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Acoustic waveguiding rods with graded velocity profiles
Acoustic waveguiding rods with graded velocity profiles C.K. Jen, Z. W a n g , A. Nicolle*, J.F. Bussiere, E.L. A d l e r * and K. A b e t Industrial Materials Institute, National Research Council, Boucherville, Quebec, Canada J4B 6Y4 * Department of Electrical Engineering, McGill University, Montreal, Quebec, Canada H3A 2A7 f National Optics Institute, Sainte-Foy, Quebec, Canada G1P 4N8
Received 25 July 1991; revised 25 November 1991 Experimental investigations of waveguiding in graded acoustic index rods are presented. These rods, made by ion-exchange and chemical vapour deposition methods, have graded acoustic velocity profiles in the radial direction that are shown to have an ability to focus acoustic waves. Effects of different acoustic profiles on focusing behaviour are demonstrated. The focusing behaviour is visualized with a Schlieren system.
Keywords: acoustic lens; buffer rod; acoustic imaging Introduction In the past decade there has been a considerable effort made to develop further various acoustic lenses operating at very high frequencies such as 1 to 1000 MHz. Most of these lenses use a spherical 1 or cylindrical z concave surface, but some lenses with planar geometries such as acoustic Fresnel zone plates 3'4 have also been used. This paper reports on investigations of waves in rods having acoustically graded index (velocity) profiles which exhibit acoustic focusing behaviours similar to those exhibited optically by G R I N rods s. The two end surfaces of such G R I N rods are flat. We name them 'acoustic graded index ( G R I N ) rods or lenses '6. For lens applications, as an analogy, the phase velocity at the centre of either optical or acoustic G R I N rod must be less than at the edge. One application of acoustic G R I N rods is for long acoustic imaging probes 7. The fabrication methods such as ion exchange, chemical vapour deposition, sol-gel techniques etc. for optical G R I N rods may also be used to produce acoustic G R I N rods. Optically opaque materials may also be employed for acoustic rods. Many doped silica rods with graded index profiles were used in the experiments. Some of these rods have been made with lengths up to 40 cm. In other work 6, G R I N rods made by a modified chemical vapour deposition ( M C V D ) method 8 were used. In this paper, G R I N rods were also fabricated by an ion-exchange and a vapour axial deposition (VAD) method 9. We demonstrate the effects of different profiles on acoustic focusing and waveguiding in G R I N rods.
Acoustic velocity profile measurements Since it is very difficult to obtain the radial distribution of bulk longitudinal, VL, and shear wave velocity, Vs, we have used a 225 M H z line-focus-beam reflection scanning acoustic microscope and the V(z) technique to obtain leaky 0041 624X/92/020091-04 © 1992 Butterworth-Heinemann Ltd
surface acoustic wave (LSAW) and leaky surface-skimming compressional wave (LSSCW) velocities 2'6'7. Because LSAW and LSSCW have predominantly shear and longitudinal wave components, respectively, their velocity profiles can be regarded to be essentially the same as those for the shear vs and longitudinal VLwaves. For fused silica US/ULASW ~- 1.102 and UL/ULSSC W = 1.014. First, an optical G R I N glass rod commercially purchased from Nippon Sheet Glass America Inc., Somerset, N J, was used as the sample. It was fabricated by the exchange of thallium or caesium ions for potassium or ~odium ions in a silicate glass 5. Its LSAW and LSSCW velocity profiles shown in Figures la and lb, respectively, exhibit very smooth graded shapes. The scan-line was across the rod centre. This rod is designed for an optical wavelength of 0.83/~m with a refractive index profile which follows the relationship n(r) = no[1 - (Q/2)r 2] where n o = 1.5986, , , ~ = 0.202 and the rod diameter d = 2r = 3 mm. It is noted that G R I N rods made by the M C V D method 8 have a centre dip in the acoustic velocity profile 6 and that those made by the ion exchange method do not. Analogy to optical G R I N rods 5 the velocity profile of a G R I N acoustic rod can be given as 1
rE(r)
--
1
1
(1 - Q a r = ) 1/2 ~ " (1 - Q a r " / 2 ) Vto rE0
(1)
where Q, is a positive constant, m is the order of the power of the radius and VL0 is the longitudinal velocity at the rod centre. For a G R I N acoustic rod with a parabolic profile (i.e. m = 2), acoustic rays have been shown to behave similarly to optical rays in optical G R I N rods 6 in that a ray incident on the front surface follows a sinusoidal path along the rod lens. The period of this sinusoidal path is called the 'pitch' of the lens and is an important parameter in G R I N lens for use in imaging. The pitch is given by P = 2rC/,,~aa.
