Abstract:
A method of ultrasonic testing comprising conditioning a radiation wave from a laser source by efficiently converting the radiation wave&#39;s wavelength to a mid-IR wavelength for enhanced ultrasonic testing of a composite. The method includes passing the radiation wave through a first optical frequency converter where the radiation wave is converted into a signal wave and an idler wave, where the idler wave is at a mid-IR wavelength. The method further includes directing the signal and idler waves to a second optical frequency converter where the signal wave wavelength is converted to a mid-IR wavelength which combines with the idler wave to form a generation wave. The generation wave is directed at a composite surface to be tested.

Description:
BACKGROUND 
     1. Field of Invention 
     The invention relates generally to the field of non-destructive testing. More specifically, the present invention relates to a method and system for forming a generation laser beam in a mid-IR wavelength. 
     2. Description of Prior Art 
     Recent developments in creating composite materials have expanded the use of composite materials into a wide variety of applications. Because of its high strength and durability combined with its low weight, composites are replacing metals and metal alloys as the base material for certain load bearing components. For example, composites are now commonly used as a material for body parts and structure in vehicles such as automobiles, watercraft, and aircraft. However, to ensure composite mechanical integrity, strict inspections are required. The inspections are typically required upon fabrication of a component made from a composite and periodically during the life of the component. 
     Laser ultrasound is one example of a method of inspecting objects made from composite materials. The method involves producing ultrasonic vibrations on a composite surface by radiating a portion of the composite with a pulsed laser. A detection laser beam is directed at the vibrating surface and scattered by the surface vibrations. Collection optics receives the scattered detection laser light and directs it for processing. Scattered laser light processing is typically performed by an interferometer coupled to the collection optics. Information concerning the composite can be ascertained from the scattered light processing, the information includes the detection of cracks, delaminations, porosity, and fiber information. 
     SUMMARY OF INVENTION 
     Disclosed herein is a method of ultrasonic testing comprising directing a radiation wave from a pump laser to a first optical converter, wherein the first optical converter converts the radiation wave to a signal wave and an idler wave, wherein the idler wave wavelength is in a mid-IR range, directing the signal wave and idler wave to a second optical converter, wherein the second optical converter converts the signal wave wavelength to a mid-IR range and the idler wave passes through the second optical converter substantially unchanged, and wherein the idler wave combines with the converted signal wave to form a single output wave, and directing the single output wave at an inspection surface of an inspection object for ultrasonic testing of the inspection object. 
     The output wave may be a generation wave for generating ultrasonic displacements on the inspection surface and/or for detecting ultrasonic displacements on the inspection surface. The inspection surface may comprise a composite. In one embodiment, the first optical converter is an optical parametric oscillator. In one embodiment the second optical converter can be an optical parametric converter or a difference frequency generator. Optionally, the first and second optical converters are combined into a single crystal. The first optical converter and second optical converter may be segregated into different portions of the crystal, optionally the first optical converter and second optical converter are integrated within a single crystal. 
     In one optional embodiment of a method of ultrasonic testing, the pump laser wave wavelength is about 1.064 microns. In one optional embodiment of a method of ultrasonic testing the signal wave wavelength is about 1.594 microns. In one optional embodiment of a method of ultrasonic testing, the idler wave wavelength is about 3.2 microns. The output wave wavelength of the present method may range from about 3 to about 4 microns. Optionally, in one embodiment of the present method of ultrasonic testing, the output wave wavelength is about 3.2 microns. 
     Disclosed herein is a method of laser ultrasonic testing a test object comprising converting an input laser wave having a wavelength of about 1.064 microns to a signal wave having a wavelength of about 3.2 microns and an idler wave having a wavelength of about 1.594 microns, converting the signal wave wavelength to about 3.2 microns, and producing ultrasonic vibrations on the target surface of a target object by directing the idler wave and the converted signal wave to a target surface as a combined wave. The method may further include generating a second combined wave, directing the second combined wave on the vibrating target surface, and detecting target surface displacement with the second combined wave. The step of converting the input laser wave may involve directing the input wave to an optical parametric oscillator. The step of forming a converted signal wave may involve directing the signal and idler waves to a frequency converter, where the frequency converter may be an optical parametric oscillator and a difference frequency generator. 
     The present disclosure also includes a laser ultrasonic testing system that includes an input laser source, a first optical frequency converter coupled to receive an input wave from the input laser source, the first optical frequency converter and configured to convert the input wave to an idler wave and a signal wave, wherein the idler and signal waves have different wavelengths. Also includable with the testing system is a second optical frequency converter coupled to receive the idler wave and signal wave and configured to convert the signal wave wavelength to substantially the same wavelength of the idler wave and emit a combined output wave comprising the converted signal wave and idler wave, wherein the combined output wave is directable to the target surface of a target object to produce an ultrasonic vibration on the target surface. The system may further include a detection laser directable to the target surface and configured to register a target surface vibration. 
