Abstract:
Fiber optic probe scatterometers for spectroscopy measurements are disclosed. An example device includes an optically transparent illumination tube, an opaque tube, an inner surface of the opaque tube being adjacent an outer surface of the illumination tube and the illumination tube being disposed within the opaque tube, and an optical fiber disposed within and spaced a first distance from the illumination tube, wherein the opaque tube is to be coupled to a spectrometer and an illumination source to provide a light signal along the illumination tube and to collect a scattered light signal via the optical fiber for the spectrometer.

Description:
RELATED APPLICATIONS 
     This patent arises from a continuation of U.S. patent application Ser. No. 12/372,698, filed on Feb. 17, 2009 (now U.S. Pat. No. 8,218,142), the entirety of which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure generally relates to spectroscopy measurement methods and apparatus including at infrared (IR) wavelengths, and more particularly Fiber Optic Probes for making Non-Destructive spectroscopy measurements including evaluation of the condition of organic containing materials, including fiber reinforced composite materials, such as aircraft structural composite materials. 
     BACKGROUND 
     IR spectroscopy measurements may be useful for a variety of purposes including aerospace, automotive and industrial applications, as well as biological and bio-medical applications. For example, infrared (IR) radiation is readily absorbed by organic materials in association with relative motions (vibrations) of atoms such as carbon, hydrogen, oxygen and nitrogen. As such, IR spectroscopy measurements may indicate a condition of a wide variety or organic materials. 
     For example, organic polymer materials such as resin-fiber composites or adhesives may change over time due to a variety of reasons including heat exposure. Chemical changes to a polymer containing structure may affect the desired properties of the polymer containing structure including structural integrity such as strength of a composite or the adhesive properties of an adhesive. 
     One problem with prior art approaches to making IR Spectroscopy measurements of polymer containing materials is that a signal-to-noise ratio may be insufficient to determine relative changes in chemistry of the material. For example, prior art Fiber Optic Probes have failed to address the problem of Fresnel reflections from a surface of a sample which may obscure molecular absorption and/or fluorescence spectral data that may be present in the scattered light signal from within a sample. 
     In addition, prior art devices and methods for making IR Spectroscopy measurements of polymer containing materials have the drawback that they may only be able to measure the outer surface of the material. For example, prior art IR Spectroscopy approaches typically require destruction of a material in an ex-situ setting. 
     Accordingly, there is a need for an improved spectroscopy non-destructive testing device and method for using the same to non-destructively determine a condition of organic containing materials, including fiber reinforced composite materials, over small sampling areas and/or in hard-to-access configurations with a suitable signal-to-noise ratio. 
     SUMMARY 
     In one embodiment, a device for making spectroscopy measurements with reduced or eliminated surface reflections is provided, the device including an elongated member including an outermost opaque thin walled enclosure; an optically transparent thin-walled enclosure adjacent an inner surface of said outermost thin walled enclosure; one or more optical fibers centrally and axially disposed and spaced apart a distance B with respect to the optically transparent thin-walled enclosure; wherein the elongated member is adapted to be coupled to a spectrometer and an illumination source to provide a light signal from the illumination source along said optically transparent thin-walled enclosure and collect a scattered light signal from the sample by said one or more optical fibers to provide to the spectrometer. 
     In another embodiment, A method of non-destructively determining the condition of an organic containing material sample with reduced or eliminated surface reflections is provided, the method including providing an elongated member including an outermost opaque thin walled enclosure; providing an optically transparent thin-walled enclosure adjacent an inner surface of said outermost thin walled enclosure; providing one or more optical fibers centrally and axially disposed and spaced apart a distance B with respect to the optically transparent thin-walled enclosure; positioning said distal end of said optically transparent thin-walled enclosure adjacent said organic containing material sample; providing an interrogating light signal from an illumination source to said sample along said optically transparent thin-walled enclosure; and collecting a scattered light signal from said sample by said one or more optical fibers and providing said scattered light signal to a spectrometer. 
     These and other objects, aspects and features of the disclosure will be better understood from a detailed description of the preferred embodiments of the disclosure which are further described below in conjunction with the accompanying Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE ILLUSTRATIONS 
         FIG. 1  is a side view of a portion of a fiber optic probe scatterometer assembly in a spectroscopy measurement configuration according to an embodiment. 
