CA1210128A - Efficient laser generation of surface acoustic waves - Google Patents
Efficient laser generation of surface acoustic wavesInfo
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- CA1210128A CA1210128A CA000448413A CA448413A CA1210128A CA 1210128 A CA1210128 A CA 1210128A CA 000448413 A CA000448413 A CA 000448413A CA 448413 A CA448413 A CA 448413A CA 1210128 A CA1210128 A CA 1210128A
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Abstract
TITLE
EFFICIENT LASER GENERATION
OF SURFACE ACOUSTIC WAVES
INVENTORS
Paolo G. Cielo Jean Bussière ABSTRACT OF THE DISCLOSURE
The surface acoustic waves are generated by laser beam that is focussed onto a surface to irradiate it in an arcuate pattern as a par-tial annulus or as a still or moving fringe pattern. The arcuate pattern may be formed by a transmitting or a reflecting axicon, while the fringe pattern may be formed by splitting the laser beam into two beams and di-recting the two beams to the surface. In addition, a lens or a frequency shifting device may be placed in the path of one of the split beams to form a circular fringe or a moving fringe, respectively.
EFFICIENT LASER GENERATION
OF SURFACE ACOUSTIC WAVES
INVENTORS
Paolo G. Cielo Jean Bussière ABSTRACT OF THE DISCLOSURE
The surface acoustic waves are generated by laser beam that is focussed onto a surface to irradiate it in an arcuate pattern as a par-tial annulus or as a still or moving fringe pattern. The arcuate pattern may be formed by a transmitting or a reflecting axicon, while the fringe pattern may be formed by splitting the laser beam into two beams and di-recting the two beams to the surface. In addition, a lens or a frequency shifting device may be placed in the path of one of the split beams to form a circular fringe or a moving fringe, respectively.
Description
~%~
Background of the Invention This invention is directed to ultrasonic techniques for detect-ing flaws on a surface, and in particular, to a me~hod and laser appara-tus for generating surface acoustic waves. P
Conventional ultrasonic and eddy current techniquea have been used for detecting flaws in various types of objects. However, these techniques requir~ contact with the object and, therefore, cannot be used when non-contact ls desirable or necessary, as with ob~ects at h~gh temperatures.
Non-contact techniques have been developed in which bulk acous-tic waves are generated in a material by a laser and the acoustic waves reflected by the inner flaws are detected by interferometry. The genera- l tion of bulk acoustic waves is described by W. Kaule et al in United States Patent 4,121,469 which issued on October 24, 1978; by R.L. Melcher et al in United States Patent 4,137,991 which issued on February 6, 1979;
and by W. Kaule et al in United States Patent 4,169,662 whlch issued on October 29 1979. The interferometric method of detecting acoustic waves in a material is described by C.M. Penney in United States Patent 3,978,713 which issued on September 7, 1976; and by E. Primback in United States Patent No. 4,180,324~ which issued on December 25, 1979. This non-contact technique has the advantages of better repetitivity of the measurement because of the absence o~ a coupling liquid, ease of scan-nlng, access to concave or irregular surfaces, and large and flat fre~
quency response leading to an improved spatial and temporal resolution.
It has been found, however, that the detection of cracks or flaws in surfaces may best be carried out by using sur~ace or Rayleigh acoustic waves, as described in the publication by B.R. Tittman et al, "Fatigue Lifetime Prediction with the Aid of Surface Acoustic Wave NDE", Journal N.D.E., 1, 123 (1980). Surface acoustic waves are difficult to generate by conventional pie~oelectric methods, especially at high re-quencies, because of precise angular alignment and need for a liquid couplant which strongly attenuates surface ~aves. The laser generation of surface acoustic waves has not been very successul to date as out-lined in the publieation by A.M. Aindow et al "Laser-Generated Ultrasonic Pulses at Free Metal Surfaces", J. Acoust. Soc. Am., 69, 449, 1981, because of low efficiency, low frequency and difficulty to discriminate against bulk waves.
