CA2489714C - System and method for ultrasonic laser testing using a laser source to generate ultrasound having a tunable wavelength - Google Patents

Abstract

Laser radiation is produced with a tunable wavelength and temporal shape. A first laser beam is generated having a first wavelength. The first wavelength of the first laser beam is shifted to a second wavelength. The second wavelength of the first laser beam is modulated.

Description

SYSTEM AND METHOD FOR ULTRASONIC LASER TESTING USING A LASER
SOURCE TO GENERATE ULTRASOUND HAVING A TUNABLE WAVELENGTH
This application is a division of co-pending Canadian Patent Application No.

2,363,928 filed on July 27, 2000.

TECHNICAL FIELD
This invention relates generally to the field of optical information processing and more particularly to a method and apparatus for ultrasonic laser testing. More specifically, the present invention provides an improved laser source for generation of ultrasound. Even more particularly, the laser source of the present invention can significantly improve the characteristics of the ultrasound generated by modulating the optical and temporal characteristics of the generation laser to optimize the characteristics of the generated ultrasound.
BACKGROUND ART
In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries.
While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation ("NDE") methods are required to assess the structural integrity of composite structures, for example, to detect inclusions, delaminations and porosities. Conventional NDE methods are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures. Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One method discloses the use of a pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece. A disadvantage associated with this approach is that the first pulsed laser beam is not optimized for the generation of ultrasound in the workplace.
Prior solutions describe operable techniques for optically detecting transient motion from a scattering surface, which techniques are useful for ultrasonic composite materials non-destructive test and evaluation, these techniques have numerous failings.
Known techniques provide the ability to perform common mode noise cancellation. By using a single laser signal, the known techniques cannot perform differential mode operation.
An adverse consequence of this characteristic is the inability to use known ultrasonic systems in factory or industrial settings where ambient light noise levels exceed certain threshold levels.
This problem prevents the proper detection of scatter signals from the composite materials.
Another limitation associated with none ultrasonic test and evaluation techniques relates to their broad scanning approaches to determine the existence of a transient, thereby indicating a defect. Because broad scanning occurs, both the amount of data is excessive and the degree of accuracy is lower.
Another limitation associated with the known systems relates to their ability to process ultrasonic data in real-time. This limitation makes such systems only marginally useful for testing and evaluating composite materials.
Other limitations associated with existing systems relate to general inflexibility of such systems, which may hold all distances low, result in small depth of field performance and only minimal extraction of information from the back scattered signals. These limitations make industrial application of the ultrasonic testing method generally impractical.
Therefore, it would be desirable for a new method and apparatus for ultrasonic laser testing that overcomes the disadvantages and deficiencies of the prior art.

