An interferometric gauge for high precision absolute and incremental measurements of distances of up to at least 100 meters. An accuracy of 0.02 Angstrom has been demonstrated, free of the ˜10 Angstrom polarization-leakage errors of conventional gauges. The absolute distance measurement requires no a priori information, and multiple synthetic wavelengths need not be used. The gauge can be used as the sensor for a null servo. There are reduced-cost versions of lower accuracy. The gauge can employ a resonant cavity for greater sensitivity, and can be built with no transmissive elements in the measured path. Without additional hardware, the gauge readout can be in the form of a frequency or an analog electrical signal, in addition to a digital count of an integral number of wavelengths. The gauge employs a high-sensitivity null sensor. Feedback to an optical frequency shifter maintains the null condition; the frequency shift is measured electronically, providing a high sensitivity distance readout. The gauge keeps track of the whole and fractional number of the optical waves travelled. Absolute distance measurement is based on free spectral range determination, employing the high sensitivity and freedom from polarization leakage of this design.

System for measuring distance between two points using a variable frequency coherent source

The combined optical spectrum of a pair of multimode laser diodes is composed of a large number of well-defined wavelengths. This work reports the use of three of these wavelengths in a phase-modulated interferometer to measure absolute distance over 360-μm intervals with a resolution of 0.5 nm. The laboratory demonstration system is composed of a three-wavelength source coupled by single-mode fiber to a compact interferometric probe. This system has been used for displacement measurement and profiling of optical surfaces.

Three-color laser-diode interferometer.

The phase ambiguity in conventional interferometers can be removed by using two laser diodes of different optical frequencies to generate a synthetic wavelength. However, the stability requirements for a two-color interferometric laser gauge that must provide unambiguous determination of the optical fringe order over a large distance can be severe. We derive upper limits on the optical wavelength uncertainty and express them as a function of optical path difference between the object and reference beams, phase measurement errors, and the synthetic wavelength. A simple stabilization arrangement is proposed, involving simultaneous servo control of both lasers with a single Fabry-Perot etalon. The experimental implementation of the proposed system demonstrates its effectiveness for long-term (16-h) stabilized two-color interferometry over a distance of 250 mm, with a 15-mm synthetic wavelength and a repeatability of 40 nm. For periods of < 1000 s, the repeatability was 8 nm.

Synthetic wavelength stabilization for two-color laser-diode interferometry

This paper describes the principle and design of a first prototype of an absolute distance interferometer using a variable synthetic wavelength. The major components of the prototype are two piezoelectrically tunable diode lasers with an external cavity, and a mechanical dimensional standard. Measurements made over distances of several metres in a workshop environment with an averaging time of 5 s gave an uncertainty of . Cet article presente le premier prototype d'interferometre mesurant une distance absolue a partir d'une longueur d'onde variable synthetique. Les composants principaux sont deux diodes lasers piezo-electriques a cavite externe et un etalon de longueur mecanique. Les mesures realisees en laboratoire sur des distances de plusieurs metres pour des temps moyens de 5 s, ont demontre une incertitude de .

Absolute interferometric distance measurements applying a variable synthetic wavelength

A new synthetic interferometric distance-measuring (SIDM) system is described using two-wavelength simultaneous (synthetic) interferometry with a CO2 laser. The measuring procedure of the SIDM system is based on phase measurement such as with conventional light-modulation distance-measuring instruments. The SIDM system is excellent in length resolution since it uses the synthetic wavelengths of 5.5 mm and 300 μm and a regular single wavelength, interferometrically. It is applicable to the measurement of distances up to 100 m with an accuracy of 1 × 10−7. Its performance is confirmed by comparing it with an HP laser interferometer using a 50-m movable carriage in the NRLM 300-m optical tunnel.

Synthetic interferometric distance-measuring system using a CO2 laser.

A basis for making absolute distance measurements to an accuracy of 0.025 pm over 0-1.5 m intervals is reported. Extensions of this technology will permit distances of 50 m and greater to be measured to the same accuracy. Two-color, synthetic Michelson interferometry using a CO2 laser source capa-ble of generating four sets of R- and P-line pairs is employed. This allows reduction of the very large ambiguity exhibited by conventional Michelson interferometers as well as the resolution of difficulties which would otherwise arise due to instabilities in the measurement arm of the interferometer. This latter effect is a practical rather than a fundamental consideration, but is nonetheless important if the interferometer is to emerge from the laboratory as an effective, workable instrument. Distance is determined in terms of a denumerable number of precisely known wavelengths and fractions thereof.

Absolute Distance Interferometry

This paper describes the results of recent research undertaken to examine the feasibility of employing laser interferometry to precisely measure absolute distance over extended ranges. Data are presented that show a resolution of 0.03 um (rms) for measurements over distances up to 10 meters. The technology developed for achieving these results is based on two-color, synthetic Michelson interferometry employing a new CO2 laser source. Indeed, the new laser is the key element in this process: it was specifically designed to sequentially switch between four sets of stable R- and P-line pairs and thereby provide a basis for forming simultaneous equations which were employed to greatly reduce the half-wavelength ambiguity typical of single wavelength interferometers. Potential applications to future optical telescopes--particularly the large, multipanel telescopes under consideration for 10-15 years hence--their initial alignment and control, are suggested.

Progress In Absolute Distance Interferometry

This paper describes the results of recent research undertaken to examine the feasibility of employing laser interferametry to precisely measure absolute distance over extended ranges. Results are presented which show a resolution of 0.03 μm (RMS) for measurements over distances up to 10 meters. The technology developed for achieving these results is based on two-color, synthetic Michelson interferometry employing a new CO2 laser source. Indeed, the new laser is the key element in this process: it was specifically designed to sequentially switch between four sets of stable R- and P-line pairs and thereby provide a basis for forming simultaneous equations which were employed to greatly reduce the half wavelength ambiguity typical of single wavelength interferometers. Potential applications to future optical telescopes - particularly the large, multipanel telescopes under consideration for 10-15 years hence - their initial alignment and control, are suggested.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A basis for making absolute distance measurements to an accuracy of 0.025 μm over 0-1.5 m intervals is reported. Extensions of this technology will permit distances of 50 m and greater to be measured to the same accuracy. Two-color, synthetic Michelson interferometry using a CO2 laser source capable of generating four sets of R- and P-line pairs is employed. This allows reduction of the very large ambiguity exhibited by conventional Michelson interferometers as well as the resolution of difficulties which would otherwise arise due to instabilities in the measurement arm of the interferometer. This latter effect is a practical rather than a fundamental consideration, but is nonetheless important if the interferometer is to emerge from the laboratory as an effective, workable instrument. Distance is determined in terms of a denumerable number of precisely known wavelengths and fractions thereof.

C. W. Gillard

Papers

Absolute Distance Interferometry

Progress In Absolute Distance Interferometry

Progress In Absolute Distance Interferometry

Absolute Distance Interferometry