Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1989-02-06
1990-03-13
Willis, Davis L.
Optics: measuring and testing
By particle light scattering
With photocell detection
356358, G01B 902
Patent
active
049078860
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for measuring changes to a variable interference section of an interferometer using two-wavelength interferometry and optical heterodyne processes according to the preamble of claim 1, as well as the use of the method and an apparatus for performing the same.
For the optical measurement of distances of more than a few kilometers use is conventionally made of light pulses, for distances between a few kilometers and a few meters measurement generally takes place by phase measurement on sinusoidally modulated light and for distances below a few meters a coherent light-operated interferometer is normally used. The optical measurement of distances with light pulses or sinusoidally modulated light permits a measuring accuracy of approximately 1 mm for a measuring duration of a few seconds. In the case of interferometric measurement, the measuring section during the measuring time of 10 to 100 ms must be precisely stable within a wavelength of the light, i.e. to approximately 0.1 .mu.m. Under this condition heterodyne interferometry known e.g. from No. EP-0194941 permits the attainment of a measuring accuracy of approximately 10.sup.-3 wavelength, i.e. approximately 1 nm.
For measuring distances up to a few hundred meters with an accuracy and sensitivity of fractions of a millimeter and a measuring duration of 10 to 100 ms, it is not possible to use any of the aforementioned methods. Fundamentally the problem of optical range finding with a reduced precision compared with conventional interferometry can be solved by so-called two-wavelength interferometry where, unlike in the case of conventional interferometry, resolution does not take place by the wavelength of the light, but through the equivalent wavelength corresponding to the difference of the light frequencies of the two wavelengths. In the known constructions, such as the apparatus described e.g. by R. Dandliker in "Optoelektronik in der Technik, Vortrage des 5. Internationalen Kongresses Laser 1981" (Springer-Verlag, 1982, pp. 52-53), the measuring section during the measuring time must, as for conventional interferometry, be stable within a wavelength of the light, which is not ensured in most practical cases.
The problem of the invention is therefore to achieve a measuring precision and sensitivity of fractions of a millimeter with a two-wavelength interferometer, without the measuring section having to be more stable during the measuring time that the desired measuring precision. In addition, the measurement must be rapidly performable, i.e. requires a measuring time of 10 to 100 ms.
This problem is solved by the inventive method characterized by the combination of method steps given in claim 1. A use of this method and an apparatus for performing it, advantageous further developments of the method and its use, as well as advantageous constructions of the apparatus can be gathered from the dependent claims.
In the inventive method use is made of two laser beams of frequencies n.sub.1 and n.sub.2, which are in turn split into two partial beams of frequencies n.sub.1, n.sub.2, n.sub.1 +f.sub.1 and n.sub.2 +f.sub.2. Beat f.sub.1 -f.sub.2 is formed and its phase evaluated, in that the difference of the phase of said beat is determined before and after the passage through the Michelson interferometer. Changes to the path length in the Michelson interferometer leads to a change of said phase difference. Unlike in the known methods, in the method according to the invention this change to the phase difference is not directly dependent on the frequencies n.sub.1, n.sub.2, but is only dependent on their difference n.sub.1 -n.sub.2. Therefore during the measuring period the measuring section only has to be stable with respect to the equivalent wavelength of the differential frequency n.sub.1 -n.sub.2 and not with respect to the much smaller wavelengths of the optical frequencies n.sub.1 and n.sub.2. In addition, both with a pair of laser diodes and with a laser emitting on two frequencies it is easier, or can be achieved with a
REFERENCES:
patent: 3458259 (1969-07-01), Bagley et al.
patent: 4295741 (1981-10-01), Palma et al.
patent: 4688940 (1987-08-01), Sommargren et al.
Koren Matthew W.
Wild Heerburgg, AG
Willis Davis L.
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