NMIJ line standard interferometer has been modified for measurement of a linear scale module with analog output. The interferometer was developed for line standard calibration. The light source is a stabilized He-Ne laser. Before the modification, a line standard can be calibrated with an uncertainty of about 0.2 micrometer for the total length of 500 mm (k=2). After the modification, a linear scale module can be calibrated as well as a line standard. A linear scale is set in a support on a moving carriage. The displacement of the moving carriage is measured by the interferometer with a sampling frequency of 30 kHz to 300 kHz while the electronic output of the linear scale module is sampled with the same timing. The analog output of the line scale module is used instead of digital output because it is important to assure the simultaneous sampling of the displacement and the scale output.
Optical measurement technique is widely used in length measurement, such as laser interferometers and electronic distance meters. The correction of air refractive index is necessary because the optical path length should be converted to the geometrical length in most vases. In order to obtain an average refractive index through the optical path, Edlen's formulae have been widely used with environmental measurements of air temperature, pressure, humidity and carbon dioxide concentration. In recent days, more precise formulae for calculating a refractive index of air have been desired in IR region. However, the measurement accuracy for near IR region is not sufficient.
Sinusoidal modulation method is utilized in optical distance meters. In this method, the phase delay of the returned beam is measured with high resolution. One of the dominant error factors in the phase measurement is cyclic error. We have already reported on a cyclic error compensation of an optical waveguide distance meter using a preliminary phase measurement with mechanical displacement. In this paper, a new technique of the cyclic error compensation is described, which is based on the modulation frequency scanning of the waveguide distance meter around 14 GHz. We have compared two types of cyclic error curves; one was obtained from the data during the displacement of the target and the other was obtained from the data during the scanning of the modulation frequency. The curve fitting technique was used to determine the amplitude and the phase of the cyclic error component for the two curves. We have obtained almost the same values for both parameters of the amplitude and the phase for the two cyclic error curves. Therefore, the cyclic error curve obtained from the data of frequency scanning can be utilized for the cyclic error compensation instead of the displacement of the target.
A laser tracking system has been developed to trace a moving target for outdoor measurements of 3D position. Beam direction of a 670 nm laser diode is controlled with two mirrors mounted on Galvano scanners, for horizontal ((phi) ) and vertical ((theta) ) directions, respectively, in order to trace a target made of reflection sheet. The beam reflected by a the target is detected at a quadrant photodiode to estimate the position of the target. The distance D between the apparatus and the target was measured with a range finder of 830 nm LED, the light beam from which is superposed on the beam for tracking by a beam combiner. Finally 3D position of the target is estimated from two azimuth angles of the 2 scanners and the measured distance. The whole system was set on a crane to measure the position of a weight suspended by the crane's arm. Up to about 100 m, this system traced the target and detected the distance. The 3D position of the target is measured with a resolution of 10 mm.
A high resolution distance meter has been made using optical wave-guide modulators. The modulation frequencies for the optical modulators are around 3.5 GHz, while the modulation frequencies of optical intensity are around 7 GHz because the modulation bias voltages are set to peak points of the electro-optic characteristic. The distance meter was evaluated in the NRLM tunnel. The measurement resolution is 15 micrometers of a standard deviation for a distance range of 0.5 m to 5 m after a correction of a cyclic error.