Abstract:
A mid infrared range laser source for ultrasound inspection that comprises a high energy laser coupled with one or more harmonic generation devices. The high energy laser may be a CO2 laser and tuned to emit laser light at a single wavelength. The harmonic generation devices convert the laser beam into the mid infrared range for optimal ultrasound inspection.

Description:
BACKGROUND 
       [0001]    1. Field of Invention 
         [0002]    The invention relates generally to the field of non-destructive testing. More specifically, the present invention relates to a system to create a mid-range infrared generation laser beam using a carbon dioxide (CO 2 ) laser and generating a harmonic of the CO 2  laser output. 
         [0003]    2. Description of Prior Art 
         [0004]    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. 
         [0005]    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 generation laser. A detection laser beam is directed at the vibrating surface and scattered, reflected, and phase modulated by the surface vibrations to produce phase modulated light. Collection optics receives the phase modulated laser light and directs it for processing. Processing is typically performed by an interferometer coupled to the collection optics. Information concerning the composite can be ascertained from the phase modulated light processing, the information includes the detection of cracks, delaminations, porosity, and fiber information. Currently known laser ultrasonic detections systems used for analyzing composite target objects have a limited energy and repetition rate (frequency). At a target surface, typical laser beam energy values for a mid infrared generation laser beam are about 10 milli-Joules (mJ) with a corresponding frequency of about 10 Hertz (Hz). 
       SUMMARY OF INVENTION 
       [0006]    Disclosed herein is a method of ultrasonic testing a target object using a high energy generation laser beam comprising, providing a CO2 laser beam, producing a harmonic of the CO2 laser beam, directing the CO2 laser beam harmonic to the target object, thermo-elastically exciting a surface of the target object to produce ultrasonic displacements on the target object, and measuring the ultrasonic displacements. The CO2 harmonic may be a second or a third harmonic. An optical fiber may be coupled to the CO2 laser beam. The CO2 laser beam harmonic energy at the target object may be at least 50 milli-joules, at least 75 milli-Joules, at least 100 milli-Joules, or at least 100 Hz. The CO2 laser beam harmonic frequency at the target object may be at least 200 Hz, or at least 400 Hz. The CO2 laser beam harmonic wavelength may range from about 3 microns to about 4 microns or be about 3.2 microns. 
         [0007]    Also disclosed herein is an ultrasonic detection system. In one embodiment the system comprises, a CO2 laser, a harmonic beam generation system, a CO2 laser beam emitted from CO2 laser and directed to the harmonic beam generation system; and a harmonic beam of the CO2 laser beam emitted from the harmonic beam generation system and directed to a target object. In an embodiment, the harmonic beam thermo-elastically expands a portion of the target object thereby producing displacements on the target surface, the detection system further comprises a detection beam directed at the displacements, wherein the detection beam is phase modulated and reflected by the displacements. The harmonic generation system may be a second harmonic generator or a third harmonic generator. The CO2 laser beam energy emitted from the CO2 laser can be at least about 4.5 Joules or at least about 1 Joule. The harmonic laser beam energy at the target can be at least about 50 milli-Joules or at least about 100 milli-Joules. The harmonic laser beam wavelength can range from about 3 microns to about 4 microns or can be about 3.2 microns. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    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: 
           [0009]      FIG. 1  is a schematic representation of a laser ultrasonic detection system. 
           [0010]      FIG. 2  is a schematic view of a mid range infrared ultrasonic laser source in accordance with the present disclosure. 
           [0011]      FIG. 3  is a schematic view of a mid range infrared ultrasonic laser source in accordance with the present disclosure. 
       
    
    
