Sensor system

Details Sensor system Sensor system

1998-09-16

2000-10-24

Kim, Robert H.

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrometer

356345, G10D 902

Patent

active

06137573&

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a sensor system for measuring strain and temperature.
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 strain and temperature acting on the fibres. Information contained in these modulations may be extracted by interferometric means. This is particularly attractive for accurate applications as the phase sensitivity of optical fibres to physical influences such as strain and temperature is high.
The fundamental problem of strain and temperature measurement by optical means is one of deconvolving the contributions from each parameter. Moreover, a further problem may arise in monochromatic interferometry because phase differences of multiplies of 2.pi. are lost. This latter situation is known as phase ambiguity.
Light propagating along a fibre will traverse it in a time .DELTA..tau. which is dependent on the optical path length (OPL) of the fibre. The OPL is defined as the physical length of the fibre multiplied by the refractive index at the wavelength of light propagating therein. Thus physical influences acting to change the OPL of a fibre may affect one or both of the fibre length or refractive index.
Strain in an optical fibre may arise from, among other factors, stress (elasticity) or an electric field (piezoelectricity). In any case the external influence causes displacement of points in a body with magnitude and orientation dependent on position within the body. In general, the strain induced is determined by the magnitude and orientation of the influence as well as by the physical properties and symmetry of the material. The physical distortion within a strained material alters both its physical and electronic structure and hence affects its optical properties. In particular a longitudinal strain component, defined to be parallel to the length of an optical fibre (and hence propagation direction of light therein), will lengthen the fibre inducing a change in OPL via physical length. Transverse components however can only affect the OPL through a change in electronic structure and hence birefringence of the material.
The refractive index of an unconstrained, bare fibre is temperature dependent and so heating such a fibre will result in a change in OPL. The thermal expansion of optical fibre material (typically fused silica) is low and any consequent change in OPL through a change in fibre length will be negligible. The cross-sensitivity of temperature and strain both affecting the OPL in a bare-fibre case is of the order 10 microstrain per Kelvin. An optical fibre embedded within a host material will however be affected by the thermal expansion of the host material. Differing thermal expansivities of host and embedded fibre will result in the host extending a stress on the fibre. Such a fibre used as a sensor will therefore indicate a fictitious strain, due to temperature change, in the absence of an applied stress. This indicated strain is known as the thermal-apparent strain of the sensor.
Despite allowance made for host thermal expansion, a sensor constructed from an embedded fibre will still exhibit temperature cross-sensitivity which restricts its use to environments with known temperatures or to situations of time-varying stresses.
Temperature-independent strain measurement has important applications in the construction and engineering industries. Civil engineering requires strains within buildings to be monitored over time and higher accuracy sensors are required for the high tech construction industry e.g. aeroplanes, helicopters and space equipment. For example, it is important in the aircraft industry to determine the operating strains experienced by panels of aircraft fuselage. One fibre arm of an interferometer embedded in a panel enables strains in that panel to be monitored over its life. Furthermore a sensor arm subdivided into a series of sensor elements by a series of partially-reflective mirrors enables the strain experienced by each sensor element t

REFERENCES:
Optics Letters, vol. 19, No. 24, Dec. 1994, Washington US, pp. 2164-2166, XP000485818 V.Gusmeroli et al.: "Nonincremental interferometric fiber-optic measurement method for simultaneous detection of temperature and strain" see the whole document.
Proceedings of the SPIE, vol. 2861, Aug. 8-9, 1996, Denver, Colorado, pp. 26-31, XP000676843 D.G. Luke et al.: "Composite-embedded highly-birefringent optical fibre strain guage with zero thermal-apparent strain" see the whole document.
Optics Letters, vol. 19, No. 24, Dec. 15, 1994, Washington US, pp. 2167-2169, XP000485819 D.A. Flavin et al.: "Combined temperature and strain measurement with a dispersive optical fiber Fourier-transform spectrometer" see the whole document.
Optics Letters, vol. 20, No. 3, Feb. 1, 1995, Washington US, pp. 333-335, XP000482486 Sotiris E. Kanellopoulos et al.: Simultaneous strain and temperature sensing with photogenerated in-fiber gratings: see the whole document.
Proceedings of the SPIE, vol. 1367, Sep. 17-19, 1990, San Jose, California, pp. 249-260, XP000676040 Ashish M. Vengsarkar et al.: "Fiber optic sensor for simultaneous measurement of strain and temperature" see the whole document.

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