Tech Briefs

A Short-Range Distance Sensor with Exceptional Linearity

Potential uses exist in the areas of micromachining and nanotechnology.

A sensor has been demonstrated that can measure distance over a total range of about 300 microns to an accuracy of about 0.1 nm (resolution of about 0.01 nm). This represents an exceptionally large dynamic range of operation — over 1,000,000. The sensor is optical in nature, and requires the attachment of a mirror to the object whose distance is being measured.

Distance Sensor is based on the wavelength variations of the light transfer through a Michelson interferometer. Collimated white light is launched into an interferometer composed of a fixed mirror and a translating mirror aligned with a beamsplitter. Light reflected from each mirror makes its way to a small spectrometer where the optical intensity can be measured as a function of wavelength." class="caption" align="right">This work resulted from actively developing a white light interferometric system to be used to measure the depths of defects in the Space Shuttle Orbiter windows. The concept was then applied to measuring distance. The concept later expanded to include spectrometer calibration.

In summary, broadband (i.e., white) light is launched into a Michelson interferometer, one mirror of which is fixed and one of which is attached to the object whose distance is to be measured. The light emerging from the interferometer has traveled one of two distances: either the distance to the fixed mirror and back, or the distance to the moving mirror and back. These two light beams mix and produce an interference pattern where some wavelengths interfere constructively and some destructively. Sending this light into a spectrometer allows this interference pattern to be analyzed, yielding the net distance difference between the two paths.

The unique feature of this distance sensor is its ability to measure accurately distance over a dynamic range of more than one million, the ratio of its range (about 300 microns) to its accuracy (about 0.1 nanometer). Such a large linear operating range is rare and arises here because both amplitude and phasematching algorithms contribute to the performance. The sensor is limited by the need to attach a mirror of some kind to the object being tracked, and by the fairly small total range, but the exceptional dynamic range should make it of interest.

This work was done by Stephen Simmons and Robert Youngquist of Kennedy Space Center. For more information, contact the Kennedy Space Center Technology Transfer Office at (321) 867-7171. KSC-13382