Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1979-09-13
1982-11-23
Corbin, John K.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356358, G01B 902
Patent
active
043602711
DESCRIPTION:
BRIEF SUMMARY
This invention relates to displacement-measuring interferometers, more especially to Michelson interferometers arranged to provide an output suitable for use with an automatic reversible fringe counting system.
Use of a reversible fringe counting system allows automatic correction for any vibration or retraced motion. Conventional systems require two electrical input signals which vary sinusoidally with path difference in the interferometer, which are in phase quadrature, and which ideally have fixed amplitudes.
Usually the signals are provided by photodetectors each arranged to sense an interferogram, that is, a part of an interference fringe pattern which changes as the optical path difference in the interferometer changes.
In many prior art arrangements, interferograms are generated which each comprise a regularly alternating component, which will be referred to as an a.c. component and which is dependent on path difference between its two constituent light beams, and a component which will be referred to as a d.c. component which does not alternate regularly and does not depend on the path difference.
The magnitude of the d.c. component depends on variations in the alignment and relative sizes of the interfering beams, attenuation in one or both interferometer arms, losses due to limiting apertures, and on intensity fluctuations of the light source, so that this component cannot be removed by simple subtraction. If the d.c. component increases to a certain level, operation of the fringe counting system may not be possible.
In U.S. Pat. No. 3,771,875, Russo, an arrangement is disclosed in which three interferogram signals are provided which can be combined by sum and difference to give two purely a.c. components with no d.c. contribution. However, in the Michelson interferometer, in the measuring arm the radiation is at one direction of polarisation, and in the reference arm it is at the orthogonal direction of polarisation. The disadvantage of this is that the interferometer beam splitter must be constructed so that an area of its surface is polarising and the remaining area is non-polarising; further the non-polarising area must have values of reflectance and transmittance which are the same for each state of polarisation; this is a difficult condition to achieve.
It is an object of the invention to provide an interferometer system capable of supplying two signals which vary sinusoidally with fixed amplitude as a function of path difference, which are in phase quadrature, in which any signal level changes not related to path difference are substantially eliminated, and in which the interferometer beam splitter is relatively easy to make.
According to the invention, an improved interferometer system comprises: two reflecting means arranged to receive radiation from opposite sides of the beam splitter and to return radiation to those sides whereby two exit beams of radiation are provided, one from each side of the beam splitter; to separate the received radiation into radiation at two orthogonal directions of polarisation whereby two optical interferograms are provided; radiation at one of said orthogonal directions of polarisation whereby a third optical interferogram is provided; the second interferograms, and being in antiphase with the other of the first and the second interferograms; each orthogonal direction of polarisation reflection coefficients on its two faces which are equal, and has for radiation at one orthogonal direction of polarisation a transmission coefficient which is equal to its reflection coefficient.
The effect of this equality of the coefficients is that, as will be explained in detail below, three interferograms can be provided having the same ratio of the a.c. to the d.c. component.
It has been stated that the three interferograms must be in a phase quadrature or antiphase relationship with one another. While the system operates most effectively, so far as the signal-to-noise ratio is concerned, when the interferograms differ in phase by, ideally, 90.degree. and 180.degree., th
REFERENCES:
patent: 3529894 (1970-09-01), Hock
patent: 3601490 (1971-08-01), Erickson
patent: 3771875 (1973-11-01), Russo
patent: 3822942 (1974-07-01), Hock
patent: 3881823 (1975-05-01), De Lang
patent: 3976379 (1976-08-01), Morokuma
Harrison, "Coaxial Laser-Interferometer," IBM Tech. Discl. Bull., vol. 14, No. 7, pp. 1949-1950, 12/71.
Vyskub et al., "Beam Splitter With 90.degree. Phase Shift for Interference Displacement Measuring Instruments" Instruments & Eaptl., Technique (SU), vol. 20, No. 4/2, pp. 1151-1153, 8/77.
Downs Michael J.
Raine Kenneth W.
Corbin John K.
Koren Matthew W.
National Research Development Corporation
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