Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1993-04-16
1994-11-08
Turner, Samuel A.
Optics: measuring and testing
By particle light scattering
With photocell detection
385 12, G01B 902
Patent
active
053631919
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a fibre optic sensor reading device.
In order to decode the data coming from a network of coherently multiplexed sensors it is necessary to compensate the phase shifts with the aid of an interferometric reading device. Fibre optic interferometers of Mach-Zehnder type or interferometers of Michelson type are used depending on the architecture of the sensor network. In the particular case of weakly coherent multiplexing, the use of non birefringent optical fibres is tricky given the accuracy required for the adjusting of the length of the fibre segments used in passive interferometers of Mach-Zehnder type. In the general case, the Michelson interferometer is widely used in the laboratory given its great versatility and ease of use. However, use outside the laboratory, for example at an industrial site, is very tricky because, in particular, of the fragility of the movable mirrors of this interferometer.
The subject of the invention is a fibre optic sensor reading device which is of small size, easy to regulate and to use, which can be employed in a hostile environment (industrial surroundings, on-board hardware etc.), which offers a sensitivity at least equal to that of known interferometers whilst retaining a good dynamic range, and which has the shortest possible response time.
The fibre optic sensor reading device according to the invention comprises an integrated optics interferometer with two arms of different lengths, followed by a detector and by a circuit for slaving the interferometer, the measuring device receiving the negative-feedback signal of the slaving circuit as the signal to be measured.
According to another characteristic of the invention, at least one of the arms of the interferometer comprises a phase modulator connected to the slaving circuit. Thus, according to the invention, the interferometer with two arms of different lengths permits compensation for the average delay between the two wave trains produced from the optical wave of a weakly coherent source, this delay being a function of the magnitude to be measured, the measurement consisting in determining a magnitude necessary for the slaving of the interferometer. Thus, the measurement is carried out without mechanical displacement, resulting in a better sensitivity of the reader, whilst retaining a good dynamic range, this reader being of short size and offering a short response time.
The present invention will be better understood on reading the detailed description of several embodiments, taken by way of non-limiting examples and illustrated by the attached drawing in which:
FIG. 1 is a graph of a signal collected by a detector associated with a coherent sensor device and with a reader of the prior art.
FIG. 2 is a graph of a signal collected by a detector associated with a coherent sensor device and with a reader according to the invention,
FIG. 3 is a graph explaining the principle of operation for reading signals with large delay, with the aid of a reader according to the invention,
FIG. 4 is a block diagram of a reading device according to the invention,
FIG. 5 is a block diagram of a variant of a part of the reading device of FIG. 4.
FIG. 6 is the simplified block diagram of a reading device according to the invention for the reading of multiplexed signals,
FIGS. 8 and 10 are diagrams of interferometers used by the invention and,
FIGS. 7, 9 and 11 are graphs of wave shapes used in the slaving circuit of the invention.
The principle of the measuring of physical magnitudes with the aid of fibre optic sensors operating coherently will firstly be described. Operating coherently means that the datum relating to the physical magnitude to be measured is contained in the delay (that is to say the phase shift) between two optical wave trains generated from a single wave produced by a weakly coherent source (coherence wavelength less than a few hundreds of microns).
A few orders of magnitude permit the advantage in coherence methods to be specified in relation to the other techniques (modulation of th
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patent: 4759627 (1988-07-01), Thylen et al.
patent: 4799797 (1989-01-01), Huggins
patent: 4889986 (1989-12-01), Kersey et al.
patent: 4989979 (1991-02-01), Buckman
Patent Abstracts of Japan, vol. 10, No. 15 (P-422)(2072) Jan. 21, 1986 Japanese Appln. 60-170723 (Yokokawa Hokushin Denki KK) Sep. 4, 1985.
Refregier Philippe
Turpin Marc
"Thomson-CSF"
Turner Samuel A.
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