         FIG. 2  is a cross sectional view of the measuring end of the fiber optic probe scatterometer according to an embodiment. 
         FIG. 3  is a process flow diagram including several embodiments of the disclosure including using the IR fiber optic needle probe. 
         FIG. 4  is a flow diagram of an aircraft and service methodology according to an embodiment. 
         FIG. 5  is a block diagram of an aircraft according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present disclosure achieves the foregoing objects, aspects and features by providing a fiber optic probe scatterometer for accessing small sampling areas and/or hard-to-access or normally inaccessible areas and surfaces for performing non-destructive spectroscopy measurements. 
     It will be appreciated that the fiber optic probe scatterometer of the present disclosure may be suitably used to non-destructively evaluate any material using any suitable interrogating wavelength of light, but is particularly advantageous for non-destructively evaluating by infrared (IR) spectroscopy, organic containing materials, including fiber reinforced composite materials. The fiber optic probe scatterometer is particularly useful in obtaining spectral data where the sample size desired is on the order of the diameter or width of the fiber optic probe scatterometer, or where the desired sampling surface is accessible through a small opening. 
     It will further be appreciated that although the fiber optic probe scatterometer of the present disclosure is explained with exemplary use with respect to a carbon fiber-resin composite material, such as a layered carbon composite structure, that the fiber optic probe scatterometer and method of using the same may be equally applicable to the measurement of any organic material having a small sample size and/or accessible through only a small opening, including applications in aerospace, automotive, and industrial fields, as well as biological, medical, and bio-medical fields. 
     Referring to  FIG. 1  is shown a side view of the fiber optic probe scatterometer assembly according to an embodiment of the disclosure. A fiber optic probe scatterometer  10  may be coupled to one or more fiber optic cables e.g.,  11 A,  11 B, which may in turn be respectively coupled to a spectrometer, e.g.,  12 A, and an illumination source  12 B. The spectrometer  12 A may be any spectrometer that may be interfaced with fiber optics, including a hand-held spectrometer. It will be appreciated that the illumination source  12 B and the spectrometer  12 A may be housed together in a single instrument and that the signal interrogating e.g.,  11 B and signal collection cable, e.g.,  11 A may be housed as a single cable including coaxial signal carrying capability. 
     In one embodiment, the spectrometer  12 A may have the ability to make infrared (IR) spectroscopic reflectance measurements including a multi-frequency broadband infrared detection capability including near-IR, midwave-IR, and far-IR wavelengths and the illumination source  12 B may have the ability to provide a broadband of interrogating IR wavelengths including near-IR, midwave-IR, and far-IR wavelengths. In one embodiment, the illumination source  12 B and the spectrometer  12 A may have an ability to make IR spectroscopy measurements over the wavelength region of about 500 to about 4000 nanometers. 
     In some embodiments, the spectrometer used to make the measurement may use measurement techniques such as reflectance including specular and/or diffuse reflectance. The illumination source  12 B may include a multi-frequency infrared source and the spectrometer  12 A may include an infrared detector that includes multi-frequency infrared detection capability. 
     In one embodiment, the diameter of a measuring end (distal end)  10 A of the fiber optic probe scatterometer  10 , may have a diameter that enables the measuring end  10 A to fit through a slightly larger sized hole e.g.,  15  within a polymer containing material, such as a fiber (e.g., carbon) reinforced composite structure e.g.,  14  in order to access an interior portion such as an interior layer e.g.,  14 A. 
     For example, in some embodiments, the fiber optic probe scatterometer measuring end  10 A may have a diameter (shown below in  FIG. 2  as C) of less than about 2 mm, more preferably less than about 1.5 mm, and even more preferably about 1 mm in diameter or less. It will be appreciated that the ‘small opening’ through which the measuring end may be inserted may be larger than the measuring end diameter and that the sampled size may be smaller than the measuring end diameter. 
