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Phase-preserving imaging of high frequency surface acoustic wave fields
Materia&Science and Engineering, A 122 (1989) 43-46
43
Phase-preserving Imaging of High Frequency Surface Acoustic Wave Fields G. SOLKNER,A. GINTER and H.-P. GRABL
Research Laboratories, SiemensAG., Otto-Hahn-Ring6, D-8000Munich 83 (F.R.G.) (Received May 30, 1989)
Abstract
Measurements of the acoustic wave field of high frequency surface acoustic wave (SAW) devices have to be performed in order to develop and improve the computer modelling of SA W propagation. We introduce a method which detects the angle deflection of a laser beam due to the mechanical undulation of the substrate surface and in addition has a high temporal resolution capability by the use of a picosecond laser source. The shortness of the optical pulses (3 ps) in the time domain transforms to a large bandwidth (greater than 10 GHz) of the measurement limited only by the jitter of the driving electronics. 1. Introduction Surface acoustic wave (SAW) filters have been widely used in telecommunication equipment such as television sets, radio relay systems or radar systems. They offer signal-processing capabilities incorporating small size and costeffective mass production. The design of SAW filters require exact modelling of the wave propagation on the substrates. Efficient computational methods for accounting for the two-dimensional wave propagation have been described in the literature [1]. Simulations for our filters show deviations from the measurement down to 0.1 dB in the filter passband for the in-line configuration [2]. Filter configurations using a multistrip coupler yield, in general, better stop band attenuation but are more complicated to model and the electrical measurement of the device does not give information about the two-dimensional acoustic wave field. To verify the computer simulations, quantitative measurements of the acoustic wave field are essential. The measurement capabilities must include frequencies from 10 MHz up to 5 GHz 1)921-5093/89/$3.50
with respect to recent high frequency applications of SAW filters. 2. Optical detection scheme A large variety of optical detection methods for SAWs were reported [3-5]. All of them are somehow based on interferometry. Interferometry in the near field [3, 5] has proved to be both simple and effective. So, for our scheme we adopted the "Balanced photodiode" method [5]. Therein the surface displacement caused by the propagating SAW changes the angle of reflection of a focused laser beam. The focusing lens transforms the deviation in angle into a parallel shift of the reflected beam which can be detected as the differential output of a pair of photodiodes (Fig. 1 ). Since the angular changes of the reflected beam are small (about 10-5 rad), the weak photodiode signal has to be processed in a signal chain with high gain amplification, which is impractical at frequencies beyond 100 MHz. Therefore the measurement of high frequency devices implies the use of some sort of stroboscopic illumination of the wave field in order to freeze the fast surface displacement for the detection. Usually this is done by amplitude modulating a continuouswave (CW) laser (e.g. in an electro-optic
~ IGHz
[
SAW-Device
hotodiodes
]
Fig. 1. Principle of the "'Balanced photodiodes", the reflected laser probe is deviated in angle ct by the surface undulation caused by the propagating surface wave. © Elsevier Sequoia/Printedin The Netherlands
44
modulator). The modulation frequency has to be at or near the frequency of the SAWs. However, at frequencies beyond 1 GHz it is very difficult to find electric driving sources for electro-optic modulators which can provide the necessary high voltage levels (typically 100 V). Our proposal is to use a picosecond laser as a stroboscopic light source. Picosecond lasers generate ultrashort pulses (about lps) at fixed repetition rates. Other than with CW lasers there are no pulses sliced out of a continuous energy stream but the available energy is concentrated into short pulses. This gives a twofold advantage over the use of CW lasers: firstly, 100% modulation depth is obtained and, secondly, the available laser power is more effectively used.
3. Harmonic mixing
~f Envelope:
"'~.
I .....] .............i_ 0
to
2fo
3fo
Fouriertransf of 3ps-pulse
nlo n ~ 100
A f <_fo/2
When using a modulated light source for the illumination of the SAW device surface the signal at the photodiodes (see Fig. 1) shows harmonic mixing of the modulation frequency and the device frequency. If we assume the surface undulation Us to be harmonic at a fixed frequency fs,
Fig. 2. (a) Intensity spectrum of the SAW device driven harmonically at is. (b) Intensity frequency spectrum of a mode-locked laser (mode-locked frequency f0). The envelope function corresponds to an optical pulsewidth of about 3 ps. (c) Frequency spectrum of the harmonic mixing of (a) and (b), indicating a mixing component at Afwhich is used for phasepreserving detection.
