Remote sensing of physical variables with fiber optic systems

• US 5,363,463 A
• Filed: 01/02/1992
• Issued: 11/08/1994
• Est. Priority Date: 08/06/1982
• Status: Expired due to Fees

• Current Assignee: Marcos Y. Kleinerman
• Original Assignee: Marcos Y. Kleinerman
• Inventors: Kleinerman, Marcos Y.
• Primary Examiner(s): Healy, Brian
• Application Number:
  US07/815,741
• Time in Patent Office: 1,041 Days
• Field of Search: 385/12, 385/13, 385/27, 385/28, 385/29, 385/30, 385/31, 385/39, 385/38, 385/123, 385/125, 385/126, 385/127, 385/147, 359/115, 359/124, 359/127, 250/227.11, 250/227.16, 250/227.18, 250/227.22, 250/227.23, 250/458.1, 250/459.1, 372/6, 356/43, 356/44, 356/73.1, 356/300, 356/301, 356/319, 356/320, 356/432, 356/445
• US Class Current: 385/123
• CPC Class Codes: A61B 2017/00084 Temperature
• CPC Class Codes: G01D 5/35358 using backscattering to detect the measured quantity
• CPC Class Codes: G01D 5/3538 using a particular type of fiber, e.g. fibre with several cores, PANDA fiber, fiber with an elliptic core or the like
• CPC Class Codes: G01J 5/0025 Living bodies ear thermometers G01J5/0011; detecting, measuring or recording for diagnostic purposes A61B5/00
• CPC Class Codes: G01J 5/08 Optical arrangements
• CPC Class Codes: G01J 5/0818 Waveguides
• CPC Class Codes: G01J 5/0821 Optical fibres
• CPC Class Codes: G01J 5/0896 using a light source, e.g. for illuminating a surface
• CPC Class Codes: G01J 5/52 using comparison with reference sources, e.g. disappearing-filament pyrometer
• CPC Class Codes: G01J 5/602 using selective, monochromatic or bandpass filtering
• CPC Class Codes: G01K 11/32 using changes in transmittance, scattering or luminescence in optical fibres
• CPC Class Codes: G01K 11/3213 using changes in luminescence, e.g. at the distal end of the fibres
• CPC Class Codes: G01L 1/243 using means for applying force perpendicular to the fibre axis
• CPC Class Codes: G06F 2203/04109 FTIR in optical digitiser, i.e. touch detection by frustrating the total internal reflection within an optical waveguide due to changes of optical properties or deformation at the touch location
• CPC Class Codes: G06F 3/041 Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
• CPC Class Codes: G06F 3/042 by opto-electronic means
• CPC Class Codes: G06F 3/0421 by interrupting or reflecting a light beam, e.g. optical touch-screen

Abstract

The invention relates to methods and devices for measuring physical parameters by converting a fraction of the intensity of the interrogating light into a positive optical signal with wavelengths and/or light propagation modes different from those of the interrogating light, and having at least one characteristic which is a known function of the physical parameter being measured. The invention is adapted to the measurement of distributed forces and/or temperatures along a continuous length of optical fiber, and to the non-invasive coupling of information into an optical fiber from the side at any point or a multiplicity of points. The optical fiber is so designed that information signals coupled into it simultaneously at different points are separable at one end of the fiber and measurable without interference from each other.

Claims

1. A light guiding optical fiber having a proximal end and a distal end and comprising a first core having an index of refraction n₁ and at least one light-guiding region including a pre-selected organic dye characterized by absorbing light of pre-selected wavelengths λ

s to which said core is substantially transparent, and by emitting fluorescence radiation when exposed to said light of wavelengths λ

_s, said fluorescence radiation having wavelengths λ

_f different from λ

_s and a decay time of the order of 10^-8 seconds or shorter, wherein said light guiding region including said organic dye can e a cladding around said first core or a second core separated from said first core by a clear cladding common to both cores.

3. An optical fiber as claimed in claim 1 wherein said light-guiding region is a fluorescent cladding around said core and also including a clear cladding around and in contact with said core and surrounded and in contact with said fluorescent cladding, said clear cladding being essentially transparent to said light of wavelengths λ

_s and having an index of refraction n₂ lower than n₁ and higher than the index of refraction n₃ of the fluorescent cladding.

6. A device for sensing physical changes in the environment of an optical fiber, comprising:

(a) an optical fiber as claimed in claim 1, the fiber being further so characterized that, when interrogating light of wavelengths within an absorption band of said fluorescent dye propagates along the fiber core, the physical changes to be sensed in said environment cause changes in the intensities of the fluorescence emitted by said fluorescent dye.

(b) light source means for generating and launching said interrogating light into the proximal end of said fiber; and

(c) fiberoptic means for directing a fraction of the intensity of the fluorescence emitted by said fluorescent dye to photodetector means.