Ultrasonics 1992 Vol 30 No 2
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Acoustic waveguiding rods with graded velocity profiles." C.K. Jen et al. 2600 ~ a
01I ii
A
E < d
-1.5
-1.0
-0.5
0.0
1.0
0.5
1.5
Position (mm) 4650
b 4600 4550
E
4500
¢_1 u
2~
4450
doped G R I N silica rods with maximum GeO2 concentrations of 8, 17 and 31%. The measurements indicated that the LSAW and LSSCW velocity profiles exhibit graded shapes, but have centre dips; that the GeO 2 dopant decreases the acoustic wave velocity of the pure fused silica; and that for a given radius a G R I N rod having a higher concentration of GeO 2 has a shorter pitch length P (i.e. higher Q~). Here, we investigate the effects of different G R I N profiles on focusing behaviour. G R I N rods with nearly the same length but with different Qa values in Equation (1) were then used as samples. Figures 2a and 2b show the beam exiting from 12 mm long G R I N rods with 8% (a small Q,) and 17% (a large Q,) GeO 2 dopants (acoustic profiles can be found Ref. 6), respectively. From Figures 2a and 2b and other similar measurements, we find that the focal length decreases when there is a steeper (larger Qa) G R I N velocity profile or an increasing GeO 2 dopant concentration, provided that the length of the rod is less than one-quarter of the 'pitch' of the rod (i.e. p,'4). To study the effects of different operating frequencies on focusing behaviour we have used a 9 m m 17% GeO2-doped G R I N rod. It should be noted that this rod has ~ 5 mm diameter core and ~ 2.4 mm thick cladding. At l0 MHz, the beam exiting the rod did not seem to focus, but the focus is just observable at 20 MHz. Above 30 MHz we can clearly see the focal region. The beam patterns at 30 and 50 MHz are shown in Fiqures 3a and 3h, respectively.
4400 4350i - - ' ,5
I
I
-1.0
i
I
I
-0.5
I
I
0.0
I
0.5
I
I
1.0
The e f f e c t of t h e c e n t r e dip in t h e index profi l e on a c o u s t i c f o c u s i n g
I
1.5
Position (ram) Figure I Acoustic velocity profiles of a Nippon Sheet Glass GRIN rod. (a) VLSAW; (b) VLSSCW
Visualization of focusing behaviour of GR I N rods Since we are interested in the acoustic field intensity distribution of acoustic G R I N rods, a Schlieren technique was used to visualize the acoustic field 1°. In an early experiment we found that, due to the cylindrical shape of the rod and the presence of fine structure in the index profile of G R I N rods made by the chemical vapour deposition method ~', it was very difficult to observe focusing inside the G R I N rod because of beam scattering. Acoustic waves exiting the G R I N rod into a water bath were therefore used to evaluate the focusing behaviour. Because nearly all the G R I N rods used for the experiments were fabricated with a 3 5 mm core diameter (gradedindex) and -~ 2.5 mm pure silica (uniform index) cladding thickness, a planar acoustic wave front can be assumed to be launched into the G R I N core region if a high frequency ultrasonic transducer is used. From consideration of the available ultrasonic transducers and instrumerts, and an acceptable acoustic attenuation in the measurement system, a 5 0 M H z transducer having a diameter of 6.35 mm was chosen first. At 50 MHz the longitudinal acoustic wavelength in the G R I N rods is around 120 ~tm. We previously reported 6'v the use of a modified chemical vapour deposition method 8 to fabricate GeO2-
92
Ultrasonics 1992 Vol 30 No 2
For our previous work 6 and this study we obtained G R I N rods made by the MCVD method 8. The acoustic profiles with different Q~ values could be obtained, but only with a centre dip present 6. Figure 4 shows the refractive index profile of a G R I N rod showing the centre dip made by a MCVD method 8. This dip also exists in the corresponding
