Fiber optic gas sensor

A gas sensor includes an in-fiber resonant wavelength device provided in a fiber core at a first location. The fiber propagates a sensing light and a power light. A layer of a material is attached to the fiber at the first location. The material is able to absorb the gas at a temperature dependent gas absorption rate. The power light is used to heat the material and increases the gas absorption rate, thereby increasing sensor performance, especially at low temperatures. Further, a method is described of flash heating the gas sensor to absorb more of the gas, allowing the sensor to cool, thereby locking in the gas content of the sensor material, and taking the difference between the starting and ending resonant wavelengths as an indication of the concentration of the gas in the ambient atmosphere.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sensors for sensing the presence of gasses such as hydrogen, and more specifically to fiber optic gas sensor that employs an in-fiber resonant wavelength device, such as an FBG, wherein performance is improved using an in-fiber power light.

2. Description of Related Art

Fiber optic components, such as, without limitation, Fiber Bragg Gratings (FBGs), fiber interferometers, and Fabry-Perot cavities (FPs) are well known and are key components used in many optical communication and sensing applications. For example, such components are often utilized in constructing multiplexers and de-multiplexers used in wavelength division multiplexing (WDM) optical communications systems, and in constructing optical strain sensors, temperature sensors, pressure or vibration sensors, chemical sensors and accelerometers. In-fiber optic components, meaning those provided in or as part of an optical fiber, offer several important advantages over other optical and electronic devices, including low manufacturing cost, immunity to electromagnetic radiation and changing (often harsh) ambient conditions, an explosive-proof and in-vivo safe nature, long lifetime, and high sensitivity.

Historically, in-fiber optic components have been passive, meaning they cannot be actively adjusted and/or reconfigured once deployed to, for example, adopt new network topologies or adjust sensing parameters including sensitivity, set point, triggering time, dynamic range and responsivity. In addition, passive in-fiber optic components require delicate and costly packaging to eliminate temperature drifting. These facts have, despite the advantages described above, limited the performance and use of in-fiber components. As a result, work has been done to develop tunable in-fiber optic components, such as a tunable FBG. As is known in the art, an FBG consists of a series of perturbations, forming a grating from periodic variations in the index of refraction along the length of an optical fiber, that will be here termed “grating elements”. An FBG reflects a spectral peak of a light back through the fiber toward the light source, and the particular spectral peak (called the resonance wavelength) that is reflected depends upon the grating spacing. A corresponding valley is transmitted forward though the fiber. Thus, changes in the length of the fiber due to heat, tension or compression will change the spacing of the grating index of refraction variations (and to a lesser extent, the grating component indices of refraction) and thus the wavelength of the light that is reflected.

A typical prior art implementation of an FBG is shown inFIG. 1, and includes optical fiber5having core10surrounded by cladding15, wherein the core10is provided with a grating20. The light transmitted through optical fiber5and reflected by grating20is shown by the arrow inFIG. 1. The grating20shown inFIG. 1has a constant period, Λ, meaning the grating elements are evenly spaced, and is referred to as a uniform FBG. FBGs may also include gratings that have a varying period. Such FBGs are referred to as chirped FGBs, and reflect multiple spectral peaks or a wide spectrum of light. Long period gratings, in which the spacing is large compared to the core diameter, and apodized gratings are also useful. Tuning mechanisms (for changing the fiber length and other characteristics such as refractive index) that have been previously explored for FBGs and other in-fiber optic components include on-fiber electrical heating, piezoelectric actuators, mechanical stretching and bending, and acoustic modulation. The problem has been that each of these tuning mechanisms requires an energy source for operation, which, to date, has been electrical. In particular, electrical cable must be run with the optical fiber to provide current for on-fiber heating elements, to supply voltages to drive piezoelectric actuators, to drive stepper motors to stretch and bend the fibers, or to initialize acoustic waves. Additional cabling of this sort is problematic, as it, among other things, typically increases manufacturing costs, is bulky, is not immune to electromagnetic radiation, is difficult to embed in materials and structures, and typically has a shorter lifetime than the associated, normally durable optical fibers.

Thus, there is a need for a mechanism for powering and tuning in-fiber optic components that does not require additional electrical cabling. Such a mechanism would allow fiber optic systems to take advantage of the improved performance and functionality of in-fiber optic components without the disadvantages and drawbacks presented by electrical cabling.

Moreover, hydrogen is becoming an attractive alternative fuel source for use in clean-burning engines and power plants. Some mission-critical applications such as the Space Shuttle engine already employ liquid hydrogen as a fuel. Unfortunately, the use of highly flammable liquid H2also introduces a number of safety concerns due to its rapid evaporation rate and low explosive limit. In order to mitigate the high risk of explosion due to leaks in hydrogen fueled systems, an efficient system of H2leak detection is needed. Such a system should allow detection well below the 4% mass concentration explosion limit of hydrogen.

Recently, various electrical sensors based on the change of resistivity of palladium (Pd) have been developed including some nano-scale devices. Examples of such sensors are described in A. D'Amico et al, “Palladium-surface acoustic wave interaction for hydrogen detection,” Appl. Phys. Lett., vol. 41, pp. 300-301, (1982); I. Lundstrom et al., “A hydrogen-sensitive MOS field-effect transistor,” Appl. Phys. Lett., vol. 26, pp. 55-57, (1975); and M. C. Steele et al., “Palladium/cadmium-sulfide Schottky diodes for hydrogen detection,” Appl. Phys. Lett. vol. 28, pp. 687-688, (1976). Furthermore, because of their explosion proof nature, the desirability of fiber optic sensors has been recognized in recent years and more emphasis has been placed on the development of optical sensors such as those described in M. Tabib-Azar et al., “Highly sensitive hydrogen sensors using palladium coated fiber optics with exposed cores and evanescent field interactions,” Sens. Act., vol. B 56, pp. 158-163, (1999); J. Villatoro et al., “Fast detection of hydrogen with nano fiber tapers coated with ultra thin palladium layers,” Optics. Exp., vol. 13, pp. 5087-5092, (2005); J. Villatoro et al., “In-Line Highly Sensitive Hydrogen Sensor Based on Palladium-Coated Single-Mode Tapered Fibers,” IEEE Sens. Journal, vol. 3, pp. 533-537 (2003); and X. Bevenot et al., “Surface plasmon resonance hydrogen sensor using an optical fibre,” IOP Meas. Sci. Technol., vol. 13, pp. 118-124, (2002). Of particular interest are optical sensors that are of the type that can be interrogated remotely over long fibers, such as those described in X. Bevenot et al., “Hydrogen leak detection using an optical fibre sensor for aerospace applications,” Sens. Act., vol. B 67, pp. 57-67, (2000); A. Trouillet et al., “Fibre gratings for hydrogen sensing,” Measurement Science & Technol., vol. 17(5), pp. 1124-1128, (2006); and J. A. Guemes et al., “Comparison of three types of fibre optic hydrogen sensors within the frame of CryoFOS project,” Third International Conference on Experimental Mechanics and Third Conference of the Asian Committee on Experimental Mechanics, Proceedings of the SPIE, Vol. 5855, pp. 1000-1003 (2005). Another significant advantage of fiber-based hydrogen sensors is the capability of providing numerous sensing points in order to generate data regarding the location of the leak itself.