Us = l ul cos{2Jrfst +~(x)}
obtained by tuning a high stability frequency source to Af or by mixing the device frequency and the laser spectrum in an electronic mixer and low pass filtering the component at Aft The latter scheme has been adopted for our measurement set-up.
(1)
where ~(x) is a position-dependent phase factor. The frequency spectrum of eqn. (1) is given in Fig. 2(a). The frequency spectrum of a picosecond laser is made up of a series of delta functions equally spaced at multiples of the laser repetition frequency. The envelope function is given by the Fourier transform of the optical pulse width and, because of the shortness of the pulses, extends to a harmonic number of at least 100 (!)(Fig. 2(b)). The spectrum, which is detected by the photodiodes, shows an amplitude modulation at each harmonic of the repetition frequency f~ (Fig. 2(c)). There is especially one component which can be understood as mixing of the device frequency fs with its nearest harmonic of the laser. In the mixed spectrum this component can be found at the lowest frequency Af but, as a result of the harmonic mixing process, carries all the amplitude and phase information of the propagating surface wave. Thus the actual signal at fs can be said to be frequency scaled and phase preserved to a signal at Af, which is a sufficiently low frequency to be easily dectectable with a phase-sensitive receiver (e.g. a lock-in amplifier). The reference signal for such a receiver can be
4. Experimental results In Fig. 3 some wave field measurements are shown. The device under test is a 110 MHz bandpass filter used in digital direct radio systems. Figure 3(a) shows a 100 p m x 100/~m section of the surface wave field measured just in front of the transmitting transducer. The device is driven with an acoustic power of 1.8 mW which transforms into a maximum surface displacement of 0.13 nm. The laser intensity at the specimen surface has to be reduced to 5 mW below the level of thermal excitation of surface waves by the laser probe. The integration time per measurement point is 50 ms which allows a fast scanning speed limited only by the positioning time of our translation stages. This compares favourably with the dwell time per point obtainable with other methods [5]. To demonstrate the high frequency capabilities of our set-up, we have driven the 110 MHz filter
45 a
.
.
.
.
.
.
, !!LIi013o i:iiiiiiiiiI005o
On the assumption of a laser spot diameter of 1 ~m as a typical value in our set-up in the case of LiNbO 3 as the substrate material, a frequency range of about 2 GHz can be inferred. A further improvement of the frequency range seems feasible, e.g. by the use of picosecond UV light and high numerical aperture objectives or by the adoption of schemes that enhance the lateral resolution such as confocal imaging. The minimum detectable surface displacement is determined by the noise of the measurement setup. Among a number of noise sources there are two that give considerable contributions: shot noise; 1If noise (amplitude noise) of the laser. From geometrical considerations the photodiode signal current can be deduced to be AU
_ i~ig = 27r L/: 7PI..... _N~,2~,c
(2)
0
Fig. 3. (a) Acoustic wave field of a 110 M H z bandpass filter, driven at 110 M H z with 1.8 m W acoustic power (scan field, 100/~m x 100/~m; x/y step width, 2 ~um; integration time per point, 50 ms. (b) Acoustic wave field of the same device as in (a), driven at 332 MHz with 0.3 m W acoustic power (scan field, 90 ~ m × 90 k~m; x/y step width, 3/~m; integration time per point, 100 ms). The structure in the upper left-hand corner is due to the higher reflectivity of a transducer finger.
at its third harmonic, at 332 MHz. Figure 3(b) shows the measured acoustic wave field of a 90 ~ m x 9 0 ~m section at the transmitting transducer. The acoustic power in this case was 0.3 mW which corresponds to a maximum surface excursion of 0.05 nm. The structure in the upper left-hand corner of Fig. 3(b) is due to the reflectivity change at one of the transducer fingers for which, in this measurement, no correction was made. 5. F r e q u e n c y range and m i n i m u m d e t e c t a b l e surface a c o u s t i c wave a m p l i t u d e
As can be seen in Fig. 2(b), harmonics off~ in the picosecond laser spectrum exist up to a number of about 100. Thus, device frequencies up to about 10 GHz can harmonically be mixed to baseband. Therefore the frequency range of our method limited by the harmonic mixing process is about 10 GHz. However, for phasepreserving measurements a second condition has to be met: the laser spot diameter d has to be less than half the acoustic wavelength A.