DISCLOSURE OF INVENTION
In accordance with one aspect of the present invention there is provided a method for producing laser radiation with a tunable wavelength and temporal shape comprising: generating a first laser beam having a first wavelength; shifting the first wavelength of the first laser beam to a second wavelength; and modulating the second wavelength of the first laser beam.
In accordance with another aspect of the present invention there is provided a system for producing laser radiation with a tunable wavelength and temporal shape comprising: a first pulsed laser to generate a first pulsed laser beam having a first wavelength;
a wavelength shifting device to shift the first wavelength of the first laser beam to a second wavelength; and a modulator to modulate the laser beam having the second wavelength.
In accordance with yet another aspect of the present invention there is provided a method for generating ultrasonic displacements at a remote target, comprising the steps of:
generating a first laser having a first wavelength; shifting the first wavelength of the first laser beam to create a second laser having a second wavelength; generating ultrasonic displacements at the remote target using the second laser beam; and modulating the second wavelength of the second laser beam to alter a characteristic of the ultrasonic displacements.
The present invention provides an important technical advantage in that a laser-generated ultrasonic wave can be formed with a desired frequency content and temporal shape.
Thus, for certain materials that require specific ranges of acoustic frequency to adequately 2a perform a non-destructive evaluation of the material, an optimal set of ultrasonic displacements can be determined. In turn, the wavelength shifting device is used to shift the first wavelength to a wavelength, which generates the optimal laser pulse to produce the desired range of acoustic energy in the ultrasonic wave. Therefore, depending on the thickness or composition of the materials, the desired ultrasonic displacements can be generated to produce the best resolution for inspection.
Additionally, the attenuation of the ultrasound can be controlled, allowing a user to optimize the inspection techniques for the defects to be searched for.
Furthermore, by understanding the attenuation characteristics of the ultrasound generated in the target, the scanning technique can be optimized based on these characteristics to reduce or eliminate over sampling and thcrefore increase the speed and efficiency of the inspection which decreasing cost.
The present invention provides another technical advantage in that a system is provided for flexrble, accurate, and cost-effective methoiic for inspecting coniplcx composite structures. Tlre present invention is able to optimize a scan and test a large size composite structure based on empirical data associated with the composite strvcture or data modeled on the composition of the structure.
$]2TFF DESCRTFTiON OF D AVJ~GS
A more complete undersninding of the present invention and the advantages thereof may be acquired by referring to the following description, taken in conjunction with the accompanying drawings in which Itlce reference numbers indicate like features and wherein:
FIGURE 1 provides a fu-st embodiment of the present invention;
FIGURE 2 illustrates an embodiment of the present invention utilizing a control system;
FIGURE 3 illustrates a second embodiment of the present invention wherein wavelength shifting is accomplished using a second laser FIGURE 4 provides a process to reduce noise associated with the generation laser of the present invention;
FIGiJRE 5 provides a flow chart of the present invention; and FIGURE 6 provides a block diagram of the present invention wherein the present invention is utilized in conjunction with a laser detection system for detecting uhrasonic displacements.

hj ODES FOR .ARRYIN C, O 1T' Prefen;ed embodiments of the present invention are illustrated in the FIGUREs, like numerals being used to refer to like and corresponding parts of various drawings.
'I'he present invention provides a system and method for generating laser radiation with a tunable wavelength and temporal shape that substantially eliminates or reduces di~Tdvantages and problems associated with previously developed systems and methods for laser inspection.
