Remote sensing of physical variables with fiber optic systems

Optical time division multiplexing apparatus for sensing respective values of a physical parameter at different locations on an optical fiber system throughout a region of interest, said apparatus being essentially an optical time domain reflectometer comprising:

• (a) an optical fiber, for deployment throughout said region of interest, the fiber being so characterized that, when interrogated with optical radiation of suitable wavelength or wavelengths and a given intensity, it converts a fraction the intensity of the interrogating radiation propagating along the fiber at each of said different locations thereof into an optical radiation of wavelengths .lambda..sub.f different from the wavelength or wavelengths of the interrogating radiation and having an intensity indicative of the value of the physical parameter at said locations;
• (b) source means for launching interrogating pulsed or AC-modulated optical radiation into said optical fiber system at one end thereof, to generate therein said optical radiation of wavelengths .lambda..sub.f at said different locations thereof, the intensity of said radiation of wavelengths 80 .sub.f at each location being indicative of the value of the physical parameter at said location;
• (c) fiberoptic means for directing a fraction of the intensity of said radiation of wavelengths .lambda.hd f generated at said different locations to said one end and to photodetector means located at or near said one end;
• and
• (d) photodetector and associated optical means for selectively receiving, from said one end of the optical fiber, said radiation of wavelengths .lambda..sub.f directed from said locations to said one end, and for producing electrical output signals indicative of the intensity of said radiation of wavelengths .lambda..sub.f.

Optical time division multiplexing method of sensing respective values of a physical parameter at different locations, comprising the steps of:

• (a) laying out an optical fiber along a path including said locations, the fiber having a core with an index of refraction n.sub.1, a first cladding around and in contact with said core and having an index of refraction n.sub.2 lower than n.sub.1, and a second cladding around and in contact with said first cladding and having an index of refraction n.sub.3 lower than n.sub.2,the fiber being so characterized that, when interrogated with light pulses of submicrosecond duration and a wavelength or wavelengths within a suitable pre-selected spectral region launched into core and said first cladding, the relative distribution varying as a function of the value of the physical parameter acting on the fiber at that location and generating pulses of backscattered light the intensity of which is indicative of the value of the physical parameter at that location;
• (b) launching said interrogating light pulses of suitable wavelength or wavelengths into the fiber at one end thereof;
• and
• (c) directing a fraction of the intensity of said pulse of backscattered light returned to said one end of the fiber to photodetector means, thus producing electrical output signals indicative of the values being sensed.

Optical time division multiplexing device for sensing respective values of a physical parameter at different locations throughout a region of interest, said apparatus comprising:

• (a) an optical fiber for deployment throughout said region of interest, the fiber having a core with an index of refraction n.sub.1, a first cladding around and in contact with said core and having an index of refraction n.sub.2 lower than n.sub.1, and a second cladding around and in contact with said first cladding and having an index of refraction n.sub.3 lower than n.sub.2, the fiber being so characterized that, when interrogated with light pulses of submicrosecond duration and of a wavelength or wavelengths within a suitable pre-selected spectral region launched into said core, the intensity of the interrogating light pulses propagating at each location along the fiber is distributed between said core and said first cladding, the relative distribution varying as a function of the value of the physical parameter acting on the fiber at that location and generating pulses of backscattered light the intensity of which is indicative of the value of the physical parameter at that location;
• (b) source means for launching interrogating light pulses into said core of said optical fiber at one end thereof, to generate said pulses of backscattered light;
• and
• (c) photodetector means for receiving, from said one end of the optical fiber, said pulses of backscattered light returned from said locations to said one end, and for producing electrical output signals indicative of the intensity of said backscattered light.

Optical time division multiplexing arrangement for sensing respective magnitude of physical forces acting at different locations on an optical fiber system throughout a region of interest, said apparatus comprising:

• (a) an optical fiber for deployment throughout said region of interest, the fiber including a core A having an index of refraction n.sub.1 and a second core B having an index of refraction n.sub.2 higher than n.sub.1, said two cores being disposed with respect to each other within a common cladding in such a manner that, when pulses of interrogating light of pre-selected wavelengths .lambda..sub.s and submicrosecond duration are propagating within core A, a force acting on the fiber at any location causes the coupling of a fraction .alpha. of the intensity of said interrogating light to core B, thereby generating an optical signal the intensity of which is a function of the magnitude of the force acting on the fiber at that location, said signal reaching at least one fiber end separated in the time domain from optical signals generated by forces at other locations along the fiber.
• (b) source means for launching interrogating pulsed or AC-modulated optical radiation into said optical fiber system at one end thereof;
• and
• (d) photodetector means for receiving the optical signals generated at different locations along the fiber.

Optical time division multiplexing apparatus for sensing variations of the values of a physical parameter at different locations along an optical fiber system, said apparatus comprising:

• (a) source means for launching interrogating pulsed or AC-modulated optical radiation into said optical fiber system at one location thereof, to generate at said different locations therein pulsed or AC-modulated radiation including wavelengths .lambda..sub.f different from the wavelength or wavelengths of the interrogating radiation and having an intensity indicative of the value of the physical parameter at said locations;
• `(b) fiberoptic means for directing a fraction of the intensity of said radiation of wavelengths .lambda..sub.f generated at said different locations to photodetector means;
• and
• (c) photodetector and associated optical means for selectively receiving, from said one location of the optical fiber, said radiation of wavelengths .lambda..sub.f returned from said locations to said one location, and for producing electrical output signals indicative of the intensity of said backscattered radiation of wavelengths .lambda..sub.f.