with 7 the photodiode efficiency, P~..... the average laser power, N a the numerical aperture of the focusing lens, 2ac the acoustic wavelength and A U the surface displacement. The shot-noiselimited minimum detectable SAW amplitude for a detection bandwidth of 1 Hz, an N a of 0.36 and a laser power of 5 mW at the 110 MHz filter device then is 3 × 1 0 - 4 n m . However, the amplitude noise of the picosecond laser which is probably due to Raman scattering in the optical pulse compressor dominates the shot noise by about 25 dB. The noise-limited minimum detectable surface displacement taking the same parameters as above is now 5 × 10 3 nm. The minimum detectable SAW amplitude has also been determined experimentally by reducing the driving power to the device to obtain a oneto-one signal-to-noise ratio. At 18 /~W acoustic power we obtained a minimum detectable SAW amplitude of 6.4 x 10 - 3 nm which is in agreement with the amplitude-noise-limited value. From these results the dynamic range of our measurement set-up can be inferred; if the SAW devices are modestly driven with 20 dBm electrical power, the dynamic range is 40 dB. In conclusion we demonstrated that the imaging of the acoustic wave field of SAW devices with a picosecond laser allows for both a high frequency bandwidth and a high signal-to-noise ratio. Measurement times for the acquisition of extended wave field images are thus greatly reduced.
46
Acknowledgments The authors wish to thank C. Kappacher for providing the SAW devices and for valuable discussions, J. Pomplitz for his assistance and E Wolfgang for his general support. References 1 G. Visintini, Differential analysis with extended angular spectrum of waves formalism, Proc. Ultrasonics Symp., IEEE, New York, 1987, p. 145.
2 R. Ganss, C. Ruppel and H. R. Stocker, Spectrum shaping SAW filters for high bit rate digital ratio, 1EEE Trans. Sonics Ultrason., 35 (6) (1988) 673. 3 R. Adler, A. Korpel, and P. Desmares, An instrument for making surface waves visible, IEEE Trans. Sonics Ultrason., 15 (3)(1968) 157. 4 R. M. de la Rue, R. F. Humphreys, I. M. Mason and E. A. Ash, Acoustic-surface-wave amplitude and phase measurements using laser probes, Proc. Inst. Electr. Eng., 119(2) (1972) 117. 5 H. Engan, A phase sensitive laser probe for pulsed SAW measurements, IEEE Trans. Sonics .Ultrason., 29 (5) (1982)281.
43
Phase-preserving Imaging of High Frequency Surface Acoustic Wave Fields G. SOLKNER,A. GINTER and H.-P. GRABL
Research Laboratories, SiemensAG., Otto-Hahn-Ring6, D-8000Munich 83 (F.R.G.) (Received May 30, 1989)
Abstract
Measurements of the acoustic wave field of high frequency surface acoustic wave (SAW) devices have to be performed in order to develop and improve the computer modelling of SA W propagation. We introduce a method which detects the angle deflection of a laser beam due to the mechanical undulation of the substrate surface and in addition has a high temporal resolution capability by the use of a picosecond laser source. The shortness of the optical pulses (3 ps) in the time domain transforms to a large bandwidth (greater than 10 GHz) of the measurement limited only by the jitter of the driving electronics. 1. Introduction Surface acoustic wave (SAW) filters have been widely used in telecommunication equipment such as television sets, radio relay systems or radar systems. They offer signal-processing capabilities incorporating small size and costeffective mass production. The design of SAW filters require exact modelling of the wave propagation on the substrates. Efficient computational methods for accounting for the two-dimensional wave propagation have been described in the literature [1]. Simulations for our filters show deviations from the measurement down to 0.1 dB in the filter passband for the in-line configuration [2]. Filter configurations using a multistrip coupler yield, in general, better stop band attenuation but are more complicated to model and the electrical measurement of the device does not give information about the two-dimensional acoustic wave field. To verify the computer simulations, quantitative measurements of the acoustic wave field are essential. The measurement capabilities must include frequencies from 10 MHz up to 5 GHz 1)921-5093/89/$3.50