One of the most important requirements for any leak detection system, particularly one for detecting hydrogen leaks, is the ability to operate over a large range of temperatures (e.g., for use near extremely cold liquid-H2tanks and pipes, as well as in much warmer environments). In addition, with any leak-detection system, response time is paramount to successfully averting disaster. Although a number of sensing solutions have been developed based on the use of a Pd-coating as described above, those solutions share a common problem, namely, due to palladium's slow hydrogen absorption rate at low temperature (e.g., on the order of 20 degrees C. and lower), the sensors exhibit an extremely low sensitivity and slow response time at low temperatures. Thus, there is a need for a fiber optic sensing solution that exhibits improved sensitivity and response time at low temperatures.

SUMMARY OF THE INVENTION

The present invention, in one embodiment, provides a sensor for sensing a gas, such as hydrogen, that includes an optical fiber having a core and a wavelength resonant in-fiber optic component, such as a fiber Bragg grating or Fabry-Perot filter, provided in the core at a first location. The optical fiber propagates a sensing light and a power light, with the sensing light being propagated in the core. The wavelength resonant in-fiber optic component receives the sensing light and reflects a reflected light having a resonance wavelength that is dependent on a characteristic, such as a grating spacing, of the wavelength resonant in-fiber optic component. The reflected light is used to determine at least one of a presence of and a concentration of the gas. In addition, at least one layer of a certain material is attached to the optical fiber in proximity to the first location. In particular, the material is a material, such as palladium or a palladium alloy, that is able to absorb the gas being sensed (e.g., hydrogen) at a temperature dependent gas absorption rate that increases when a temperature of the material is increased up to some limiting high temperature that is dependent on environmental conditions. The material induces a strain in the optical fiber when the material absorbs the gas, with a magnitude of the strain being dependent upon an amount of the gas that is absorbed. The strain in the optical fiber changes the characteristic, e.g., grating spacing. Finally, the optical fiber is structured to allow at least a portion of the power light to be used to heat the material. In one particular embodiment, the optical fiber is structured to allow at least a portion of the power light to be released from the optical fiber at the first location and be absorbed by the material. In another embodiment, at least a portion of the power light may be absorbed by another portion of the fiber, such as the inner or out cladding, generate heat that in turn heats the material. The absorbed power light heats the material and increases the gas absorption rate. As a result, the sensitivity and response time of the sensor is improved, particularly at low temperatures.

In another embodiment, the invention provides a method of sensing a gas, such as hydrogen, that includes providing an optical fiber, wherein the optical fiber has a core, a wavelength resonant in-fiber optic component, such as a fiber Bragg grating or a Fabry-Perot filter, provided in the core at a first location, and at least one layer of a material, such as palladium or an alloy of palladium, attached to the optical fiber in proximity to the first location. The material is able to absorb the gas at a temperature dependent gas absorption rate that increases when the temperature of the material is increased. The material induces a strain in the optical fiber when the material absorbs the gas, with a magnitude of the strain being dependent upon an amount of the gas that is absorbed by the material. The strain changes a characteristic, e.g., the grating spacing, of the wavelength resonant in-fiber optic component. The method further includes propagating a sensing light in the core, wherein the wavelength resonant in-fiber optic component receives the sensing light and reflects a reflected light having a resonance wavelength that is dependent on the characteristic of the wavelength resonant in-fiber optic component. The method still further includes propagating a power light in the optical fiber, and using the power light to directly or indirectly heat the material and therefore increase the gas absorption rate. This may include causing at least a portion of the power light to be released from the optical fiber at the first location and be absorbed by the material. The absorbed power light heats the material and increases the gas absorption rate. Finally, the method includes using the reflected light to determine at least one of a presence of and a concentration of the gas.

In yet another embodiment, the invention provides a method of sensing a gas at an ambient temperature that includes providing an optical fiber, wherein the optical fiber has a core, a wavelength resonant in-fiber optic component provided in the core at a first location, and at least one layer of a material attached to the optical fiber in proximity to the first location. The material is able to absorb the gas at a temperature dependent gas absorption rate that increases when a temperature of the material is increased. The method further includes propagating a first sensing light in the core at the ambient temperature, wherein the wavelength resonant in-fiber optic component receives the first sensing light and reflects a first reflected light having a first resonance wavelength that is dependent on an ambient characteristic of the wavelength resonant in-fiber optic component. Also, the method includes propagating a power light in the optical fiber for a defined period of time during which at least a portion of the power light is used to directly or indirectly heat the material. In one embodiment, at least a portion of the power light is released from the optical fiber at the first location and absorbed by the material, wherein the absorbed power light heats the material. Alternatively, the power light may be absorbed by another portion of the fiber, such as a cladding layer or even the core, which in turn causes heat to be generated which heats the material. The heating of the material causes the material to induce a strain in the optical fiber that changes the ambient characteristic to a changed characteristic. The method still further includes allowing the material to cool to a temperature substantially equal to the ambient temperature after the defined period of time has expired, and propagating a second sensing light in the core after the material is allowed to cool, wherein the wavelength resonant in-fiber optic component receives the second sensing light and reflects a second reflected light having a second resonance wavelength that is dependent on the changed characteristic. Finally, the method includes determining a difference between the first resonance wavelength and the second resonance wavelength, and using the difference to determine at least one of a presence of and a concentration of the gas.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to various systems and methods for providing active in-fiber optic components that are powered by in-fiber light. Specifically, as described in greater detail herein, various optical fibers are provided that propagate both a sensing or signal light and a power light wherein the power light is used to provide the energy required to tune the in-fiber optic component.

FIGS. 2A and 2Bare side views (2A in partial cross-section) andFIG. 3is a cross-sectional end view of optical fiber30according to one embodiment of the present invention. As seen most readily inFIG. 3, optical fiber30includes a core35, inner cladding40, outer cladding45and protective layer50. Preferably, core35, inner cladding40and outer cladding45are made of light propagating materials, wherein core35has an index of refraction that is greater than the index of refraction of inner cladding40, which in turn is greater than the index of refraction of outer cladding45. Except as otherwise described herein, establishing the relative indices of refraction in this manner causes light propagating in core35to be confined therein, and light propagating in inner cladding40to be confined therein. Inner cladding40and outer cladding45may be formed by a number of known techniques, such as modified chemical vapor deposition (MCVD). The thickness of outer cladding45is preferably about 10 μm to allow for convenient optical tap region fabrication as described elsewhere herein. In addition, inner cladding40and outer cladding45should be highly transparent.

In one particular embodiment, core35is made of a glass material such as fused silica that is doped with germanium and/or boron to increase the index of refraction thereof, inner cladding40is made of fused silica, and outer cladding45is made of fused silica that is doped with fluorine (preferably 3-mole % fluorine) to decrease the index of refraction thereof. Preferably, core35is doped to achieve an N.A. value of about 0.16, which is slightly higher than the N.A. value of 0.13 for standard telecommunications fibers. Both pure silica and fluorine-doped silica are transparent to UV light, which allows uniform penetration of KrF 248 nm laser radiation for in-fiber FBG fabrication. In one particular example of this particular embodiment, core35has an index of refraction equal to about 1.45, inner cladding40has an index of refraction equal to about 1.445, and outer cladding45has an index of refraction equal to about 1.44. In addition, in another specific example, core35has an elliptical shape with an 8 μm major axis and a 5 μm minor axis, inner cladding40has an outer diameter equal to about 105 μm, and outer cladding45has an outer diameter equal to about 125 μm. Alternatively, outer cladding45may be made of a polymer such as clear silicone or PFA. Preferably, core35is a polarization-maintaining ellipse with a beat length of less than 4 mm at 1550 nm to ensure high sensing sensitivity and to reduce polarization phase noise. Other core and cladding sizes and shapes are possible without limitation.