Summary of the Invention r It is therefore an ob;ect of this invention to provide a method and apparatus for efficiently producing surface acoustic waves using a laser beam.
This and other ob~ects are achieved ln a method for generating surface acoustic waves which comprises providing a laser generated optical beam, and directing the beam to the surface in order to irradiate it in an arcuate pattern.
In accordance with one a~pect of the invention, the arcuate pattern may be obtained by focussing the beam into a partial or complete annulus on the surface by an axicon lens located on the beam path or by an a~icon reflector for reflecting the beam to the surface.
In accordance with another aspect of the invention, the surface acoustic waves may be generated by an optical fringe pattern irradiated on the surface, the fringe pattern being formed from the laser beam which is split into two beams. In addition, a lens may be placed in the path oE one of the split beams to form a circular fringe, or a frequency shifting element may be placed in the path of one of the split beams to form a moving fringe.
Many other objects and aspects of the invention will be clear from the detailed description of the drawlngs.
Brief Description of the Drawlngs In the drawings:
Figure 1 illustrates apparatus for generating surface acoustic waves;
Figure 2 illustrates the radiated area on the surface to be inspected;
Figures 3 and ~ illustrate alternate apparatus for generating the focussed laser beam;
Figure 5 illustrates a fringe method for generating surface acoustic waves; and Figure 6 illustra~es a method for producing a moving fringe.
Detailed DescrlpLion Figure l illustrates one embodiment by which a surface acoustic wave may be generated efficiently on the surface 1 of a material 2 that is to be inspected. The apparatus lncludes an infrared laser 3 that generates a coherent beam. Laser 3, which may be a Q-switched Nd:Y~G
laser, provides a pulsed beam 4 that is focussed by a lens 5 on the sur- -face 1. In addltion, an axicon 6 or conical lens is used to refract the beam 4 such that the focussed area has a partial annulus 7 subtending an angle ~ and havlng a width w, as shown in figure 2.
In this method, a thermal-stress surface wave ls produced by the absorption in the partial annulus 7 illuminated by the shaped laser pulse beam 4. This acoustic wave moves off to the right, expanding and dissipating. At the same time, a wave also moves off to the left where it converges to a narrow focal region 8, -as illustrated by the dotted lines. ~ny bulk acoustic waves generated by the heated area 7 also diverge rapidly, resulting in acoustic echoes of very low power compared to the converging surface wave. The radius of the converging wave can be varied by moving the axicon 6 in a vertical direction towards or away from laser 3. On the other hand, a displacement of the axicon 6 in a horlzontal direction into or out of the beam 4, will change the converg-ing angle ~. Large values of ~, i.e. up to 360, ma~ be desirable when the orientatlon of a surface crack is not known and the surface is being scanned. Moreover, a large aperture a implies better focussing of the acoustic wave, because of the diffraction laws. In generating surface acoustic waves in this manner, the average acoustic wavelength ~ i9 equal to twice the width w of the heated area. Widtb w for a Q~switched, single transverse mode Nd:YAG la6er can be made in the order of 100 ~m.
This results in a cross-6ection of the focused surface wave of the order of 0.2 to 1 mm, dependlng on the value of the angle ~. The increase in the efficiency, i.e. the increase of the amplitude of the detected signal with respect to the signal obtained by a conventional unfocussed tech-nique is thus of the order of 100 if the radius of the converging acous-tic wave is 1 cm.
The surface wave in the focal region 8 may be detected by con-ventional interferometric techniques as illustrated in figure 1. Thls - 4 - .
technique includes the use of a ~ichelson interferometer 10 which pro-vides a probing beam ll generated by a laser 12. The interferometer 10 further includes a beam splltter 14 whlch allows part of the laser 12 beam to pass through to a lens 15 to produce beam ll. The remaining part 5 of the laser 12 beam is reflected to a mirror 16. The returning beams are directed to a detector l7 whlch produces a signal for the readout apparatus 18. Beam 11 i6 deflected by a dichroic mirror 13 to be focus-sed on the focal region 8 of the surface wave in order to take advantage of the ~arge signal amplitude produced by the concentration of the acous-lO tic wave in that region. The presence of a crack 9 ln the path of the acoustic wave from lts point of orlgln, area 7, will strongly reduce the p amplitude of the detected signal ln region 8 since the crack 9 would cause part of the surface wave to be reflected and another portion to be diffracted. Thus9 cracks or flaws may be detected by scanning the sur-15 face l of materlal 2. In order to further increase the power of laser 12 and thus the signal to noise ratio of the system, a diode laser 12 that is pulsed in synchronism with laser 3, may be used.
