APPARATUS, METHOD AND SYSTEM FOR DETECTING PRESENCE OF A FLUID

Abstract

An apparatus, method and system are set forth for detection of fluids using Bragg grating sensors, wherein the Bragg grating sensing element comprises an optical fiber having a Bragg grating inscribed therein characterized by optical properties that are dependent upon the periodicity and effective refractive index of the grating, and a package for subjecting the Bragg grating to a change in strain when contacted by a fluid such that periodicity and effective refractive index of the grating changes, whereby when interrogated with laser light any such change in periodicity and effective refractive index may be detected.

Claims

1. A method for detecting presence of a fluid, comprising: mounting a sensor array proximate a conduit for said fluid, said sensor array comprising a plurality of Bragg grating sensing elements within an optical waveguide, each Bragg grating sensing element being encapsulated in a package for subjecting the Bragg grating sensing element to a change in strain when contacted by said fluid; and interrogating said sensor array with laser light to detect any change in the optical properties of the Bragg grating sensing elements due to the change in strain.

2. The method of claim 1, wherein said package increases the strain to which the Bragg grating sensing element is subjected when contacted by said fluid.

3. The method of claim 1, wherein said package encapsulates the Bragg grating sensing element under an initial strain and decreases the strain to which the Bragg grating sensing element is subjected when contacted by said fluid.

4. The method of claim 2 or 3, wherein the strain is one of either compression or tension.

5. The method of claim 1, wherein the sensor array is interrogated using at least one of wavelength division multiplexing or time division multiplexing.

7. The method of claim 1, wherein the change in strain comprises a change in one of either tension, compression, torsion, shear or bending.

8. The method of claim 1, wherein the Bragg grating sensing elements are inscribed through the protective polymer coating of an optical fiber with a laser having a pulse duration less than 5 ps.

9. A fluid detecting Bragg grating sensing element, comprising: an optical fiber having a Bragg grating inscribed therein characterized by optical properties that are dependent upon the periodicity and effective refractive index of the grating; and a package for subjecting the Bragg grating to a change in strain when contacted by a fluid such that periodicity and effective refractive index of the grating changes, whereby when interrogated with laser light any such change in periodicity and effective refractive index may be detected.

10. The sensing element of claim 9, wherein the optical fiber is connected to the package and comprises a core, a cladding and a protective polymer coating.

11. The sensing element of claim 10, wherein the package comprises a first attachment point and a second attachment point, the optical fiber being attached under strain to the first and second attachment points via adhesive, with the Bragg grating disposed being between the attachment points.

12. The sensing element of claim 11, wherein the adhesive is insoluble in water but soluble in hydrocarbon-based fluids.

13. The sensing element of claim 12, wherein the adhesive is UV cured epoxy.

14. The sensing element of claim 11, wherein the first attachment point and second attachment point are made of different materials such that expansion of one of the first and second attachment points counteracts variation in strain caused by expansion of the other one of the first and second attachment points so as to be temperature independent.

15. The sensing element of claim 10, wherein the package is fabricated from one of either thermally formed polystyrene or rubber-based heat-shrink tubing wherein a compressive strain is applied to the Bragg grating by the thermal formation of the package such that the compressive strain is released when the package is exposed to hydrocarbon-based solvents.

16. The sensing element of claim 10, wherein the package comprises anchors that are one of either bonded or crimped to the protective polymer coating on either side of the Bragg grating, a region between the anchors being filled with a material that swells when exposed to said fluid such that the anchors constrain swelling of the material so as to apply a tensile strain to the Bragg grating resulting in a variation in optical properties of the Bragg grating.

17. The sensing element of claim 16, wherein the fluid is oil and the material comprises one of either polyolefin or petrogel.

18. The sensing element of claim 10, wherein the package comprises anchors that are one of either bonded or crimped to the protective polymer coating on either side of the Bragg grating, a region between the anchors being filled with a material that contracts when exposed to said fluid such that the anchors conform to the contraction of the material so as to apply a compressive strain to the Bragg grating sufficient to break the optical fiber.

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Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1a is a schematic illustration of an exemplary Bragg fiber grating sensor for use in a distributed sensor array, packaged under tension with adhesives that are insoluble in water but soluble in oil.

[0023] FIG. 1b is a schematic illustration of an alternative exemplary Bragg fiber grating sensor where the package is designed to compensate for variations in temperature of the environment.

[0024] FIG. 1c is a schematic illustration of a further alternative exemplary Bragg fiber grating sensor for use in a distributed sensor array, packaged under a compressive strain rather than a tensile strain

[0025] FIG. 1d is a cross-sectional view of the package depicted in FIG. 1c.

[0026] FIG. 1e is a schematic illustration of yet a further alternative exemplary Bragg fiber grating sensor packaged without any initial strain, wherein the package is capable of reacting to oil but not to water such that, upon exposure to oil, the package applies a detectable strain to the grating.

[0027] FIG. 2a shows a WDM based sensing system comprising an FBG interrogator and a distributed sensor array with individual FBG elements packaged, in accordance with an embodiment.