Referring toFIGS. 2A and 2B, optical fiber30includes an in-fiber optic component55such as, for example, an FBG (shown inFIG. 2A). In addition, an optical transducing element is located in a position that is proximate to a portion of optical fiber30. In the embodiment shown inFIGS. 2A and 2B, the optical transducing element is a light absorbing thermal coating60provided around at least a portion of and preferably the entirety of the outer circumference of optical fiber30(FIG. 2Ashows thermal coating60in partial cut-away to enable viewing of the other components of optical fiber30). According to one particular embodiment, thermal coating60is a metal film, such as, for example, a silver, nickel, titanium or other light absorbing metal film, that is deposited on the outside of optical fiber30(protective layer50is removed at this location for reasons that will be clear below and thermal coating60is applied to outer cladding45) by any one of many known coating methods such as plating, sputtering and e-beam thermal evaporation. The thickness of thermal coating60is preferably on the order of about 10 nm to tens of microns. A key characteristic of thermal coating60is that is heats up and radiates and/or conducts heat when exposed to certain types of light from inside optical fiber30. Other suitable materials such as, without limitation, light absorbing polymers, carbon, semiconductors, ceramics, light absorbing doped glasses, metal films of any kind, metal oxides, metal nitrides, and metal carbides, may be used for thermal coating60.

Referring toFIG. 2A, optical fiber30also includes an optical tap region65located in a portion of optical fiber30that is proximate to thermal coating60. Optical tap region65is a region of optical fiber30that will allow certain light, as described in greater detail below, that is propagating through optical fiber30to leak out of (i.e., be released from) optical fiber30and be absorbed by thermal coating60. Optical tap region65may be created in a number of ways. For example, laser techniques or ion-implantation techniques may be used to, in effect, damage inner cladding40in a selected region and thereby alter its index of refraction such that the power light75will leak out of inner cladding40at optical tap region65. It is estimated that an index change of about 5×10−3to 1×10−2through outer cladding45will be sufficient to release the power light75.

Preferably, a combination of deep UV laser radiation and ion implantation are used to fabricate optical tap region65. Compaction produced by deep UV laser radiation in germanium doped cores such as core35produces stress in the core-cladding interface, which eventually damages the interface region and produces leaking light. For example, a combination of 157 nm F2vacuum UV lasers and 248 nm KrF deep UV lasers based on type II photosensitivity response may be used to fabricate long-period grating type optical taps. KrF lasers are well suited for fabrication of uniform optical taps for optical tap region65due to the relatively weak absorption of such laser light by germanosilica waveguide cores such as core35. The type II photosensitivity can be enhanced by using known hydrogen loading techniques. In addition, anisotropic optical taps for optical tap region65with highly directional leaking light can readily be fabricated with 157 nm F2laser radiation. Anisotropic optical taps are convenient for leaking light collection and refocusing. For example, highly anisotropic diffused light can be easily line-focused to generate acoustic waves for active ultrasonic sensing.

Furthermore, using an amplitude mask, the pulse fluence of the optical tap fabricating laser(s) can be tailored along fiber30to achieve a uniform leaking light intensity. Angular uniformity of an optical tap fabricated by deep UV laser radiation can be obtained by rotating fiber30during laser exposure. By adjusting pulse fluence and accumulated fluence of the lasers, optical taps can be fabricated with desired tap lengths, leaking percentages, and emitting directionalities.

With respect to ion implantation, the magnitude and location of an index of refraction change in a fiber can be precisely controlled by the selected ion species, ion energy, and total ion dose. As such, the optical damage can be localized between the surface and the interface of the inner cladding40and the outer cladding45. It has been learned, for example, that 21-MeV Si and 12-MeV C ions produce uniform vacancy profiles and thus uniform index profiles, while) 0.8-MeV H ions produce vacancies that are concentrated at the end of the implantation trajectory.

Alternatively, blazed Bragg gratings may be used to implement optical tap region65by providing a blazed grating in a core of a fiber such as core35shown inFIGS. 2A and 2Bor in the core of a single mode fiber similar to single mode fiber portion85in a location that is near the thermal coating such as thermal coating60or115. In such an application, both the power light and the sensing/signal light are propagated through the same core. As is known in the art, blazed Bragg gratings are fiber gratings that have grating planes that are at an angle (<90 degrees) with respect to the longitudinal axis of the fiber in which they are created. The angled nature of the gratings causes the light reflected by the blazed Bragg grating (the resonance wavelength) to be reflected at an angle with respect to the longitudinal axis of the fiber. This light (the power light) will be coupled out of the fiber core, into the surrounding cladding, and out of the fiber, where it may be absorbed by a thermal coating such as thermal coating60or115. The particular blazing angles and the degree of change of the index of refraction will determine the out-coupling efficiency, and the period of the grating will determine the out-coupling wavelength.

In operation, as illustrated inFIGS. 2A and 2B, a sensing light70is directed through and propagates through core35. Sensing light70may be, for example, 1300 to 1700 nm light generated by a diode laser, such as a swept tunable laser, or a broadband source. Although the term sensing light is used herein for illustrative purposes, it will be appreciated that sensing light70may also be a signal propagating light used in, for example, a fiber optic communication system, and the term sensing light or sensing/signal light is intended to include signal propagating or similar lights. As is known in the art and as described elsewhere herein, sensing light70will propagate through core35and encounter in-fiber optic component55, which, in the embodiment shown inFIG. 2A, is an FBG, and a particular resonance wavelength will be reflected back in the opposite direction. This functionality is essentially the same as described in connection with the prior art FBG shown inFIG. 1. According to an aspect of the present invention, power light75is simultaneously directed through both core35and inner cladding40(although power light75is shown propagating in a direction opposite the sensing light70, it may also be propagated in the same direction as sensing light70. Power light75may be, for example, light generated from a high-power diode laser array (not shown). Preferably, power light is on the order of 0.1 to hundreds of watts with a wavelength of between 600 nm and 1600 nm. Power light75will, as seen inFIGS. 2A and 2B, propagate through core35and inner cladding40and will be confined therein until it reaches optical tap region65. When power light75reaches optical tap region65, at least a portion of power light75will leak out of inner cladding40and into outer cladding45. The portion of power light75that has leaked into outer cladding45will then be transmitted substantially radially outwardly therefrom and will ultimately be absorbed by thermal coating60. The absorption of power light75will cause the temperature of thermal coating60to increase. As a result, thermal coating60will then radiate heat that is transmitted/conducted through outer cladding45and inner cladding40and into core35. The heat in core35heats the in-fiber optic component55. As is known, this heat will change (increase) the index or indexes of refraction of in-fiber optic component55and will, to an extent, change the size of (make larger) optical fiber30, each of which will alter the characteristics of in-fiber optic component55. In the case of an FBG as shown inFIG. 2A, these changes, resulting from the power light75, will alter the resonance wavelength of the FBG. Thus, power light75may be used to power and tune the in-fiber optic component55provided in optical fiber30. As will be appreciated, the intensity and/or duration of power light75may be controlled to selectively heat thermal coating60to produce particular changes in the in-fiber optic component55(e.g., particular resonance wavelengths).

