Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Patent
1987-06-11
1989-04-25
Willis, Davis L.
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
G01B 902
Patent
active
048252620
DESCRIPTION:
BRIEF SUMMARY
The invention relates to Fabry-Perot interferometers.
A typical Fabry-Perot interferometer comprises a pair of substantially parallel reflective surfaces which are spaced apart to define a gap, at least one of the surfaces being movable relatively to the other to vary the size of the gap. In use, radiation comprising a number of different wavelengths impinges on the interferometer and passes into the gap and is then reflected between the two reflective surfaces. Constructive and destructive interference takes place leading to certain well defined wavelengths being transmitted through the interferometer while the remaining wavelengths are not transmitted. In typical Fabry-Perot interferometers a series of well defined transmission peaks are obtained corresponding to wavelengths which are transmitted, the wavelengths at which the peaks are situated being adjustable by varying the width of the gap.
Fabry-Perot interferometers have been used to a large extent to define laser cavities but also find widespread use as multiple wavelength filters.
It is important that the reflective surfaces of the interferometer are as parallel as possible and it is also desirable to be able to change the separation between the reflective surfaces over a wide range.
The most common form of Fabry-Perot interferometer currently in use comprises two glass flats securely mounted on a stable support with facing surfaces of the flats being highly polished and having suitable coatings to define the reflective surfaces. The size of the gap may vary between one millimeter and several centimeters and is varied by using microadjusters and/or piezoelectric translation elements. This is a cumbersome and expensive arrangement and has a relatively large overall size, typically in the order of inches.
Another form of Fabry-Perot interferometer comprises a single solid glass flat, the opposite faces of which are polished and suitably coated to define the reflective surfaces. The only practical way in which the spacing or gap between the surfaces can be changed is by heating the flat to cause thermal expansion. This construction suffers from the disadvantage that the variation in separation obtainable is small and the disadvantage that it is very difficult to obtain accurately parallel surfaces.
In accordance with the present invention, in a Fabry-Perot interferometer one of the reflective surfaces is provided on a diaphragm mounted by a hinge assembly to a support.
This invention improves upon the known interferometers by making use of a diaphragm to provide one of the reflective surfaces and mounting the diaphragm by a hinge assembly to a support so that the position of the diaphragm can be easily changed. This enables the size of the gap to be easily and rapidly changed. For example, the interferometer can be used to demultiplex an incoming wavelength division multiplexed signal in which a number of different channels are carried by different wavelength signals. In this application, it is often necessary to retune rapidly from one channel to another and this can easily be achieved using an interferometer according to the invention.
Preferably, the diaphragm and the hinge assembly are integral with the support. This leads to a compact and secure construction which is much cheaper to manufacture than known devices and involves far fewer components.
Conveniently, a single crystal such as silicon is used for the support, diaphragm and hinge assembly. This is particularly advantageous since conventional micromachining techniques such as anisotropic etching can be used to form the hinge assembly and diaphragm. Such techniques include masking and etching and laser etching. (It is also believed that these techniques will enable the orientation of the reflective surfaces to be accurately controlled thus making it easier to arrange the one reflective surface parallel with the other.)
The interferometer may further comprise control means responsive to control signals to cause the diaphragm to move relatively to the support towards and away from the other
REFERENCES:
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patent: 3387531 (1968-06-01), Hesse
patent: 3635562 (1972-01-01), Catherin
patent: 4668093 (1987-05-01), Cahill
Patents Abstracts of Japan, vol. 4, No. 146 (P-31) (628), 15th Oct. 1980, & JP, A. 5596903 (Mitsubishi Denki K.K.), 23 Jul. 1980.
Optical Engineering, vol. 20, No. 6, Nov./Dec. 1981 (Society of Photo-Optical Instrumentation Engineers, Bellingham, Washington, U.S.), P. D. Atherton et al.: "Tunable Fabry-Perot filters", pp. 806-814.
Optical Engineering, vol. 15, No. 1, Jan./Feb. 1976 (Society of Photo-Optical Instrumentation Engineers, Palos Verdes, U.S.), E. G. Lierke: "Infrared Modulator for Optical Sensors Based on a Vibrating Fabry-Perot Interferometer", pp. 35-38.
IEE Journal of Quantum Electronics, vol. QE-17, No. 12, Dec. 1981 (IEEE, New York, U.S.), H. J. Eichler et al: "Optical Tuning of a Silicon Fabry-Perot Interferometer by a Pulsed 1.06um Laser", pp. 2351-2355.
British Telecommunications public limited company
Koren Matthew W.
Willis Davis L.
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