[0028] FIG. 2b shows a WDM based sensing system including an array of FBG sensors for detecting an oil spill when the spill results in either a release or application of strain to at least one sensor, in accordance with an embodiment.

[0029] FIG. 3a shows a TDM based sensing system comprising an FBG interrogator and a distributed sensor array with individual FBG elements packaged, in accordance with an embodiment.

[0030] FIG. 3b shows a TDM based sensing system including an array of FBG sensors for detecting an oil spill when the spill results in either a release or application of strain to at least one sensor, in accordance with an embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] As used in this specification, a fluid is a phase of matter and includes liquids, gases and plasmas. Thus, detection of fluids includes, inter alia, detection of petrochemical fluids such as oil, methane gas, etc. Also, as used in this specification, strain means, inter alia, tension, compression, torsion, shear, bending and any geometrical measure of deformation.

[0032] According to an aspect of this disclosure, an array of fiber Bragg gratings (FBGs) is provided, each FBG having a different resonance wavelength inscribed through a hard protective polymer coating of an optical fiber, such as polyimide, with a femtosecond laser and a phase mask, such that the FBGs are written in the Type I regime. Each grating element is then individually mounted on a package, for example as illustrated schematically in FIG. 1a. An optical fiber 100 having a core 101, a cladding 102 and a protective polymer coating 103 containing FBG region 104, is attached to a package 105. The package has two attachment points 106 and 107. Optical fiber 100 is attached under strain to attachment points 106 and 107 using bonds 108 and 109, with FBG region 104 being disposed between the bonds. Bonds 108 and 109 are an epoxy or other adhesive that is insoluble in water but soluble in hydrocarbon-based fluids (ex. UV cured epoxy and toluene). The entire array comprises a series of n individually packaged FBG elements may be further packaged in a tube (not shown) to afford mechanical protection, wherein the tube is perforated to allow flow of fluids transversally through the tube.

[0033] Alternatively, the package can be an athermal package as depicted in FIG. 1b. The package has portions 110 and 111 that are made of different materials. Optical fiber 100 is attached under strain to portions 110 and 111 using bonds 108 and 109, with FBG region 104 being disposed between the bonds. Bonds 108 and 109 are an epoxy or other adhesive that is insoluble in water but soluble in hydrocarbon-based fluids (ex. UV cured epoxy soluble in toluene). The package is athermal, where expansion of the material portion 110 exactly counteracts the variation in strain caused by expansion of the portion 111 and the inherent temperature dependence of the FBG.

[0034] As discussed above, each FBG has a resonance wavelength and is inscribed through a protective polymer coating of the optical fiber with a femtosecond laser and a phase mask. In one embodiment, the electromagnetic radiation has a pulse duration of less than or equal to 5 picoseconds, and a characteristic wavelength in the range of from 150 nm to 2.0 microns, the electromagnetic radiation incident on the optical waveguide being sufficiently intense to cause a permanent change in an index of refraction within the core of the optical fiber (i.e. creating an interference pattern).

[0035] According to a further embodiment, the package may apply a compressive strain instead of applying a tensile strain. The package 112 depicted in FIG. 1c, constricts about the FBG 104 during fabrication. This results in a shift in the FBG resonance toward shorter wavelengths. As in the aforementioned tensile design, the package is insoluble in water but soluble in hydrocarbon-based solvents. Such a constricting package can, for example, be thermally formed polystyrene or rubber-based heat-shrink tubing. The compressive strain is applied by the thermal formation of the package, and exposure to hydrocarbons releases the compressive strain.

[0036] In FIG. 1e, the package has portions 114 that are bonded or crimped to the fiber coating 103 beyond the location of the FBG. The anchors 114 may be surrounded by an optional perforated membrane cylinder 115. The region between the membrane and the fiber coating is filled with a material 113 that swells or hardens when exposed to oil (ex. polyolefin or petrogel). Without exposure to a hydrocarbon, the FBG sensor is not subjected to any strain. The perforated cylinder 115 and anchors 114 constrain the swelling material 113 such that it applies a tensile strain to the fiber grating which results in a variation in the sensor's wavelength.

[0037] Depending on the initial thickness of the swelling material surrounding the fiber, for example ethylene propylene diene monomer rubber (EPDM), and the adhesive strength of the anchors to the fiber, sufficient tensile strain can be applied such that the fiber will reach its breakage point, creating an optical ‘fuse’. Using an OTDR, the location of the breakage point in the fiber can be determined.

[0038] The breakage strength of the fiber with grating can be controlled by the laser exposure conditions used to fabricate the Bragg grating. Gratings written through polymer coatings of the fiber that in the Type I regime can withstand tensile strain levels up to that of the pristine optical fiber. By increasing the intensity such that Type II gratings are formed, the breakage strength can be reduced up to a factor of 5. By controlling the exposure conditions, beam intensity and number of superimposed laser pulses during FBG inscription, the resultant breakage strength of the fiber can be accurately controlled.

