US 4,699,513 A · Distributed sensor and method using coherence multiplexing of fiber-optic interferometric sensors

Abstract

A distributed sensor system including an optical source having a short coherence length for optionally continuously monitoring each sensor in the system. In one preferred embodiment, an array of fiber-optic sensors are organized in a ladder configuration, with the sensors positioned in spaced relation and defining the rungs of the ladder. Light transmitted through the sensors is multiplexed onto a return arm of the ladder, with sensor spacing being such that interference between light from different sensors is prevented. The multiplexed signals are received by an optical fiber receiver which couples the multiplexed light with an interfering optical reference signal to produce a phase difference signal representing conditions influencing selected sensors. Embodiments are disclosed for use of either pulsed or continuous wave light sources. In another preferred embodiment, the sensors comprise a plurality of interferometers connected in series configuration by a common optical fiber, which provides multiplexed output signals from the sensors to another plurality of interferometers comprising receivers. Functional equivalents of the series configuration provide sensors and receivers comprising birefringent or two-mode fiber. The optical path length differences between each pair of sensor arms are selected to prevent interference between the multiplexed sensor output signals from the various sensors. The optical path lengths through the sensors and receivers are structured so that each receiver produces a phase difference signal relating to conditions affecting light transmission through a specific sensor. A phase and amplitude modulation technique is disclosed for providing heterodyned output signals from the distributed sensor system.

Claims45

Dependent claims

An apparatus for remotely sensing environmental effects on a pair of sensors, comprising:

a light source;
a plurality of optical waveguide segments which are optically coupled to receive light transmitted from the light source, for conducting light from the source along first, second, third and fourth light paths at least first and second ones of said waveguide segments configured such that said first and second light paths are different in at least one sensing region and at least one of said first and second light paths are influenced by environmental effects in said sensing region, at least third and fourth ones of said waveguide segments configured such that said third and fourth light paths are different in at least one receiving region, with the difference between the first and second path lengths in the sensing region being substantially equal to the difference between the third and fourth path lengths in the receiving region, with said third and fourth light paths not subject to the same environmental effect in the receiving region as said first and second light paths in the sensing region, with at least a fifth one of said waveguide segments being optically coupled at its first end to an output end of at least one of said first and second wavguide segments, and being optically coupled at its second end to an input end of at least one of said third and fourth waveguide segments to conduct light between said first, second, third and fourth waveguide segments along a single, common propagation path comprising a portion of said first, second, third and fourth light paths;
and
means optically coupled to an output end of said third and fourth waveguide segments for combining light from said first, second, third and fourth light paths at a location on said paths where coherence exists only for light which has traveled paths selected to yield interference between the light in the paths.

1 more element

An apparatus for remotely sensing environmental effects on a pair of sensors, comprising:

a light source;
first and second optical interferometers, each having an input end and an output end, said input end being optically coupled to receive light transmitted from the light source and each having first and second interferometer arms, at least one of said arms in each of the first and second interferometers being sensitive to an environmental effect and influencing light propagating in said sensitive arm in response to said environmental effects;
at least a third optical interferometer having an input end and an output end and having first and second interferometer arms configured such that optical path lengths of the first and second arms of the third interferometer have a difference which is substantially equal to the optical path length difference between the arms of one of the first and second interferometers, said third interferometer not being subject to the same environmental effect as either of said first and second interferometers;
an optical waveguide optically coupled to receive light from the output end of each of said first and second interferometers and to provide said light to the input end of said third interferometer so that light from said first and second interferometers is propagated from the first and second interferometers to the third interferometer through said optical waveguide only;
and
wherein said at least a third interferometer includes means for combining light from said first and second arms of said third interferometer, so that light signals which interfere in said combining means produce a signal representative of environmental influence on said sensitive arm of one of the first and second interferometers.

