DEVICE AND METHOD FOR LASER-ULTRASONIC FREQUENCY CONTROL USING OPTIMAL WAVELENGTH TUNING

Abstract: A system and method for generating a desired acoustic frequency content in a laser-generated ultrasonic wave emitted from a target in response to a laser pulse. The method includes generating a generation laser pulse (12) using a laser source (10). An optimal wavelength lambda 0 for the generation laser pulse is determined using a computer. The optimal wavelength data is determined from material-specific, empirically calculated data stored in a storage device (21) that is accessible to the computer. An optimal laser pulse (19) is generated by shifting the generation laser pulse to the optimal wavelength lambda 0. The optimal laser pulse is directed to the target (16) to generate the laser-generated ultrasonic wave with the desired frequency content.

Description: DEVICE AND METHOD FOR LASER-ULTRASONIC FREQUENCY CONTROL USING OPTIMAL WAVELENGTH TUNING TECHNICAL FIELD This invention relates generally to the field of ultrasonic laser testing, and more particularly a method and system for optimizing the frequency content of laser-generated ultrasonic waves for target inspection RACK GROUND ART The use of advanced composite structures has experienced tremendous growth m the aerospace, automotive and other commercial industries Non-destructive evaluation (NDE) methods are often employed to detect inclusions, delaminations and porosities in an effort to ascertain the structural integrity of the composite structures One method of NDE is laser ultrasound Laser ultrasound involves the use of lasers for generation and detection of ultrasound in mateπals such as composites The technique offers the potential of rapid, non-contact mspection Typically, a laser source produces ultrasonic surface displacements on the surface of a remote target A second probe laser beam can detect the ultrasonic surface displacement on the surface of the remote target Collection optics and instrumentation can then be used to process the probe laser beam and output data representing the ultrasonic surface displacements on the surface of the target FIGURE 1 illustrates a conventional laser ultrasound mspection method FIGURE 1 employs pulse laser 10 to mspect an object, such as aircraft 16 Pulse laser 10 emits generation pulse 12 which is directed towards scanning mirror 14 Scanning mirror 14 steers generation pulse 12 to mspect aircraft 16 Vaπous lenses for focusing may also be employed A CO z pulse laser may be employed as pulse laser 10 Pulse laser 10 deposits generation pulse 12 over a certain depth m an area on ancraft 16 Generation pulse 12 is converted to heat and causes expansion of an mspected area on aircraft 16 The expansion of the inspected area on aircraft 16 generates laser-generated ultrasonic waves The frequency content of the laser-generated ultrasonic waves contributes to the success of laser ultrasound as an NDE method Higher frequencies yield better spatial resolution However, for certain materials such as composites, high frequencies are attenuated more rapidly than lower frequencies The thicker and more attenuative the component to be mspected, the lower the ultrasonic frequency content must be to avoid large attenuation Consequently, with conventional methods the resolution and accuracy of defect detection tends to be limited for more attenuative mateπals DISCLOSURE OF INVENTION In light of the above, a need exists for a system and method that generates a desired frequency content m laser-generated ultrasonic waves used for NDE The present mvention provides a system and method for ultrasonic laser mspection that substantially eliminates or reduces disadvantages and problems associated with previously developed systems and methods for ultrasonic laser inspection More specifically, the present invention provides a method for generatmg a desired acoustic frequency content m a laser-generated ultrasonic wave emitted from a remote target m response to a generation laser pulse The method mcludes generatmg the generation laser pulse usmg a laser source An optimal wavelength λ 0 for the laser pulse is determined usmg a computer The optimal wavelength λ 0 is determined from material-specific, empirically calculated data stored m a storage device that is accessible to the computer An optimal laser pulse is generated by shifting the generation laser pulse to the optimal wavelength λ 0 The optimal laser pulse is directed to the remote target to generate the laser-generated ultrasonic wave with the desired frequency content The present invention provides an important technical advantage m that a laser-generated ultrasonic wave can be generated with a desired frequency content Thus, for certain materials that require a specific range for frequencies to adequately mspect the material, an