     Referring to  FIG. 2 , is shown an enlarged view of a portion of the measuring end  10 A of the fiber optic probe scatterometer  10 . In some embodiments, the measuring end  10 A of the IR fiber optic probe scatterometer may be of different lengths, depending on the application, e.g., the distance required to access a normally inaccessible organic material containing surface (e.g., the surface of interior layer  14 A of composite material  14 ). For example, in some embodiments, the length of the measuring end of the fiber optic probe scatterometer  10 A may be from about 1 to about 10 inches in length. 
     The fiber optic probe scatterometer  10  may include one or more signal receiving optical fibers  16  located axially and centrally (coaxially) with respect to a first outer thin walled tube  18  (jacket) and a second inner concentric thin walled tube  20  (illumination tube). In one embodiment, a single signal receiving optical fiber  16  is provided axially and centrally (coaxially) with respect to the outer tubes  18  and  20  to collect a scattered light signal. 
     In some embodiments the one or more axially and centrally located optical fibers  16  have a diameter of about 100 microns to about 500 microns, more preferably from about 100 microns to about 300 microns, more preferably from about 150 microns to about 250 microns. As shown, the one or more optical fibers  16  collects a scattered light optical signal from the interior of the probed sample e.g.,  14 A over a signal collection volume, e.g.,  16 B while reducing or eliminating collection of sample surface reflections. The one or more optical fibers may be formed of an IR transparent material such as fused silica, preferably low-OH fused silica (dehydroxylated fused silica). Optical fibers which transmit further into the IR, such as silicon fibers and chalcogenide glass fibers, are known in the art. The one or more optical fibers may be coated with a low refractive index cladding as is known in the art. 
     In one embodiment, an interrogating optical signal from the illumination source e.g.,  12 B is provided through the second inner concentric thin walled tube  20  (illumination tube). For example, the illumination tube  20  is preferably transparent to the wavelength of interrogating illumination used and may be coated with a low refractive index cladding as is known in the art that allows propagation of light through the illumination tube by total internal reflection. In one embodiment, the illumination tube  20  may be formed of an IR transparent material such as fused silica, preferably low-OH fused silica (dehydroxylated fused silica). In one embodiment the illumination tube  20  may have a wall thickness of from about 10 microns to about 500 microns. 
     The jacket (outermost) tube  18  may be any structurally stiff and opaque material, and in one embodiment, may be a metal tube, and in another embodiment may be a steel tube, such as a stainless steel tube. Preferably, the illumination tube  20  fits snugly and concentrically within the jacket tube  18 . In one embodiment the jacket tube  18  may have a wall thickness of from about thickness of about 10 microns to about 500 microns. 
     In another embodiment, a structural filler material, e.g.,  22  may be included to fill the gap between the one or more optical fibers  16  and the illumination tube  20 . The filler material may be an opaque material, such as one or more of a powder metal oxide, glass, or polymer material. 
     In one embodiment, the one or more optical fibers  16  have a tip (distal end)  16 A that is terminated within (axially set back from) a plane defined by the distal ends  18 A of the outermost tube  18  which may be co-planar with a sample in contact with the distal ends  18 A of the outermost tube  18 . In some embodiments, the tip  16 A may be axially set back from the distal end of the outermost tube  18 A by a distance A, of about 100 to about 500 microns, more preferably from about 200 to about 300 microns, even more preferably about 250 microns. In other embodiments, the tip  16 A may be axially set back from the distal end of the outermost tube  18 A by between about 1 and about 2 diameters of the one or more optical fibers  16 . In another embodiment, the distal end e.g.,  20 A of the illumination tube  20  and the tip  16 A may be axially set back from the distal end of the outermost tube  18 A by about the same distance A. 
     Thus, in one embodiment, the distance A may be selected in order to improve a signal-to-noise ratio by reducing or eliminating surface reflected (Fresnel reflections e.g., specular or diffuse) light from entering the one or more signal collection optical fibers  16 . For example, the amount of surface reflected light that undesirably contributes to the signal may be reduced or eliminated by decreasing the setback distance A, e.g., from the tip of one or more optical fibers  16 A to a plane that is co-planar with a sample surface. In addition, the setback distance A allows the tip  16 A of the one or more optical fibers to be protected from contact with the sample while allowing the distal end  18 A of the outermost tube  18  to contact the sample. 
