Optical sensor and method of use
First Claim
1. An interferometer apparatus for an optical fibre system, the apparatus comprising:
- an optical coupler having an input port and first and second ports coupled to optical fibres which define first and second optical paths;
first and second Faraday rotator mirrors arranged respectively at the ends of the first and second optical paths to reflect light propagating in the first and second optical paths back to the optical coupler to generate an interference signal;
wherein the optical coupler is configured to direct first and second interference signal components of the interference signal respectively to first and second detector ports, and is configured to direct a third interference signal component of the interference signal towards the input port, and the apparatus comprises means for introducing a phase shift between the first, second and third interference signal components;
first and second photodetectors connected to first and second detector ports of the optical coupler and configured to measure an intensity of the first and second interference signal components at respective phase shifts;
a third photodetector connected to a non-reciprocal optical device and configured to measure the intensity of the third interference signal component directed back towards the input port;
an incoherent light source configured to propagate light in the interferometer apparatus; and
calibration circuitry configured to;
determine an electrical offset for each of the photodetectors from the outputs of the photodetectors in a first condition, in which no light is input to the input port and the incoherent light source is switched off so that no light propagates in the interferometer apparatus; and
determine at least one of a photodetector offset, a relative photodetector gain, and a coupling ratio of the first and second optical paths from the outputs of the photodetectors in a second condition, in which the incoherent light source is switched on such that light propagates in the interferometer apparatus.
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Abstract
An interferometer apparatus for an optical fiber system and method of use is described. The interferometer comprises an optical coupler and optical fibers which define first and second optical paths. Light propagating in the first and second optical paths is reflected back to the optical coupler to generate an interference signal. First, second and third interference signal components are directed towards respective first, second and third photodetectors. The third photodetector is connected to the coupler via a non-reciprocal optical device and is configured to measure the intensity of the third interference signal component directed back towards the input fiber. Methods of use in applications to monitoring acoustic perturbations and a calibration method are described.
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Citations
29 Claims
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1. An interferometer apparatus for an optical fibre system, the apparatus comprising:
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an optical coupler having an input port and first and second ports coupled to optical fibres which define first and second optical paths; first and second Faraday rotator mirrors arranged respectively at the ends of the first and second optical paths to reflect light propagating in the first and second optical paths back to the optical coupler to generate an interference signal; wherein the optical coupler is configured to direct first and second interference signal components of the interference signal respectively to first and second detector ports, and is configured to direct a third interference signal component of the interference signal towards the input port, and the apparatus comprises means for introducing a phase shift between the first, second and third interference signal components; first and second photodetectors connected to first and second detector ports of the optical coupler and configured to measure an intensity of the first and second interference signal components at respective phase shifts; a third photodetector connected to a non-reciprocal optical device and configured to measure the intensity of the third interference signal component directed back towards the input port; an incoherent light source configured to propagate light in the interferometer apparatus; and calibration circuitry configured to; determine an electrical offset for each of the photodetectors from the outputs of the photodetectors in a first condition, in which no light is input to the input port and the incoherent light source is switched off so that no light propagates in the interferometer apparatus; and determine at least one of a photodetector offset, a relative photodetector gain, and a coupling ratio of the first and second optical paths from the outputs of the photodetectors in a second condition, in which the incoherent light source is switched on such that light propagates in the interferometer apparatus. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22)
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23. A method of monitoring acoustic perturbations, the method comprising:
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providing a light source, an optical fibre deployed in an environment to be monitored and coupled to the light source, and an interferometer configured to receive a pulsed optical signal from the optical fibre, the interferometer comprising at least two optical paths having Faraday rotator mirrors at the ends thereof and at least three photodetectors; receiving, in the interferometer, backscattered light from the optical fibre, and generating an interference signal in dependence thereon; introducing a phase shift between first, second, and third interference signal components of the interference signal, and directing the first, second, and third interference signal components to first, second, and third photodetectors respectively; measuring the intensity of the first, second, and third interference signal components at respective phase shills to provide first intensity data, second intensity data, and third intensity data; processing the first, second, and third intensity data to determine an optical phase angle of the optical signal to thereby provide optical phase angle data and optical phase angle modulation data and; identifying acoustic perturbations to which the optical fibre has been exposed from the optical phase angle modulation data; determining a normalisation factor for at least one of a photodetector offset, a relative photodetector gain, and a coupling ratio of the interferometer optical paths, by inputting light from an incoherent light source to the interferometer and measuring the outputs of the photodetectors; wherein the determining further comprises; determining the electrical photodetector offset for each of the photodetectors from the outputs of the photodetectors in a first condition, in which transmitted, reflected or backscattered light propagating through the optical fibre is decoupled from the interferometer and the incoherent light source is switched off so that no light is input to the interferometer; and determining at least one of the photodetector offset, the relative photodetector gain, and the coupling ratio of the interferometer optical paths from the outputs of the photodetectors in a second condition, in which transmitted, reflected or backscattered light propagating through the optical fibre is decoupled from the interferometer and the incoherent light source is switched on to input light to the interferometer. - View Dependent Claims (24, 25, 26, 27, 28, 29)
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Specification