       [0012]    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 
       [0013]    The present invention will now be described more filly 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. 
         [0014]    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. 
         [0015]      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 ultrasonic 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 ultrasonic unit  12 . The collection optics  23  are shown separate from the laser ultrasonic unit  12  and in communication with the laser ultrasonic unit  12  through the arrow Al, however the collection optics may be included with the laser ultrasonic unit  12 . 
         [0016]    With reference now to  FIG. 2 , one example of a mid infrared laser system  30  is shown in a schematic view. The system  30  produces a mid IR beam that may be used as the generation beam  14  of  FIG. 1 . The mid IR laser system  30  comprises a CO 2  laser  32  used to form a CO 2  laser beam  44 . Schematically illustrated within the CO 2  laser  32  is a mirror  34  and an output coupler  38  operatively disposed within the laser  32 . A cavity  36  is provided between the mirror  34  and the output coupler  38 . Energy input into the CO 2  laser  32 , combined with the operative coupling of the mirror and a reflective surface of the output coupler  38 , generate a beam between these two reflective surfaces. A diffraction grating  42  is provided within the cavity  36  and configured to permit therethrough photons having a particular wavelength. The diffraction grating  42  within the cavity  36  thus forms a single wavelength beam  40  in the cavity  36 . 
         [0017]    Some photons of the single wavelength beam  42  escape from the CO 2  laser  32  through the output coupler  38  to form a CO 2  beam  44 . The embodiment of  FIG. 2  illustrates a harmonic generator  46  disposed in the path of the CO 2  beam  44 . The harmonic generator  46  converts the CO 2  beam  44  to a harmonic to create a harmonic beam  48  passing from the harmonic generator  46 . Optionally, an optical fiber  54  is shown receiving the harmonic beam  48  for generation and direction of the generation beam  14 . The harmonic beam  48  may be at the second harmonic of the fundamental wavelength with CO 2  beam  44 . Optionally, the harmonic beam  48  may be at the third harmonic, or some other harmonic of the fundamental wavelength of the CO 2  beam  44 . 
         [0018]      FIG. 3  provides an alternate embodiment of a mid IR laser system  30   a  wherein the CO 2  beam  44  is conditioned by more than one harmonic generator. In  FIG. 3 , the CO 2  beam  44  is directed to a second harmonic generator  47  that changes the CO 2  beam  44  to a second harmonic thereby forming the second harmonic beam  49 . The second harmonic beam  49  is directed to a third harmonic generator  50  that emits a third harmonic beam  52  having a wavelength substantially equal to the third harmonic of the fundamental wavelength of the CO 2  beam  44 . The embodiment of  FIG. 3  is not limited to the two harmonic generators shown, but can include additional harmonic generators disposed in the path of the laser beam. The third harmonic beam  52  can also be used as the generation beam  14  of  FIG. 1  emitted from the laser ultrasonic source  12 . The third harmonic generator  50  can produce the third harmonic of the CO 2  laser beam  44  either by direct conversion or can convert the fundamental wavelength and mix it with the second harmonic wavelength to form the third harmonic of the CO 2  laser beam fundamental wavelength. 
         [0019]    In one embodiment of a detection or testing system disclosed herein, the CO 2  laser beam can be harmonically processed to have a wavelength of between about 3 microns up to about 5.5 microns. Optionally, the CO 2  laser beam can have a wavelength in the entire mid infrared range. Optionally, the CO 2  laser beam can have a wavelength of from about 3 microns to about 4 microns. Optionally, the CO 2  laser beam can have a wavelength of about 3.2 microns. 
         [0020]    One of the many advantages of employing a CO 2  laser for the formation of a laser beam used for ultrasonic displacement testing of target objects is the high energy available with the CO 2  laser. The increased energy correspondingly produces displacements with higher amplitudes; this provides more discrete measurements and precision in the recorded testing data. 
         [0021]    The CO 2  laser produces a beam whose wavelength extends from about 9 microns to about 11 microns and has a usual wavelength of about 10.6 microns. Laser beams at this wavelength have a relatively shallow optical depth when directed to composite materials which concentrates laser beam energy at a composite surface. Composites can be damaged by CO 2  lasers if too much energy is applied to the surface or the beam is allowed to contact the surface for a protracted period of time. However, laser beams in the mid IR range, i.e., from about 3 microns to about 4 microns, have an increased optical depth thereby allowing more laser energy into the composite surface without the danger of surface ablations. Thus, an additional advantage of laser ultrasonic testing of a composite with a CO 2  beam harmonic is that the laser beam can be used having a higher energy level which corresponds to higher amplitude displacements on the testing surface. 
         [0022]    The CO 2  laser  30  can be designed to emit its corresponding laser beam  44  at various values of energy. Values of at least one Joule up to and in excess of about 4.5 Joules may be realized for a CO 2  laser  30  design. Additionally, the CO 2  laser  30  can be configured such that its corresponding beam  44  has an energy value of any number between 1 Joule and 4.5 Joules. Thus, depending on the conversion efficiency of the harmonic generators, the energy value of the generation beam contacting the target surface can be multiples of the current value of 10 milli-Joules of currently available ultrasonic laser testing systems. Thus, the method and system disclosed herein can provide a generation laser beam having a value at target surface contact of at least about 50 milli-Joules, at least about 75 milli-Joules, at least about 100 milli-Joules, and at least about 300 milli-Joules. Additionally, the frequency of the generation beam  14  can be higher than the 10 Hz currently available. The frequency values can be at least about 100 Hz, at least about 200 Hz, at least about 300 Hz, at least about 400 Hz, at least about 500 Hz, and at least up to about 1000 Hz. 
         [0023]    In one embodiment, the harmonic generators ( 46 ,  47 ,  50 ) may be crystals and may be critical phase matched or quasi-phase matched configurations. In one example, the crystals may be made from the following compounds: AgGaS 2 , AgGaSe 2 , GaAs, GaSe, ZnGeP 2 (ZGP), AgGa1-x1nxSe 2 , Tl 3 AsSe3(TAS), CdGeAs 2 (CGA), and combinations thereof. 
         [0024]    An additional advantage of using a harmonic laser beam formed by a CO 2  laser for laser ultrasonic testing is the harmonic laser beam is less likely to damage a composite surface during testing of the target object. Additionally, the high energy of the CO 2  laser can be used to create higher and more readily measurable displacements within the target surface. Yet another advantage is the ability of coupling the CO 2  laser beam with fiber optics for enhanced transmissibility of the laser beam. 
         [0025]    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.