More specifically, the present invention provides a system for gernrai}W
lascr,radiation with a tunable wavelength and tcmporal shape. This systcm includes a fust pulsed laser to generate a first pulse laser beam. The first pulse laser beam is produced having a first wavelength. A wavelength shifting device shifts the first wavelength of the first laser beam to a second wavelength. A modulator then modulates the laser beam having the second wavelength.
FIGURE 1 shows in a block diagram the basic components of system 100 for generating laser radiation with a tunable wavelength and temporal shape. System 100 comprises a pulsed laser 10 which will generate a laser beam 12 having a first wavelength X0. The first laser beam 12 will have a path of propagation to a a=avolength shifting device 14. This wavelength shifting device will shift the first wavelength X0 to a second wavelength XI, producing a laser beam 16 having a second wavelength X1. Laser beam 16 may then be modulated by an optical modulator 18 to produce a laser bcam 20 having a tunable temporal shape. In the present invention, wavelength shifting device 14 may be an optical parametric oscillator, RAMAN cell, difference frequency gencrators, or a device combirang a second laser beam with the first to generate a laser having wavelength X1.
Pulsed laser 10 may be a C02 laser, ncodynaium YAG laser (Nd:Yag laser), alexandrite laser, titattitun sapphirc laser, diode pumped laser, lamp pumped laser, or other laser known by those slrilled in the art. The second wavelength chosen may be chosen in order to be cornpatible with an optical carrier used to transmit the laser to a remote target, or may be selected to an optical wavelength ll.
In one cmbodiment of the present nivention, a second wavelcngth tnay be chosen in order to optimize characteristics of the interaction of the laser beam with a target material, such as in the generation of ultrasound in a tnaterial for NDE inspcction. For example, a laser-generated ultrasonic wave can be generated by the laser with a desired frequency content and penetrati.on depth, and wherein the ultrasonic displacements have a desired frequency content. Therefore, depending on the thickness of the material or material composition to be tested, the desired frequencies and ultrasonic displacements can be generated to produce the best resolution for the specific NDE inspection at hand. This second wavelength may be an optimal wavelength that is determined from either matcrial-specific data or empirically calculated data associated with the material to be tested. This data may vary for different materials and thiclmesses of'thesse materials. A specific embodiment of the present invention uses a diode purnped laser with an OPO for operation at 3.5 microns. In another example, the first wavelength may be 1064 nrn and the second wavelength may be 3500 nm, or the second wavelength may be chosen between 1000 mn and 4000 nm.
The fust lascr may be transmitv.d to the wavelength shifting device by an optical fiber mechanism. This optical fiber mechanisni may deliver C02 laser radiation operating at 10 microns. After the wavelength of laser 12 has been shifted from a first wavelength X0 to a second wavelength 7.1, a modulator 18 tnay be used to encode a long-source laser pulse such as a tone burst, chirp, more complex modulation schemes to control the generated ultrasonic displacements spectral or content, or provide a basis for match 5lin4ateetion.
FIGURE 2 shows an additional embodiment of the present invention wherein a computer control system 22 is coupled to the wavelength shifting device 14.and modulator 18. This cont;ol system 22 may be utilized to dynamically control the wavele:ngth shifting device 14 and modulator 18 such that an optimal second wavelength al and the temporal shape of the laser 20 can be dynamically altered during an inspection to optimize an NDE
test.
FIGURE 3 is similar to FIGURE 1. However, the wavelength shifting device may combine a second laser 24 with first laser 12 to produce a uew laser beam 16 having a second wavelength X I.
FIGURE 4 again is similar to F'IGURE 1, however FIGURE 4 incorporates a filter to filter laser beam 12 prior to shifting the wavelength of laser beam 12 to X 1 in order to improve a signal-to-noise ratio of the first laser beam.