[0039] It is possible that the material 113 upon exposure to hydrocarbon-based solvents such as oil will constrict thus applying a compressive strain. This would be advantageous if an optical fuse is not desired as optical fibers can withstand much higher compressive strains when compared with tensile strains.

[0040] An important common factor of each embodiment shown in FIGS. 1a to 1e, is that strain is either released or applied to the FBG as a result of interaction of the package with a chemical to be detected. As discussed above, the strain can be but is not limited to tension, compression, torsion, shear, bending etc.

[0041] FIG. 2a illustrates a system for detecting presence of a fluid, such as an oil leak using the sensors discussed above with reference to FIGS. 1a to 1e. In the system of FIG. 2a, an optical source/detector system or fiber Bragg grating interrogator 200, such as the SM125 Optical Sensing Interrogator from Micron Optics, Atlanta, Ga., interrogates a fiber Bragg grating array, which is comprised of an optical fiber 100 and a series of n fiber grating sensor elements, with a first element 201 having a resonant wavelength λ1 and a last sensing element 203 having a resonant wavelength λ.sub.n. All gratings have reflectivities >25% in order to enhance the signal to noise ratio of the detection. Each grating element is packaged under strain, as discussed above, for producing a reflection spectrum 204 of reflected intensity versus wavelength at the interrogator 200. FIG. 2b depicts an example of a Bragg resonance shift when packaged grating element 205 having a Bragg resonance at λ2, is exposed to an oil leak 206. For the case of the packages denoted in FIGS. 1a and 1b, volatile organics in the oil dissolve the adhesive bonds of the package that locally apply strain to the grating element 205. With the adhesive dissolved, the strain on the grating 205 is released resulting in a shifting of the Bragg resonance λ2, 207, to a shorter wavelength 208. With specific Bragg resonances correlated with specific positions along the length of the fiber, the leak 206 can be localized. By packaging individual sensors in athermal packages, such as the package of FIG. 1b, wavelength shifts due to temperature variations are minimized. For the case of the packages illustrated in FIGS. 1c and 1d, which apply a compressive strain to the FBG, the volatile organics in the fluid leak 206 dissolve the constraining package resulting in a release of the compressive strain and shift to higher wavelengths 209 of the grating element 205.

[0042] Alternatively, the interrogation system can be based on a WSTDM approach, as taught by Bo Dong et al. in U.S. Pat. No. 9,677,957. In this case, each of the FBG sensing elements, instead of having a unique Bragg resonance, has an identical Bragg resonance and reflectivity less than 0.1%. In FIG. 3a, the WSTDM 300 comprises a continuous wave tunable laser 301, three fiber amplifiers 302, an electro-optic modulator 303, a pulse driver 304, an optical circulator 305, a bandpass filter 306, a lightwave detector 307, and a computer oscilloscope 308. Light from the tunable laser 301 is amplified by the first fiber amplifier 302 and then launched in the electro-optic modulator 303 to produce an optical pulse with a desired pulse duration and repetition rate. To compensate for the high loss of electro-optic modulator 303, the output optical signal is further amplified by the second fiber amplifier 302 before being passed through the optical circulator 305 and launched into the array of weak FBGs with identical wavelength 310. The incident pulse is partially reflected by each of the serial FBGs. The magnitude of the pulse reflected by an FBG is determined by the pulse wavelength and the FBG reflection spectrum. The successive pulses reflected by the FBG array are amplified by the third fiber amplifier 302, and bandpass filter 306 is used to suppress amplified spontaneous emission from the three fiber amplifiers before the signal is detected by the lightwave detector 307. This process is repeated for each wavelength increment of the tunable laser that is needed to scan the spectral range where an FBG reflection may appear. The oscilloscope 308 then measures the time of light of the returning pulses.

[0043] FIG. 3b shows detection of an event when the wavelength of a single device 205 shifts away from the nominal same resonant wavelength 311 of the remaining FBGs. These same wavelength resonance values of the remaining FBGs can be distributed about the nominal wavelength value 311 differing in resonance values from 0.001 nm to 10 nm. Identification of the individual sensor that is tripped by the leak is determined by the time of flight of the pulse having the resonant wavelength of the tripped FBG sensor. If the sensor is packaged under tension, Bragg resonance of the sensing element 205 shifts to the lower wavelength 208. Alternatively, if the sensor is packaged under compression, resonance of the sensing element 205 shifts to higher wavelengths 209. If the sensors are packaged with anchors as depicted in FIG. 1e) and all the Bragg resonances are contained in a defined wavelength range 311, then when one sensor is activated by the fluid, the package will determine the magnitude of the wavelength shift beyond the said wavelength range where all the other wavelengths are confined. The new resonance wavelength is then determined by the FBG interrogator and the position determined by the time of flight measurement of an interrogating laser pulse.

[0044] The many features and advantages of the invention are apparent from the detailed specification and, thus, it is intended by the appended claims to cover all such features and advantages of the invention that fall within the true spirit and scope of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.