optimal optical penetration depth μ 0 can be determined Based on the necessary optimal optical penetration depth μ 0 , an optimal wavelength λ 0 for the optimal laser pulse can be determined In turn, the optimal wavelength SO to generate the optimal laser pulse wavelength that products the desired range of frequencies m the laser-generated ultrasonic wave is obtained by tuning the source laser wavelength or by shifting the generation pulse laser wavelength usmg a shifting device such as an optical parametric oscillator Therefore, dependmg on the thickness of the mateπal or material composition, the desired frequencies can be generated to produce the best resolution for mspection Additionally, the attenuation of the ultrasound can be controlled allowmg a user to optimize their mspection techniques for the defects to be searched for Furthermore, by understanding the attenuation characteristics of the ultrasound generated m the target, the scanning technique can be optimized based on these characteristics to reduce or eliminate over samplmg and therefore mcrease the speed and efficiency of the mspection BRIEF DESCRIPTION OF DRAWINGS A more complete understandmg of the present invention and the advantages thereof may be acquired by referring to the following description, taken m conjunction with the accompanying drawings m which like reference numbers mdicate like features and wherem FIGURE 1 illustrates a conventional laser ultrasound mspection method, FIGURE 2 is a flow diagram of one embodiment of the present invention, FIGURE 3 represents the frequency spectra of the first back wall echoes and a reflection configuration calculated usmg a sophisticated model for a graphite- epoxy composite, FIGURE 4 represents experimental photo-acoustic spectroscopy data showmg optical penetration depth μ in an epoxy used as a matrix m graphite-epoxy composites, FIGURE 5 represents a more detailed flow diagram of step 23 m Figure 2, and FIGURE 6A and 6B are system block diagrams illustrating one embodiment of the present invention MODES FOR CARRYING OUT THE INVENTION Preferred embodiments of the present mvention are illustrated m the FIGURES, like numerals being used to refer to like and corresponding parts of various drawings The present mvention provides a system and method for laser-ultrasonic frequency control usmg optimal wavelength tuning FIGURE 2 is a flow diagram illustrating one embodiment of the present invention At step 22, an optimal wavelength λ 0 for the generation laser pulse is determined using a computer At step 23, a generation laser pulse is generated usmg a laser source The optimal wavelength data can be determined from matenal- specific, empirically calculated data stored m a storage device in or accessible to the computer At step 24, an optimal laser pulse is generated by shifting the generation laser pulse to the optimal wavelength λ 0 The generation laser pulse is generated from a laser source which may mclude neodynium laser, erbium laser, holrmum laser, thulium laser, excimer laser, chromium laser, fosteπte laser, alexandrite laser, dye laser, titanium sapphire laser, diode laser, limp pumped laser or other laser known to those skilled m the art The mateπal specific data values may vary for different mateπals such as various composites, as well as for different thicknesses of materials At step 26, the optimal laser pulse can be directed toward the target The following discussion in reference to FIGURES 3 and 4 is presented to provide the basis for understandmg the type of data that can be stored m the storage device and how the data can be used to determine the optimal wavelength λ 0 The optimal wavelength λ 0 is the wavelength of the optimal laser pulse necessary to generate the desired frequency content m the laser-generated ultrasonic wave The frequency content of the laser-generated ultrasonic waves is dictated by a combmation of the optical penetration depth μ at the generation laser optical wavelength λ and the temporal profile of the generation laser pulse If the ultrasonic propagation delay of the laser-generated ultrasonic wave mside the optical penetration depth μ is much shorter than the generation laser pulse duration, the frequency content of the laser-generated ultrasonic waves is directly related to the duration of the generation laser pulse On the other hand, if the generation laser pulse duration is much shorter than the ultrasonic propagation delay mside the optical penetration depth μ, the frequency content is directly related to optical penetration depth μ In the latter case, by controlling the optical penetration depth μ, the frequency content of the laser-generated ultrasonic wave may also be controlled FIGURE 3 represents the frequency spectra of the first back wall echoes m a reflection configuration calculated usmg