a
b
Figure 2 Images of acoustic waves exiting a 12 mm long (a) 17% and (b) 8% GeO2-doped GRIN silica rod. Arrow indicates the location of the rod-water interface
Acoustic waveguiding rods with graded velocity profiles: C.K. Jen et al. a
b~
Figure 3 Images of acoustic waves exiting a 9mm long 17% GeO2-doped GRIN rod at (a) 30 and (b) 50 MHz. Arrow indicates the location of the rod-water interface
G R I N rod without a centre dip has a better acoustic energy confinement near the rod axis. In addition, the Schlieren system was used to visualize an acoustic beam exiting from long buffer rods. Because of the attenuation in the long rod and in water, the operating frequency chosen was 5 MHz. The aim was to observe the acoustic guiding effect due to the presence of the centre dip and the cladding. The two rods tested were 66 m m long GeO2-doped G R I N rods as shown in Figures 4 and 5. The rod with the refractive index profile shown in Figure 4 was made by the M C V D method and it has a centre dip. In the other rod, shown in Figure 5, it was made by a VAD method and this has no centre dip. Figures 7a and 7b show the beams exiting from the rod into the water. They demonstrate that not only is the acoustic energy guided in the core, but also that the G R I N rod without a centre dip gives better energy confinement in the core than the one with a centre dip. Therefore, for buffer rod applications, G R I N rods without the centre index dip should have better performance than those with a centre index dip.
Conclusions 0
Acoustic characterization and focusing mechanisms of rods with graded acoustic velocity profiles fabricated by several different methods have been presented. Such rods
X
"=
0.010
core1cladding 1
2
3
4
5
Radius (mm) Figure 4 Refractive index profile of a GRIN rod made by the MCVD method
/
I I I, acoustic profile. Since our optical measurement has a better spatial resolution 6'7 than the acoustic measurement, only the refractive index profile is given here. Because of the abrupt index change at the rod centre, the acoustic wave energy in the rods made by the M C V D method is not concentrated at the rod centre implying that the focusing beams are not formed by a solid cone but rather by a hollow one. Therefore in Figures 2a, 2b, 3a and 3b we should expect to see a stronger beam intensity away from the centre. In order to verify this hypothesis we used a 5 mm long G R I N rod having profile without a centre dip as shown in Figure 5. This rod, which is also a GeOz-doped silica glass, was made by a VAD method which can be used to fabricate G R I N rods with long lengths and large diameters. Measurements of the beam exiting this rod into the water at 30 M H z are shown in Figure 6. The solid focusing cone in Figure 6 clearly indicates that the
I11 I'
~
', =T,
core
~
cladding
,, : ,, ;..,,
1
2
3
,, ,, ,,
4
5
Radius (ram) Figure 5 method
Refractive index profile of a GRIN rod made by the VAD
Cladding
Core Cladding Figure 6 Image of 30 MHz acoustic waves exiting a 5 mm long GeO2-doped GRIN rod made by the VAD method, as shown in Figure 5. Arrow indicates the location of the rod-water interface
Ultrasonics 1992 Vol 30 No 2
93
Acoustic waveguiding rods with graded velocity profiles. C.K. Jen et al. fl
Claddin Core
anisms. An interesting application of these acoustic focusing rods is a long, low-noise, buffer rod v which could be used in hostile environments such as high temperature and pressure. They can also be made of optically opaque materials such as metals and ceramics.
Acknowledgement
Claddir
Financial support from the N a t u r a l Sciences and Engineering Research Council of Canada is acknowledged. The authors would like to thank Dr A. Iino of Furukawa Electric Co. for providing the G R I N rod made by the VAD method.
