Patent ID: 12259296

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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.

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 inFIG.1a. An optical fiber100having a core101, a cladding102and a protective polymer coating103containing FBG region104, is attached to a package105. The package has two attachment points106and107. Optical fiber100is attached under strain to attachment points106and107using bonds108and109, with FBG region104being disposed between the bonds. Bonds108and109are 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.

Alternatively, the package can be an athermal package as depicted inFIG.1b. The package has portions110and111that are made of different materials. Optical fiber100is attached under strain to portions110and111using bonds108and109, with FBG region104being disposed between the bonds. Bonds108and109are 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 portion110exactly counteracts the variation in strain caused by expansion of the portion111and the inherent temperature dependence of the FBG.

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).

According to a further embodiment, the package may apply a compressive strain instead of applying a tensile strain. The package112depicted inFIG.1c, constricts about the FBG104during 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.

InFIG.1e, the package has portions114that are bonded or crimped to the fiber coating103beyond the location of the FBG. The anchors114may be surrounded by an optional perforated membrane cylinder115. The region between the membrane and the fiber coating is filled with a material113that 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 cylinder115and anchors114constrain the swelling material113such that it applies a tensile strain to the fiber grating which results in a variation in the sensor's wavelength.

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.

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.

It is possible that the material113upon 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.

An important common factor of each embodiment shown inFIGS.1ato1e, 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.

FIG.2aillustrates a system for detecting presence of a fluid, such as an oil leak using the sensors discussed above with reference toFIGS.1ato1e. In the system ofFIG.2a, an optical source/detector system or fiber Bragg grating interrogator200, such as the SM125 Optical Sensing Interrogator from Micron Optics, Atlanta, Ga., interrogates a fiber Bragg grating array, which is comprised of an optical fiber100and a series of n fiber grating sensor elements, with a first element201having a resonant wavelength λ1and a last sensing element203having a resonant wavelength λn. 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 spectrum204of reflected intensity versus wavelength at the interrogator200.FIG.2bdepicts an example of a Bragg resonance shift when packaged grating element205having a Bragg resonance at λ2, is exposed to an oil leak206. For the case of the packages denoted inFIGS.1aand1b, volatile organics in the oil dissolve the adhesive bonds of the package that locally apply strain to the grating element205. With the adhesive dissolved, the strain on the grating205is released resulting in a shifting of the Bragg resonance λ2,207, to a shorter wavelength208. With specific Bragg resonances correlated with specific positions along the length of the fiber, the leak206can be localized. By packaging individual sensors in athermal packages, such as the package ofFIG.1b, wavelength shifts due to temperature variations are minimized. For the case of the packages illustrated inFIGS.1cand1d, which apply a compressive strain to the FBG, the volatile organics in the fluid leak206dissolve the constraining package resulting in a release of the compressive strain and shift to higher wavelengths209of the grating element205.

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%. InFIG.3a, the WSTDM300comprises a continuous wave tunable laser301, three fiber amplifiers302, an electro-optic modulator303, a pulse driver304, an optical circulator305, a bandpass filter306, a lightwave detector307, and a computer oscilloscope308. Light from the tunable laser301is amplified by the first fiber amplifier302and then launched in the electro-optic modulator303to produce an optical pulse with a desired pulse duration and repetition rate. To compensate for the high loss of electro-optic modulator303, the output optical signal is further amplified by the second fiber amplifier302before being passed through the optical circulator305and launched into the array of weak FBGs with identical wavelength310. 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 amplifier302, and bandpass filter306is used to suppress amplified spontaneous emission from the three fiber amplifiers before the signal is detected by the lightwave detector307. 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 oscilloscope308then measures the time of light of the returning pulses.

FIG.3bshows detection of an event when the wavelength of a single device205shifts away from the nominal same resonant wavelength311of the remaining FBGs. These same wavelength resonance values of the remaining FBGs can be distributed about the nominal wavelength value311differing 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 element205shifts to the lower wavelength208. Alternatively, if the sensor is packaged under compression, resonance of the sensing element205shifts to higher wavelengths209. If the sensors are packaged with anchors as depicted inFIG.1e) and all the Bragg resonances are contained in a defined wavelength range311, 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.

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.