the sophisticated model for a graphite-epoxy composite with optical penetration depth μ of lOOμm, 40μm, and 15μm Along the y-axis is the amplitude of the back wall echoes, while along the x-axis is the frequency of the back wall echoes In the cases presented m FIGURE 3, if the ultrasonic frequency of 80 MHz is used, the wavelength λ 15 correspondmg to a optical penetration depth μ=15μm is approximately two times more efficient than the wavelength λ 40 correspondmg to the optical penetration depth μ=40μm and four tunes more efficient than the wavelength λ 100 correspondmg to the optical penetration depth μ=100μm However, if the ultrasonic frequency of 10 MHz is required, the wavelength λ ]0 o coπesponding to an optical penetration depth μ=100μm is approximately two times more efficient than the wavelength λ 0 correspondmg to the optical penetration depth μ=40μm and four tunes more efficient than the wavelength λ 15 corresponding to the optical penetration depth μ=15μm In an mtermediate case, a wavelength λ correspondmg to an optical penetration depth between 15μm and lOOμm would be the most efficient wavelength In the frequency spectrum presented in FIGURE 3, if the desired frequency content of the laser-generated ultrasonic waves must be higher than 40 MHz, the optimal wavelength λ 0 correspondmg to an optimal optical penetration depth μ 0 of 15μm should be chosen For an ultrasonic frequency content between 20 and 40 MHz, an optimal wavelength λ 0 correspondmg to an optimal an optical penetration depth μ 0 around 40μm is appropriate Finally, for an ultrasonic frequency content below 20 MHz, the optimal wavelength λ 0 coπesponding to an optimal optical penetration depth μ 0 of lOOμm should be chosen FIGURE 4 represents experimental photo-acoustic spectroscopy data showmg optical penetration depth μ in an epoxy used as a matrix m graphite-epoxy composites Along the y-axis is the optical penetration depth μ and along the x-axis is the wavelength λ of the laser pulse needed to achieve the correspondmg optical penetration depth μ The dotted lmes indicate the range of operation for a laser generation system with tunable wavelength At 3 4 μm wavelength λ, the optical penetration depth μ=15μm, and at 4 μm wavelength λ, the optical penetration depth μ > lOOμm Therefore, FIGURE 2 illustrates that a system capable of tuning the generation laser wavelength between 3 4 and 4 micrometers has access to optical penetration depth between 15μm and lOOμm Therefore, the generation laser wavelength can be tuned to an optimal wavelength λ 0 correspondmg to an optimal optical penetration depth μ 0 The optimal optical penetration depth μ 0 can generate the desired frequency content in the laser-generated ultrasonic wave Reference is made to the laser generation system with tunable wavelength disclosed in U S Patent application Serial No 09/361,769 filed on 27 July 1999 to Lorraine, et al , entitled "System And Method For Ultrasonic Laser Testmg Usmg A Laser Source To Generate Ultrasound Havmg A Tunable Wavelength," hereafter "LORRAINE " FIGURE 5 represents a more detailed representation of step 23 in FIGURE 2 At step 28, the optimal optical penetration depth μ 0 is determined usmg a computer from empincal, material-specific frequency data A database or storage device accessible to the computer may store the empincal, mateπal-specific frequency data The empirical, mateπal-specific wavelength data can be of the type illustrated in FIGURE 3 At step 30, the optimal wavelength λ 0 that will produce the optimal optical penetration depth μ 0 (determined at step 28), is determined from empincal, material-specific wavelength data The empincal, material-specific wavelength data can be stored m a storage device on or accessible to a computer and can be of the type illustrated m FIGURE 4 FIGURES 6A and 6B illustrate one embodiment of the present mvention employmg an optical parametric oscillator (OPO) 18 to shift the wavelength of the laser generation pulse to generate optimal laser pulses for two different regions of a target, such as aircraft 16 The example illustrated m FIGURES 6A and 6B mclude OPO 18, but any apparatus for tuning the wavelength of generation laser pulse 12 can be used In FIGURE 6A, pulsed laser or tunable pulse laser 10 emits generation pulse 12 Generation pulse 12 is shifted usmg OPO 18 conjunction with computer 20 and database 21 to produce optimal laser pulse 19 for inspecting aircraft 16, an optical parametric oscillator, a Raman cell, a Bπlloum cell, a difference frequency mix g setup, a sum frequency mixing setup, a harmonic generation setup, the wavelength tuning of the said pulse laser source, or other wavelength shifting device as known to those skilled m the art Scanning mirror 14 can direct optimal laser pulse 19 to mspect aircraft 16 Vanous lenses for