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Fiber optic sensor with enhanced immunity to random environmental perturbations

  • US 4,442,350 A
  • Filed: 08/17/1981
  • Issued: 04/10/1984
  • Est. Priority Date: 08/17/1981
  • Status: Expired due to Fees
First Claim
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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:

  • 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.

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