Fiber optic sensor with enhanced immunity to random environmental perturbations
First Claim
1. A method of sensing a field condition with minimum disturbance from low frequency environmentally induced perturbations comprising the steps of:
- propagating polarized light through an optical fiber made birefringent to have two orthogonally disposed modes oriented respectively along fast and slow axes;
allowing a field condition to stress a length of the optical fiber in axes directions for changing its birefringence;
splitting one portion of the light emanating from the optical fiber into first and second components of linearly polarized light;
phase shifting another portion of the light emanating from the optical fiber by 90° and
splitting it into a third component of left circularly polarized light and a fourth component of right circularly polarized light;
monitoring the intensities of the first and second linearly polarized orthogonal light components to produce respective first and second electrical signals and combining the first and second electrical signals to filter out low frequency perturbations to provide a third electrical signal proportional to the field condition induced stresses in the optical fiber;
at the same time monitoring the intensities of the third and fourth circularly polarized light components to produce respective fourth and fifth electrical signals and combining the fourth and fifth electrical signals to filter out low frequency perturbations to provide a sixth electrical signal proportional to the field condition induced stresses in the optical fiber, and which the sixth electrical signal is shifted in phase by 90°
relative to the third electrical signal; and
,combining the third and sixth electrical signals to produce an output electrical signal which is independent of low frequency environmentally induced perturbations.
1 Assignment
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Accused Products
Abstract
A sensor is provided for sensing or detecting the presence of an environmental field condition such as acceleration, temperature change, magnetic or acoustic fields. The field is sensed by interference between two mutually orthogonal polarized eigenmodes in a single monomode optical fiber which may be disposed either linearly or wound on a mandrel made of compliant material for sensing an acoustic field, or made of magnetostrictive material for sensing a magnetic field. Polarized light propagated through the optical fiber is detected at its outlet independent of environmentally induced low frequency variations whereby the sensor may be maintained at quadrature and maximum sensitivity.
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Citations
18 Claims
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1. A method of sensing a field condition with minimum disturbance from low frequency environmentally induced perturbations comprising the steps of:
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propagating polarized light through an optical fiber made birefringent to have two orthogonally disposed modes oriented respectively along fast and slow axes; allowing a field condition to stress a length of the optical fiber in axes directions for changing its birefringence; splitting one portion of the light emanating from the optical fiber into first and second components of linearly polarized light; phase shifting another portion of the light emanating from the optical fiber by 90° and
splitting it into a third component of left circularly polarized light and a fourth component of right circularly polarized light;monitoring the intensities of the first and second linearly polarized orthogonal light components to produce respective first and second electrical signals and combining the first and second electrical signals to filter out low frequency perturbations to provide a third electrical signal proportional to the field condition induced stresses in the optical fiber; at the same time monitoring the intensities of the third and fourth circularly polarized light components to produce respective fourth and fifth electrical signals and combining the fourth and fifth electrical signals to filter out low frequency perturbations to provide a sixth electrical signal proportional to the field condition induced stresses in the optical fiber, and which the sixth electrical signal is shifted in phase by 90°
relative to the third electrical signal; and
,combining the third and sixth electrical signals to produce an output electrical signal which is independent of low frequency environmentally induced perturbations. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A sensor for sensing a field condition with minimum disturbance from environmentally induced perturbations comprising:
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a sensing region defined by a length of single mode birefringent optical fiber made birefringent to have two orthogonally disposed eigenmodes oriented respectively along fast and slow axes, said optical fiber being disposed on a carrier; means for launching linear polarized light into one end of the optical fiber at 45°
to its fast and slow axes for propagation therethrough along its length and emanating at the other end;said carrier adapted to undergo dimensional changes in response to the field condition to strain the birefringent fiber relative to its axes for changing the birefringence thereof; means for splitting one portion of the light emanating from the optical fiber other end into first and second components of linearly polarized light which are at 45°
to the optical fiber fast and slow axes;a quarter wave plate in another portion of the polarized light emanating from the optical fiber end for introducing a differential 90°
phase shift therein;means for splitting the phase shifted light into a third component of left circularly polarized light and a fourth component of right circularly polarized light; means for monitoring the intensities of the first and second linearly polarized light components to produce first and second electrical signals respectively and for combining the first and second electrical signals to filter out low frequency perturbations to provide a third electrical signal proportional to stresses in the optical fiber induced by the field condition; means for monitoring the intensities of the third and fourth circularly polarized light components to produce fourth and fifth electrical signals respectively and for combining the fourth and fifth electrical signals to filter out low frequency perturbations to provide a stress induced sixth electrical signal which is phase shifted by 90°
relative to the third electrical signal;means for combining the third and sixth electrical signals to produce an output electrical signal which is independent of low frequency environmentally induced perturbations. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18)
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Specification