Figure 3 illustrates an alternate embodlment of the surface J
wave generator in accordance with the pre~ent lnvention. Beam 34 passes 20 through the axlcon 3~ and then through a partially reflecting mirror 33, a lens 35 and a rotating mirror 32 which reflects the focussed beam 34 onto the surface 31 to ba inspected. The rotating mirror 32 allows the beam 34 to scan the surface 31. The interfero~etric beam 37 18 reflected by the mlrror 33 through lens 35 and onto mirror 3~ to scan beam 37 25 across the surface 31 in synchronism with beam 34.
In a ~urther embodiment shown in figure 4, the acoustic wave generatlng beam 44 i~ directed to a reflecting axicon 46 whlch produces the curved heated area.
The interferometric signal obtained from the above apparatus 30 may be analysed slmply for the detecting of cracks. On the other hand, more complex signal processlng may be utili~ed. For instance, a spectro-scopic analysis of the detected signal may be made taking advantage of the selective reflectivity by the crack of shorter acoustic wavelengths as well as time delay of the wave following the crack profile. Such an 35 analysis ls described by C.P. Burger et al, "Rayleigh Wave Spectroscopy 11 2i~ r to Measure the ~epth of Surface Cracks , 13 Symposium NDE, San Antonio, April 1981. Similar techniques can be used to analyse dispersive surface features other than cracks. For example, the thlckness of a coating can be evaluated from the phase delays of the different spectral components ~, 5 of the detected signal. Other possible application~ are the measurement of the acoustic velocity and attenuation of the material.
The maximum frequency of the acoustic wave which can be gener-ated by the method and apparatus described above is limited by the width w of the laser-heated area 7 to approximately 30 ~UIz. A narrower heated 10 area 7 could be produced by increasing the aperture oE the optical sys-tem, but this would make scanning more difficult. High-frequency acous-tic waves may be required in some cases, such as when thln cracks must be detected, or thin coatings must be inspected. These h~gh frequency sur-face acoustic waves may be generated efficiently ln the apparatus 15 illustrated in figure 5. A laser 51 generates a beam 52, which is focus- -sed by a lens 53 onto a beam splitter 54 that produces beams 52' and 52 which are reflected by mirrors 55 and 56 onto the surface 57 of the material 58 being teste~. The beams 52' and 52 are directed to the same irradiated region 59 in order to obtain an interference fringe on the 20 surface 57 of the material 58. An optional lens 60 is positioned in the ~, path of beam 52' such that circular fringes occur which will produce a converging surface wave travelling towards a probing spot in region 59.
In addition, laser 51 is amplitude modulated with a period TaC following the resonance condition v = ~aC/Tac where v is the surface-acoustic~wave phase velocity and ~ac is the surface acoustic wavelength which is equal to the interfringe of the fringe pattern. Typical values for a mode-locked laser 51 and a meta11ic surface are TaC ~ 5 nsec and ~ac ~ 15 llm, which correspond to a frequency of the surface acoustic wave of 200 Mnlz.
The probing syseem cou]d be similar to the one described with respect to figure 1, if the electronics of the interferometer 10 are suf-ficiently fast. Another probing technique, which is also well known in the llterature, ~ay be used as shown sche~atically in figure 5. The probing laser 61 beam 62 is focussed by a lens 63 onto a probing area 64 which is larger than ~ac~ Beam 62 is diffracted by the surface-wave train as it mo~es through area 64 towards a detector 65 with its readout ~2~ 2~
66. This probing technique relaxes the electronics speed requirements, but it has a lower temporal resolution and requires a smoother surface than the probing technique described with respect to figure 1.