     In another embodiment, additionally or alternatively to selecting the distance A, a gap distance B, e.g., radial distance B between the inner diameter of the illumination tube  20  and a total outer diameter of the one or more signal collection optical fibers  16  may be selected to improve a signal-to-noise ratio by reducing or eliminating surface reflected light from entering the one or more optical fibers  16 . By the term ‘total outer diameter’ of the one or more optical fibers is meant a minimum outer diameter necessary to enclose the one or more optical fibers. For example, the amount of surface reflected light that undesirably contributes to the signal may be reduced or eliminated by increasing a radial gap distance B. 
     In operation, the illumination tube may provide a cone of illumination e.g.,  24 A into the sample e.g.,  14 A, and the scattered light optical signal from within the sample e.g.,  24 B may be collected by the one or more signal collection optical fibers  16  which receive the scattered light signal within a conical field of regard e.g.,  16 B. Thus, by controlling one or more of the distances A and B, as well as the size of the signal collection volume within the sample  16 B, the signal to noise ratio may be improved to a level sufficient to allow molecular (chemical) changes within a sample to be more accurately determined. In one embodiment, the size of the signal collection volume  16 B may be controlled by selecting the radial gap distance B and the setback distance A such that a width or diameter of the conical field of regard  16 B of the signal collection fiber or fibers  16  will intersect with the illumination cone of light projected from the end of the illumination tube  20  only within the interior of the sample, in a definable and controllable manner. Thus, the signal collection field of regard  16 B of the signal collection optical fibers  16  may not encompass scattered or reflected light from the upper surface of the sample lying directly under the illumination tube, thereby reducing or eliminating collection of surface reflected light by the one or more optical fibers  16 . 
     It will be appreciated that the distal ends of the outermost tube  18 A of the optical scatterometer probe may be placed in contact with a surface of a sample, e.g.,  14 A to be measured which may serve to provide stability and a repeatable and known distance between the signal collection optical fiber end e.g.,  16 A and the sample surface, thereby allowing comparison of collected spectra to comparable spectra collected on a sample of a known chemical and/or physical condition (relative calibration spectra). 
     Referring again to  FIG. 1 , in exemplary operation, the measuring end  10 A of the fiber optic probe scatterometer  10  is inserted into a small opening  15  (e.g., about 1 mm or less) in an external surface of a fiber (e.g., carbon) reinforced composite panel  14  (which may be a structural portion of an aircraft e.g., fuselage or wing), where the hole  15  may be slightly larger than the measuring end  10 A of the fiber optic probe scatterometer  10 . The tip of the fiber optic probe scatterometer  10 , such as the distal ends of the jacket tube  18 A, may be in contact or proximate to a surface, to be measured, such as an inner layer of fiber reinforced composite panel  14 A. In some embodiments, it will be appreciated that the measurement may be non-destructive and may be made in-situ, e.g., in the field without removing the structural component. It will be appreciated that industry (aircraft) specific requirements may limit the size of the hole or opening that may be permissible in a structural component to not more than (0.040 inches) (e.g., not more than about 1.0 mm). 
     Referring again to  FIG. 2 , in operation, an interrogating light signal of a selected band of wavelengths e.g.,  24 A is provided to the sample surface by the illumination tube, a portion of which propagates into the interior the sample, where it is absorbed and reemitted e.g.,  24 B into the field of view (within signal collection volume  16 B) of a signal collection fiber e.g.,  16 . As will be appreciated, by reducing or eliminating collection of light reflected from the surface of the sample, the signal strength of re-emitted absorbed light from within the volume of the sample may be improved, thereby allowing more accurate and detailed interrogation of molecular changes occurring within the sample. The absorbed and re-emitted light e.g.,  24 B is then collected by the one or more signal collection optical fibers e.g.,  16  and transferred to the spectrometer  12 A for spectral analysis. 
     In one embodiment, the spectrometer  12 A may include appropriate software e.g.,  12 C either in memory or in storage media accessible by a microprocessor included in or separate from the spectrometer  12 A, for comparing the spectral signal of the illumination source and changes imparted by absorption of light by the sample. The software may further include spectral storage capabilities (able to access memory or storage media accessible by a microprocessor included in or separate from the spectrometer  12 A) to track relative spectral changes in a sample over time. 