7. A device as claimed in claim 6 wherein said fluorescence is evanescent fluorescence generated within a thickness of said cladding of the order of magnitude of a wavelength of optical radiation to which the core is transparent.

8. A device as claimed in claim 6 and adapted to sense said changes at a plurality of locations using a single optical fiber probe disposed along a path including said locations, the device additionally comprising:

(a) means for generating from said light source interrogating light pulses of submicrosecond duration launched into said proximal end of said fiber, thereby generating fluorescent light pulses;

(b) photodetector means disposed near said proximal end for receiving the fluorescence light pulses generated at each of said plurality of locations and back-propagated toward said proximal end; and

(c) electronic means for processing said fluorescence light pulses into electrical signals indicative of said changes.

9. A device as claimed in claim 8 and adapted to sense physical forces at a plurality of locations, wherein said fiber includes a first core having an index of refraction n₁, a transparent first cladding around and in contact with said core and having an index of refraction n₂ lower than n₁, and a light-guiding around and in contact with said first cladding, said light-guiding region including a pre-selected organic dye characterized by absorbing light of pre-selected wavelengths, λ

_s to which said core is substantially transparent, and by emitting fluorescence radiation when exposed to said light of wavelengths, λ

_s, said fluorescence radiation having wavelengths λ

_f different from λ

_s, the fiber being further so characterized that, at each location where the fiber is under the action of a mechanical force, a fraction α

of the intensity of the interrogating light pulses propagating along the fiber core is deflected from said core to said fluorescent cladding and generates therein fluorescence light pulses of an intensity proportional to the value of α

, the value of α

varying as a function of the magnitude of the force.

11. A device for sensing forces at a plurality of locations, comprising:

(a) an optical fiber as claimed in claim 1;

(b) light source means for generating and launching said light of wavelengths λ

_s into the proximal end of said fiber;

(c) photodetector means optically coupled to said fiber for sensing changes of the intensity of said light of wavelengths λ

_f generated at locations along said fiber by forces acting at said points.

2. An optical fiber having a proximal end and a distal end and comprising a first core having an index of refraction n₁, and at least one light-guiding region including a pre-selected material characterized by absorbing light of pre-selected wavelengths λ

_s to which said core is substantially transparent, and also including a clear cladding around and in contact with said core and in contact with said light-guiding region, said clear cladding being essentially transparent to said light of wavelengths λ

_s and having an index of refraction n₂ lower than n₁ and higher than the index of refraction n₃ of said light-guiding region, wherein said light guiding region including said pre-selected material can be a second cladding around said clear cladding or a second core separated from said first core by said clear cladding.

4. An optical fiber as claimed in claim 2 wherein said light-guiding region is a second core comprised of photoluminescent material characterized by emitting luminescence radiation upon absorption of said light of wavelengths λ

_s, said second core and said first core being disposed within said clear cladding.

5. An optical fiber as claimed in claim 4 wherein said second core is comprised of laser material.

10. A device for sensing mechanical forces comprising:

(a) an optical fiber as claimed in claim 2;

(b) light source means for generating and launching said light of wavelengths λ

_s into the proximal end of said fiber;

(c) photodetector means optically coupled to said fiber for sensing changes of the intensity of said light of wavelengths λ

_s transmitted by said fiber caused by a force.

21. An optical fiber as claimed in claim 2 wherein said light-guiding region including said emitter of luminescence radiation is a cladding around said core and additionally includes a second luminescent material characterized by emitting luminescence radiation at wavelengths different from λ

_f and λ

_s.

22. An optical fiber as claimed in claim 4 wherein said second core is comprised of glass containing dissolved therein ions of at least one rare earth element chosen from the group of rare earth elements comprising neodymium, erbium, ytterbium and holmium.

12. An optical time division multiplexing method of sensing respective values of a physical parameter at different locations along an optical fiber, the fiber being so characterized that, when interrogated with optical radiation of suitable wavelength or wavelengths and a given intensity, it converts a fraction of the intensity of the interrogating radiation propagating along the fiber at each of said locations into an optical radiation of wavelengths λ

_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, the method comprising the steps of;

(a) launching said interrogating optical radiation of suitable wavelength or wavelengths into the fiber at one end thereof; and

selectively directing a fraction of the intensity of the said optical radiation of wavelengths λ

_f returned to said one end of the fiber to photodetector means, thus producing electrical output signals indicative of the values being sensed.

13. An optical time division multiplexing method as claimed in claim 12 and additionally comprising the steps of:

(a) directing backscattered optical radiation of the same wavelength or wavelengths of the interrogating radiation from said locations to photodetector means, thus producing electrical output signals from said locations indicative of the intensity of said interrogating radiation at said locations; and

(b) measuring the relative intensities of said radiation of wavelengths λ

_f and said radiation of the same wavelength or wavelengths as those of the interrogating radiation backscattered from each of said locations, said relative intensity being indicative of the values of the physical parameter.