focusing may also be employed The need for shifting generation laser pulse 12 to different wavelengths might arise from different polymers in the composite or different thickness in different regions of aircraft 16. Therefore, computer 20 can determine the optimal optical wavelength λ 0 from stored empirically calculated, material-specific data. This data can be stored in a storage device, such as database 21. Database 21 can contain data of the type illustrated in FIGURES 3 and 4 for Various materials and thicknesses. FIGURE 6B represents the same system as in FIGURE 6 A, yet generation laser pulse 12 is shifted to a different optimal wavelength λ 0 for inspection of an alternate region of aircraft 16 optimal for that region. The alternate region may be composed of a different material or have a different thickness. Pulsed laser or tunable pulse laser 10 may be a C0 2 laser, Nd:YAG laser, alexandrite laser, titanium sapphire laser or any other laser suitable to wavelength shifting. This embodiment may be combined with the techniques described in LORRAINE to provide control of the acoustic frequency content by either optical wavelength control or temporal modulation. A further extension is the combination of optical wavelength shifting spatial modulation or patterning of the laser source. The present invention provides an important technical advantage in that a laser-generated ultrasonic wave can be generated with a desired frequency content. Thus, for certain materials that require a specific frequency range to adequately inspect the material, an optimal penetration depth μ 0 can be determined. Based on the optimal penetration depth μ 0 , an optimal wavelength λ 0 for optimal laser pulse 19 can be determined. OPO 18 can shift generation laser pulse 12 to the optimal wavelength λ 0 to generate optimal laser pulse 19. Optimal laser pulse 19 generates the desired frequency content in the laser-generated ultrasonic wave. Therefore, depending on the thickness of the material or the material composition, the desired laser-generated ultrasonic wave can be generated and in turn produce increased resolution for target inspection. Although the present invention has been described in detail herein with reference to the illustrative embodiments, it should be understood that the description is by way of example only and is not to be construed in a limiting sense. It is to be further understood, therefore, that numerous changes in the details of the embodiments of this invention and additional embodiments of this invention will be apparent to, and may be made by, persons of ordinary skill in the art having reference to this description. It is contemplated that all such changes and additional embodiments are within the spirit and true scope of this invention as claimed below.

Claims: WE CLAIM 1 A method for generating a desired acoustic frequency content m a laser-generated ultrasonic wave emitted from a target m response to a laser pulse, said method comprising implementing a computer to determine an optimal wavelength for said laser pulse, usmg mateπal- specific, empirically calculated data stored m a storage device, generatmg said laser pulse at a first wavelength usmg a laser source, generatmg an optimal laser pulse from said laser pulse by shiftmg said laser pulse to said optimal wavelength, and directing said optimal laser pulse to said target to generate said laser-generated ultrasonic wave with said desned frequency content 2 The method of Claim 1, wherem the step of implementing a computer to determine said optimal wavelength further comprises the steps of determining an optimal optical penetration depth from empirically calculated frequency data that produces said laser-generated acoustic wave with a greatest amplitude and said desired frequency content, and determining said optimal wavelength that produces said optimal optical penetration depth usmg empmcally calculated wavelength data 3 The method of Claim 2, wherem said empmcally calculated frequency data is stored m database accessible to said computer 4 The method of Claim 3, wherem said empirically calculated frequency data comprises individual frequency data for a plurality of materials 5 The method of Claim 2, wherein said empmcally calculated wavelength data is stored in a database accessible to said computer 6 The method of Claim 5, wherem said empmcally calculated wavelength data comprises individual wavelength data for a plurality of materials 7 The method of Claim 1 , wherem said laser source is selected from the group consistmg of C0 2 laser, Nd YAG laser, alexandrite laser, or titanium sapphne laser 8 The method of Claim 1, wherein shiftmg said laser pulse at said first wavelength to said optimal wavelength is completed with a wavelength shiftmg device selected from a group consistmg of an optical