Shigeo Iwasaki

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.

A two-color heterodyne interferometer for measuring the refractive index of air using an optical diffraction grating

A new two-color heterodyne interferometer using an optical grating was proposed for measuring the refractive index of air. The heterodyne modulation is realized by moving the diffraction grating. The method allows us to measure the refractive index with high accuracy by using a 1:06 lm YAG laser and its second-harmonic wave because two beams (the fundamental and the second-harmonic wave) are in common path completely. Moreover, good signal-to-noise ratio (SNR) can be obtained by the heterodyne interference. Using the refractive index calculated by Edl e equation as a reference, that measured by present method was experimentally evaluated having an accuracy of about 1 � 10 � 6 within a distance of 100 mm. 2002 Elsevier Science B.V. All rights reserved.

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.

Laser interferometer for calibration of a line scale module with analog output

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.

Compact I2-Stabilized Frequency-Doubled Nd:YAG Laser for Long Gauge Block Interferometer

A compact I2-stabilized frequency-doubled Nd:YAG laser has been developed for a long gauge block interferometer. The laser frequency stabilization is realized by the third-harmonic technique, where a fast frequency modulation (20 kHz) is applied to the piezoelectric transducer attached on the laser crystal. The relative uncertainty of the laser frequency has reached 3.6×10-12 for a 0.01-s averaging time. It is theoretically indicated that the phase error in the interferometric measurement due to the optical frequency modulation is sufficiently small to measure the long gauge block up to 1000 mm. Long gauge block measurement up to 1000 mm was successfully demonstrated with the developed I2-stabilized frequency-doubled Nd:YAG laser.

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.

Beamsplitter path equalization for white-light interferometry

In a two-beam, white-light, or polychromatic-Iight interferometer, any path-length difference in the beamsplitter introduces wavelength- dependent path delay because of the glass dispersion. A technique to align balanced path is described. The method is based on two- or three-wavelength simultaneous interferometry. Difference in intensity of fringes placed symmetrically around the zero interference order (ZIO) is used to measure and equalize the beamsplitter path (BP). As a polychromatic source of light. 660-, 840-, and 780-nm wavelength mixed beams of multimode laser diodes are applied. The technique allows us to balance the path difference with an accuracy better than ± 0.2 μm. Theoretical background and experimental verification of the method is presented.

Verification of the Accuracy of a CO2 Laser Interferometer for Distance Measurement

The accuracy of a 10.6 μm CO2 laser interferometer for distance measurements is determined experimentally. The frequency of a commercial wavelength-stabilized CO2 laser is determined by comparison with a reference CO2 laser. A refractometer is used to measure the refractive index of moist air in the 10 μm wavelength region, within an uncertainty of 5 × 10-8. The performance of the CO2 system for length measurement is then verified by comparison with a wavelength-calibrated 0.63 μm He-Ne system, over distances up to 5 m. The result shows that the CO2 laser interferometer can measure distances with an accuracy of better than 5 × 10-8 in atmospheric air.

Touchless interferometric dimension comparator

A touchless interference method that can be used to calibrate gauge blocks or length bars with lengths from 0.01 mm up t o1mi s proposed. The method is based on a comparison of two gauges, with one of them used as a reference. Polychromatic synthesized light from three laser diodes is used to determine end surface positions. Distances referring to these positions are measured by a wavelength-stabilized- laser interferometer. Error sources are analyzed in this method. The ac- curacy of the comparison depends on the accuracy of the interferometer used to measure displacement. In the case of long gauge blocks, accu- racy is limited mainly by uncertainty in the gauge block temperature.