with respect to recent high frequency applications of SAW filters. 2. Optical detection scheme A large variety of optical detection methods for SAWs were reported [3-5]. All of them are somehow based on interferometry. Interferometry in the near field [3, 5] has proved to be both simple and effective. So, for our scheme we adopted the "Balanced photodiode" method [5]. Therein the surface displacement caused by the propagating SAW changes the angle of reflection of a focused laser beam. The focusing lens transforms the deviation in angle into a parallel shift of the reflected beam which can be detected as the differential output of a pair of photodiodes (Fig. 1 ). Since the angular changes of the reflected beam are small (about 10-5 rad), the weak photodiode signal has to be processed in a signal chain with high gain amplification, which is impractical at frequencies beyond 100 MHz. Therefore the measurement of high frequency devices implies the use of some sort of stroboscopic illumination of the wave field in order to freeze the fast surface displacement for the detection. Usually this is done by amplitude modulating a continuouswave (CW) laser (e.g. in an electro-optic
~ IGHz
[
SAW-Device
hotodiodes
]
Fig. 1. Principle of the "'Balanced photodiodes", the reflected laser probe is deviated in angle ct by the surface undulation caused by the propagating surface wave. © Elsevier Sequoia/Printedin The Netherlands
44
modulator). The modulation frequency has to be at or near the frequency of the SAWs. However, at frequencies beyond 1 GHz it is very difficult to find electric driving sources for electro-optic modulators which can provide the necessary high voltage levels (typically 100 V). Our proposal is to use a picosecond laser as a stroboscopic light source. Picosecond lasers generate ultrashort pulses (about lps) at fixed repetition rates. Other than with CW lasers there are no pulses sliced out of a continuous energy stream but the available energy is concentrated into short pulses. This gives a twofold advantage over the use of CW lasers: firstly, 100% modulation depth is obtained and, secondly, the available laser power is more effectively used.
3. Harmonic mixing
~f Envelope:
"'~.
I .....] .............i_ 0
to
2fo
3fo
Fouriertransf of 3ps-pulse
nlo n ~ 100
A f <_fo/2
When using a modulated light source for the illumination of the SAW device surface the signal at the photodiodes (see Fig. 1) shows harmonic mixing of the modulation frequency and the device frequency. If we assume the surface undulation Us to be harmonic at a fixed frequency fs,
Fig. 2. (a) Intensity spectrum of the SAW device driven harmonically at is. (b) Intensity frequency spectrum of a mode-locked laser (mode-locked frequency f0). The envelope function corresponds to an optical pulsewidth of about 3 ps. (c) Frequency spectrum of the harmonic mixing of (a) and (b), indicating a mixing component at Afwhich is used for phasepreserving detection.
Us = l ul cos{2Jrfst +~(x)}
obtained by tuning a high stability frequency source to Af or by mixing the device frequency and the laser spectrum in an electronic mixer and low pass filtering the component at Aft The latter scheme has been adopted for our measurement set-up.
(1)
where ~(x) is a position-dependent phase factor. The frequency spectrum of eqn. (1) is given in Fig. 2(a). The frequency spectrum of a picosecond laser is made up of a series of delta functions equally spaced at multiples of the laser repetition frequency. The envelope function is given by the Fourier transform of the optical pulse width and, because of the shortness of the pulses, extends to a harmonic number of at least 100 (!)(Fig. 2(b)). The spectrum, which is detected by the photodiodes, shows an amplitude modulation at each harmonic of the repetition frequency f~ (Fig. 2(c)). There is especially one component which can be understood as mixing of the device frequency fs with its nearest harmonic of the laser. In the mixed spectrum this component can be found at the lowest frequency Af but, as a result of the harmonic mixing process, carries all the amplitude and phase information of the propagating surface wave. Thus the actual signal at fs can be said to be frequency scaled and phase preserved to a signal at Af, which is a sufficiently low frequency to be easily dectectable with a phase-sensitive receiver (e.g. a lock-in amplifier). The reference signal for such a receiver can be
4. Experimental results In Fig. 3 some wave field measurements are shown. The device under test is a 110 MHz bandpass filter used in digital direct radio systems. Figure 3(a) shows a 100 p m x 100/~m section of the surface wave field measured just in front of the transmitting transducer. The device is driven with an acoustic power of 1.8 mW which transforms into a maximum surface displacement of 0.13 nm. The laser intensity at the specimen surface has to be reduced to 5 mW below the level of thermal excitation of surface waves by the laser probe. The integration time per measurement point is 50 ms which allows a fast scanning speed limited only by the positioning time of our translation stages. This compares favourably with the dwell time per point obtainable with other methods [5]. To demonstrate the high frequency capabilities of our set-up, we have driven the 110 MHz filter
45 a
.
.
.
.
.
.