Ihe fringe generating method of generating surface acoustic ;
waves is more complex and more difficult to scan than the earlier de-scribéd focussing method, however, it is potentially more efficient because very little power is coupled to bulk acoustic waves. The resonance conditlon V = ~aC/Tac can be satisfied either for the surface waves or for the bulk waves but not for both, because these two kinds of waves have different velocit~es and wavelengths. Thus, nearly 50% of the acoustic power goes into each of the counter-propagating surface waves, providing a higher signal together with a lower spurious acoustic noise.
An even larger coupling efficiency is possible by eliminating the counter-propagating surface wave. This can be done by scanning the fringe pattern on the surface 57 to be inspected at the same speed a~ the surface wave velocity. This would require a mirror rotating at very high speed in order to follow the acoustic wave which travels at a speed in the order of 3,000 m/sec on the surface. Alternately as shown in fig. 6, two interfering beams 67 and 68 of different frequencies may be used to obtain a displacement of the interference fringes within the laser-irra-diated region 69 without any physically moving part~J The two coherent beams 67, 68, of frequencies ~1 and ~2~ respectively, are superposed on the surface 69 so as to produce an interference fringe pattern. If ~2> ~1 the intersection between the wavefronts, which corresponds to the position of a bright fringe, moves towards the right as the wave pro-gresses, as can be seen in fig. 6. If the speed of ~he bright fringe ls the same as the velocity of the acoustic wave, a single acoustic wave propagating towards the right will be generated. This condition can be ac/ op (A2 ~ where fac is the frequency of the acoustic wave and fpp is the frequency of the optical wave. Typlcal values are fOp~ 3.10 Hz and fac= 5.10 Hz, which gives an interfringe ~ac ~ 60 ~m and a wavelength shift (~2~ 1.7.10 . This can be obtained, for example, by shifting the frequency of one of the two beams 52',52 with a Bragg cell inserted in the path of one of the beams 52', 52 . A typical Bragg cell with a carrier frequency of 50 M~lz would be suitable.
~2~ 2~ .
The above methods of generating surface acoustic waves provide many advantages. For example, they provide 1~ an increased optlcal detectability because of the self-amplificatlon of the convergent surface acoustic wave as well as highly efficient frlnge generation of the sur-5 face wave; 2) a reduced acoustic noise because of the enhancement of the ;' surface wave signal together with a reduction of the coupling efficiency to spurious bulk acoustic waves; 3) an increased resolution becàuse of the narrow cross-section of the focussed acoustic wave; and 4) a reduced .
heating of the laser-irradiated surface because of the relatively larg~e heated surface with respect to cross-section of the focussed acoustic wave.
Many modifications in the above described embodiments of the invention can be carried out without departing from the scope thereof and therefore the scope of the present invention is intended to be limited only by the appended claims.
''``` i .~
Background of the Invention This invention is directed to ultrasonic techniques for detect-ing flaws on a surface, and in particular, to a me~hod and laser appara-tus for generating surface acoustic waves. P
Conventional ultrasonic and eddy current techniquea have been used for detecting flaws in various types of objects. However, these techniques requir~ contact with the object and, therefore, cannot be used when non-contact ls desirable or necessary, as with ob~ects at h~gh temperatures.
Non-contact techniques have been developed in which bulk acous-tic waves are generated in a material by a laser and the acoustic waves reflected by the inner flaws are detected by interferometry. The genera- l tion of bulk acoustic waves is described by W. Kaule et al in United States Patent 4,121,469 which issued on October 24, 1978; by R.L. Melcher et al in United States Patent 4,137,991 which issued on February 6, 1979;
and by W. Kaule et al in United States Patent 4,169,662 whlch issued on October 29 1979. The interferometric method of detecting acoustic waves in a material is described by C.M. Penney in United States Patent 3,978,713 which issued on September 7, 1976; and by E. Primback in United States Patent No. 4,180,324~ which issued on December 25, 1979. This non-contact technique has the advantages of better repetitivity of the measurement because of the absence o~ a coupling liquid, ease of scan-nlng, access to concave or irregular surfaces, and large and flat fre~
quency response leading to an improved spatial and temporal resolution.