A specific embodiment of'the present invention may utilize a solid-state laser such as a Nd:YAG laser.
The optical wavelength may be shifted to match the optical attenuation properties of the material under test. A
preferred embodiment produces laser pulses between 2.5 and 4 microns in optical wavelength and even more specifically between 3.5 and 3.8 microns in optical wavelength.

5 In the present invention, an optical fiber of silver chloride/silver bromide (Ag:Br) glass auy be used to de:liver a laser pulse. More specific:ally, this optical fiber may be used to deliver a 10 micron C02 laser pulse, or other far IR laser pulse for ultrasonic purposes. T'his optical fiber has more far IR applications thatt previous materials such as ZBLAN.
FIGURE 5 presents a method of the present invention wherein laser radiation having a tunable wavelength and temporal shape is used to generate ultrasonic displacements at a remote target for NDE. At step 50, a first laser have a first wavelength is ge:ne:=rated.
At step 52, this first laser :is shifted in wavelength to a second wavelength.
This second wavelength may be: chosen in order to generate desirable characteristics in the ultrasonic displacements. This shifting the wavelength of the first laser may be accomplished using an optical paraaietric oscillator, combining the first laser with a second laser to produce a laser beam having a second wavelength, or other method as known to those skilled in the art. At step 54, the laser bcam having the second wavelength may be modulated to alter the temporal shape or frequency content of the laser. T'his modulated laser may then be appHed to a remote target in or to generate ultrasonic displacements. This is accomplisbed in step 56.
T'he ulttasonic displacemcnts gencrated at the target may be detected to dete:mnine flaws, porosities, or other defects (as known by those slrillcd in the art) in a remote target One sucb method may involve using a piilsed laser beam applied coaxially with the ultrasound generating laser to detect the ultrasonic displacements at tb.e remote target. T'he ultrasonic displacements at the remote target will create phase-modulated light that is scattered and reflected from the coaxially applied laser. This phase-modulated light may be processed in order to obtain data re:presentative of the ultrasonic displacements at the remote target The data associated with the ultrasonic displacements may be processed to discover flaws, defects, or other porosities in the mat.erial under test o f the remote target. T'hese f'unctions are represented by step 58 which inJdlves detecting the ultrasonic displacements at a remote target, and step 60 processing the data associated with the ultrasonic displacements in order to perform a non-destructive evaluation: of the remote targe:t FIGURE 6 describes a system for generating ultrasonic displacements at a remote target utilizing a laser with tunable wavelength in tempoial shape. System 100, or other embodiments of the present invention as illustrated in FIGUREs 2, 3 and 4, may be utilized to generate a laser having tunable wavelength and temporal shape. This laser may be applied by a fiber optic carrier 21 to a remote target 102. A tnate:rial such as ZBLAN, or Ag:Br glass may be selected for optical carrier 20. A long pulse may be encoded into laser 20 by encoding the long pulse prior to shifting from a fitst wavelength of laser 12 to a second wavelength of laser 16. This long pulse niay be dynamically controlled by a computer systern 26 in order to alter an inspection acoustic frequency or a depth of inspection. These encoded pulses may include a tome burst or chirp to control the inspection acoustic frequenc:y. The need to dynamically control this long pulse might arise from different polymers in a composite or differeni: thiclmcsses in different regions of remote target 102.
A detection laser 104 may be applied to the remote target 102. In one embodiment (not shown), this detection laser 104 may be applied coaxially with generation laser 20.
Generation laser 20 produces a compressional ultrasonic displacement in. the material of target 102. The ultrasonic displacement is a result of thcrmo-elastic expansion of the material of target 102 as it absorbs generation laser 20. Generation laser 20 must be of a fiequency that is readily absorbed. at the desired depth in remote target 102 without causing ablation or damage to the material of target 102. Fwrthermore, generation laser 20 must be of a long enough pulse to induce ultrasorcic deformations. Gcneration laser 20 should be absorbed into remote target 102 as heat, thereby causing thetmo~:lastic cxpansion without ablatioin. Generally, utilizing a wavelength in the ultraviolet range is undesirable because such light can potenidally damage a composite material if remote target 102 is made of such.
Detectien laser 104 must be of a frequency that does not inciuce further ultrasonic displacements at remote target 102.
Detection laser 104 interacts wilh ultrasonic displacements at remote target 102 and produces phase-modulated light 106. Some of the phasc-modulated light 106 may be captured by collection optics 108. Methods for detecting such ultrasonic displacements are disclosed in U.S. Patent 6,122,060, entitled "Method and Apparatus For Deu:cting Ultrasonic Surface Displacenunts Using Post-Collection Optical lmplemI:ntation," filed on June 29, 1999, to Thomas E. Drake, and U.S. Patent 6,633,384, entitled "Method and Apparatus For Ultrasonic Laser Testing," filed on June 29, 1999, to Thomas E. Drake.
The embodiments disclosed in the present invention may be combined with the techniques described in the above U.S. Patent Applications and U.S. Patent 6,176,13 5 , entitled "System and Method For l..aser-Ultrasonic Frequency Control Using Optical Wavelength Tuning," filed on July 27, 1999, to Marc Dubois, et. al., to provide control of the acoustic fiequency content by either optical wavelength control or temporal modulation. A further extension is a combination of optical wavelength shifting, spatial modulation, or patteming of the laser source.
The present invention provides an important technical advantage in that a laset-generated ultrasonic wave can be generated with a desired frequency coptent and temporal shape.
Thus, for certain material.s and geomenies that requ'ue specific frequency range or temporal shape to adequately inspeci the material, optimal ultrason.ic displacements may be generated by a laser having optimal optical characteristics. An optimal wavelcngth X1 for generation laser 20 may be determined. Then generation laser 20 may be used to generate the desired ultrasonic displacements at a rerrcote target to be inspected. Hence, the desired ultrasonic d.:yplacernents can be generated and in tum produce increased resolution for target inspection. Additionally, inspection scanning of a reniote target can be optimized based on the attenuation cbaracteristics of the ultrasonic displacements generated at the remote target. Although this enhances and optimizes the efficiency of the non-destructive evaluat7,on testing of materials, this reduces costs and inspection time associated with these materials.

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.

Claims (32)

1. A method for producing laser radiation with a tunable wavelength and temporal shape comprising:
generating a first laser beam having a first wavelength with a first laser;
shifting the first wavelength of the first laser beam to a second wavelength;
and modulating the second wavelength of the first laser beam to alter a characteristic of ultrasonic displacements generated with the first leaser beam.

