Tech Briefs

Optical Profilometers Using Adaptive Signal Processing

Sizes would be reduced, leading to development of hand-held profilometers.

John F. Kennedy Space Center, Florida

A method of adaptive signal processing has been proposed as the basis of a new generation of interferometric optical profilometers for measuring surfaces. Many current optical surface-measuring profilometers utilize white-light-interferometry and, because of optical and mechanical components essential to their operation, are comparable in size to desktop computers. In contrast, the proposed profilometers would be portable, hand-held units. Sizes could be thus reduced because the adaptive-signal-processing method would make it possible to substitute lower-power coherent light sources (e.g., laser diodes) for white light sources and would eliminate the need for most of the optical components of current white-light profilometers. Furthermore, whereas the height scanning ranges of current surface-measuring profilometers are of the order of millimeters, the adaptive-signal-processing method would make it possible to attain scanning ranges of the order of decimeters in the proposed profilometers.

altThe figure depicts the optical layout of a simple Michelson interferometer configured for use as a profilometer, according to the proposal, for measuring the deviation from flatness of a nominally flat surface that contains a pit. The pit can be characterized as comprising multiple facets at different depth, each producing a coherence function having signal intensity proportional to its size. As a result, the output of the photodetector in this interferometer would include a multitude of overlapping coherence functions that cannot be easily discriminated.

A complete overlapping-coherence-function profile of the surface area within the interrogating light beam would be collected by recording and processing the photodetector output as a function of height while scanning the adjustable mirror through the interrogation depth. The adjustable mirror could be mounted on a piezoelectric actuator for rapid scanning in height. Optionally, a digitally controlled micromirror device could also be used to scan the light beam laterally (horizontally in the figure) across the surface. Modern digital signal-processing hardware would be used to rapidly acquire and process the photodetector output and the overlapping coherence signals contained therein according to the adaptive method described below.

In this method, a Fourier transform of a synthetic intensity-versus-depth signal generated from a mathematical model of the surface to be measured would be subtracted from the Fourier transform of the intensity-versus-depth signal obtained by the interferometer scan of the surface to be measured. The result of the subtraction would be an error signal. The coefficients of the model, representing the sizes and depths of facets in the pit, would be adjusted to minimize the error signal. To obtain the coherence function needed for the model, it would be necessary to perform a calibration measurement, prior to operation, in which a reference mirror known to be optically smooth and flat would be substituted for the surface to be measured.

This work was done by Gregory A. Hall, Robert Youngquist, and Wasfy Mikhael of Kennedy Space Center. KSC-12647