Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1991-01-30
1992-12-15
Turner, Samuel A.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
356345, 356361, G01B 902
Patent
active
051721856
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a device for finding and stabilizing the wavelengths of coherent light from a laser light source found in a preferably gaseous ambient medium, with at least one photo-detector unit to detect interference generated by light from the laser light source in the ambient medium.
In interferometric length-measuring technology, the wavelength in the ambient medium (most often air) is the scale for the measurement. The wavelength (air wavelength) changes in free atmosphere in correlation with the refraction index of the air, which changes with the temperature, pressure and moisture, for example. Interferometers which use gas lasers (He-Ne) are frequency stabilized, since the discharge line itself is used as a frequency reference. Here, the influence of the refraction index of the air is often measured parametrically (temperature, pressure, moisture) and calculated by the Edlen formula, and the interferometric measurement value is then corrected by computer. This method requires a normal air mixture, which no longer corresponds to current circumstances in industrial and leads to errors.
It is therefore more advantageous to detect the air wavelength or the interference connected with it directly (and not the frequency and refraction index separately). Wavelength-stabilized He-Ne gas lasers are now known (Swiss Patent 461 114 and Swiss Patent 503 285) that have, however, a small adjustment range and are very expensive. (Feinwerktechnik und MeBtechnik 87, [1979], 8, pp. 368-372).
Besides laser-diode interferometers that do not detect environmental conditions whose sources of radiation are stabilized as to temperature and current, but whose frequency changes because of aging phenomena and hysterasis effects, (EP Application 135 000), there are also known laser-diode interferometers in waveguide technology in which interference generated in the ambient medium (air) is used to detect the refraction index of the air (computer correction) (De-OS 37 15 627). Apart from problems in evaluating the interference signals, a mirror, which has to be adjusted precisely, is provided there for coupling the light from the inflated reference interferometer into a light waveguide leading to the photo detectors.
It is the object of the invention to provide a device as mentioned at the outset for determining or detecting an air wavelength which has a simple, compact optical construction and still permits precise determination or stabilization of the air wavelength.
According to the invention this is accomplished by the fact that light coming from the laser light source emanates from two spaced locations, which basically constitute point laser light sources, into the ambient medium and that the light waves emitted by the basically point light sources interfere in the ambient medium forming spatial interference detected by one or more photo-detector units.
The light waves emitted by the spaced, basically point (i.e., expansion preferably smaller than 30 .mu.m, better still, smaller than 10 .mu.m) light sources overlap each other and interfere (somewhat like Young interference).
This interference phenomenon is first static, and the interference basically depend only on the geometry of the overlap of the two (or more) waves. But if the refraction index of the ambient medium changes, the interference also changes if the frequency of the laser light source is constant. The best way to detect the interference is to have two photo detectors arranged so that the detected radiation intensities change in the opposite direction of the interference moves. Here it is important that the distance between the detectors and the sources remains constant and stable. This can be achieved by a simple spacer ring, which is made of a material that is substantially uninfluenced by the parameters (especially the temperature) that influence the refraction index of the ambient medium. For this, quartz glass of glass ceramic with a thermal coefficient of expansion of .alpha.=10.sup.-8 to 10.sup.-8 K.sup.-1 is used accordingly. The precision of t
REFERENCES:
patent: 4668093 (1987-05-01), Cahill
patent: 4722603 (1988-02-01), Graebner et al.
patent: 4747688 (1988-05-01), Geary
Kerner Martin
Leuchs Gerhard
Tabarelli Warner
Turner Samuel A.
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