FIGS. 4A and 4Bare side views (4A in partial cross-section) of optical fiber80according to an alternate embodiment of the present invention. Optical fiber80includes single mode fiber portion85having core90and cladding95that is joined to, such as by fusion splicing, multi-mode fiber portion100having core105and cladding110. Single mode fiber portion85may be any known, commercially available single mode optical fiber material, and multi-mode fiber portion100may be any known, commercially available multi-mode optical fiber material. Optical fiber80includes an optical transducing element in the form of thermal coating115which is similar in structure to thermal coating60shown inFIGS. 2A and 2B. Optical fiber80also includes in-fiber optic component120, which, in the embodiment shown inFIGS. 4A and 4B, is a uniform FBG, but may also be other types of optic components as described herein. A sensing light125is directed through core90as shown. Single mode fiber portion85and multi-mode fiber portion100are joined to one another at junction130. As shown inFIGS. 4A and 4B, junction130, and in particular the differing diameters of core90and core105, acts as an optical tap region that allows power light135that is directed through and propagates through core105to leak out of core105and into cladding95, where it is ultimately absorbed by thermal coating115. As described in connection withFIGS. 2A and 2B, the absorbed power light135heats thermal coating115which in turn radiates heat that is conducted therefrom and heats in-fiber optic component120, thereby changing the operating characteristics thereof.

In one particular implementation investigated by the inventor, in-fiber optical component120consisted of several 4 mm long and 4 cm long uniform FBGs and several 4 cm long linear chirped FBGs (1 nm/cm) written into single mode fiber portion85, which consisted of an SMF-28 fiber available from Corning Incorporated of Corning, N.Y. Single mode fiber portion85in this implementation consisted of a 125 μm diameter (9 μm core) fiber, and multi-mode fiber portion100consisted of a 140 μm diameter (100 μm core) fiber. Thermal coating115consisted of a 2 μm thick silver film that was positioned approximately 5 mm from junction130. Ten watts of 910 nm laser light from a high power diode laser array was coupled into the end of multi-mode fiber portion100using a pair of 20× microscope objectives. The 910 nm light (power light135) propagated through core105of multi-mode fiber portion100and leaked into cladding95of single mode fiber portion85through junction130. The leaking power light135was absorbed by thermal coating115and raised the temperature of in-fiber optic component120. The reflection spectra of the in-fiber optic component120was monitored with an optical spectrum analyzer.FIG. 5Ashows a spectrum evolution of a 4 mm uniform FBG forming part of in-fiber optic component120heated with the power light135as just described. It was estimated that approximately 130 mW of power light135was leaked through to thermal coating115, raising the temperature of in-fiber optic component120significantly and shifting the resonance wavelength as shown in FIG.5A.FIG. 5Bshows the resonance wavelength shift as a function of the input diode laser power (power light135).

In addition, as will be appreciated, the energy of power light135leaking out of core150and being transmitted through cladding95falls exponentially with distance (assuming a constant loss coefficient). Thus, a temperature gradient is created along the length of thermal coating115. In some cases, this temperature gradient is longer than the FBGs forming a part of in-fiber optic component120. This gradient modifies the spectrum response of the FBGs and may be used to control the grating chirp and cause a spectrum stretch of the 4 cm long uniform FBG forming a part of in-fiber optic component120.FIG. 6shows a spectrum width stretch of a 4 cm long uniform FBG when 250 mW of diode laser light (power light135) was directed through core105of multi-mode fiber portion100(it was estimated that approximately 120 mW of power light135leaked).FIG. 7, on the other hand, shows a spectrum width compression of the 4 cm linear chirped grating forming part of in-fiber optic component120when a 250 mW diode laser light was utilized as power light135. In this situation, the temperature gradient created by power light135“de-chirped” the linear chirped grating and compressed the spectrum width.

According to a further alternate embodiment of the present invention, shown inFIG. 8, a small local refractive index change may be produced by generating a “hot spot” to modify the reflection spectrum of a chirped FBG. Specifically, in-fiber optical component120is, in this embodiment, a 4 cm long linear chirped FBG (1 nm/cm), and thermal coating115is an approximately 2 mm long silver film. In one implementation investigated by the inventor, the power light135was approximately 450 mW of 910 nm diode laser light. Power light135heated thermal coating115, which in turn heated in-fiber optic component120, thereby changing increasing the local refractive index of the portion of in-fiber optic component120near thermal coating115and expanding the grating period or periods. As a result, the resonance wavelength of the in-fiber optic component120at this point increased. As shown inFIG. 9, this localized heating created a notch filter in the FBG reflection spectrum.

FIGS. 10A and 10Bare side views of optical fiber system140according to still a further alternate embodiment of the present invention. As described below, optical fiber system140provides a dynamic Fabry-Perot micro-cavity resonator. Fiber optical Fabry-Perot micro-cavity resonators are utilized as an important optical component in fiber optic communication networks and fiber optic sensing systems. The present invention, among other applications, may be utilized to either stabilize the reflection spectrum of a Fabry-Perot micro-cavity resonator from random temperature or stress-induced drifting or to introduce a periodic phase change to the cavity for phase-locking signal detection. As seen inFIGS. 10A and 10B, optical fiber system140includes first single mode fiber portion145having core150and cladding155. Optical fiber system140also includes second single mode fiber portion160having a core165and cladding170that is joined to, such as by fusion splicing, multimode fiber portion190having core195and cladding200. Optical fiber system140includes an optical transducing element in the form of thermal coating190which is similar in structure to thermal coating60shown inFIGS. 2A and 2B. Thermal coating190is provided around at least a part of, and preferably the entirety of, the circumference of second single mode fiber portion160.

As is known in the art, a Fabry-Perot filter is a high-spectral resolution (narrow-band-pass) optical filtering device that operates on the property of destructive light interference. A Fabry-Perot filter includes a cavity bounded on each side by two generally flat, transparent plates that have a partially reflective coating provided thereon. Typically, the cavity is filled with a dielectric material, which may include, without limitation, air. Incident light is passed through the two coated reflecting plates. The distance between the reflective coatings determines which wavelengths will destructively interfere and which wavelengths will be allowed to pass through the coated plates. In addition, the optical transmission spectrum of a Fabry-Perot filter typically shows multiple peaks with narrow passband width. The spacing between neighboring peaks is primarily determined by the gap between the two reflecting plates that form the cavity and the dielectric function of the material present in the cavity. As seen inFIGS. 10A and 10B, a partially reflective plate215A is provided at the end of the first single mode fiber portion145and a partially reflective plate215B is provided at the end of the second single mode fiber portion160opposite partially reflective plate215A such that a cavity220is provided therebetween.

A sensing light225is directed through core150as shown inFIGS. 10A and 10B. Second single mode fiber portion160and multimode fiber portion190are joined to one another at junction205. As shown inFIGS. 10A and 10B, junction205, and in particular the different diameters of core165and core195, act as an optical tap region that allows power light230that is directed through and propagates through core195to leak out of core195and into cladding170, where it is ultimately absorbed by thermal coating210. The absorbed power light230heats thermal coating210, which in turn heats second single mode optical fiber160. The heating of second single mode fiber160causes its length to increase, thereby decreasing the width of cavity220and changing the characteristics of the Fabry-Perot filter implemented by partially reflective plates215A and215B and cavity220. In particular, the addition of power light230will cause the reflection spectrum of sensing light225to be shifted as demonstrated inFIG. 11.

