It is easy to measure length accurately by means of optical interferometry. However, the measurement range of a single-frequency interferometer is limited to less than half the wavelength without a moving carriage to count interference fringes. To resolve this problem, a frequency scanning method had been developed. However, its phase resolution is not so high that the integer part of the order of the interference fringe can be accurately determined. We proposed a method to measure any length absolutely and accurately, by combining a high-resolution phase measurement technique with a frequency scanning technique. In this paper, this method was investigated by using a frequency scanning heterodyne interferometer. With heterodyne phase measurement, we achieved high resolution, higher than quarter wavelength, using the wide-range frequency scanning method. This means that we can measure the absolute length with nanometre accuracy, since the integer part of the order of the interference fringe for a wavelength is determined with the frequency scanning. We measured distances up to about 4 mm with an accuracy of about 3 nm.
We have established a compact and transportable I 2 -stabilized Nd:YAG laser for international comparisons of laser frequency. The root Allan variance of the portable laser has reached 3 9 10 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 ( NRLM-Y1JILA-W ) was 2 5 kHz, when the cold-finger temperatures of NRLM-Y1 and JILA-W were kept at 10 C and 15 C, respectively. The averaged frequency offset between two NRLM lasers ( NRLM-Y1 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.Index Terms-International comparison, laser frequency stabilization, molecular iodine, Nd:YAG laser, optical frequency standard.
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 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.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.