geiger-mode detectors

For a direct-detection laser radar that uses a Geiger-mode detector, theory shows that the single-pulse detection probability is reduced by a factor exp(-K), where K is the mean number of primary electrons created by noise in the interval t between detector turn-on and arrival of laser photons reflected from the target. The corresponding false-alarm probability is at least 1 - exp(-K). For fixed-rate noise, one can improve the detection and false-alarm probabilities by reducing t. Moreover, when background-light noise is significant and dominates dark-current noise and when the laser signal is of the order of ten photoelectrons or more, the probabilities can be improved by reducing the amount of light falling on the detector, even if the laser signal is reduced by the same factor as the background light is. Additional analytical calculations show that identifying coincidences in data from as few as three pulses canreduce the false-alarm probability by orders of magnitude and, for some conditions, can also improve the detection probability.

Three-dimensional laser radar with APD arrays

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

Thermal-blooming compensation: experimental observations using a deformable-mirror system

A laboratory experiment has demonstrated the effectiveness of compensating for forced-convection-dominated cw thermal blooming by using a deformable mirror to add phase corrections to the laser beam. In agreement with theoretical predictions, the peak focal-plane irradiance has been increased by a factor of 3 under severely bloomed conditions.

Control of intensity distribution and spectra through resonator design for the SABLE propagation experiments

Characterization of atmospheric turbulence and thermal blooming for high energy laser propagation has been conducted for the Scaled Atmospheric Blooming Experiment (SABLE) under controlled experimental conditions. To enhance thermal blooming with a high brightness, moderate power laser beam, a hydrogen fluoride (HF) chemical laser, producing six major lines, P1(7), P1(8), P1(9), P2(7), P2(8), and P2(9), was utilized. This paper summarizes design options and the design and operation of an X-folding scheme and the resulting quad-pass resonator (QPR), which produced a spectrum shift of 2 J-lines, a near-field irradiance distribution that was more uniform along the flow direction, had less line-to-line variation in near-field irradiance distribution, and produced twice the far-field power after atmospheric propagation, when compared to a more conventional double-pass resonator (DPR).

Scaled atmospheric blooming experiment

The SABLE experiment investigated the ability of phase-conjugate adaptive-optics to compensate strong thermal blooming and turbulence encountered during atmospheric propagation of a high-power laser beam. The experiments utilized a 10 kW hydrogen fluoride laser, selected because its spectrum is strongly absorbed by the atmosphere. Characteristics of the beam at both the transmitter and receiver, of the atmosphere along the propagation path, and of the adaptive optics were measured and recorded during the tests and used to test the accuracy of a time-dependent computer propagation code. Wind variations along the propagation path were shown to significantly improve system performance and to suppress the phase conjugate instability.