Real-time gas and liquid flow sensing has many important applications in, for example, aerodynamics, combustion engine design, medical devices (such as respiratory devices) and chemical analysis. At present, state-of-the-art flow sensors are mostly based on MEMS technology. Although MEMS-based devices have been found to be effective, the packaging cost is relatively high, the packaged devices are typically relatively bulky, and they rely on external electrical power. As a result, the implementation of MEMS-based flow sensors in small diameter flow tubes (as required in respiratory devices) and in harsh environments is currently not feasible.

As an alternative, according to another aspect of the present invention, a tunable (active) optical fiber system including an FBG type in-fiber optic component powered by in-fiber light such as is shown inFIGS. 2A and 2Bor4A and4B may be utilized to sense real-time gas and liquid (fluid) flow. In particular, if the FBG comprising in-fiber optic component55(FIGS. 2A and 2B) or in-fiber optic component120(FIGS. 4A and 4B) is heated as described herein such that the temperature thereof is higher than the surrounding environment, and if a gas or liquid is caused to flow past the associated optical fiber30or80, the thermal energy removed from the FBG (in-fiber optic component55or120) as represented by the resulting temperature change will depend on the flow rate of the surrounding gas or liquid. As a result, the flow rate can be measured by measuring the resonance wavelength shift(s) of the FBG that, as described above, are dependent upon FBG temperature changes. Resonance wavelength shifts due to fluid flow may be correlated to flow rates using known methods.

In one example implemented by the inventor, optical fiber30was provided with an in-fiber optic component55consisting of a 5 mm uniform FBG having a resonance wavelength of about 1553.7 nm at room temperature as shown inFIG. 12. Power light75consisting of 384 mW diode laser light was then provided, causing the resonance wavelength of the FBG to shift to about 1558.2 nm as shown inFIG. 12. Air was then caused to flow around optical fiber30at about 2.84 m/s, which cooled optical fiber30down and removed heat from the FBG, thereby causing another shift in the resonance wavelength to about 1556.6 nm as shown inFIG. 12.

In another example implemented by the inventor, optical fiber30was provided with an in-fiber optic component55consisting of a 5 mm uniform FBG having a resonance wavelength of about 1554.9 nm at room temperature as shown inFIG. 13. Power light75consisting of 384 mW diode laser light was then provided, causing the resonance wavelength of the FBG to shift to about 1558.2 nm as shown inFIG. 13. N2was then caused to flow around optical fiber30in a direction perpendicular to the FBG at about 2.63 m/s, which cooled optical fiber30down and removed heat from the FBG, thereby causing another shift in the resonance wavelength to about 1556.5 nm as shown inFIG. 13.FIG. 14shows the resonance wavelength shifts of the 5 mm uniform FBG as a function of flow velocity using a power light75at two different levels, 384 mW and 557 mW. As seen, the resonance wavelength shifts closely follow a simple exponential decay function of the N2velocity. Similar flow measurements were also carried out on an in-fiber optic component55consisting of a 1.7 cm uniform FBG having a resonance wavelength of about 1538.3 nm at room temperature. As discussed elsewhere herein, the magnitude of leaking power light75falls off exponentially with distance from the junction130, thereby causing a temperature gradient in longer FBG such as the 1.7 cm uniform FBG just described.FIG. 15shows the spectral evolution of such a 1.7 cm uniform FBG heated with a 442 mW power light75under N2flow velocities of 0.88 m/s and 2.63 m/s.FIG. 16shows the spectral width of the 7 cm uniform FBG as a function of flow velocity using a power light75at two different levels, 345 mW and 442 mW. As seen inFIG. 16, the spectral widths decrease exponentially with the increase in flow velocity.

In addition, as will be appreciated, such a flow sensor may be operated in a constant power mode or a constant wavelength mode. In the constant power mode, a power light75or135having a constant power level is provided, and, as just described, flow rate is measured based on resonance wavelength shifts. In contrast, in the constant wavelength mode, flow rate is measured based on the power level(s) of power light75or135that is/are required to keep the resonance wavelength of the FBG constant (equal to some pre-set, pre-flow value) when liquid or gas flows thereby.FIGS. 17 and 18demonstrate operation in the constant wavelength (variable power) mode. Specifically,FIG. 17shows the power levels of power light75that are required to maintain a constant resonance wavelength at various flow velocities with initial power light levels of 152 mW and 335 mW using the 1.7 cm uniform FBG described above. Similarly,FIG. 18shows the power levels of power light75that are required to maintain a constant resonance wavelength at various flow velocities with initial power light levels of 249 mW, 348 mW and 538 mW using the 5 mm uniform FBG described above. As seen inFIGS. 17 and 18, the power levels needed to maintain a constant, pre-set resonance wavelength follow linear functions with the flow velocity.

As a further alternative, according to yet another aspect of the present invention, a tunable (active) optical fiber system including an FBG type in-fiber optic component powered by in-fiber light such as is shown inFIGS. 2A and 2Bor4A and4B may be utilized as a liquid level sensor to monitor the level of a liquid in a container, such as, for example, and without limitation, the level of liquid hydrogen in cryogenic fuel tanks for space missions. One example of such an implementation is shown inFIG. 19. As seen inFIG. 19, an optical fiber80as described in connection withFIGS. 4A and 4Bis used to monitor the level of water250contained in tank255. Optical fiber80includes single mode fiber portion85(having a core and cladding (not shown)) that is joined to, such as by fusion splicing, multi-mode fiber portion100(having a core and cladding (not shown)) at a junction130. Optical fiber80also includes thermal coating115and an in-fiber optic component (surrounded by thermal coating115and not shown inFIG. 19) in the form of a uniform 5 mm FBG. A sensing light125is directed through the core of single mode fiber portion85and a power light135(comprising ten watts of 910 nm laser light from a high power diode laser array) is directed through the core of multi-mode fiber portion100. As described in connection withFIGS. 4A and 4B, junction130acts as an optical tap region that allows a portion of power light135to leak out of the core and into of multi-mode fiber portion100, where it is ultimately absorbed by thermal coating115. The absorbed power light135heats thermal coating115which in turn radiates heat that is conducted therefrom and heats in-fiber optic component120.FIG. 20shows the spectral response of the FBG of the optical fiber80ofFIG. 19in: (i) water with no power light135, (ii) in water with about 115 mW of power light135, and (iii) in air with about 115 mW of power light135.

To determine whether the FBG is submersed inside the water250inside tank255(level sensing), the 910 nm laser was turned on to inject 115 mW power light into multi-mode fiber portion100to heat the FBG. When the grating is submersed under the water250, the resonance peak (solid trace) of the heated FBG is shifted about 60-pm from the unheated peak (dotted trace). When the FBG is pulled above the level of water250, the FBG reflection peak rapidly shifted over about 1.4 nm from 1541 nm to 1542.4 nm as shown in theFIG. 20. This dramatic thermal response for the heated FBG thus provides unambiguous detection whether or not the FBG is immersed in the water250. As will be appreciated, such an optical fiber80may be used to determine whether a liquid in a container such as tank255has fall below or risen above a particular level by positioning the FBG at the level of interest, heating the FBG with a power light135, and monitoring the spectral response of the FBG.