parametric oscillator, a Raman cell, a Bπllouin cell, a difference frequency mixmg setup, a sum frequency mixmg setup, and a harmonic generation setup 9 The method of Claim 1 , wherem said computer is operable to communicate with a database 10 The method of Claim 1 , wherem said target is composed of a composite material 11 The method of Claim 1 , wherein said target is composed of multiple types of composite matenal 12 A method for generatmg a desired acoustic frequency content m a laser-generated ultrasonic wave emitted from a target m response to a laser pulse, said method compπsmg unplementmg a computer to determme, usmg empmcally calculated frequency data, an optimal optical penetration depth that produces said laser-generated acoustic wave with a greatest amplitude and said desired frequency content, implementing said computer to determme, usmg empmcally calculated frequency data, an optimal wavelength that produces said optimal optical penetration depth, generatmg said laser pulse at a first wavelength usmg a laser source, shiftmg said laser pulse to said optimal wavelength to create an optimal laser pulse, and directing said optimal laser pulse to said target to generate said laser-generated ultrasonic wave with said desired frequency content 13 The method of Claim 12, wherem said empirically calculated frequency data is stored m a database accessible to said computer 14 The method of Claim 13, wherem said empirically calculated frequency data compnses individual frequency data for a plurality of matenals 15 The method of Claim 12, wherem said empmcally calculated wavelength data is stored m a database accessible to said computer 16 The method of Claim 15, wherem said empmcally calculated wavelength data comprises individual wavelength data for a plurality of materials 17 The method of Claim 12, wherem said laser source is selected from the group consisting of C0 2 laser, Nd YAG laser, alexandrite laser, or titanium sapphne laser 18 The method of Claim 12, wherem shifting said laser pulse at said first wavelength to said optimal wavelength is completed with an optical parametric oscillator 19 The method of Claim 12, wherem said computer is operable to communicate with a database 20 The method of Claim 12, wherem said target is composed of a composite mateπal 21 The method of Claim 12, wherem said target is composed of multiple types of composite mateπal 22 A system for generating a desired acoustic frequency content m a laser-generated ultrasonic wave emitted from a target m response to a laser pulse, said system compπsmg a computer for determining an optimal optical penetration depth that produces said laser-generated acoustic wave with a greatest amplitude and said desired frequency content usmg empirically calculated frequency data, and an optimal wavelength that produces said optimal optical penetration depth usmg empirically calculated wavelength data, a laser source for generatmg said laser pulse at a first wavelength, and a wavelength shifting device for shifting said laser pulse to said optimal wavelength to create an optimal laser pulse used to generate said laser-generated ultrasonic wave with said desired frequency content 23 The system of Claim 22, wherem said empmcally calculated frequency data is stored m a database accessible to said computer 24 The system of Claim 23, wherem said empmcally calculated frequency data compπses individual frequency data for a plurality of mateπals 25 The system of Claim 22, wherem said empirically calculated wavelength data is stored m a database accessible to said computer 26 The system of Claim 25, wherem said empirically calculated wavelength data compnses individual wavelength data for a plurality of materials 27 The system of Claim 22, wherem said laser source is selected from the group consistmg of C0 2 laser, Nd YAG laser, alexandrite laser, or titanium sapphire laser 28 The system of Claim 22, wherem shiftmg said laser pulse at said first wavelength to said optimal wavelength is completed with an optical parametric oscillator 29 The system of Claim 22, wherem said computer is operable to communicate with a database 30 The system of Claim 22, wherein said target is composed of a composite mateπal 31 The system of Claim 22, wherem said target is composed of multiple types of composite matenal

Inventors: BAUCO ANTHONY S, DUBOIS MARC, FILKINS ROBERT J, LORRAINE PETER W, VENCHIARUTTI BARBARA, BAUCO, ANTHONY, S., DUBOIS, MARC, FILKINS, ROBERT, J., LORRAINE, PETER, W., VENCHIARUTTI, BARBARA
Applicants: BAUCO ANTHONY S, DUBOIS MARC, FILKINS ROBERT J, LOCKHEED CORP, LORRAINE PETER W, VENCHIARUTTI BARBARA, BAUCO, ANTHONY, S., DUBOIS, MARC, FILKINS, ROBERT, J., LOCKHEED MARTIN CORPORATION, LORRAINE, PETER, W., VENCHIARUTTI, BARBARA
Application year: 2000
Main classification: G01N29
IPC codes: G01N29