, !!LIi013o i:iiiiiiiiiI005o
On the assumption of a laser spot diameter of 1 ~m as a typical value in our set-up in the case of LiNbO 3 as the substrate material, a frequency range of about 2 GHz can be inferred. A further improvement of the frequency range seems feasible, e.g. by the use of picosecond UV light and high numerical aperture objectives or by the adoption of schemes that enhance the lateral resolution such as confocal imaging. The minimum detectable surface displacement is determined by the noise of the measurement setup. Among a number of noise sources there are two that give considerable contributions: shot noise; 1If noise (amplitude noise) of the laser. From geometrical considerations the photodiode signal current can be deduced to be AU
_ i~ig = 27r L/: 7PI..... _N~,2~,c
(2)
0
Fig. 3. (a) Acoustic wave field of a 110 M H z bandpass filter, driven at 110 M H z with 1.8 m W acoustic power (scan field, 100/~m x 100/~m; x/y step width, 2 ~um; integration time per point, 50 ms. (b) Acoustic wave field of the same device as in (a), driven at 332 MHz with 0.3 m W acoustic power (scan field, 90 ~ m × 90 k~m; x/y step width, 3/~m; integration time per point, 100 ms). The structure in the upper left-hand corner is due to the higher reflectivity of a transducer finger.
at its third harmonic, at 332 MHz. Figure 3(b) shows the measured acoustic wave field of a 90 ~ m x 9 0 ~m section at the transmitting transducer. The acoustic power in this case was 0.3 mW which corresponds to a maximum surface excursion of 0.05 nm. The structure in the upper left-hand corner of Fig. 3(b) is due to the reflectivity change at one of the transducer fingers for which, in this measurement, no correction was made. 5. F r e q u e n c y range and m i n i m u m d e t e c t a b l e surface a c o u s t i c wave a m p l i t u d e
As can be seen in Fig. 2(b), harmonics off~ in the picosecond laser spectrum exist up to a number of about 100. Thus, device frequencies up to about 10 GHz can harmonically be mixed to baseband. Therefore the frequency range of our method limited by the harmonic mixing process is about 10 GHz. However, for phasepreserving measurements a second condition has to be met: the laser spot diameter d has to be less than half the acoustic wavelength A.
with 7 the photodiode efficiency, P~..... the average laser power, N a the numerical aperture of the focusing lens, 2ac the acoustic wavelength and A U the surface displacement. The shot-noiselimited minimum detectable SAW amplitude for a detection bandwidth of 1 Hz, an N a of 0.36 and a laser power of 5 mW at the 110 MHz filter device then is 3 × 1 0 - 4 n m . However, the amplitude noise of the picosecond laser which is probably due to Raman scattering in the optical pulse compressor dominates the shot noise by about 25 dB. The noise-limited minimum detectable surface displacement taking the same parameters as above is now 5 × 10 3 nm. The minimum detectable SAW amplitude has also been determined experimentally by reducing the driving power to the device to obtain a oneto-one signal-to-noise ratio. At 18 /~W acoustic power we obtained a minimum detectable SAW amplitude of 6.4 x 10 - 3 nm which is in agreement with the amplitude-noise-limited value. From these results the dynamic range of our measurement set-up can be inferred; if the SAW devices are modestly driven with 20 dBm electrical power, the dynamic range is 40 dB. In conclusion we demonstrated that the imaging of the acoustic wave field of SAW devices with a picosecond laser allows for both a high frequency bandwidth and a high signal-to-noise ratio. Measurement times for the acquisition of extended wave field images are thus greatly reduced.
46
Acknowledgments The authors wish to thank C. Kappacher for providing the SAW devices and for valuable discussions, J. Pomplitz for his assistance and E Wolfgang for his general support. References 1 G. Visintini, Differential analysis with extended angular spectrum of waves formalism, Proc. Ultrasonics Symp., IEEE, New York, 1987, p. 145.
2 R. Ganss, C. Ruppel and H. R. Stocker, Spectrum shaping SAW filters for high bit rate digital ratio, 1EEE Trans. Sonics Ultrason., 35 (6) (1988) 673. 3 R. Adler, A. Korpel, and P. Desmares, An instrument for making surface waves visible, IEEE Trans. Sonics Ultrason., 15 (3)(1968) 157. 4 R. M. de la Rue, R. F. Humphreys, I. M. Mason and E. A. Ash, Acoustic-surface-wave amplitude and phase measurements using laser probes, Proc. Inst. Electr. Eng., 119(2) (1972) 117. 5 H. Engan, A phase sensitive laser probe for pulsed SAW measurements, IEEE Trans. Sonics .Ultrason., 29 (5) (1982)281.