Infrared Laser Interferometer for Measuring Air-bearing Separation

The design and characteristics are presented for an infrared instrument capable of measuring air-bearing separation distances over a mechanical bandwidth ranging from dc to 30 kHz. The measurement technique involves monitoring optical intensity variations of the interferometric cavity formed by two air-bearing surfaces. This intensity varies between a minimum at zero separation and a maximum at a distance equal to one-quarter of the optical wavelength. For air-bearing distances less than 1 µm, a convenient source is the 3.391 µm infrared line of the helium-neon laser. By continually monitoring a fraction of the intensity of the optical source, a real-time analog division can be performed on the spacing signals to produce an output independent of laser intensity variations. Room-temperature indium arsenide detectors were selected for their high responsivity and rapid rise time.