     The input laser wave may have a wavelength of about 1.064 microns. The idler wave and converted signal wave may have a wavelength ranging from about 3 microns to about 4 microns. The idler wave and the converted signal wave may have a wavelength of about 3.2 microns. The first optical converter may be an optical parametric oscillator. The second optical converter may be an optical parametric converter or a difference frequency generator. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the features and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an ultrasonic inspection system. 
         FIG. 2  illustrates a schematic representation of an embodiment of an optical source for ultrasonic testing. 
         FIGS. 3   a  and  4   a  are schematic representations of alternative embodiments of an optical source for ultrasonic testing. 
         FIGS. 3   b  and  4   b  are schematic views of poled crystals for use in an ultrasonic testing optical source. 
         FIG. 5  is schematic representation of an alternative embodiment of an optical source for ultrasonic testing. 
     
    
    
     While the invention will be described in connection with the preferred embodiments, it will be understood that it is not intended to limit the invention to that embodiment. On the contrary, it is intended to cover all alternatives, modifications, and equivalents, as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout. For the convenience in referring to the accompanying figures, directional terms are used for reference and illustration only. For example, the directional terms such as “upper”, “lower”, “above”, “below”, and the like are being used to illustrate a relational location. 
     It is to be understood that the invention is not limited to the exact details of construction, operation, exact materials, or embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. In the drawings and specification, there have been disclosed illustrative embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. Accordingly, the invention is therefore to be limited only by the scope of the appended claims. 
       FIG. 1  provides a side perspective view of one embodiment of a laser ultrasonic detection system  10 . The detection system  10  comprises a laser ultrasonic unit  12  formed to emit a generation beam  14  and directed to an inspection target  15 . The generation beam  14  contacts the inspection target  15  on an inspection surface  16 . The generation beam  14  thermo-elastically expands the inspection surface  16  to produce corresponding wave displacements  18  on the inspection surface  16 . In one embodiment, the generation beam  14  is a pulsed laser configured to produce the wave displacements  18  on the inspection surface  16 . A detection beam  20  is also illustrated emanating from the laser ultrasonic unit  12  and is shown coaxial around the generation beam  14 . Although emanating from the same laser ultrasonic unit  12 , the detection and generation beams ( 14 ,  20 ) are generated by different sources. However, the detection beam  20  may optionally originate from a different unit as well as a different location. As is known, the detection beam  20  comprises a detection wave that is scattered, reflected, and phase modulated upon contact with the wave displacements  18  to form phase modulated light  21 . The phase modulated light  21  from the detection beam  20  is then received by collection optics  23  and processed to determine information about the inspection target  15 . The generation and detection beams ( 14 ,  20 ) may be scanned across the target  15  to obtain information regarding the entire surface  16 . A mechanism (not shown) used to scan the beams ( 14 ,  20 ) may be housed within the laser unit  12 . A processor (not shown) for controlling the mechanism and optionally for processing the data recorded by the collection optics, may also be housed in the laser unit  12 . The collection optics  23  are shown separate from the laser unit  12  and in communication with the laser unit  12  through the arrow A, however the collection optics may be included with the laser unit  12 . 
     With reference now to  FIG. 2 , one embodiment of a mid-IR generator  22  is illustrated in schematic view. As will be described in further detail below, the mid IR generator  22  generates an output wave that may be used for one of the generation laser beam  14  of  FIG. 1 . In the embodiment shown, the mid-IR generator  22  comprises a pump laser  24  that emits a pump laser beam  26  directed to a first optical frequency converter  30 . The first optical frequency converter  30  converts the single pump input wave into two waves: (1) an idler wave  32  and a (2) a signal wave  36 . Some amount of the remaining pump wave  34  passes through the converter  30 . Each wave ( 32 ,  34 ,  36 ) is at a different wavelength. A converter operates below 100% efficiency and allows passage therethrough of a small portion of the energy from the pump laser beam. 
     The waves emitting from the first optical frequency converter  30  are directed to a second optical frequency converter  38 . The second optical frequency converter  38  has been configured to allow free passage of the idler waves  32  without affecting any of its wave properties, such as frequency wavelength and energy. The signal wave  36  wavelength however, is converted within the second optical frequency converter  38  to be substantially the same as the idler wave  32  wavelength. Thus, the idler wave  32  and signal wave  36  are combined into a single output wave  40  having a specified wavelength and an energy level greater than the idler wave  32  energy level. Accordingly, the mid infrared generator  22  is configured to create an output wave  40  having a desired wavelength for ultrasonic testing. 
