David G. Kocher

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.

Satellite vibration measurements with an autodyne CO 2 laser radar

Vibration signatures of the Low Power Atmospheric Compensation Experiment satellite were obtained with a ground-based CO(2) laser radar. The laser radar operated in a cw mode and used autodyne receivers to extract relative target velocity information between a germanium retroreflector located at the base of the satellite and a retroreflector array located at the tip of an extended forward boom. Time-frequency analysis algorithms were applied to the vibration data to investigate the correlation between excitations and modal structure. The resultant analysis suggests that vibration modes of an on-orbit spacecraft can be suppressed with simple open-loop techniques.

MIT/LL development of broadband linear frequency chirp for high-resolution ladar

The development of a high-resolution laser radar (ladar) exhibiting sub-mm resolution would have a great impact on standoff identification applications. It would provide biometric identification capabilities such as three-dimensional facial recognition, interrogation of skin pore patterns and skin texture, and iris recognition. The most significant technical challenge to developing such a ladar is to produce the appropriate optical waveform with high fideltiy. One implementation of such a system requires a 1.5-THz linear frequency sweep in 75 &mgr;s. Previous demonstrations of imaging with such waveforms achieved a 1 THz sweep in > 100 ms, and required additional corrections to compensate for sweep nonlinearity. The generation of high fidelity, temporally short frequency-swept waveforms is of considerable interest to the DoD community. We are developing a technique that utilizes a novel method to generate a 1 THz sweep in 50 &mgr;s from a mode-locked laser. As a proof-of-principle demonstration of this technique we have successfully generated a 20 GHz sweep in 1 µs with a fidelity sufficient to produce better than -20 dB sidelobes for a range measurement without using any additional corrections. This method is scalable to produce the entire 1 THz sweep in 50 &mgr;s.

Automated Foucault test for focus sensing

A chopper wheel, a bicell photodetector, and an electronic phase detector are used in a mechanization of the Foucault optical test to obtain a linear focus error voltage for an isolated point image. Geometrical and diffraction analyses of the technique are included which show the sensitivity of the technique to aberrations of higher order than focus.

Time-multiplexed optical waveform generation for high-resolution imaging

We have developed a time-multiplexed technique for controlling the amplitude and phase of the individual frequency components of a mode-locked laser to generate a precisely linear chirped waveform. We have demonstrated a 20-GHz, 1-mus chirp.

Optical Waveform Generation for Coherent High-Resolution Imaging

We have developed a time-multiplexed technique for controlling the amplitude and phase of the individual frequency components of a mode-locked laser to generate a precisely linear chirped waveform. We have demonstrated a 20-GHz, 1-us chirp.

Point Image Focus Sensing Using An Automated Foucault Test

An electromechanical focus sensing technique for isolated point or small spot images is described. The technique is a mechanization of the Foucault knife-edge test, and it works with either laser or white light. The focus sensor provides an output voltage vs. focus discriminant which is linear in the vicinity of the best focus and monotonic over a wide range, making it suitable as a sensor for servo focus control applications. The focus sensor uses a chopping wheel at the optical focus and a field lens and bicell photodetector behind the focus. The relative phase of the zero-crossings of the AC-coupled bicell signals is detected electronically to obtain the voltage vs. focus discriminant. Two electronic detection algorithms are described and discriminants obtained with than in a typical implementation of the technique are presented. Also described are two techniques that have been used to compensate the detector signals for rapid fluctuations in the source intensity, if the fluctuations are rapid enough to interfere with the phase detection process. This focus sensing technique is sensitive primarily to focus, but some sensitivity to certain other wave-front ahprrations also exists. These sensitivities have been computed for two particular cases, the resolved spot source (with a geometrical analysis) and the unresolved point source (with a diffraction analysis). These results are tabulated for the Zernike aberrations up to m = n = 8.

A Multiple-Spark Light Source and Camera for Schlieren and Silhouette Photography

A multiple-spark light source and camera utilizing the Cranz-Schardin optical method for schlieren and silhouette photography is described. Each light source is a spark in air, with an effective duration of 10-7 second and a peak of light of 24,000 candle power. The interval between successive sparks is independently adjustable from 10-5 to 10-7 second, permitting a variable picture rate to be used in a single sequence of photographs. The equipment contains ten spark light sources and camera lenses, permitting a ten picture sequence of high speed phenomena. Schlieren and silhouette photographs demonstrating the performance of the equipment are presented.