Abstract

A laser radar prototype has been developed for determining an object’s shape and estimating its surface scattering properties. In this system the target models are flood illuminated with a frequency scanning Ti:sapphire laser. As the frequency changes, the speckle pattern in the backscattered light appears to boil; the time-varying intensity carries information about the object’s range-resolved laser radar cross-section U(z). If the frequency is scanned linearly in time, the power spectrum of the intensity fluctuation at a single point provides a speckled estimate of the autocorrelation of U(z). The speckle is smoothed by low-pass filtering, scan averaging, or averaging estimates from multiple detectors. In a laboratory setting U(z) can be obtained directly from the power spectrum measurement by placing a reflective reference plane behind the object. In general, U(z) can be reconstructed from a higher-order spectrum, the bispectrum. In either case, U(z) is determined uniquely and provides a fingerprint for the object at the given aspect angle. The main advantage of this type of laser radar is its high range resolution, which can be >1 mm. Given U(z) as a function of aspect angle, the object shape can be reconstructed by reflective tomography. The result is a function of position representing the object shape, i.e., zero valued outside of the object bounds. Its value inside the object is related to the surface scattering properties, e.g., the reconstruction tends to fill in more uniformly as the scattering becomes more diffuse. Illustrative measurements are presented for simple shapes including cones, spheres, and cylinders. Parameters, such as sphere radius and cone half angle, are extracted.

© 1991 Optical Society of America