14. 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 λ

_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 λ

_f at said different locations thereof, the intensity of said radiation of wavelengths 80 _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 λ

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 λ

_f directed from said locations to said one end, and for producing electrical output signals indicative of the intensity of said radiation of wavelengths λ

_f.

15. Optical time division multiplexing apparatus as claimed in claim 14 additionally comprising means for sensing variations of the intensity of optical radiation having the same wavelength or wavelengths of the interrogating radiation and backscattered from said locations to said one end of the optical fiber.

16. Optical time division multiplexing apparatus as claimed in claim 14 apparatus wherein said radiation of wavelengths λ

_f is fluorescent radiations generated by partial absorption of the interrogating light at each of said locations.

17. Optical time division multiplexing apparatus as claimed in claim 16 wherein said fiber has a core into which the interrogating radiation is launched, and a cladding around said core and having incorporated therein a fluorescent dye, the fiber being further so characterized that, when transmitting pulses of interrogating optical radiation along said core, a lateral force acting on the fiber at one of said locations causes the deflection of a fraction of the intensity of said interrogating radiation from said core to said cladding and the generation at said cladding of pulses of fluorescence radiation of said wavelengths λ

_f.

18. Optical time division multiplexing apparatus as claimed in claim 14 wherein said fiber is doped along its length with laser material having an optical absorption band at wavelengths including the wavelength or wavelengths of the interrogating radiation, and wherein said interrogating optical radiation has an intensity sufficiently high to produce a population inversion of the laser material and a mplification of otpical radiation of wavelength λ

_f within a laser band of said material at said locations, the magnitude of said population inversion and the amplification gain being a function of the value of the physical parameter at said locations, the apparatus additionally comprising;

(a) source means for launching a continuous beam of optical radiation of wavelength λ

_f at the second end of said fiber, to propagate in a direction counter to the direction of propagation of the interrogating radiation; and

(b) means for measuring the amplification gain of said continuous beam of optical radiation of wavelengths λ

_f at said locations.

19. 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 n₁, a first cladding around and in contact with said core and having an index of refraction n₂ lower than n₁, and a second cladding around and in contact with said first cladding and having an index of refraction n₃ lower than n₂,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.

20. 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 n₁, a first cladding around and in contact with said core and having an index of refraction n₂ lower than n₁, and a second cladding around and in contact with said first cladding and having an index of refraction n₃ lower than n₂, 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.

23. 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 n₁ and a second core B having an index of refraction n₂ higher than n₁, 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 λ

_s and submicrosecond duration are propagating within core A, a force acting on the fiber at any location causes the coupling of a fraction α

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.

24. Optical time division multiplexing arrangement as claimed in claim 23 wherein said core B is comprised of photoluminescent material and said fraction α

of the intensity of the interrogating light coupled from core A to core B is converted into luminescence light including wavelengths λ

_f different from λ

_s, the value of α

varying as a function of the magnitude of said force.

25. 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 λ

_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 λ

_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 λ

_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 λ

_f.

26. Optical time division multiplexing apparatus as claimed in claim 25 and adapted to sense respective values of a physical parameter at different locations along an optical fiber system having a light-guiding region doped along its length with fluorescent material so characterized that, when exposed to said interrogating optical radiation, it converts at least a fraction of the intensity of the interrogating radiation into pulsed or AC-modulated fluorescence radiation including said wavelengths λ

_f.

27. Optical time division multiplexing apparatus as claimed in claims 25 and adapted to sense respective values of a physical parameter at different locations along an optical fiber doped along its length with laser material having an optical absorption band at wavelengths including the wavelength or wavelengths of the interrogating radiation, wherein said interrogating radiation has an intensity sufficiently high to produce a population inversion of the laser material and amplification of optical radiation of wavelength λ

_f within a laser band of said material, the magnitude of said population inversion and the amplification gain being a function of the value of the physical parameter at said locations, the apparatus additionally comprising source means for injecting into the fiber a counterpropagating beam of optical radiation of wavelength λ

_f for propagating in a direction counter to the direction of propagation of the interrogating radiation.

28. Optical time division multiplexing apparatus as claimed in claim 25 and adapted to sense respective values of a physical parameter at different locations along an optical fiber doped along its length with Raman-scattering material, wherein said interrogating optical radiation has wavelengths λ

_s and an intensity sufficiently high to produce amplification of counterpropagating optical radiation of the Raman-shifted wavelengths λ

_f, the magnitude of said population inversion and the amplification gain being a function of the value of the physical parameter at said locations, the apparatus additionally comprising source means for injecting into the fiber a counterpropagating beam of optical radiation of wavelengths λ

_f for propagating in a direction counter to the direction of propagation of the interrogating radiation.