2. The method of Claim 1, wherein the step of shifting the first wavelength of the first laser beam to the second wavelength is realized with an optical parametric oscillator.

3. The method of Claim 1, wherein the first wavelength is 1064 nm and the second wavelength is 3500 nm.

4. The method of Claim 1, wherein the second wavelength is 1000 to 4000 nm.

5. The method of Claim 1, wherein the second wavelength is compatible with a flexible optical conduit.

6. The method of Claim 5, wherein the flexible optical conduit is a fiber optic carrier.

7. The method of Claim 1, further comprising the step of encoding a long source laser pulse prior to shifting the first wavelength of the first laser beam to the second wavelength.

8. The method of Claim 7, wherein the step of encoding a long source laser pulse source is dynamically controlled by a computer system.

9. The method of Claim 8, wherein the long source laser pulse is encoded with a tone burst.

10. The method of Claim 8, wherein the long source laser pulse is encoded with a chirp.

11. The method of Claim 8, wherein the first laser has a repetition rate greater than 400 Hz.

12. The method of Claim 1, wherein the first laser is selected from the group consisting of CO2 Laser, Nd:YAG laser, alexandrite laser, titanium sapphire laser, lamp pumped laser, and diode pumped laser.

13. The method of Claim 1, wherein the first laser is a solid state laser.

14. The method of Claim 6, wherein the fiber optic carrier is constructed from Zblan.

15. The method of Claim 6, wherein the fiber optic carrier is constructed from silver chloride/silver bromide glass.

16. The method of Claim 6, further comprising reducing peak power of the first laser by temporally controlling pulse width.

17. The method of Claim 1, wherein the step of shifting the first wavelength of the first laser beam to the second wavelength is realized by combining the first laser with a second laser to produce a new laser beam at the second wavelength.

18. The method of Claim 1, further comprising the step of filtering the first laser beam prior to tuning the first wavelength of the first laser beam to improve a signal to noise ratio of the first laser beam.

19. A system for producing laser radiation with a tunable wavelength and temporal shape comprising:
a first pulsed laser to generate a first pulsed laser beam having a first wavelength;
a tunable wavelength shifting device to shift the first wavelength of the first pulsed laser beam to a second wavelength; and a modulator to modulate the first pulsed laser beam having the second wavelength.

20. The system of Claim 19, wherein the tunable wavelength shifting device is an optical parametric oscillator.

21. The system of Claim 19, wherein the first wavelength is 1064 nm and the second wavelength is 3500 nm.

22. The system of Claim 19, wherein the second wavelength is 1000 to 4000 nm.

23. The system of Claim 19, wherein the second wavelength is compatible with a flexible optical conduit.

24. The system of Claim 23, wherein the flexible optical conduit is a fiber optic carrier.

25. The system of Claim 19, further comprising an encoder to encode a long pulse laser prior to shifting the first wavelength of the first pulsed laser beam to the second wavelength.

26. The system of Claim 25, further comprising a computer system to dynamically control the encoding of the long pulse laser.

27. The system of Claim 19, wherein the first pulsed laser is selected from the group consisting of CO2 Laser, Nd:YAG laser, alexandrite laser, titanium sapphire laser, pumped diode and lamped pumped laser.

28. The system of Claim 19, wherein the first pulsed laser is a solid state laser.

29. The system of Claim 19, further comprising a fiber optic carrier constructed from Zblan coupled to the first pulsed laser to deliver the first pulsed laser beam to a point on a remote target.

30. The system of Claim 29, wherein a peak power of the first pulsed laser is reduced by temporally controlling pulse width.

31. The system of Claim 19, wherein the tunable wavelength shifting device combines the first pulsed laser with a second laser to produce a new laser beam at the second wavelength.

32. The system of Claim 19, further comprising an optical filter to filter the first pulsed laser beam prior to tuning the first wavelength of the first pulsed laser beam to improve a signal to noise ratio of the first pulsed laser beam.