References 1 2 3
Figure 7 Images of 5 MHz acoustic waves exiting a 66 mm long GeO2-doped GRIN rod made by (a) the MCVD method, as shown in Figure 4, and (b) the VAD method, as shown in Figure 5. Arrow indicates the location of the r o d - w a t e r interface
4
can be used as lenses to focus the acoustic energy. We have found that the focal length decreases with a steeper G R I N velocity profile (larger Q~) or an increasing GeO2dopant concentration, provided that the length of the rod is less than P/4, where P is the pitch of the lens. We also demonstrated that the G R I N rod without a centre index dip has better acoustic energy confinement near the rod axis. Due to experimental limitations, only experiments involving longitudinal acoustic waves were performed. However, we believe that inside a G R I N rod, shear acoustic waves will exhibit similar focusing mech-
7
94
Ultrasonics 1992 Vol 30 No 2
5
6
X
9 10
Lemons, R.A. and Quate, C.F. Acoustic Microscopy, in: Physical Acoustics Vol 14 (Ed Mason, W.P. and Thurston, R.N.), Academic Press, New York (1979) 2 92 Kushibiki, J. and Chubachi, N. Material characterization by line focus beam acoustic microscope IEEE Trans Sonics Ultrason (1985) SU-32 189 212 Farnow, S.A and Auld, B.A. Acoustic Fresnel zone plate transducers Appl Phys Lett (1974) 25 681 682 Yamada, K. and Shimizu, H. Planar-structure focusing lens for acoustic microscope Proc IEEE Ultrasonics Syrup ( 1985 ) 755 758 Marchand, E.W. Gradient Index Optics Academic Press, Ne~ York (1978) Jen, C.K., Wang, Z., Nieolle, A., Neron, C., Adler, E.L. and Kushihiki, J. Acoustic graded-index lenses Appl Phys Lett( 1991 59 1398 1400 Jen, C.K., Neron, C., Adler, E.L., Farnell, G.W., Kushibiki, J. and Abe, K. Long acoustic imaging probes Proc IEEE Uhrasonic,~ Syrup 11990) 875 880 French, W.G., Jaeger, R.E., Macchesney, J.B., Nagel, S.R., Nassau, K. and Pearson, A.D. Fiber preform preparation, in: Optical Fiher Telecommunications (Eds Miller, S.E. and Chynoweth, A.G.), Academic Press, New York (1979) 233 261 Takahashi, H. and Sugimoto, I. Preparation of germanate glass by vapor-phase axial deposition .1 Am Ceram Soc (I983) 66 C66 C67 Nicolle, A. Two novel ultrasonic lenses. M Eng Thesis, Department of Electrical Engineering, McGill University, Montreal, Canada ( 1991 )
Received 25 July 1991; revised 25 November 1991 Experimental investigations of waveguiding in graded acoustic index rods are presented. These rods, made by ion-exchange and chemical vapour deposition methods, have graded acoustic velocity profiles in the radial direction that are shown to have an ability to focus acoustic waves. Effects of different acoustic profiles on focusing behaviour are demonstrated. The focusing behaviour is visualized with a Schlieren system.
Keywords: acoustic lens; buffer rod; acoustic imaging Introduction In the past decade there has been a considerable effort made to develop further various acoustic lenses operating at very high frequencies such as 1 to 1000 MHz. Most of these lenses use a spherical 1 or cylindrical z concave surface, but some lenses with planar geometries such as acoustic Fresnel zone plates 3'4 have also been used. This paper reports on investigations of waves in rods having acoustically graded index (velocity) profiles which exhibit acoustic focusing behaviours similar to those exhibited optically by G R I N rods s. The two end surfaces of such G R I N rods are flat. We name them 'acoustic graded index ( G R I N ) rods or lenses '6. For lens applications, as an analogy, the phase velocity at the centre of either optical or acoustic G R I N rod must be less than at the edge. One application of acoustic G R I N rods is for long acoustic imaging probes 7. The fabrication methods such as ion exchange, chemical vapour deposition, sol-gel techniques etc. for optical G R I N rods may also be used to produce acoustic G R I N rods. Optically opaque materials may also be employed for acoustic rods. Many doped silica rods with graded index profiles were used in the experiments. Some of these rods have been made with lengths up to 40 cm. In other work 6, G R I N rods made by a modified chemical vapour deposition ( M C V D ) method 8 were used. In this paper, G R I N rods were also fabricated by an ion-exchange and a vapour axial deposition (VAD) method 9. We demonstrate the effects of different profiles on acoustic focusing and waveguiding in G R I N rods.