It has been found, however, that the detection of cracks or flaws in surfaces may best be carried out by using sur~ace or Rayleigh acoustic waves, as described in the publication by B.R. Tittman et al, "Fatigue Lifetime Prediction with the Aid of Surface Acoustic Wave NDE", Journal N.D.E., 1, 123 (1980). Surface acoustic waves are difficult to generate by conventional pie~oelectric methods, especially at high re-quencies, because of precise angular alignment and need for a liquid couplant which strongly attenuates surface ~aves. The laser generation of surface acoustic waves has not been very successul to date as out-lined in the publieation by A.M. Aindow et al "Laser-Generated Ultrasonic Pulses at Free Metal Surfaces", J. Acoust. Soc. Am., 69, 449, 1981, because of low efficiency, low frequency and difficulty to discriminate against bulk waves.
Summary of the Invention r It is therefore an ob;ect of this invention to provide a method and apparatus for efficiently producing surface acoustic waves using a laser beam.
This and other ob~ects are achieved ln a method for generating surface acoustic waves which comprises providing a laser generated optical beam, and directing the beam to the surface in order to irradiate it in an arcuate pattern.
In accordance with one a~pect of the invention, the arcuate pattern may be obtained by focussing the beam into a partial or complete annulus on the surface by an axicon lens located on the beam path or by an a~icon reflector for reflecting the beam to the surface.
In accordance with another aspect of the invention, the surface acoustic waves may be generated by an optical fringe pattern irradiated on the surface, the fringe pattern being formed from the laser beam which is split into two beams. In addition, a lens may be placed in the path oE one of the split beams to form a circular fringe, or a frequency shifting element may be placed in the path of one of the split beams to form a moving fringe.
Many other objects and aspects of the invention will be clear from the detailed description of the drawlngs.
Brief Description of the Drawlngs In the drawings:
Figure 1 illustrates apparatus for generating surface acoustic waves;
Figure 2 illustrates the radiated area on the surface to be inspected;
Figures 3 and ~ illustrate alternate apparatus for generating the focussed laser beam;
Figure 5 illustrates a fringe method for generating surface acoustic waves; and Figure 6 illustra~es a method for producing a moving fringe.
Detailed DescrlpLion Figure l illustrates one embodiment by which a surface acoustic wave may be generated efficiently on the surface 1 of a material 2 that is to be inspected. The apparatus lncludes an infrared laser 3 that generates a coherent beam. Laser 3, which may be a Q-switched Nd:Y~G
laser, provides a pulsed beam 4 that is focussed by a lens 5 on the sur- -face 1. In addltion, an axicon 6 or conical lens is used to refract the beam 4 such that the focussed area has a partial annulus 7 subtending an angle ~ and havlng a width w, as shown in figure 2.
In this method, a thermal-stress surface wave ls produced by the absorption in the partial annulus 7 illuminated by the shaped laser pulse beam 4. This acoustic wave moves off to the right, expanding and dissipating. At the same time, a wave also moves off to the left where it converges to a narrow focal region 8, -as illustrated by the dotted lines. ~ny bulk acoustic waves generated by the heated area 7 also diverge rapidly, resulting in acoustic echoes of very low power compared to the converging surface wave. The radius of the converging wave can be varied by moving the axicon 6 in a vertical direction towards or away from laser 3. On the other hand, a displacement of the axicon 6 in a horlzontal direction into or out of the beam 4, will change the converg-ing angle ~. Large values of ~, i.e. up to 360, ma~ be desirable when the orientatlon of a surface crack is not known and the surface is being scanned. Moreover, a large aperture a implies better focussing of the acoustic wave, because of the diffraction laws. In generating surface acoustic waves in this manner, the average acoustic wavelength ~ i9 equal to twice the width w of the heated area. Widtb w for a Q~switched, single transverse mode Nd:YAG la6er can be made in the order of 100 ~m.