     In another embodiment, spectroscopic measurements may be made by determining relative differences and/or similarities in measured spectra with respect to spectra from a relative calibration of control samples, such as samples that have been exposed to a known amount and/or type of environmental stress and whose material and/or chemical properties are known, e.g., determined by separate physical property and/or chemical testing. 
     It will be appreciated that an Absorbance at one or more wavelengths may be calculated according to well known equations based on the intensity of reflected IR light measured, e.g., a diffuse reflectance measurement. It will also be appreciated that depending on the wavelength of the region interrogated, that the absorbance peaks represent complex motions of organic materials including the relative motions (vibrations) of atoms such as carbon, hydrogen, oxygen and nitrogen. Thus, depending on the chemical and/or material property changes associated with spectral changes in a material, a determination as to whether the changes represent acceptable or unacceptable chemical and/or material property changes may be made e.g., by associating a particular absorbance (or reflectance) at one or more wavelengths with a particular acceptable and/or unacceptable absorbance (or reflectance) threshold. 
     For example, evaluation of the IR spectroscopy measurement may be made in-situ (in the field) automatically by a controller included in or connected to a hand-held or portable IR spectrometer according to a preprogrammed series of steps including providing an indication (e.g., alarm or signal) indicating unacceptable IR spectroscopy measurement values above or below a predetermined threshold. Alternatively, or in addition, the IR spectroscopy measurement results may be stored in memory included in or connected to the IR spectrometer for later analysis. 
     Referring to  FIG. 3  is shown a process flow diagram including several embodiments of the present disclosure. In step  301 , an opening suitable for inserting the fiber optic probe scatterometer  10  may be provided in a first surface in order to access a normally inaccessible organic material containing second surface interior with respect to the first surface. In process  303 , a measuring end of the fiber optic probe scatterometer may be inserted through the opening and positioned proximate the organic material containing surface. In process  305 , the fiber optic probe scatterometer may be coupled to an IR spectrometer and one or more wavelengths of IR light provided through the fiber optic probe scatterometer to the organic material containing surface through a probe illumination tube. In step  307  reflected IR light (spectra) (e.g., with minimal or no surface reflected light) may be collected by one or more optical fibers central and coaxial with respect to the illumination tube and provided to the IR spectrometer. In step  309 , a condition of the organic material may be determined based on relative changes in the spectra compared to reference spectra including a known condition of the material. 
     Referring next to  FIGS. 4 and 5 , embodiments of the disclosure may be used in the context of an aircraft manufacturing and service method  78  as shown in  FIG. 4  and an aircraft  94  as shown in  FIG. 5 . During pre-production, exemplary method  78  may include specification and design  80  of the aircraft  94  and material procurement  82 . During production, component and subassembly manufacturing  84  and system integration  86  of the aircraft  94  takes place. Thereafter, the aircraft  94  may go through certification and delivery  88  in order to be placed in service  90 . While in service by a customer, the aircraft  94  may be scheduled for routine maintenance and service  92  (which may also include modification, reconfiguration, refurbishment, and so on). 
     Each of the processes of method  78  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 5 , the aircraft  94  produced by exemplary method  78  may include an airframe  98  with a plurality of systems  96  and an interior  100 . Examples of high-level systems  96  include one or more of a propulsion system  102 , an electrical system  104 , a hydraulic system  106 , and an environmental system  108 . Any number of other systems may be included. Although an aerospace example is shown, the principles of the embodiments may be applied to other industries, such as the automotive industry. 
     The apparatus embodied herein may be employed during any one or more of the stages of the production and service method  78 . For example, components or subassemblies corresponding to production process  84  may be fabricated or manufactured in a manner similar to components or subassemblies produced while the aircraft  94  is in service. Also, one or more apparatus embodiments may be utilized during the production stages  84  and  86 , for example, by substantially expediting assembly of or reducing the cost of an aircraft  94 . Similarly, one or more apparatus embodiments may be utilized while the aircraft  94  is in service, for example and without limitation, to maintenance and service  92 . 
     While the embodiments illustrated in the Figures and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. The disclosure is not limited to a particular embodiment, but extends to various modifications, combinations, and permutations as will occur to the ordinarily skilled artisan that nevertheless fall within the scope of the appended claims.