Acoustic velocity profile measurements Since it is very difficult to obtain the radial distribution of bulk longitudinal, VL, and shear wave velocity, Vs, we have used a 225 M H z line-focus-beam reflection scanning acoustic microscope and the V(z) technique to obtain leaky 0041 624X/92/020091-04 © 1992 Butterworth-Heinemann Ltd
surface acoustic wave (LSAW) and leaky surface-skimming compressional wave (LSSCW) velocities 2'6'7. Because LSAW and LSSCW have predominantly shear and longitudinal wave components, respectively, their velocity profiles can be regarded to be essentially the same as those for the shear vs and longitudinal VLwaves. For fused silica US/ULASW ~- 1.102 and UL/ULSSC W = 1.014. First, an optical G R I N glass rod commercially purchased from Nippon Sheet Glass America Inc., Somerset, N J, was used as the sample. It was fabricated by the exchange of thallium or caesium ions for potassium or ~odium ions in a silicate glass 5. Its LSAW and LSSCW velocity profiles shown in Figures la and lb, respectively, exhibit very smooth graded shapes. The scan-line was across the rod centre. This rod is designed for an optical wavelength of 0.83/~m with a refractive index profile which follows the relationship n(r) = no[1 - (Q/2)r 2] where n o = 1.5986, , , ~ = 0.202 and the rod diameter d = 2r = 3 mm. It is noted that G R I N rods made by the M C V D method 8 have a centre dip in the acoustic velocity profile 6 and that those made by the ion exchange method do not. Analogy to optical G R I N rods 5 the velocity profile of a G R I N acoustic rod can be given as 1
rE(r)
--
1
1
(1 - Q a r = ) 1/2 ~ " (1 - Q a r " / 2 ) Vto rE0
(1)
where Q, is a positive constant, m is the order of the power of the radius and VL0 is the longitudinal velocity at the rod centre. For a G R I N acoustic rod with a parabolic profile (i.e. m = 2), acoustic rays have been shown to behave similarly to optical rays in optical G R I N rods 6 in that a ray incident on the front surface follows a sinusoidal path along the rod lens. The period of this sinusoidal path is called the 'pitch' of the lens and is an important parameter in G R I N lens for use in imaging. The pitch is given by P = 2rC/,,~aa.
Ultrasonics 1992 Vol 30 No 2
91
Acoustic waveguiding rods with graded velocity profiles." C.K. Jen et al. 2600 ~ a
01I ii
A
E < d
-1.5
-1.0
-0.5
0.0
1.0
0.5
1.5
Position (mm) 4650
b 4600 4550
E
4500
¢_1 u
2~
4450
doped G R I N silica rods with maximum GeO2 concentrations of 8, 17 and 31%. The measurements indicated that the LSAW and LSSCW velocity profiles exhibit graded shapes, but have centre dips; that the GeO 2 dopant decreases the acoustic wave velocity of the pure fused silica; and that for a given radius a G R I N rod having a higher concentration of GeO 2 has a shorter pitch length P (i.e. higher Q~). Here, we investigate the effects of different G R I N profiles on focusing behaviour. G R I N rods with nearly the same length but with different Qa values in Equation (1) were then used as samples. Figures 2a and 2b show the beam exiting from 12 mm long G R I N rods with 8% (a small Q,) and 17% (a large Q,) GeO 2 dopants (acoustic profiles can be found Ref. 6), respectively. From Figures 2a and 2b and other similar measurements, we find that the focal length decreases when there is a steeper (larger Qa) G R I N velocity profile or an increasing GeO 2 dopant concentration, provided that the length of the rod is less than one-quarter of the 'pitch' of the rod (i.e. p,'4). To study the effects of different operating frequencies on focusing behaviour we have used a 9 m m 17% GeO2-doped G R I N rod. It should be noted that this rod has ~ 5 mm diameter core and ~ 2.4 mm thick cladding. At l0 MHz, the beam exiting the rod did not seem to focus, but the focus is just observable at 20 MHz. Above 30 MHz we can clearly see the focal region. The beam patterns at 30 and 50 MHz are shown in Fiqures 3a and 3h, respectively.