This results in a cross-6ection of the focused surface wave of the order of 0.2 to 1 mm, dependlng on the value of the angle ~. The increase in the efficiency, i.e. the increase of the amplitude of the detected signal with respect to the signal obtained by a conventional unfocussed tech-nique is thus of the order of 100 if the radius of the converging acous-tic wave is 1 cm.
The surface wave in the focal region 8 may be detected by con-ventional interferometric techniques as illustrated in figure 1. Thls - 4 - .
technique includes the use of a ~ichelson interferometer 10 which pro-vides a probing beam ll generated by a laser 12. The interferometer 10 further includes a beam splltter 14 whlch allows part of the laser 12 beam to pass through to a lens 15 to produce beam ll. The remaining part 5 of the laser 12 beam is reflected to a mirror 16. The returning beams are directed to a detector l7 whlch produces a signal for the readout apparatus 18. Beam 11 i6 deflected by a dichroic mirror 13 to be focus-sed on the focal region 8 of the surface wave in order to take advantage of the ~arge signal amplitude produced by the concentration of the acous-lO tic wave in that region. The presence of a crack 9 ln the path of the acoustic wave from lts point of orlgln, area 7, will strongly reduce the p amplitude of the detected signal ln region 8 since the crack 9 would cause part of the surface wave to be reflected and another portion to be diffracted. Thus9 cracks or flaws may be detected by scanning the sur-15 face l of materlal 2. In order to further increase the power of laser 12 and thus the signal to noise ratio of the system, a diode laser 12 that is pulsed in synchronism with laser 3, may be used.
Figure 3 illustrates an alternate embodlment of the surface J
wave generator in accordance with the pre~ent lnvention. Beam 34 passes 20 through the axlcon 3~ and then through a partially reflecting mirror 33, a lens 35 and a rotating mirror 32 which reflects the focussed beam 34 onto the surface 31 to ba inspected. The rotating mirror 32 allows the beam 34 to scan the surface 31. The interfero~etric beam 37 18 reflected by the mlrror 33 through lens 35 and onto mirror 3~ to scan beam 37 25 across the surface 31 in synchronism with beam 34.
In a ~urther embodiment shown in figure 4, the acoustic wave generatlng beam 44 i~ directed to a reflecting axicon 46 whlch produces the curved heated area.
The interferometric signal obtained from the above apparatus 30 may be analysed slmply for the detecting of cracks. On the other hand, more complex signal processlng may be utili~ed. For instance, a spectro-scopic analysis of the detected signal may be made taking advantage of the selective reflectivity by the crack of shorter acoustic wavelengths as well as time delay of the wave following the crack profile. Such an 35 analysis ls described by C.P. Burger et al, "Rayleigh Wave Spectroscopy 11 2i~ r to Measure the ~epth of Surface Cracks , 13 Symposium NDE, San Antonio, April 1981. Similar techniques can be used to analyse dispersive surface features other than cracks. For example, the thlckness of a coating can be evaluated from the phase delays of the different spectral components ~, 5 of the detected signal. Other possible application~ are the measurement of the acoustic velocity and attenuation of the material.
The maximum frequency of the acoustic wave which can be gener-ated by the method and apparatus described above is limited by the width w of the laser-heated area 7 to approximately 30 ~UIz. A narrower heated 10 area 7 could be produced by increasing the aperture oE the optical sys-tem, but this would make scanning more difficult. High-frequency acous-tic waves may be required in some cases, such as when thln cracks must be detected, or thin coatings must be inspected. These h~gh frequency sur-face acoustic waves may be generated efficiently ln the apparatus 15 illustrated in figure 5. A laser 51 generates a beam 52, which is focus- -sed by a lens 53 onto a beam splitter 54 that produces beams 52' and 52 which are reflected by mirrors 55 and 56 onto the surface 57 of the material 58 being teste~. The beams 52' and 52 are directed to the same irradiated region 59 in order to obtain an interference fringe on the 20 surface 57 of the material 58. An optional lens 60 is positioned in the ~, path of beam 52' such that circular fringes occur which will produce a converging surface wave travelling towards a probing spot in region 59.