The thermal responses of a heated grating, such as the FBG of optical fiber80shown inFIG. 19, in air, water, and liquid nitrogen as a function of input laser power (power light) are characterized inFIG. 21. As expected, the heated grating when exposed to air produced the largest resonance wavelength shift. The peak shift of the heated grating in ambient room temperature air follows a linear variation with input laser power with a slope of 15 pm per mW. A 10-mW laser input will produce a 150 pm reflection peak shift. This is in contrast to a 20 pm shift in water and a five pm shift in liquid nitrogen. The grating wavelength shifts in water and liquid nitrogen were re-plotted using a reduced vertical scale as shown in the inlet ofFIG. 21.

FIG. 22is an alternate embodiment of an optical fiber80utilized as a liquid level sensor. The optical fiber ofFIG. 22is similar to the optical fiber80ofFIG. 19except that it includes four thermal coatings115(each surrounding a 5 mm uniform FBG) spaced 3 cm apart from one another along single mode fiber portion85. For convenience, the FBGs shall be referred to as sensor1, sensor2, sensor3and sensor4, with sensor4being located about 3 cm from junction130. As seen inFIG. 22, sensor1has the shortest resonance wavelength and is the topmost FBG, and sensor4has the longest resonance wavelength is the lowest FBG. The optical fiber80ofFIG. 22can thus be used to sense the presence of liquid at four different locations, and as a result can sense four different liquid levels within a container such as tank255.

FIGS. 23Athrough D show the reflection spectrum of each FBG (sensors1-4) in an unheated condition (no power light135) in dotted line form. When the optical fiber80is pulled out from the water250, sensor1is the first grating to rise above the water level and sensor4is the last to emerge.FIG. 23Ashows the reflection spectrum of the heated FBG when sensor1is pulled out from the water with 600-mW input laser power (power light135). The resonance peak for sensor1shifted 350 pm from 1535.7 nm to 1536.05 nm. Due to the much larger specific heat and thermal convection rates of water than those of air, reflection peaks for heated gratings remaining in the water shifted less than 10 pm. The dramatic resonance peak shift for sensor1above the liquid surface provides unambiguous detection of the liquid level. As shown inFIGS. 23B-23D, similar behavior is observed when sensors2,3and4are pulled out from the water250in Succession.FIGS. 23C and 23Dalso show non-uniform peak shifts for the different FBGs while the input laser power (power light135) is reduced to 550 mW and then to 400 mW. This is due to the non-uniform leakage profile in single mode fiber portion85. The power is reduced to avoid the spectral deformation of sensor4due to overheating.

The level sensing applications have been described herein using optical fiber80as shown inFIGS. 4A and 4B. It should be understood, however, that this is for illustrative purposes and that other embodiments of the present invention, such as the optical fiber30shown inFIGS. 2A and 2Bmay also be used in level sensing applications.

Finally, the particular embodiments described above in connection withFIGS. 2-12have been based on the conversion of the in-fiber power light (75,135,230) to thermal energy using an optical transducing element comprising a light absorbing thermal coating (60,115,210). However, the concept of the present invention is not limited to the conversion of the in-fiber power light to thermal energy. The in-fiber power light may also be converted to other energy types (that are then used to tune an in-fiber optic component), such as mechanical, acoustic, electrical, magnetic and optical (at other wavelengths) energy using various types of alternative transducing elements and energy conversion/harvesting mechanisms. For example, recent developments in the area of photo-mechanics have shown that polymer membranes containing light-sensitive molecules undergo rapid photo-contraction or expansion under weak polarized light radiation. In particular, recent investigations have shown that liquid-crystal membranes containing azobenzene chromeophore can be repeatedly bent without apparent fatigue. Such membranes may be provided on an optical fiber containing an in-fiber optic component, and bending, twisting, stretching and/or compressing of such membranes using in-fiber power light may be utilized to tune the in-fiber optic component (e.g., to change the spacing of the grating of an FBG or to deform a micro-mirror to adjust the Q-value and finesse of a micro-optical resonator). In this case, the membrane acts as an on-fiber actuator. In addition, such a membrane may be attached to a piezo actuator membrane to provide on-fiber optical-to-electrical conversion, without wires attached from the light transmitting end of the fiber. In addition, laser micromachining of a thermal coating such as thermal coatings60and115will enable in-fiber optical energy to induce periodic index modulation to produce long period fiber grating filters for in-fiber power equalization.

FIG. 24is a side isometric view (in partial cross-section) of a hydrogen sensor300according to a further embodiment of the present invention. As seen inFIG. 24, the hydrogen sensor300includes an optical fiber305that includes a core310, an inner cladding315, an outer cladding320and a protective layer325. Preferably, and as described elsewhere herein in connection with the optical fiber30shown inFIGS. 2A and 2B, the core310, inner cladding315and outer cladding320are made of light propagating materials, wherein the core310has an index of refraction that is greater than the index of refraction of the inner cladding315, which in turn is greater than the index of refraction of the outer cladding320. Except as otherwise described herein, establishing the relative indices of refraction in this manner causes light propagating in core310to be confined therein, and light propagating in inner cladding315to be confined therein.

In one particular embodiment, the core310is made of a glass material such as fused silica that is doped with germanium and/or boron to increase the index of refraction thereof, the inner cladding315is made of fused silica, and the outer cladding320is made of fused silica that is doped with fluorine to decrease the index of refraction thereof. Particular dopant levels and indices of refraction may be as described elsewhere herein, such as in connection with the optical fiber30shown inFIGS. 2A and 2B. Alternatively, outer cladding320may be made of a polymer material such as clear silicone or PFA.

As seen inFIG. 24, the optical fiber305includes an FBG330written into the core310in a suitable manner, such as, without limitation, any of the manners described elsewhere herein. Alternatively, another type of wavelength resonant in-fiber optic component (i.e., a component that reflects particular wavelengths dependent on the characteristics of the component), such as, without limitation, a Fabry-Perot filter, may be employed instead of FBG330. In addition, a palladium layer335is provided around at least a portion of and preferably the entirety of the outer circumference of the outer cladding320(the protective layer325has been removed at this location) in a position that is proximate to the position of the FBG330(FIG. 24shows thermal coating60in partial cut-away to enable viewing of the other components of optical fiber30). The palladium layer335is a layer of material that includes palladium, and may be palladium only or a palladium alloy. The palladium layer335acts as a hydrogen absorbing material for the reasons described below. As is known, when palladium is in the presence of ambient hydrogen, a chemical reaction occurs wherein palladium hydride is formed. In addition, during this reaction, the material expands. As a result, in the presence of ambient hydrogen, the palladium layer335will form palladium hydride and will expand and thereby induce strain in the optical fiber305at a location adjacent thereto. The magnitude of the expansion and thus the strain is dependent upon the amount of hydrogen that is absorbed. As is also known, palladium does not normally adhere well to glass materials. Thus, in the preferred embodiment, an intermediate layer340that adheres well to both glass materials and palladium, such as a glue metal, is first provided around at least a portion of and preferably the entirety of the outer circumference of the outer cladding320, and then the palladium layer335is provided over the intermediate layer340as shown inFIG. 24. The palladium layer335and the intermediate layer340may be applied by any one of many known coating methods such as sputter coating, and may be thickened economically by electro or electroless plating.