4400 4350i - - ' ,5
I
I
-1.0
i
I
I
-0.5
I
I
0.0
I
0.5
I
I
1.0
The e f f e c t of t h e c e n t r e dip in t h e index profi l e on a c o u s t i c f o c u s i n g
I
1.5
Position (ram) Figure I Acoustic velocity profiles of a Nippon Sheet Glass GRIN rod. (a) VLSAW; (b) VLSSCW
Visualization of focusing behaviour of GR I N rods Since we are interested in the acoustic field intensity distribution of acoustic G R I N rods, a Schlieren technique was used to visualize the acoustic field 1°. In an early experiment we found that, due to the cylindrical shape of the rod and the presence of fine structure in the index profile of G R I N rods made by the chemical vapour deposition method ~', it was very difficult to observe focusing inside the G R I N rod because of beam scattering. Acoustic waves exiting the G R I N rod into a water bath were therefore used to evaluate the focusing behaviour. Because nearly all the G R I N rods used for the experiments were fabricated with a 3 5 mm core diameter (gradedindex) and -~ 2.5 mm pure silica (uniform index) cladding thickness, a planar acoustic wave front can be assumed to be launched into the G R I N core region if a high frequency ultrasonic transducer is used. From consideration of the available ultrasonic transducers and instrumerts, and an acceptable acoustic attenuation in the measurement system, a 5 0 M H z transducer having a diameter of 6.35 mm was chosen first. At 50 MHz the longitudinal acoustic wavelength in the G R I N rods is around 120 ~tm. We previously reported 6'v the use of a modified chemical vapour deposition method 8 to fabricate GeO2-
92
Ultrasonics 1992 Vol 30 No 2
For our previous work 6 and this study we obtained G R I N rods made by the MCVD method 8. The acoustic profiles with different Q~ values could be obtained, but only with a centre dip present 6. Figure 4 shows the refractive index profile of a G R I N rod showing the centre dip made by a MCVD method 8. This dip also exists in the corresponding
a
b
Figure 2 Images of acoustic waves exiting a 12 mm long (a) 17% and (b) 8% GeO2-doped GRIN silica rod. Arrow indicates the location of the rod-water interface
Acoustic waveguiding rods with graded velocity profiles: C.K. Jen et al. a
b~
Figure 3 Images of acoustic waves exiting a 9mm long 17% GeO2-doped GRIN rod at (a) 30 and (b) 50 MHz. Arrow indicates the location of the rod-water interface
G R I N rod without a centre dip has a better acoustic energy confinement near the rod axis. In addition, the Schlieren system was used to visualize an acoustic beam exiting from long buffer rods. Because of the attenuation in the long rod and in water, the operating frequency chosen was 5 MHz. The aim was to observe the acoustic guiding effect due to the presence of the centre dip and the cladding. The two rods tested were 66 m m long GeO2-doped G R I N rods as shown in Figures 4 and 5. The rod with the refractive index profile shown in Figure 4 was made by the M C V D method and it has a centre dip. In the other rod, shown in Figure 5, it was made by a VAD method and this has no centre dip. Figures 7a and 7b show the beams exiting from the rod into the water. They demonstrate that not only is the acoustic energy guided in the core, but also that the G R I N rod without a centre dip gives better energy confinement in the core than the one with a centre dip. Therefore, for buffer rod applications, G R I N rods without the centre index dip should have better performance than those with a centre index dip.
