Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2001-06-25
2003-07-22
Kim, Robert H. (Department: 2882)
Optical waveguides
Optical waveguide sensor
Reexamination Certificate
active
06597821
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of optical pressure measurement. It is based on a fiber-optic laser according to the preamble of claims
1
and
14
.
BACKGROUND OF THE INVENTION
In oil production it is necessary to monitor boreholes with regard to pressure and temperature. ID the borehole, the liquid pressures can be up to approximately 100 MPa (1000 bar) and the temperatures can be up to above 200° C. Electrical sensors, such as e.g. piezoresistors, piezoelectric elements, capacitive probes or crystal resonators, or optical pressure sensors, such as e.g. Fabry-Perot resonators, or elasto-optical sensors, are often used for pressure measurement up to approximately 170° C.
A polarimetric fiber laser sensor in accordance with the preamble is disclosed in the article by H. K. Kim et al., “Polarimetric fiber laser sensors”, Optics Letters 18 (4), pp. 317-319 (1993). An Nd-doped fiber with a round core and dichroically mirrored ends which are transparent to pumping light is used as laser and pressure sensor fiber. Unidirectional lateral pressure on the fiber induces a birefringence and thus a frequency shift between the linear inherent polarizations of longitudinal modes. The resulting beat frequency in the polarimetric interference signal can be measured very easily using a frequency counter.
In accordance with the article by G. A. Ball et al., “Polarimetric heterodyning Bragg-grating fiber-laser sensor”, Optics Letters 18 (22), pp. 1976-1978, instead of dichroic mirrors, it is also possible to use two fiber Bragg-gratings written directly into the fiber core for the purpose of bounding the laser cavity.
In both arrangements, hydrostatic or all-round isotropic pressures cannot be measured. The measurement of steady-state or absolute pressures is difficult or impossible since the operating point, i.e. the beat frequency in the unloaded state, can fluctuate in an uncontrolled manner as a result of temperature fluctuations, changes in optical parameters owing to material fatigue and the like.
Serial multiplexing of passive fiber Bragg-grating sensors is disclosed e.g. in U.S. Pat. No. 4,761,073. A plurality of fiber Bragg-gratings with different reflection wavelengths are written in along a sensor fiber. Each grating can be read out wavelength-selectively and/or by means of time-resolved measurement with a pulsed light source.
U.S. Pat. No. 5,515,459 discloses a fiber-optic pressure sensor for measuring an isotropic pressure, the sensor fiber having two side-hole fiber segments with a structure that is not rotationally symmetrical. The two fiber segments are arranged such that they are rotated by 90° with respect to one another, and are exposed to the same isotropic pressure, the side holes of one fiber segment being exposed to the isotropic pressure, but the side holes of the other fiber segment not being so exposed. In this case, the rotation of the two fiber segments by 90° serve for temperature compensation.
J. P. Darkin and C. Wade, “Compensated polarimetric sensor using polarization-maintaining fiber in a differential configuration”, Electron. Lett. Vol. 20, No. 1, pages 51-53, 1984, disclose a fiber-optic temperature sensor in which two fiber segments are arranged such that they are rotated by 90° with respect to one another, in order to compensate common-mode temperature changes and isotropic pressure changes.
SUMMARY OF THE INVENTION
It is an object of the present invention to specify a fiber laser pressure sensor which is suitable for measuring differential pressures in liquids or gases and is distinguished by a simple construction, a good measurement sensitivity and a large measurement range. This object is achieved according to the invention by means of the features of claims 1 and 14.
The invention consists in arranging in the laser cavity of a fiber laser, in addition to the laser-amplifying fiber, two sensor fiber segments which are not rotationally symmetrical and have a mutually opposite pressure dependence of the birefringence, and in providing means for determining a birefringence-induced beat frequency. By virtue of the rotational asymmetry, an isotropic pressure is converted into an anisotropic birefringence in the fiber laser. The mutually opposite pressure dependence has the effect that the total birefringence and hence the induced beat frequency is proportional to the pressure difference at the fiber segments.
One exemplary embodiment shows a fiber laser differential pressure sensor with two polarimetric sensor fiber segments. By virtue of 90° rotation between the segments, it is possible to achieve a coupling between the polarization modes and, as a result, an opposite pressure sensitivity and inherent compensation of temperature effects.
Another exemplary embodiment shows a fiber laser differential pressure sensor with two spatially bimodal sensor fiber segments. The opposite pressure sensitivity and temperature compensation can be achieved by coupling between the spatial modes at a transversely offset splice between the segments.
A further exemplary embodiment represents a serial arrangement of a plurality of fiber laser differential pressure sensors with different emission wavelengths, which are fed via a common pumping light source and whose pressure-proportional beat frequencies are detected in a wavelength-selective manner.
Additional exemplary embodiments relate to pressure housings for fiber lasers, in the case of which the sensor fiber segments are in pressure exchange with two media and, if appropriate, the laser-amplifying fiber and the fiber Bragg-gratings effective as laser mirrors are shielded from the pressures.
Further embodiments, advantages and applications of the invention emerge from the dependent claims and from the following description with reference to the figures.
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“Polarimetric fiber laser sensors”, Kim, et al., Optics Letters 18, Feb. 15, 1993, No. 4, New York, New York, USA, pp. 317-319.
“Compensated Polarimetric Sensor using Polarisation-Maintaining Fibre in a Differential Configuration”, Dakin, et al., Electronics Letters, Jan. 5, 1984, vol. 20, No. 1, pp. 51-53.
Bohnert Klaus
Brändle Hubert
ABB Research Ltd
Burns Doane Swecker & Mathis L.L.P.
Kim Richard
LandOfFree
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