In addition, laser 51 is amplitude modulated with a period TaC following the resonance condition v = ~aC/Tac where v is the surface-acoustic~wave phase velocity and ~ac is the surface acoustic wavelength which is equal to the interfringe of the fringe pattern. Typical values for a mode-locked laser 51 and a meta11ic surface are TaC ~ 5 nsec and ~ac ~ 15 llm, which correspond to a frequency of the surface acoustic wave of 200 Mnlz.
The probing syseem cou]d be similar to the one described with respect to figure 1, if the electronics of the interferometer 10 are suf-ficiently fast. Another probing technique, which is also well known in the llterature, ~ay be used as shown sche~atically in figure 5. The probing laser 61 beam 62 is focussed by a lens 63 onto a probing area 64 which is larger than ~ac~ Beam 62 is diffracted by the surface-wave train as it mo~es through area 64 towards a detector 65 with its readout ~2~ 2~
66. This probing technique relaxes the electronics speed requirements, but it has a lower temporal resolution and requires a smoother surface than the probing technique described with respect to figure 1.
Ihe fringe generating method of generating surface acoustic ;
waves is more complex and more difficult to scan than the earlier de-scribéd focussing method, however, it is potentially more efficient because very little power is coupled to bulk acoustic waves. The resonance conditlon V = ~aC/Tac can be satisfied either for the surface waves or for the bulk waves but not for both, because these two kinds of waves have different velocit~es and wavelengths. Thus, nearly 50% of the acoustic power goes into each of the counter-propagating surface waves, providing a higher signal together with a lower spurious acoustic noise.
An even larger coupling efficiency is possible by eliminating the counter-propagating surface wave. This can be done by scanning the fringe pattern on the surface 57 to be inspected at the same speed a~ the surface wave velocity. This would require a mirror rotating at very high speed in order to follow the acoustic wave which travels at a speed in the order of 3,000 m/sec on the surface. Alternately as shown in fig. 6, two interfering beams 67 and 68 of different frequencies may be used to obtain a displacement of the interference fringes within the laser-irra-diated region 69 without any physically moving part~J The two coherent beams 67, 68, of frequencies ~1 and ~2~ respectively, are superposed on the surface 69 so as to produce an interference fringe pattern. If ~2> ~1 the intersection between the wavefronts, which corresponds to the position of a bright fringe, moves towards the right as the wave pro-gresses, as can be seen in fig. 6. If the speed of ~he bright fringe ls the same as the velocity of the acoustic wave, a single acoustic wave propagating towards the right will be generated. This condition can be ac/ op (A2 ~ where fac is the frequency of the acoustic wave and fpp is the frequency of the optical wave. Typlcal values are fOp~ 3.10 Hz and fac= 5.10 Hz, which gives an interfringe ~ac ~ 60 ~m and a wavelength shift (~2~ 1.7.10 . This can be obtained, for example, by shifting the frequency of one of the two beams 52',52 with a Bragg cell inserted in the path of one of the beams 52', 52 . A typical Bragg cell with a carrier frequency of 50 M~lz would be suitable.
~2~ 2~ .
The above methods of generating surface acoustic waves provide many advantages. For example, they provide 1~ an increased optlcal detectability because of the self-amplificatlon of the convergent surface acoustic wave as well as highly efficient frlnge generation of the sur-5 face wave; 2) a reduced acoustic noise because of the enhancement of the ;' surface wave signal together with a reduction of the coupling efficiency to spurious bulk acoustic waves; 3) an increased resolution becàuse of the narrow cross-section of the focussed acoustic wave; and 4) a reduced .
heating of the laser-irradiated surface because of the relatively larg~e heated surface with respect to cross-section of the focussed acoustic wave.