Conclusions 0
Acoustic characterization and focusing mechanisms of rods with graded acoustic velocity profiles fabricated by several different methods have been presented. Such rods
X
"=
0.010
core1cladding 1
2
3
4
5
Radius (mm) Figure 4 Refractive index profile of a GRIN rod made by the MCVD method
/
I I I, acoustic profile. Since our optical measurement has a better spatial resolution 6'7 than the acoustic measurement, only the refractive index profile is given here. Because of the abrupt index change at the rod centre, the acoustic wave energy in the rods made by the M C V D method is not concentrated at the rod centre implying that the focusing beams are not formed by a solid cone but rather by a hollow one. Therefore in Figures 2a, 2b, 3a and 3b we should expect to see a stronger beam intensity away from the centre. In order to verify this hypothesis we used a 5 mm long G R I N rod having profile without a centre dip as shown in Figure 5. This rod, which is also a GeOz-doped silica glass, was made by a VAD method which can be used to fabricate G R I N rods with long lengths and large diameters. Measurements of the beam exiting this rod into the water at 30 M H z are shown in Figure 6. The solid focusing cone in Figure 6 clearly indicates that the
I11 I'
~
', =T,
core
~
cladding
,, : ,, ;..,,
1
2
3
,, ,, ,,
4
5
Radius (ram) Figure 5 method
Refractive index profile of a GRIN rod made by the VAD
Cladding
Core Cladding Figure 6 Image of 30 MHz acoustic waves exiting a 5 mm long GeO2-doped GRIN rod made by the VAD method, as shown in Figure 5. Arrow indicates the location of the rod-water interface
Ultrasonics 1992 Vol 30 No 2
93
Acoustic waveguiding rods with graded velocity profiles. C.K. Jen et al. fl
Claddin Core
anisms. An interesting application of these acoustic focusing rods is a long, low-noise, buffer rod v which could be used in hostile environments such as high temperature and pressure. They can also be made of optically opaque materials such as metals and ceramics.
Acknowledgement
Claddir
Financial support from the N a t u r a l Sciences and Engineering Research Council of Canada is acknowledged. The authors would like to thank Dr A. Iino of Furukawa Electric Co. for providing the G R I N rod made by the VAD method.
References 1 2 3
Figure 7 Images of 5 MHz acoustic waves exiting a 66 mm long GeO2-doped GRIN rod made by (a) the MCVD method, as shown in Figure 4, and (b) the VAD method, as shown in Figure 5. Arrow indicates the location of the r o d - w a t e r interface
4
can be used as lenses to focus the acoustic energy. We have found that the focal length decreases with a steeper G R I N velocity profile (larger Q~) or an increasing GeO2dopant concentration, provided that the length of the rod is less than P/4, where P is the pitch of the lens. We also demonstrated that the G R I N rod without a centre index dip has better acoustic energy confinement near the rod axis. Due to experimental limitations, only experiments involving longitudinal acoustic waves were performed. However, we believe that inside a G R I N rod, shear acoustic waves will exhibit similar focusing mech-
7
94
Ultrasonics 1992 Vol 30 No 2
5
6
X
9 10
Lemons, R.A. and Quate, C.F. Acoustic Microscopy, in: Physical Acoustics Vol 14 (Ed Mason, W.P. and Thurston, R.N.), Academic Press, New York (1979) 2 92 Kushibiki, J. and Chubachi, N. Material characterization by line focus beam acoustic microscope IEEE Trans Sonics Ultrason (1985) SU-32 189 212 Farnow, S.A and Auld, B.A. Acoustic Fresnel zone plate transducers Appl Phys Lett (1974) 25 681 682 Yamada, K. and Shimizu, H. Planar-structure focusing lens for acoustic microscope Proc IEEE Ultrasonics Syrup ( 1985 ) 755 758 Marchand, E.W. Gradient Index Optics Academic Press, Ne~ York (1978) Jen, C.K., Wang, Z., Nieolle, A., Neron, C., Adler, E.L. and Kushihiki, J. Acoustic graded-index lenses Appl Phys Lett( 1991 59 1398 1400 Jen, C.K., Neron, C., Adler, E.L., Farnell, G.W., Kushibiki, J. and Abe, K. Long acoustic imaging probes Proc IEEE Uhrasonic,~ Syrup 11990) 875 880 French, W.G., Jaeger, R.E., Macchesney, J.B., Nagel, S.R., Nassau, K. and Pearson, A.D. Fiber preform preparation, in: Optical Fiher Telecommunications (Eds Miller, S.E. and Chynoweth, A.G.), Academic Press, New York (1979) 233 261 Takahashi, H. and Sugimoto, I. Preparation of germanate glass by vapor-phase axial deposition .1 Am Ceram Soc (I983) 66 C66 C67 Nicolle, A. Two novel ultrasonic lenses. M Eng Thesis, Department of Electrical Engineering, McGill University, Montreal, Canada ( 1991 )