Interferometer gas flow system

Details Interferometer gas flow system Interferometer gas flow system

1991-01-16

1992-09-08

Turner, Samuel A.

Optics: measuring and testing

By particle light scattering

With photocell detection

356358, 356361, G01B 902

Patent

active

051462842

DESCRIPTION:

BRIEF SUMMARY
An interferometer for measuring displacements of a movable structural component, comprising a light source for emission of a light beam, providing a beam splitting device for separation of the light into two partial beams, of which at least one portion of one of the partial beams traverses a reference path, and at least one part of the other partial beam traverses a measurement path of variable optical length, whereby it is guided by an adjustable reflector system attached to a movable structural component, and with a photoelectric detector device that analyses the intensity of the interfering partial beams which interfere after respective traversal of the reference and measurement path, whereby the optical components are arranged together with measurement and reference paths in a sealed housing which also contains a wavelength measuring device for measuring the wavelength of the partial beam which travels through the measurement path in a gaseous medium.
With such interferomic measurement, the length of the determined displacement path is in units of the wavelength existing in the gaseous medium of the employed light beam, whereby this wavelength depends upon the instantaneous refractive index in the medium (and naturally upon the frequency of the light beam.) The precise knowledge of the "instantaneous" refractive index in the medium, especially the measurement path, as well as of the frequency of the light beam or of the wavelength existing in the medium, is a prerequisite for the coveted precise knowledge of a moveable component's path in metric units.
With such an interferometer, with low relative uncertainty in measurement, it is therefore necessary in practice to determine the particular light wavelength in the medium using an apparatus for evaluating the specific wavelength. In principle, when knowing the frequency, procedures for determining the refraction index may be used, for example the "parameter method" (values for pressure, temperature and other parameters and formula-related calculation of the refraction indices) or the "refractometer method" (determination of the refraction index through comparison of the optical length of path of equal geometric distances in air in one case and in vacuum in the other case). If the frequency of the light is not exactly known, as the case may be with laser diodes, the possibility arises for interferometric comparison to a mechanical embodiment of a measure of specific length, whereby the wavelength of a lightbeam in medium can be directly determined (without separately determining the values of refraction index n and frequency f).
It is already known in production of interferometers, to arrange all the optical components in a sealed housing including the measuring and reference paths. Furthermore, one provides for arrangement of a wavelength determining device or the sensors(measuring-section)thereof within the housing. In such housing, a certain protection effect against impurities (dust, oil vapor, etc.) is initially created. These impurities can alter the refraction index and thus the wavelength. Furthermore, the housing avoids sudden sporadic changes in the medium condition, especially the temperature, thereby suppressing streaks in the measuring path that can be present in "open" measuring paths, and with which the real time measurement of the instantaneous wavelength of traversing partial-light beams in the measuring path, is controllable.
By the known methods there is, however, the possibility that the avoidance of variable refraction index is not achieved. On the contrary, fluctuation in pressure and thus fluctuation in the refractive index (which is dependant thereon) is intentionally generated in order to move the measuring pin. Also with the inward movement of the measuring pin, local pressure gradients (differential pressures) are created, that reduce the entire accuracy of measurement despite the conducted measuring of the refraction index. By these and other known Interferometer arrangements with wavelength or refraction index measuring devic

REFERENCES:
patent: 4643577 (1987-02-01), Roth et al.
patent: 4765741 (1988-08-01), Detro et al.
patent: 4814625 (1989-03-01), Yabu

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