3 more elements

An apparatus for remotely sensing environmental effects on a pair of sensors comprising:

a light source;
first and second optical interferometers, each having an input end and an output end, said input end of said first interferometer being optically coupled to receive light transmitted from the light source, said output end of the first interferometer being optically coupled to the input end of the second interferometer, each said first and second interferometer having first and second interferometer arms, wherein optical path length differences between the first and second arms in each interferometer are greater than one source coherence length of the light source, and wherein said optical path length difference of the first and second arms in the first interferometer is different than said optical path length difference of the first and second arms in the second interferometer by at least one source coherence length of the light source, at least one of the arms in each of said first and second interferometers being sensitive to an environmental effect and influencing light propagating in said sensitive arm in response to said environmental effect;
at least one third optical interferometer having an input end and an output end and having first and second interferometer arms, wherein the difference in optical path lengths between the first and second interferometer arms in said third interferometer is substantially equal to the difference in optical path lengths of the interferometer arms in at least one of said first and second interferometers, said third interferometer not being subject to the same environmental effect as either of said first and second interferometers;
an optical waveguide optically coupled to receive light from the output end of said second interferometer and to provide said light to the input end of said third interferometer so that light from said first and second interferometers is propagated from the first and second interferometers to the third interferometer through said optical waveguide only;
and
wherein said at least a third interferometer includes means for coupling light from the third interferometer, thereby providing an optical signal representative of the environmental effect influencing light propagating through the sensitive arm in one of the first and second interferometers whose optical path length difference between its pair of interferometer arms substantially corresponds to the optical path length difference between the pair of interferometer arms in said third interferometer at any time.

3 more elements

A distributed sensor system comprising:

a light source;
a plurality of fiber-optic sensing interferometers, each said sensing interferometer having first and second interferometer arms, and at least a portion of one of said first and second arms in each said interferometer having light transmission characteristics which vary in response to environmental conditions, with each said sensing interferometer having input and output terminals which are optically coupled together to form a ladder network, said input terminals being optically coupled to the light source, and wherein the sensing interferometers are optically spaced from one another and the difference in the optical path lengths of the first and second interferometer arms in each said sensing interferometer are different for each interferometer by an amount such that light transmitted through said interferometers from the light source and distributed from any one output terminal is optically incoherent with respect to all other light distributed from any other output terminal at a corresponding time;
and
at least one fiber-optic receiving interferometer having an input terminal optically coupled to the output terminals of the sensing interferometers and having first and second interferometer arms whose optical path length difference substantially matches an optical path length difference of the first and second interferometer arms in a selected sensing interferometer, such that portions of an optical signal transmitted from the light source through the sensor system will interfere at an output of the receiving interferometer providing an optical signal representative of conditions causing change in light transmission characteristics of said selected sensing interferometer.

1 more element

A distributed sensor system comprising:

a light source;
an optical fiber optically coupled at an input end to the light source, said optical fiber defining first and second orthogonal modes for propagating light therethrough;
a plurality of means positioned at selected locations on said optical fiber for acting on said fiber to couple light between said orthogonal modes at said selected locations, wherein a first pair of means for acting are positioned on said optical fiber to define a first propagation length difference in the orthogonal modes therebetween which is greater than one coherence length of the light source, with the optical fiber between said first pair of acting means being sensitive to an environmental effect so as to influence light propagating in said orthogonal modes, and wherein a second pair of acting means are positioned on said optical fiber to define a second propagation length difference in the orthogonal modes therebetween which is substantially equal to the first propagation length difference, with the optical fiber between the second pair of acting means not subject to the same environmental effect as the optical fiber between the first pair of acting means;
and
means for receiving light from the orthogonal modes in the optical fiber between the second pair of acting means and for coupling said received light, thereby providing an optical signal which is produced by light which interferes in the coupling means, said optical signal being representative of the environmental effect influencing light propagating through the orthogonal modes in said optical fiber between the first pair of acting means.

2 more elements

Dependencies are inferred from the claim text.

Distributed sensor and method using coherence multiplexing of fiber-optic interferometric sensors

Life of this patent

Distributed sensor and method using coherence multiplexing of fiber-optic interferometric sensors

Life of this patent

Abstract

Claims45