Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-06-25
2001-03-13
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S231100, C385S012000, C356S035500
Reexamination Certificate
active
06201237
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to fiber optic sensors, and more particularly to a method and apparatus for independently measuring the temperature and axial strain of an optical fiber.
2. Technical Background
Fiber optic sensors, and in particular distributed fiber sensors, are of interest for smart structures and other monitoring applications. Smart structures are often composite structures that may incorporate electrical communication devices for monitoring or actively controlling the operation of the structure. A sensor is required to monitor the conditions the smart structure is subjected to. A fiber sensor, for example, can be embedded within the layers of the composite structure to determine strain and temperature. There are other industrial applications that require knowledge of the environment in order to control both the quality and productivity of the process. Interest has peaked recently with the encouraging results obtained using Bragg gratings distributed along the length of the sensing fiber. One issue that arises with fiber optic sensors relates to their sensitivity to both temperature and strain. In one approach that has been considered, a combined strain and temperature sensor using polarization-maintaining fibers was developed. Unfortunately, it was determined that the temperature and the strain values obtained by the sensor were dependent upon one another. Thus, the values measured by the sensor were inherently skewed.
A sensor that can measure temperature without being adversely affected by a strain component, or conversely, a sensor that is able to measure strain without a temperature component is therefore desired.
In another approach, a first polarization-maintaining fiber having an elliptical core is fused to a second polarization-maintaining fiber having an elliptical core. The major axis of the second fiber is rotated 90° with respect to the first fiber. When a polarized light signal is transmitted through the fibers, the temperature and strain affect the phase of the light signal differently. This relationship is characterized by the following equations:
&Dgr;&phgr;
1
=A
1
L
1
&Dgr;T+B
1
&Dgr;L
1
, (1)
&Dgr;&phgr;
2
=A
2
L
2
&Dgr;T+B
2
&Dgr;L
2
. (2)
wherein &Dgr;&phgr;
1
is the change in phase difference in the first fiber, A
1
is the temperature coefficient for the change in temperature of the first fiber, L
1
is the length of the first fiber, &Dgr;T is the change in temperature, B
1
is the strain coefficient for the change in strain of the first fiber, &Dgr;L
1
is the change in the length of the first fiber due to strain, &Dgr;&phgr;
2
is the change in phase difference in the second fiber, A
2
is the temperature coefficient for the change in temperature of the first fiber, L
2
is the length of the second fiber, &Dgr;T is the change in temperature, B
2
is the strain coefficient for the change in strain of the second fiber, &Dgr;L
2
is the change in the length of the second fiber due to strain.
In order to “de-couple” temperature and strain, the two fibers must be selected such that either their strain coefficients are equal, or that their temperature coefficients are equal, such that:
B
1
&Dgr;L
1
=B
2
&Dgr;L
2
, or (3)
A
1
L
1
&Dgr;T=A
2
L
2
&Dgr;T.
(4)
Thus, when the phase differences of the two fibers are subtracted,
&Dgr;&phgr;=&Dgr;&phgr;1−&Dgr;&phgr;2 (5)
The variable having equal coefficients is eliminated. Thus, a single variable is obtained. However, there are disadvantages to this approach. First, the two have fibers must be precisely selected to equalize the phase difference between the first and second fibers caused by either strain or temperature. Secondly, it is understood from equations 3, 4, and 5 that the sensor is limited to detecting either temperature or strain. It cannot detect both simultaneously.
Thus, a need exists for a fiber optic sensor that has the ability to accurately measure strain on a fiber without that measurement being affected by the temperature, while simultaneously being able to accurately measure the temperature of the fiber's environment without the temperature measurement being affected by the applied strain.
SUMMARY OF THE INVENTION
The existing problems discussed above are solved with the present invention. The present invention includes a pair of fibers each having a plurality of polarization-maintaining fiber segments and a phase shifter disposed therein. Because the strain and temperature response curves of the phase shifters deployed in each fiber are so different, temperature and strain can be measured independently and simultaneously.
One aspect of the invention relates to an optical fiber that propagates a light signal characterized by a center wavelength. The optical fiber is disposed in an environment and used for measuring a plurality of environmental parameters. The optical fiber includes a plurality of polarization-maintaining fiber segments, each of which has a cladding and an elliptical core. The major axis of each of the plurality of polarization-maintaining fiber segments is rotated 45° with respect to a preceding fiber segment, and optically connected to that preceding fiber segment. The optical fiber also includes a sensing element disposed within the plurality of polarization-maintaining fiber segments. The sensing element shifts the center wavelength of the light signal at a predetermined rate in response to the plurality of environmental parameters.
Another aspect of the invention relates to a Mach-Zehnder device that couples a light signal characterized by a center wavelength. The Mach-Zehnder device is disposed in an environment and used to measure a plurality of environmental parameters. The Mach-Zehnder device includes a first polarization-maintaining fiber for propagating the light signal. The first polarization maintaining fiber includes a first elliptical core, a first cladding, and a plurality of first fiber segments, wherein each of the plurality of first fiber segments is rotated 45° with respect to a preceding first fiber segment and optically connected to the preceding first fiber segment. It also includes a second polarization maintaining fiber disposed adjacent to the first polarization-maintaining fiber. The second polarization maintaining fiber includes a second elliptical core, a second cladding, and a plurality of second fiber segments, wherein each of the plurality of second fiber segments is rotated 45° with respect to a preceding second fiber segment and optically connected to the preceding second fiber segment. A coupling region is disposed between the first polarization-maintaining fiber and the second polarization-maintaining fiber for coupling the light signal between the first and second polarization-maintaining fibers. A sensing element is disposed in the first and second polarization maintaining fibers. The sensing element shifts the center wavelength of the light signal at a first predetermined rate in the first polarization-maintaining fiber and by a second predetermined rate in the second polarization-maintaining fiber, in response to one or more of the plurality of environmental parameters.
Another aspect of the invention relates to a fiber optic sensor disposed in an environment and used for measuring a plurality of environmental parameters. The fiber optic sensor includes a polarized light source for transmitting a light signal having a center wavelength and a first polarization-maintaining fiber connected to the polarized light source. The first polarization-maintaining fiber includes a first elliptical core, a first cladding, and a plurality of first fiber segments. The fiber optic sensor also includes a second polarization-maintaining fiber disposed adjacent the first polarization-maintaining fiber. The second polarization-maintaining fiber includes a second elliptical core, a second cladding, and a plurality of second fiber segments. A coupling region is disposed be
Berkey George E.
Krol Mark F.
Nolan Daniel A.
Corning Incorporated
Lee John R.
Malley Daniel P.
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