Optical waveguides – Optical waveguide sensor
Reexamination Certificate
2002-05-17
2004-11-02
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Optical waveguide sensor
C385S013000, C385S015000, C356S032000, C250S227110, C250S227140, C250S227160
Reexamination Certificate
active
06813403
ABSTRACT:
FIELD OF INVENTION
The present invention is directed to a method of measuring physical characteristics, particularly but not exclusively strain, displacement and temperature, and in particular to such a method based on the measurement and analysis of the Brillouin scattering spectrum. The present invention is further directed to a fiber optic sensing configuration for use in measuring such physical characteristics.
DESCRIPTION OF RELATED ART
When laser light pulses are propagated down an optical fiber, light is backscattered due to changes in density and composition, as well as molecular and bulk vibrations within the fiber material. That backscattered light includes Rayleigh, Brillouin and Raman backscattered components. The Raman backscattered light, caused by thermally induced molecular vibrations, can be used to obtain information on temperature distributions along the fiber. Thus, that technique has been demonstrated for using optical fibers as sensors for leakage detection in pipelines and underground storage vessels, for example.
Brillouin scattering results from scattering of light by sound waves, which produce a periodic modulation in the fiber's index of refraction. That phenomenon is measured by the Brillouin frequency shift, given by the formula,
F
=2
nV/L
(1)
where F=the Brillouin frequency, n=index of refraction of the fiber, V=velocity of the light wave in the fiber and L=wavelength of the incident light in the fiber. Thus the application of mechanical strain and/or temperature to the fiber results in changes in “F”. The use of Brillouin loss spectrum analysis to measure strain and temperature with single mode optical fibers is superior to using the Brillouin “gain” technique, since it has been shown that the “loss” method can be applied over longer fiber distances.
To obtain both temperature and strain, measurements of the Brillouin power as well as the Brillouin frequency shift (F) are required. The measured Brillouin strain in an optical fiber contains components associated with the fiber's temperature (reflecting both ambient temperature and that of the structure to which it is attached) and the mechanical strain applied to the structure (to which the fiber is attached), given by the following equation,
E
(
x
)=
E
t
(
x
)+
E
m
(
x
) (2)
where E (x) is the measured Brillouin total strain as obtained from measuring the Brillouin frequency shift (F) at any location “x” along the fiber, E
t
is the thermal strain component and E
m
is the mechanical applied strain. Thus the determination of E
t
allows one to calculate the mechanical strain knowing E(x).
It is known in the art to use the Brillouin frequency shift to measure optical fiber distortion, temperature along a fiber or both temperature and distortion. The use of a single laser light source to also measure temperature and distortion is also known. Limited applications of the Brillouin method utilizing buried optical fibers are disclosed in the prior art, purporting to measure earth sloping, distortion of the ground between fixed points and the motion of embedded weights attached to an optical fiber. A method of measuring a single optical fiber's distortion between two fixed points using Brillouin scattering is also known.
Brillouin instruments have been developed to measure temperature distributions over long distances using single mode optical fiber, where the fiber runs, e.g., along the bottom of a lake. It has also been shown that a Brillouin instrument can measure the concrete curing temperature distributions in a dam. One commercial Brillouin instrument using a single DFB light source is known, but it is limited to a strain accuracy of ±100 to 300 microstrain, or 0.01% (1 microstrain=10
−6
mm/mm or in/in). Such strain accuracies are not suitable for applications to bridges and pipelines, for example, where maximum operating strains are of the order of 100 microstrain.
The typical Brillouin instrument system used to measure strains and temperatures, shown schematically in
FIG. 1
as
100
, can incorporate one or two light sources. To achieve better strain measuring accuracies, it is known to use two separate “frequency tunable” laser light sources
102
,
104
operating at about 1320 nm wavelength. One laser
102
acts as a pump laser, while the other laser
104
serves as the probe laser which sends optical pulses down the fiber
106
to interact with the counter propagating laser lightwave pumped into the fiber
106
from its opposite end. Each laser
102
,
104
is in optical communication with the fiber
106
through a polarization controller
108
,
110
. In addition, a pulse generator
112
controls a modulator
114
to modulate the light from the pump laser
102
to form pulses. A circulator
116
diverts light from the fiber
106
into a signal detector
118
, whose output is applied to an oscilloscope
120
. In addition, the output of a detector
122
is applied to a spectrum analyzer
124
, whose output is applied to an oscilloscope
126
. The outputs of both of the oscilloscopes
120
,
126
are analyzed in a data acquisition system
128
. The system
100
operates in a manner which will be familiar to those skilled in the art.
It is well known to those skilled in the Brillouin technology that both lasers can be located at one end of the fiber, providing the other end has a mirror (or some other reflective optical element) to reflect the laser wavelengths. A possible configuration to perform that task is shown in
FIG. 2
as
200
, in which the light from the lasers
102
,
104
is applied to the same end of a Brillouin fiber sensor
206
. In
FIG. 2
, reference numerals
230
and
232
designate couplers and sections of single-mode optical fiber, respectively, while those reference numerals which are common to
FIGS. 1 and 2
have the same significance in both of those figures. In addition to the laser light sources, other instrumentation components include, but are not limited to, a pulse generator, a spectral analyzer and a signal detection system. The theoretical description of how that Brillouin loss technique works is known in the art.
It is known that a Brillouin system can be used to measure strain over optical fiber distances exceeding 50 km. It is also known that a Brillouin system can achieve a strain resolution of typically as low as ±20 microstrain, over gage lengths as small as 10~15 cm, and can measure temperature changes as low as ±1° C. Such measurements, based on the system shown in
FIG. 1
, obtain information on the Brillouin frequency shift and the Brillouin loss spectrum, which combine to yield simultaneous measurements of the strain and temperature over the selected gage length.
Applications of the Brillouin loss technique, as described in the published literature, are limited to laboratory materials and small test structural elements such as a steel beam and concrete beams. None of the published documents employ, or describe, in their experiments or test cases, how to apply the Brillouin loss technology to large structures such as pipelines, dams, buildings or bridges, for example. No data or design concepts on large structural applications have been reported in these documents or their related references contained in their publications. No mention is ever made of the potential use of multiple Brillouin sensors operating off a single fiber optic backbone.
SUMMARY OF THE INVENTION
It will be apparent from the above that a need exists in the art to overcome the above-noted deficiencies in the art. It is therefore a primary object of the invention to apply Brillouin loss technology to large structures.
It is another object of the invention to apply Brillouin loss technology to measure physical characteristics such as strain or displacement over large structures and also, optionally, to determine the location of the strain or displacement and to compensate for temperature.
To achieve the above and other objects, the present invention is directed to t
Blank Rome LLP
Fiber Optic Systems Technology, Inc.
Petkovsek Daniel
Ullah Akm Enayet
LandOfFree
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