Optical waveguides – Optical waveguide sensor
Patent
1994-08-15
1996-05-07
Palmer, Phan T. H.
Optical waveguides
Optical waveguide sensor
385 11, G02B 627
Patent
active
055154590
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
A variety of industrial processes require knowledge of fluid pressure in order effectively to control productivity and quality of the process. In particular, the pressures and temperatures encountered are often very different to those of ambient conditions. Examples are the chemical process industries, internal combustion engine control and oil production. A pressure sensor manufactured of a robust material such as silica can withstand extreme pressures and temperatures and thus is particularly useful.
BACKGROUND OF THE INVENTION
European Patent Application EP 0 144 509 A3 "Fiber Optic interferometer transducer" teaches how a dual path polarimetric interferometer may be constructed so that, when suitable means convert isotropic pressure to anisotropic radial forces on an optical fibre, a useful signal results, which may be used as a measure of the isotropic pressure. Suitable means are described in the paper by K. Jansen and Ph. Dabkiewicz. "High pressure fiber-optic sensor with side-hole fiber", SPIE Proceedings, Fiber Optic Sensors II, Vol. 798 pp. 56-60 1987. where a single mode optical fibre is manufactured with side holes so as to form a polarimetric sensor sensitive to isotropic pressure. The dual path polarimetric interferometer operates by obtaining a measurement of the optical path length difference between the two orthogonal polarisation modes which are guided by the fibre. The optical path length difference is made intentionally sensitive to pressure, but is also generally sensitive to temperature which is often undesirable. Thus, such polarimetric pressure sensors normally require temperature compensation to be useful.
The output of the dual-path polarimetric interferometer sensors may be expressed in the form COS(.phi.), where the optical path length difference is expressed as a phase delay .phi. such that: wavelength. L the length of fibre. A T! the inherent birefringence function of temperature, and B1 T! the pressure dependent birefringence function of temperature. When two lengths of fibre, designated 1 and 2. and of lengths L1 and L2. are joined together, and subject to pressures P1 and P2 respectively, then the combined phase delay is given by: and: angles.
In general, the temperature sensitivity of the inherent birefringence, A, is at least an order of magnitude greater than that of the pressure dependent birefringence, B, and so there is considerable advantage in miniraising the inherent component. If the two lengths are made identical, L1=L2=L. and the fibres rotated 90.degree. relative to each other about their longitudinal axis at the join, as taught by J.P. Dakin and C. Wade. "Compensated polarimetric sensor using polarisation-maintaining fibre in a differential configuration", Electron. Lett., Vol. 20, No.1, pp. 51-53, 1984. then A1 T!=A2 T!, and so the inherent birefringence component cancels to give:
If B1 T!=B2 T!, which will be the case if the two pieces of fibre are identically sensitive to pressure, then a useful signal will only be obtained if the two pieces of fibre experience different pressures, that is P1 is not equal to P2. This is the approach taught by the prior art, Dakin and Wade. However, there is an alternative approach in which a useful signal may be obtained when the two pieces of fibre experience the same pressure, that is P1 equals P2. If B1 T! may be made not equal to B2 T! and yet the expression A1 T!=A2 T! still applies, then P1 may equal P2, that is the two pieces of fibre may experience the same pressure, and yet temperature compensation of the inherent birefringence still occur. This is the basis for the present invention, where B1 T! and B2 T! are made to differ by changing the pressure experienced within side-holes formed in the fibre.
In some cases it may be advantageous to use unequal lengths of fibre in order to bias, the sensor at a convenient phase delay, especially when temperature compensation is only required over a limited range of operation. In a particular case, for example, the inherent birefringence component A1
REFERENCES:
Meltz et al. "Formation of Bragg Gratings in Optical Fiber by a Transverse Holographic Method" Optics Letters Aug. 1989 vol. 14, pp. 823-825.
Yoshino et al. "Fiber-Optic Fabry-Perot Interferometer and Its Sensor Applications", Oct. 1982 IEEE pp. 1624-1633.
Jansen et al. "High Pressure Fiber-Optic Sensor with Side-Hole Fiber" SPIE vol. 789 Fiber Optic Sensors II (1987) pp. 56-61. No month.
Lee John D.
Palmer Phan T. H.
Sensor Dynamics Inc.
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