Optical: systems and elements – Optical frequency converter
Patent
1997-02-19
1998-07-14
Lee, John D.
Optical: systems and elements
Optical frequency converter
359115, 359127, 372 23, G02F 202
Patent
active
057813345
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a stabilized multi-frequency light source for generating synthetic light wavelengths, having at least three light sources for emitting coherent light, in accordance with the preamble of claim 1.
The invention also relates to a method for generating synthetic light wavelengths by means of this stabilized multi-frequency light source.
The term "light" is to be understood here as any electromagnetic radiation in the wavelength region of ultraviolet to infrared light which can be diffracted and reflected using optical elements.
A multi-frequency light source of the type mentioned at the beginning is disclosed, for example, in the article "Distance Measurements with Multiple Wavelength Techniques" by R. Dandliker in "High Precision Navigation 91, Proceedings of the 2nd Internat. Workshop on High Precision Linkwitz & U. Hangleiter, Ferd. Dummlers Verlag, Bonn (1992), pages 159 to 170!. A stabilized helium-neon laser and three laser diodes are used therein as light sources. The Fabry-Perot resonator is stabilized owing to the fact that its optically active resonator length is locked by a tracking controller with the frequency of the helium-neon laser. For their part, the frequencies of the light of the three laser diodes are locked with the optically active resonator length of the Fabry-Perot resonator and thereby indirectly with the frequency of the helium-neon laser, and stabilized.
The principle of dual-frequency interferometry with an optical heterodyne method, in which a multi-frequency light source of the type mentioned at the beginning can be used, as well as exemplary embodiments thereof are disclosed, inter alia, in EP-B1-0314709 and U.S. Pat. No. 4,907,886 and are also mentioned in the abovementioned article by R. Dandliker. In dual-frequency interferometry, a beat frequency is formed from the frequencies of the light of two light sources and is equal to the difference between these two frequencies and corresponds to a synthetic light wavelength which, for its part, determines the resolution of the interferometric measurement. Typically, dual-frequency interferometry achieves a measuring accuracy of down to a few ppm for measured distances of up to a dozen meters.
Under these circumstances, it is the relative stability of the beat frequency which limits the relative measuring accuracy. To expand this limit, that is to say to improve the measurement results by increasing the relative measuring accuracy, it is necessary to improve the relative stability of the beat frequency down to one ppm or even fractions thereof. Two light sources have to be stabilized relative to one another in order to achieve this.
There is exactly the same stability problem when a multi-frequency light source with three laser diodes is used in order to generate two beat frequencies which together permit the ambiguity of the measurement with reference to phase to be eliminated.
It is known that electronic means nowadays permit the measurement of frequencies with a measuring accuracy of down to one ppm or even fractions thereof. On the other hand, electronic means do not presently permit frequencies of more than a few GHz to be measured. It is therefore not yet possible to measure optical beat frequencies of more than a few GHz with a measuring accuracy of down to one ppm or even fractions thereof.
Consequently, it is the object of the invention to make available a multi-frequency light source of the type mentioned at the beginning which, in the case of at least one synthetic light wavelength generated therein whose beat frequency is a multiple of the frequency which can be measured with the aid of electronic means, nevertheless permits the beat frequency to be measured with the measuring accuracy which can be achieved by means of electronic frequency measurement.
To achieve this object, a stabilized multi-frequency light source of the type mentioned at the beginning is defined according to the invention by the combination of features defined in claim 1. Preferred embodiments of the stabilized multi-frequenc
REFERENCES:
patent: 4907886 (1990-03-01), Dandliker
patent: 5347525 (1994-09-01), Faris
patent: 5576881 (1996-11-01), Doerr et al.
patent: 5663822 (1997-09-01), Fee
G. P. Barwood et al., "Laser Diodes for Length Determination Using Swept-Frequency Interferometry", Measurement of Science and Technology, vol. 4, No. 9, Sep. (1993), pp. 988-994.
P. de Groot., "Use of a Multimode Short-External-Cavity Laser Diode for Absolute-Distance Interferometry", Applied Optics, vol. 32, No. 22, Aug. (1993), pp. 4193-4198.
R. Daendliker., "Distance Measurements with Multiple Wavelength Techniques", High Precision Navigation 91, Nov. (1991), pp. 159-170.
Daendliker Rene
Thalmann Rudolf
Lee John D.
Leica AG
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