Optics: measuring and testing – Material strain analysis – By light interference detector
Reexamination Certificate
1999-11-16
2001-09-04
Font, Frank G. (Department: 2877)
Optics: measuring and testing
Material strain analysis
By light interference detector
C356S450000
Reexamination Certificate
active
06285446
ABSTRACT:
The present invention relates to an improved apparatus and method for measuring strain in materials and relates particularly to distributed sensor systems using optical fibres.
BACKGROUND OF THE INVENTION
There has been considerable interest in using optical fibres for the measurement of a wide range of physical and environmental parameters, in particular where the inherent properites of optical fibres offer significant advantages. In applications such as structural monitoring, there is a need for distributed sensor systems for the measurement of strain and temperature, particularly at serial locations. Distributed and multiplexed systems are particularly attractive as they offer monitoring of physical parameters along a length of an optical fibre with benefits of high selectivity and small dimensions enabling them to be readily deployed or embodied within the structure.
It is well known to measure or detect the strain in a structure using interferometer techniques to measure the optical path length changes along a length of optical fibre. For example, when a length of optical fibre is subjected to a strain its length increases and thus the optical path for light passing down the fibre is likewise increased.
However, the temperature variation along a length of sensing fibre can also result in changes of the optical path length of the sensing fibre and make it difficult to distinguish the temperature effects from the strain effects. To try to compensate for this temperature effect the temperature can be measured using, for example, a separate fibre or segment of fibre not subjected to the strain field. However this requires the use of extra fibres and, due to the necessary displacement of this fibre or segment of fibre from the fibre used to measure the strain, accuracy cannot be assured.
SUMMARY OF THE INVENTION
We have now devised an apparatus and a method for the simultaneous measurement of temperature and strain along an optic fibre.
The method of the invention can be used in connection with the measurement of strain in which an interferometer is used to measure the variation in length along a section of a fibre, excited by a pulse of light, so as to generate
interference signals which vary with change in length of the fibre. In this technique it is important to know the temperature at the location at which the strain is measured so that suitable correction can be made.
According to the invention there is provided a method for measuring the strain in a structure which method comprises using an optical interferometer means for measuring the strain at at least one location in the structure and substantially simultaneously measuring the loss and the temperature distributions by detecting and measuring the Raman Scatter Spectrum (RSS).
The strain can be measured along a length or segment of optical fibre by use of sensing interferometer means by sending a pulse of light down the optical fibre and detecting and measuring the signal reflected back from the interferometer means.
The strain can be measured using a sensing interferometer or a plurality of sensing interferometers positioned along a length of optical fibre so as to form a sensing network. The sensing interferometer can comprise a pair of reflective means, with the path length between the reflective means varying with changes in physical parameters such as the strain in the fibre and the temperature.
The interferometer means can be formed from two reflective surfaces positioned a suitable distance apart, which reflect only a small percentage of the incident light, e.g. less than 1% so that a plurality of interferometers can be positioned along a fibre without any substantial attenuation of the light. Preferably the optical path length delay of the interferometer is greater than the coherence length of the light transmitted down the optical fibre. In one embodiment the reflective surfaces can be formed by reflective splices in the optic fibre.
The path length variations in the interferometer means can be converted to an intensity modulation, e.g. by using the reference interferometer means and so as to provide a sensitive means of measuring the temperature and strain. The amplitude of the backscattered light and the reflected radiation from the interferometer means is directly detected to provide compensation and correction for fibre attenuation effects and variations in the reflectivities of the interferometer means.
By use of a wavelength selection means the light originating from the interferometer means and the scattered RSS light can be passed down different channels.
The temperature can be measured by RSS by detecting and measuring the RSS from a point or series of points along the fibre to give a temperature profile along the fibre.
If there is a discontinuity in the optical fibre due, e.g. to an irregularity in the fibre or due to coupling device through which the light passes this will affect the amplitude of the RSS in a discontinuous manner and so can be used to monitor such discontinuities. In one embodiment this can be used to measure both the loss and reflection of the light when it passes through a coupling or splicing junction in the optical fibre. Since the Raman backscatter light is generated due to in-elastic scattering in the optical fibre and its frequence is shifted relative to the input optical source frequency, it can be optically filtered from Fresnel reflections at a junction. In this case the RSS measured at various points along the fibre will show a discontinuity in the profile and this discontinuity will be a measure of the loss experienced by light passing through the junction. The reflection at the junction can be measured by detecting the light reflected at the input optical source frequency which undergoes elastic scattering.
The means for detecting and measuring the RSS preferably is able to detect the amplitude of the RSS and to measure the amplitude of the anti-Stokes and Stokes components. The anti-Stokes component provides the temperature information and the Stokes component provides information derived from losses from the fibre. This enables a temperature profile along the length of the fibre to be computed and thus the temperature at the interferometer or interferometers to be computed. The RSS signal can be acquired by means of a data acquisition means such as a router/multiplexer.
Preferably the light transmitted back down the optical fiber is fed to a detection processing means which comprises a wavelength selection means, a reference interferometer means, detection means and processing means. The wavelength selection means can select and separate the Stokes and anti-Stokes components of RSS. The amplitudes of Stokes and anti-Stokes can be measured and then processed to evaluate predominately the loss and the temperature along the optical fibre independent of the strain. The optical delay of the sensing interferometers, which varies with strain and temperature, can be monitored by passing a portion of light though a reference interferometer and detecting the output interference pattern. A portion of light can also be selected to measure the Rayleigh backscatter power, the interferometer reflectivities and the distance between the interferometers which can provide a coarse measure of the strain and temperature along the fibre.
The output of all the detection means can be passed to a computation means to compute the strain and temperature of the sensing interoferometer as well as overall temperature of the sensing fibre.
A sensing network formed using the present invention can be a single mode or polarisation maintaining or multimode optical fibre comprising a plurality of sensing interferometers and means of converting the magnitude of physical parameters to a change of optical path-length of the sensing interferometers.
The sensing interferometer means may be comprised of reflective means to form in-line interferometers.
A reflective means may be formed by a reflective splice or by exposing the fibre to ultra-violet light to modify the refractive index of the fibre in a singl
Baker & Botts L.L.P.
Font Frank G.
Lee Andrew H.
Sensornet Limited
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