A distance meter was built utilizing the intermode beat of a He-Ne laser instead of an optical modulater. Its performance was tested at distances between 10 and 1300 m on a 300-m baseline in a tunnel. The resolution was about 20 microm and the total uncertainty was within 0.1 mm, if the air was stable. The length of the tunnel was measured by a distance meter, and the annual periodic strain of the tunnel was observed with submillimeter precision.
The long-arm-length difference of an interferometer is measured by changing the optical frequency of a YAG laser used with a resolution of millimeters.
We discuss the measurements of the changes in air refractive indices and geometrical distances by means of a two-color phase-modulated fringe-counting interferometer. We analytically classified the optical path lengths for IR and visible light into the changes in the air refractive index and the distance by using a dispersion formula for air. The interferometer was tested over a 235-m path in the testing tunnel of the National Research Laboratory of Metrology, which has a sensor system for measuring air conditions.
The refractive-index change by the two-color interferometer agreed with the value calculated by the air conditions within an accuracy of 0.15 ppm.
Recently, high-resolution distance measurement of pm order in the range of 1-100 m is required to evaluate large-scale constructions. We have developed an optical time-of-flight distance meter, using two waveguide-type modulators and a laser diode of a wavelength 1.3 pm. In this system, the laser beam is interferometrically modulated at 3.5-GHz frequency and demodulated at (3.5 GHz + 50 kHz). The light reflected by the target is detected with a good SNR, >40 dB. This system is evaluated at distances up to 250 m, using a 50-m traveling carriage and an interferometric fringe counting system. From the results, the linearity in phase measurement was ~5 μm in standard deviation, after compensating the systematic cyclic error from the reflection on the surfaces of optical fibers and a phase sensitive phase meter.
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