     Optionally, an input coupler  28  and an output coupler  42  may be disposed on respective input and output of the first and second optical frequency converters ( 30 ,  38 ). As is known, the input and output couplers ( 28 ,  42 ) create an optical cavity increasing the conversion efficiency of converters  30  and  38 . Couplers  28  and  42  have reflection and transmission characteristics at the pump, idler, and signal wavelength, and curvature radii designed to maximize the energy in output beam  40 . The design values are determined by calculations, modeling, and experiments. The device described herein is not limited to the embodiment of  FIG. 2 , but can include several other cavity approaches. For example, alternative embodiments include three or four arm cavities that include more couplers or mirrors. An example of a four-arm cavity  53  is shown in  FIG. 5 . Here the pump laser beam  26   c  passes through the first input coupler  54  and the idler wave  32   c  leaves the cavity  53  from the output coupler  57 . The first and second frequency converters ( 30   b ,  38   b ) are in different arms of the cavity  53 . The remaining portion of the pump beam  34   c  exits from the output coupler  55  and idler and signal waves exit the second optical frequency converter  38   b  towards the mirror  56 . One advantage of multiple arm cavities consists in preventing the pump to reach the second converter, decreasing requirements on optical coatings and damage thresholds. 
     In one example of use of the mid IR generator  22  of  FIG. 2 , the pump laser beam  26  wavelength is about 1.064 microns. In this embodiment, the first optical frequency converter  30  is configured to convert the pump laser beam  26  into the idler wave  32 , where the idler wave  32  wavelength is about 3.2 microns and the signal wave  36  wavelength is about 1.594 microns. Further in this embodiment, the second optical frequency converter  38  is configured to allow free passage of the idler wave  32  while converting the signal wave  36  from about 1.594 microns to about 3.2 microns. The second optical frequency converter  38  thus creates a converted signal wave  36  that is combined with the pass-through idler wave  32  to form the output wave  40 . Accordingly, use of the second optical frequency converter  38  boosts the power of the output wave  40  by recovering energy via the converted signal wave  36 . It has been found that laser ultrasonic testing of composite materials is greatly enhanced by using laser waves whose wavelength is in the mid infrared range, i.e., of about 3 microns to about 4 microns. More specifically, enhanced detection of composite surface is realized by using laser waves whose wavelength is about 3.2 microns. Composite surface characteristics that can be evaluated with such a laser include defects, delaminations, inclusions, cracks, and fiber characteristics such as fiber orientation and fiber density. 
     Another advantage of use of the present device and method is that many well performing laser pumps operate at around 1 micron, those include Nd:YAG, Yb:YAG, and Nd:YVO4, to name but a few. Accordingly, these lasers comprise viable candidates for the pump laser  24  of a mid-IR generator  22 . In one embodiment, the first optical frequency converter  30  may comprise an optical parametric oscillator (OPO). In another embodiment, the second optical frequency converter  38  may comprise an OPO as well as a difference frequency generator (DFG). The OPO and the DFG can either be made of a perfect phase matching crystal or of a periodically poled quasi-phase matching crystal. 
       FIG. 3   a  provides an alternative embodiment of the mid-IR generator  22   a . In this embodiment, the pump laser  24   a  emits a pump laser beam  26   a  passing through the optional input coupler  28   a  towards the frequency converters. Here, the first optical frequency converter  30   a  is combined with the second optical frequency converter  38   a  in a single crystal. The front portion of the crystal comprises the first optical frequency converter  30   a  and the second portion comprises the second optical frequency converter  38   a . The combined crystal is can be made of two phase matching crystals that are fused together or, of a quasi-phase matching periodically poled crystal  44  and shown in a schematic view in  FIG. 3   b . The portion of the crystal  44  forming the first optical frequency converter  30   a  is illustrated by a series of narrow gridlines  46 . Thicker and more spaced apart wide gridlines  48  illustrate the portion of the crystal  44  that form the second optical frequency converter  38   a . These gridlines ( 46 ,  48 ) illustrate positions of periodic poling formed in well known methods. The poling of the first section of crystal  44  is designed to convert pump into idler and signal ( 30   a ) whereas the poling of the second section of the crystal ( 38   a ) is designed to convert the signal into the idler. 
     A schematic of yet another embodiment of a mid-IR generator  22   b  is shown in  FIG. 4   a . In this embodiment, the pump laser  24   b  emits a pump laser beam  26   b  through an optional input coupler  28   b  where the pump laser beam  26   b  is received into an integrated optical frequency converter  50 . The integrated optical frequency converter  50  operates in essentially the same way and performs essentially the same function as the first and second optical frequency converters ( 30 ,  38 ). The integrated optical frequency converter  50  also emits an output wave  40   b  for use as an ultrasonic laser testing beam. The integrated optical frequency converter  50  of  FIG. 4   a  is schematically portrayed in  FIG. 4   b  as an integrated periodically poled crystal  52 . Here, thin gridlines  46   a  and wide gridlines  48   a  alternate along the length of the crystal  52 . 
     It should be pointed out, however, that the final wave produced by any of the embodiments of the mid IR generator is not limited to 3.2 microns but can include from about 3 microns to about 4 microns. For purposes of discussion herein, a mid-IR range defines a wave having a wavelength of from about 3 microns to about 4 microns. 
     The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. These and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.