Sensor system for measurement of temperature or strain

Details Sensor system for measurement of temperature or strain Sensor system for measurement of temperature or strain

1996-01-25

1998-02-24

Turner, Samuel A.

Optics: measuring and testing

By particle light scattering

With photocell detection

G01B 902

Patent

active

057216150

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a sensor system for making measurements of strain and/or temperature.
2. Discussion of Prior Art
Optical fibre sensors are known in the prior art. They employ optical fibres to guide light which becomes modulated in response to external influences such as changes in temperature or strain in an environment in which the fibre is situated. Optical fibres are used as arms of interferometers such as Mach-Zehnder interferometers. In such interferometers a measurand, such as strain or temperature experienced by a fibre arm, causes changes in optical path length or polarization properties of the fibre. This in turn causes changes in an observed fringe system from the interferometer. By analysing the fringe changes, information about the measurands causing the changes can be inferred. The visibility of the interference fringes is determined partly by the coherence of the light through the interferometer system. Because of this monomode optical fibres, which maintain the spatial coherence of a guided light beam, are preferred for use in interferometers as multimode optical fibres do not maintain the spatial coherence of a guided beam.
Interferometric devices are known in which light from a source is divided to form two light beams which are input to two monomode optical fibres. One optical fibre forms a sensor which experiences the measurand and the other optical fibre forms a reference. The reference is situated in a stable environment. The two light beams pass through the respective fibres and are recombined on emergence to form a fringe system. The phase difference between the optical paths travelled by the light beams through the sensor and reference fibres can be inferred from observed output fringe intensities. The phase sensitivity of interferometers to physical influences such as temperature and strain is particularly high, which makes interferometer systems useful for sensing applications.
It is a disadvantage of optical fibre sensors that they are sensitive to all physical influences on the fibre. The sensor output therefore corresponds to a convolution of a number of physical effects. Consequently, when measuring a single parameter such as strain on the sensor, the output from an interferometer also includes the effects of other environmental factors such as temperature. Because of this cross-sensitivity between parameters it is necessary to isolate the parameter being measured. Measurements of strain are therefore made in a temperature-controlled environment, end temperature measurements in a constant-strain environment.
F Farahi et al ("Simultaneous Measurement of Temperature and Strain: Cross-Sensitivity Considerations", Journal of Lightware Technology 8(2): 138-142, February 1990) considered discrimination between strain and temperature by measuring phases at the combined output of a polarimetric and interferometric fibre device for two polarisation modes of an optical fibre sensor. However, discrimination between strain and temperature was found to be poor because the ratio of strain-dependence to temperature-dependence of optical effects on light passing through an optical fibre is similar for each polarization mode.
Discrimination between temperature and strain measurements has been achieved by A M Vengsarkar et al. ("Fibre Optic Sensor for Simultaneous measurement of Strain and Temperature", Vol. 1367 of Proc. SPIE "Fibre Optics and Laser Sensors VIII (1990)", pages 249-260). The system used is a dual technique involving the use of a dual-mode elliptical core optical fibre and light beams of two different wavelengths. One of these light wavelengths .lambda..sub.1 is below the single-mode cut-off wavelength .lambda..sub.c of the fibre and the other light wavelength .lambda..sub.2 is above it. A polarimetric technique is used for sensing with light of wavelength .lambda..sub.1 and a two-mode technique used for light of wavelength .lambda..sub.2. The two-mode technique relies on interference between the modes. The effect

REFERENCES:
patent: 4755668 (1988-07-01), Davis
patent: 4859844 (1989-08-01), Herman et al.
Simultaneous Thickness and Group Index Measurements Using Optical Low-Coherence Reflectometry, Sorin et al, IEEE Photonics Tech. Letters, 1-1992, pp. 105-107.

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