Many modifications in the above described embodiments of the invention can be carried out without departing from the scope thereof and therefore the scope of the present invention is intended to be limited only by the appended claims.
''``` i .~
Claims (12)
1. A method of generating surface acoustic waves on a surface comprising:
providing a laser generated optical beam;
directing the beam to the surface;
and focusing the beam into an annulus irradiated pattern on the surface to form a converging surface wave.
providing a laser generated optical beam;
directing the beam to the surface;
and focusing the beam into an annulus irradiated pattern on the surface to form a converging surface wave.
2. A method as claimed in claim 1 in which the annulus is a partial annulus.
3. Apparatus for generating acoustic surface waves on a surface comprising:
laser means for generating a coherent beam of optical energy;
and means for focusing the beam onto the surface in an annulus irradiated pattern to form a converging surface wave.
laser means for generating a coherent beam of optical energy;
and means for focusing the beam onto the surface in an annulus irradiated pattern to form a converging surface wave.
4. Apparatus as claimed in claim 3 wherein the annulus pattern is a partial annulus.
5. Apparatus as claimed in claim 3 or 4 wherein the focusing means includes an axicon lens located on the beam path.
6. Apparatus as claimed in claim 3 or 11 wherein the focusing means includes an axicon reflector for reflecting the beam to the surface.
7. A method of generating acoustic wave on a surface comprising:
providing a laser generated optical beam;
splitting the beam into two beams;
directing the beams to the surface; and focusing the beams to form a fringe pattern on the surface to form a converging surface wave.
CLAIMS: (continued)
providing a laser generated optical beam;
splitting the beam into two beams;
directing the beams to the surface; and focusing the beams to form a fringe pattern on the surface to form a converging surface wave.
CLAIMS: (continued)
8. A method as claimed in claim 7 which includes:
shifting the frequency of one of the two beams such that the fringe pattern moves on the surface.
shifting the frequency of one of the two beams such that the fringe pattern moves on the surface.
9. Apparatus for generating acoustic surface waves on a surface comprising;
laser means for generating a coherent beam of optical energy;
beam splitter means for splitting the beam into two beams; and reflector means for deflecting the two beams onto the surface to form a fringe pattern on the surface to form a converging surface wave.
laser means for generating a coherent beam of optical energy;
beam splitter means for splitting the beam into two beams; and reflector means for deflecting the two beams onto the surface to form a fringe pattern on the surface to form a converging surface wave.
10. Apparatus as claimed in claim 9 which further includes a lens located in the path of one of the two beams.
11. Apparatus as claimed in claim 10 which further includes frequency shifting means located in the path of one of the two beams.
12. Apparatus as claimed in claim 9 which further includes frequency shifting means located in the path of one of the two beams.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000448413A CA1210128A (en) | 1984-02-28 | 1984-02-28 | Efficient laser generation of surface acoustic waves |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000448413A CA1210128A (en) | 1984-02-28 | 1984-02-28 | Efficient laser generation of surface acoustic waves |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1210128A true CA1210128A (en) | 1986-08-19 |
Family
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000448413A Expired CA1210128A (en) | 1984-02-28 | 1984-02-28 | Efficient laser generation of surface acoustic waves |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1210128A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5613965A (en) * | 1994-12-08 | 1997-03-25 | Summit Technology Inc. | Corneal reprofiling using an annular beam of ablative radiation |
| US6063072A (en) * | 1994-12-08 | 2000-05-16 | Summit Technology, Inc. | Methods and systems for correction of hyperopia and/or astigmatism using ablative radiation |
-
1984
- 1984-02-28 CA CA000448413A patent/CA1210128A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5613965A (en) * | 1994-12-08 | 1997-03-25 | Summit Technology Inc. | Corneal reprofiling using an annular beam of ablative radiation |
| US6063072A (en) * | 1994-12-08 | 2000-05-16 | Summit Technology, Inc. | Methods and systems for correction of hyperopia and/or astigmatism using ablative radiation |
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