Referring toFIG. 24, the optical fiber305also includes an optical tap region345located in a portion of the optical fiber305that is proximate to the palladium layer335. As described elsewhere herein, the optical tap region345is a region of the optical fiber305that will allow certain light, as described in greater detail below, that is propagating through the optical fiber305to leak out of (i.e., be released from) the optical fiber305and be absorbed by the palladium layer335. As described elsewhere herein, the optical tap region345may be created in a number of ways. For example, laser techniques or ion-implantation techniques may be used to, in effect, damage the inner cladding315in a selected region and thereby alter its index of refraction such that the power light355(described below) will leak out of the inner cladding315at the optical tap region345.

In operation, as illustrated inFIG. 24, a sensing light350is directed through and propagates through the core310. As is known in the art and as described elsewhere herein, the sensing light350will propagate through the core310and encounter the FBG330, and a particular resonance wavelength will be reflected back in the opposite direction. In addition, as described above, the palladium layer335absorbs any ambient hydrogen and is caused to expand a certain amount depending upon the amount (concentration) of hydrogen that is present. The expansion of the palladium layer335will induce a strain in the optical fiber305, which strain will stretch the core310(and the entire fiber) and thereby change the spacing of the gratings of the FBG330. As also described elsewhere herein, the particular resonance wavelength that is reflected by an FBG depends upon the grating spacing. Thus, changes in the spacing of the gratings of the FBG330resulting from strain induced by the expansion of the palladium layer335will change the resonance wavelength that is reflected by the FBG330. The spectral shifts (changes in the resonance wavelength that is reflected by the FBG330) can thus be monitored and used to characterize and determine the ambient hydrogen content in the area proximate to the FBG330and palladium layer335. However, as described in the “Background of the Invention” section herein, as the ambient temperature decreases (e.g., below, for example, a range of about 20-50 degrees C.), palladium absorbs hydrogen increasingly slowly, and, as a result, and without the improvement of this embodiment of the present invention described below, sensitivity would be low and response time would be increasingly slow (below one minute, up to hours or days) at any hydrogen concentration at such low temperatures, and in particular at relatively low hydrogen concentrations (e.g., below 4%).

Thus, according to an aspect of this embodiment of the present invention, a power light355is simultaneously directed through both the core310and the inner cladding315(or, alternatively, in just the inner cladding315). Although the power light355is shown propagating in a direction opposite the sensing light350, it may also be propagated in the same direction as sensing light350. The power light355may be, for example, light generated from a high-power diode laser array (not shown). The power light355will, as seen inFIG. 24, propagate through the core310and the inner cladding315and will be confined therein until it reaches the optical tap region345. When the power light355reaches the optical tap region345, at least a portion of the power light355will leak out of the inner cladding315and into the outer cladding320. The portion of the power light355that has leaked into the outer cladding320will then be transmitted substantially radially outwardly therefrom and will ultimately be absorbed by the palladium layer335. The absorption of power light355by the palladium layer335induces heating of the palladium layer335, which increases the temperature of palladium film and in turn increases the hydrogen absorption rate (and thus decreases the hydrogen absorption time) of the palladium in the palladium layer335. As a result, the sensitivity and the response time of the hydrogen sensor300are increased, particularly at lower temperatures. In another embodiment, at least a portion of the power light may be absorbed by another part of the hydrogen sensor300, such as a portion of the inner cladding315or the outer cladding320. When the power light is so absorbed, heat will be generated which is then conducted to the palladium layer335to heat the palladium layer335and thereby increase the gas absorption rate.

Thus, as just described, the propagation of the power light355within the optical fiber305improves the performance of the hydrogen sensor300by heating the palladium layer335. In one embodiment, the propagation of the power light355is continuous, thereby providing continuous heating of the palladium layer335. However, as is known, the reaction between hydrogen and palladium that produces palladium hydride and expansion of the palladium layer335is relatively fast, on the order of a few seconds. Thus, while the continuous heating method described above will be effective, it will result in more power being used than is necessary as the heating is not required once the chemical reaction is complete. Thus according to another embodiment of the invention, a flash heating method is employed to improve the performance of the hydrogen sensor300at low temperatures (as described elsewhere herein) wherein power is conserved. That method is shown in the flowchart shown inFIG. 26.

Referring toFIG. 26, the method begins at step400, wherein the sensing light350is directed through the core310of the hydrogen sensor300at the ambient temperature (which may be a relatively low temperature on the order of −20 degrees C. or less) and the resonance wavelength that is reflected by the FBG330is measured. As described elsewhere herein, the hydrogen absorption by the palladium layer335at such low temperatures will be slow. Next, at step405, the power light355is simultaneously directed through both the core310and the inner cladding315(or, alternatively, in just the inner cladding315) for a defined period of time and then is shut off. During that time, when the power light355reaches the optical tap region345, at least a portion of the power light355will leak out of the inner cladding315and into the outer cladding320. The portion of the power light355that has leaked into the outer cladding320will then be transmitted substantially radially outwardly therefrom and will ultimately be absorbed by the palladium layer335, thereby heating the palladium layer335and increasing the hydrogen absorption rate. The amount of time that the power light355is propagated is preferably just long enough for the reaction between the ambient hydrogen and the palladium in the palladium layer335to be completed (producing palladium hydride). That time may be, for example and without limitation, on the order of 10 seconds. Next, at step410, the hydrogen sensor300is allowed to cool back down to ambient temperature. As will be appreciated by those of skill in the art, following the cool down, absorbed hydrogen is “locked in” the palladium layer335in the form of the palladium hydride. In other words, the cool down to ambient will not cause the reaction to be reversed. Then, at step415, the sensing light350is once again directed through the core310of the hydrogen sensor300at the ambient temperature (it may be left on or turned off after step400and on again in this step) and the resonance wavelength that is reflected by the FBG330is once again measured. At step420, the shift in the measured resonance wavelength that is reflected by the FBG330, if any, is determined. As described elsewhere herein, if ambient hydrogen is present, its absorption by the palladium layer335will cause the palladium layer335to expand, thereby inducing a strain in the FBG330and a resulting resonance wavelength shift. Finally, at step425, the ambient hydrogen concentration is determined based on the resonance wavelength shift (before and after the heating) due to the hydrogen absorption induced strain.

In addition, the hydrogen sensor300can be returned to its original, baseline form, so that it can be used again to make another measurement, by heating the palladium layer335using the power light355to an extent sufficient to “de-gas” the absorbed hydrogen (i.e., reverse the chemical process to convert the palladium hydride back to palladium and hydrogen). Once this is done, the palladium layer335will return to substantially its original size, thereby removing the hydrogen absorption induced strain and returning the spacing of the gratings of the FBG330to their original size (i.e., the FBG330will return to its original length). As a result, the resonance wavelength will return to its baseline level.

FIG. 25is a side isometric view (in partial cross-section) of a hydrogen sensor300′ according to an alternate embodiment of the present invention. The hydrogen sensor300′ is similar to the hydrogen sensor300except that, as seen inFIG. 25, in the hydrogen sensor300′, a portion of the outer cladding320is removed and the intermediate layer340and the palladium layer335are applied around the inner cladding315. Since a portion of the outer cladding320is removed, the power light355will be permitted to leak out of the inner cladding315at that location and will be absorbed by the palladium layer335to provide the effect described above. The hydrogen sensor300′ may be used with either of the methods described herein (continuous heating or flash heating).

