Katuo Seta

High-resolution distance meter using optical intensity modulation at 28 GHz

A high-resolution distance meter was fabricated using optical waveguide modulators. An optical intensity modulation frequency of 28 GHz was utilized for the displacement measurement when the driving frequencies to the optical modulators were around 14 GHz. The distance meter was evaluated in the National Research Laboratory of Metrology (NRLM) tunnel, which is a semi-underground tunnel. The measurement resolution is 1 and for a distance of 5 and 100 m, respectively, in one standard deviation after correction of the cyclic error. Furthermore, cyclic errors are examined for 28 GHz optical intensity modulation.

Real-time and high-resolution absolute-distance measurement using a two-wavelength superheterodyne interferometer

We present here a new method for real-time absolute-distance measurement. It is based on a two-wavelength superheterodyne interferometer. It is performed by simultaneous measurement of its synthetic wavelength and one of two wavelengths. The phase of one of two wavelengths is electrically separated from the synthetic one, thus maintaining the resolution of a one-wavelength heterodyne interferometer. Two He-Ne lasers operating at 1 = 633 nm and 2 = 612 nm are incorporated into the measuring system, as two wavelength sources, resulting in the synthetic wavelength of s = 18.4 µm. Thus, the proposed system has a range of 9.2 µm with nanometre resolution.

Portable I/sub 2/-stabilized Nd:YAG laser for international comparisons

We have established a compact and transportable I/sub 2/-stabilized Nd:YAG laser for international comparisons of laser frequency. The root Allan variance of the portable laser has reached 3.9/spl times/10/sup -14/ when the integration time is longer than 200 s. The results of an international comparison between the National Research Laboratory of Metrology (NRLM), Tsukuba, Japan and the JILA (formerly the Joint Institute for Laboratory Astrophysics), Boulder, CO, USA show that the frequency difference of the portable laser NRLM-Y1 and the JILA laser JILA-W (f/sub NRLM-Y1/-f/sub JILA-W/) was -2.5 kHz, when the cold-finger temperatures of NRLM-Y1 and JILA-W were kept at -10/spl deg/C and -15/spl deg/C, respectively. The averaged frequency offset between two NRLM lasers (f/sub NRLM-Y1/-f/sub NRLM-Y2/) was -1.1 kHz. A frequency variation of about 1.2 kHz was found for the frequency offset between two NRLM lasers, after NRLM-Y1 was taken for a round trip to Sydney for a comparison organized by the National Measurement Laboratory, (NML), Australia.

Length measurement by a two-colour interferometer using two close wavelengths to reduce errors caused by air turbulence

An improved two-colour interferometer for length measurements was developed to reduce both the errors caused by air turbulence and the effects of chromatic aberration in the optical system. The method combines a fringe-counting technique and a phase-measurement technique. The phases of interference fringes with respect to laser wavelength 1 and synthetic wavelength s are detected simultaneously by only one detector by using a band-pass filter technique. We verified this new method by measuring lengths of up to 2 m and then comparing the results with those measured by an HP 5528 interferometer. The comparison shows that the relative accuracy of this improved method is 5 × 10-7.

Quasimonochromatic white light fringe interferometer

Quasimonochromatic light sources, such as laser diodes and high power LEDs, are investigated to determine their suitability for zero path difference determination using white light fringes in a Michelson interferometer. Fringe visibility curves are theoretically determined for various combinations of light sources and compared with experimental results when used in a Michelson interferometer with a 25-m path length. A resolution of 2-3 microm was obtained for a pair of multimode laser diodes and also for a single multimode laser diode operated as an LED. This is more than adequate for the calibration of survey baselines.

Frequency stabilization of a HeNe laser using a thin film heater coated on the laser tube

Abstract The cavity length of an internal-mirror HeNe laser was controlled using a thin film heater coated on the tube. The response speed was about ten times higher than that of the usual tube wound with a ribbon heater. The high response speed brought two merits in frequency stabilization of the laser; one was improvement of stability in the short time region and the other was the realization of a thermal modulation method, by which the cavity was stabilized at Lambs dip.

Interferometric absolute distance measurement utilizing a mode-jump region of a laser diode

Abstract The frequency of a laser diode was tuned with the injection current across a mode-jump region in order to measure absolute distances with high precision. A frequency change of about 450 GHz was obtained across a mode-jump region and was measured by a Fabry-Perot etalon. Using a Michelson interferometer, this large frequency change enables a high resolution and also a good linearity to within 5 μm at a range of 50 mm to 80 mm to be obtained. No drift larger than the resolution was observed for several days.

Polychromatic light interferometer for high-accuracy positioning

Superposition of different-wavelength, multimode- and single-mode laser diode beams is investigated to locate the fringe of zero interference order in a Twyman-Green interferometer. The possibility of central fringe detection using three multimode laser diodes or one single-mode together with a pair of multimode laser diodes is shown. If a single-mode laser diode is applied, a simultaneous fringe-counting technique for displacement measurement is available. The influence of the angle between wavefronts entering the interferometer is analyzed. A repeatability of about 5 nm is shown for surface position determination when using three-beam source.

Precise measurement of the difference of the air-refractive indices between visible and near-infrared wavelengths using two-color interferometer

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, Edlens 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.

High-resolution distance meter using optical modulators at 3.5 GHz

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.