Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1987-11-12
1989-03-21
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
356367, 356351, 374161, H01J 516, G01J 400
Patent
active
048146048
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an opto-electronic method and system for remotely detecting physical magnitudes by means of sensors each of which includes a sensitive element whose spectral transmission varies as a function of the physical magnitude to be measured, said sensors being illuminated by an incoherent light beam and transmitting light signals to spectrum analyzer means.
Since optical fibers have become available, it has become common to use changes in spectral transmission in a medium for the purpose of remotely detecting variation in a physical magnitude such as temperature, chemical composition, magnetic field, etc. . . .
In particular, there exist sensors based on thermochromic solutions in which temperature changes give rise to large changes in the absorption spectrum of the sensitive medium. An example of an embodiment of such a sensor is described in the article by M. Breci et. al. entitled "Thermochromic Transducer Optical Fiber Sensor", 2nd International Conference on Optical Fiber Sensors Sept. 5-7 1984.
Sensors also exist based on a semiconductor etalon in which temperature variations in the sensitive medium cause the edge of the absorption band to move. A sensor of this type is described in the article by K. Kyuma, S. Tai, T. Sawada and M. Nunoshita, entitled "Fiber Optic Instrument for Temperature Measurement", IEEE, QE-18, 676, 1982.
There also exist colorimetric type sensors in which variations in the composition of a binary mixture give rise to variations in its tristimulus values. A sensor of this type is described in French Pat. No. 85 08 550 dated June 6, 1985.
Present methods do not make it possible to multiplex information from several identical sensors of the above-mentioned type simply on a single optical fiber in which the spectrum carriers used would be all mixed together.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a simple solution to this problem to enable spectral information using a common light source and thus a common carrier to be multiplexed on a common optical fiber.
To this end, the present invention provides an optoelectronic method of remotely detecting physical magnitudes by means of sensors each including a sensitive element whose spectral transmission varies as a function of a physical magnitude to be measured, the sensors being illuminated by an incoherent light beam, and transmitting light signals to spectrum analyzer means, the method being characterized in encoding the signals from each sensor by superposing periodic or quasi-periodic modulation of its spectrum at a frequency characteristic of the sensor under consideration onto the light transmitted by the sensitive element of each sensor, in transmitting the signals modulated in this way via an optical fiber to the said spectrum analyzer means, in decoding or demultiplexing the signals by performing the Fourier transform optically in order to isolate the signal relating to each sensor, and then in processing this signal in order to obtain the value of the corresponding physical magnitude.
Advantageously, the spectrum modulation encoding is performed in each sensor by means of a two-wave interferometer comprising, for example, a birefringent plate of determined constitution which is associated with the sensitive element of the sensor, with demultiplexing being provided by means of a two-wave interferometer comprising, for example, a birefringent plate identical to the plate of the sensor.
Thus, signals coming from different sensors are multiplexed and demultiplexed in an extremely simple manner by optical means. This method also has the advantage of being capable of being implemented in a completely static manner for each sensor.
According to another characteristic of the invention, the signal resulting from the Fourier transform is recorded for each sensor, and a mathematical operation of the inverse Fourier transform type is performed thereon by digital means.
It is thus possible to have access to the information which is representative of sensor
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patent: 4111050 (1978-09-01), Waddoups
patent: 4140393 (1979-02-01), Cetas
patent: 4493995 (1985-01-01), Adolfsson et al.
patent: 4536088 (1985-08-01), Rashleigh et al.
patent: 4629323 (1986-12-01), Matsumoto
patent: 4632551 (1986-12-01), Pavlath
patent: 4699513 (1987-10-01), Brooks et al.
"Optics Letters", vol. 11, No. 8, Aug. 1986, pp. 543-545.
Bertin & Cie, B.P.
Messinger Michael
Nelms David C.
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