In one particular embodiment of the hydrogen sensor300′, the core310of the optical fiber305was a standard 8 μm fiber core with a N.A. of 0.12, and the outer cladding was a low-index plastic jacket having a N.A. of 0.48 to provide confinement of propagation of the power light355in the inner cladding315. The protective coating325was an acrylate coating. To inscribe the FBG330, the optical fiber305was first photosensitized via hydrogen loading for 1 week at 120 bars. After stripping a portion of the protective coating325and the outer cladding320, a one-centimeter FBG330was written into the core310of the fiber305using a phase mask and a pulsed UV Excimer laser. Following grating fabrication, the fiber305was loaded into a magnetron sputtering machine. A fiber sputtering mount which allowed rotation of the fiber305during sputtering deposition was utilized to provide a uniform axial coating. To improve adhesion of the palladium forming the palladium layer to the silica surface of the inner cladding315, a 20 nm layer of a glue metal was applied to the fiber305(to form the intermediate layer340) followed by the much thicker (150-500 nm) palladium layer335.

Testing of the hydrogen sensor300′ described above was performed in a sealed temperature-regulated chamber capable of maintaining −120-120° C. through the use of encapsulating liquid nitrogen cooling or recirculating fluid heating. Fiber feed-throughs were placed at either end of the chamber for sensor access. Two mass flow controllers provided a variable hydrogen and nitrogen mix for 0-10% H2. Chamber pressure, temperature, and gas flow were monitored continuously.

The hydrogen sensor was then connected for testing and evaluation. The experimental setup can be found in K. P. Chen, L. J. Cashdollar, “Controlling Fiber Bragg Grating Spectra with In-Fiber Diode Laser Light,” IEEE Phot. Technol. Lett., vol. 16, pp. 1897-1899, (2004), and id briefly described here. A high-power 910 nm diode laser (Qphotonics QLDM-910-1.5) was used to optically heat the fiber sensor. The diode laser is pigtailed with a 110-μm core multimode fiber. One end of the double-clad fiber containing the sensor was fusion spliced to the multimode fiber output of the diode laser. The other end of the double-clad fiber was spliced to a recirculator with a broadband light source, and a spectrum analyzer (Ando 6317B) for FBG sensor monitoring.

FIG. 27shows the response of the Bragg wavelength to various hydrogen concentrations at room temperature. These measurements were performed for both 350-nm and 150-nm Pd coatings. The measurement was taken after 30 seconds of exposure to hydrogen. The thicker 350 nm Pd coated grating exhibits 0.37 nm response for 10% hydrogen, which shows 42.3% better response than 150-nm coating's response of 0.26 nm. The lowest tested hydrogen concentration was 0.5%, yielding 15 pm shift in the 350-nm Pd-coated FBG. The measurement was repeatable and showed little hysteresis at room temperature and low hydrogen concentrations. The sensor was also tested at the optimal temperature of 150° C., in which the 350-nm coated Pd-FBG shows a dramatic improvement in sensitivity and response time. The response time therein was found to be less than 9 seconds (instrument-limited). By replacing the broadband source and spectrum analyzer with a tunable laser, 0.125% hydrogen could be readily detected at 150° C.

FIG. 28explores the inherent problem associated with H2sensing at low temperatures. Herein, the 350 nm Pd coated sensor is exposed to 10% H2as the temperature is slowly raised while pausing for 30 s at each successive 10 degree increment and measuring the Bragg wavelength. The same test is repeated without hydrogen, and the two responses are compared. The response due to the presence of hydrogen is imperceptible until the temperature reaches about −20° C. This is because the absorption rate of hydrogen is extremely low at low temperatures.

This problem can be readily solved by the active fiber sensor technology described herein and is demonstrated inFIGS. 29A and 29B. In a −50° C. environment, a passive 350-nm Pd-FBG sensor does not respond to 10% hydrogen in a reasonable time frame. However, since it is known that Pd-FBG sensors respond well at room temperature, according to the present invention, high-power diode laser light can be used to locally heat the FBG sensor to a temperature region in which sensors have reasonable responsivity. This is accomplished, in one embodiment, by stripping away the double clad fiber's outer coating; and re-coating with the Ta/Pd layers that serve to absorb the power light traveling in the inner cladding. This local sensor heating facilitates rapid hydrogen gas diffusion and reaction with the Pd film to form hydrides as shown inFIG. 29A. The heated FBG shows some spectral chirp, probably due to the non-uniform heating profile. The injection of 560 mW of laser power shifted the −50° C. FBG wavelength up from 1545.65 nm to 1546.45 nm, which corresponds to a sensor temperature of ˜+10° C., determined fromFIG. 28. Using a cut-back technique, it was estimated that 53% laser power was absorbed by the 2-cm long coating. After a brief 10-second laser heating, the laser is abruptly turned off and the sensor is allowed to cool rapidly to the ambient temperature, and the absorbed hydrogen is locked temporarily into the Pd film. A wavelength shift due to the H2absorption induced strain can be used to gauge the hydrogen concentration. At −50° C. the 350-nm Pd coated FBG shows 0.25 nm shift for 10% H2inFIG. 29A, which is approximately 67% of that at room temperature. This reduced responsivity is probably due to the FBG having a smaller thermo-optical coefficient at low temperatures.FIG. 29Bserves as a comparison for this heating cycle, in which the same procedure is. performed without the presence of hydrogen. Herein, the same thermal shift is noted due to the laser heating. However, after the heating is removed, the Bragg wavelength returns to its original position at the −50° C. ambient with no noticeable shift or deformation of the FBG reflection spectrum.

FIG. 30illustrates the overall success of low temperature hydrogen leak detection using the in-fiber laser heating technique of the present invention. In this experiment, the ambient temperature is set at −50° C. while the hydrogen concentration is gradually increased. At each measured concentration, the heating laser is cycled on for 30 seconds, turned off, and the sensor response is measured. Full-scale response at −50° C. was approximately 50% to that at the room temperature, and the shape of the response curve is altered somewhat. In general, it was shown that relatively fast detection at low temperatures is possible using the laser heating technique of the present invention.

Thus, the experimental work described herein demonstrates an effective means to enhance the low-temperature performance of Pd-FBG sensors using the in-fiber optical heating technique of the present invention. By locally controlling the adsorption and degassing temperature of a Pd film, it is possible to shorten response time over a range of temperatures, thus facilitating fast operation of the hydrogen sensor at all temperatures. The technique described herein facilitates the construction of a FBG hydrogen sensor array with only one fiber and one feed-through. The elimination of electrical wires and the electricity traditionally used to heat the fiber sensor preserves all of the intrinsic advantages of fiber sensors and dramatically simplifies device packaging.

While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. For example, although two embodiments of a hydrogen sensor have been shown and described herein, it will be appreciated that the present invention may be utilized to detect gasses other than hydrogen, in which case the palladium layer would be replaced by a gas absorbing material that experiences increased absorption of the gas in question when heated by the power light as described herein. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the breadth of the claims appended in any and all equivalents thereof.