Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1988-09-29
1991-01-15
Turner, Samuel
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
G01B 902
Patent
active
049848984
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a laser interferometer for interferometric linear measurement with a semiconductor laser whose light, by way of a beam splitter, energizes a measuring interferometer with two reflectors, of which one is connected with the movable object to be covered, and a reference interferometer with a fixed reflector arrangement, which reference interferometer serves to control the laser light wavelength, is spatially adjacent to the measuring interferometer and whose interference ligh beam energizes a reference signal detector which is connected to a control circuit which readjusts the frequency of the semiconductor laser and keeps the output signal of the reference signal detector constant, while the interference light beam of the measuring interferometer containing the information regarding the length to be determined falls on an interference signal detector whose output signal energizes an evaluating device for determining the distance changes of the movable object.
Laser interferometers are used in the linear metrology to determine geometric quantities, such as lengths, distances, speeds etc. They are used especially in machine tools and coordinate measuring equipment. To obtain high measuring accuracies it is necessary to allow for the influence of the refraction index of the ambient air on the laser beam that is used as a wavelength normal. Such a refraction index caused by changes of the air temperature, air pressure and relative humidity is frequently allowed for by the so-called parameter method, where individual measurements of the air temperature, air pressure and relative humidity are used to determine the refraction index in order to correct the linear measurement with the aid of the result thus obtained. This leads not only to time delays until the correct measuring result is available, but also to problems when the effects of the refraction index are subject to quick changes, since allowing for them is then not possible.
A laser interferometer for linear measurement with a measuring interferometer and a reference interferometer serving to regulate the laser light wavelength are known from the German patent document No. 34 04 963 A1. The reference interferometer serves the stabilization of the frequency or wavelength of the semiconductor laser used and is fixed, along with the optical components of the measuring interferometer, on a base so as to obtain a compact structure that requires little space. The arm of the reference interferometer serving the stabilization of the laser light wavelength extends parallel with the base, which for compactness is small, and transverse to the direction of propagation of the light in the measuring arm of the measuring interferometer. Since the light in the reference interferometer traverses on its way to the reference section reflector, which is offset sideways relative to the measuring arm, a short air space there occurs in the laser light wavelength stabilization a compensation according to the refraction index effected by the temperature, pressure and relative humidity of the air above the base.
The problem underlying the invention is to provide a laser interferometer of the initially cited type which is distinguished by a high measuring accuracy also when the measuring arm of the laser interferometer is disturbed by local turbulences and inhomogeneities within the measuring section.
This problem is inventionally solved in that the reference section of the reference interferometer extends at a lateral distance, small in relation to the length of the reference section, across a distance which approximates the length of the measuring section, parallel with the measuring section of the measuring interferometer, and is delimited on the end of the reference section away from the semiconductor laser by a reference section reflector whose position, fixed during the measurement, is adjustable in the longitudinal direction of the measuring and reference sections.
With the aid of the reference interferometer extending along the measuring interferometer
REFERENCES:
patent: 3588254 (1971-06-01), Rhoades
patent: 4583228 (1986-04-01), Brown
patent: 4729653 (1988-03-01), Kobayashi
Wellenlangenstabilisation, 449 Feinwerktechnik & Messtechnik, vol. 87, (1) Nov./Dec. No. 8.
"Laser Interferometer for high Temperature isothermal length changes over long time periods", 8127 Review of Scientific Instruments, vol. 52 (19810, Dec., No. 12, New York, U.S.A.
Bergmann Eckhard
Hofler Heinrich
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
Turner Samuel
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