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of piezoceramic transducers. However, compared with ceramic
transducers, the piezofilm transducers have low sensitivity as
transmitters. This is primarily because the piezofilm has a
lower piezoelectric constant d33 ~ 30 pm. V " 1 compared with
200 to 600 pm. V~ l for piezoceramics. Whereas a voltage spike
of about 400 V is ample to drive the piezoceramic transducers,
700 V or more was necessary for the film transducer's output to
be visualised.
solve the edge wave echo but, for those which were, the amplitude of the edge wave was 43 to 50 dB less than that of the
direct wave, at the range giving the greatest edge-wave
intensity.
Repeating this measurement with a film transducer with
nylon backing and a plane active area of 24 mm by 9 mm, we
could find no edge wave signal. If they were present, they were
lost in electrical noise and were at least 72 dB less than the
direct-wave signal. In comparison with conventional piezoceramic probes then, the suppression of edge waves by this type
of piezofilm transducer is at least 20 dB better, probably even
better than that.
Acknowledgments: Thanks are due to the British Railways
Board for the use of the Schlieren system, and to the Open
University for support of the imaging work of which this investigation formed a part.
D. I. CRECRAFT
C. J. S. DAVIES
i~
• -
•
—i
Fig. 2 Wavefront propagated by cylindrical film transducer at ranges oj
(left) 13 mm and (right) 100 mm
10th November 1981
Faculty of Technology
The Open University
Milton Keynes MK7 6AA, England
K. G. HALL
Fig. 3a shows the wave launched by a conventional 2 MHz
piezoceramic flaw-detector probe with a 20 mm diameter plane
surface. Its lightly damped response is evident, but the feature
of more importance to us is the presence of edge waves. These
are commonly seen in visualisation work, and their effect on
near-field resolution has been explained. 10 Note that, because
the source is circular, the edge waves have a toroidal wavefront. Only a small part of this is visualised—that in the thin
slice, a few wavelengths thick, through the diameter parallel to
the plane of the photograph. We were particularly interested to
see whether or not the absence of a hard edge to a piezofilm
would result in no edge-wave generation.
Railway Technical Centre
Derby DE2 8UP, England
References
1 BUI, L., SHAW, H. j . , and ZITELLI, L. T.: 'Experimental broadband
ultrasonic transducers using PVF 2 piezoelectric film', Electron.
Lett., 1976, 12, (16), pp. 393-394
2 WOODWARD, B.: T h e suitability of polyvinylidene fluoride as an
underwater transducer material', Acustica, 1977, 38, pp. 264-268
3 SHOTTON, K. c , BACON, D. R., and QUILLIAM, R. M.: 'A pvdf mem-
brane hydrophone for operation in the range 0-5 MHz to 15 MHz',
Ultrasonics, 1980, 18, pp. 123-126
4 KOVNOVICH, s., and HARNIK, E.: 'Generation and detection of shortduration unipolar stress pulses using poled PVF 2 films', Electron.
Lett., 1980, 16, (20), pp. 776-777
5 FUKADA, E., and FURUKAWA, T.: 'Piezoelectricity and ferroelectricity
in polyvinylidene fluoride', Ultrasonics, 1981, 19, pp. 31 39
6 CRECRAFT, D. i., and DAVIES, c. i. s.: 'A time-delay ultrasonic imaging system using PVF 2 transducers'. Ultrasonics Ultrasonics International 81 Conference Proceedings, IPC Science & Technology
Press Ltd., UK, 1981, pp. 302-306
7 REDWOOD, M.: 'A study of waveforms in the generation and detection of short ultrasonic pulses', Appl. Materials Res., 1963, 2, pp.
76-84
8 HALL, K. c : 'Visualizations of acoustic waves'. Ph.D. thesis, Open
University, UK, 1975
9 HALL, K. G.: 'Observing ultrasonic wave propagation by stroboscopic visualization methods', submitted for publication in
Ultrasonics
Fig. 3
a Wavefront propagated by plane 20 mm diameter piezoceramic
probe at 40 mm range
b Wavefront propagated by plane rectangular film 30 mm x 9 mm
at 30 mm range
Fig. 3b shows the wave launched by a plane film transducer,
with the active area of film 30 mm by 9 mm. The longer side of
the film faces the camera, so the wavefront is 30 mm wide. No
edge wave can be seen, in spite of the fact that, with this rectangular source, the whole of the 9 mm length of the cylindrical
wavefront from each 9 mm edge would interact with the light
in the Schlieren system. Of course, this could be because, with
the lower sensitivity of the film transducer, any edge waves
generated were too weak to be visualised. An increase in drive
voltage to about 1 kV revealed no edge wave and, eventually,
caused failure of the transducer by rupturing the electrodes.
To settle the question of the existence of edge waves, we
turned once again to the impulse response. With a well
damped transducer situated close to a reflector, echoes of the
edge waves can be resolved from those of the direct plane wave.
The intensity of the edge waves appears greatest at a distance
of about 1-5 times the transducer radius, for a circular
source. 10 We tested a number of megahertz-frequency commercial piezoceramic flaw-detector probes placed 5 to 10 mm
from a plane metallic reflecting surface in water. In this situation, the edge wave can be identified by its timing in relation to
that of the direct-wave echo and the active diameter of the
transducer. Few probes were sufficiently well damped to reELECTRONICS LETTERS
10 WEIGHT, J. p., and HAYMAN, A. J.: 'Observations of the propagation
of very short ultrasonic pulses and their reflection by small targets',
J. Acoust. Soc. Am., 1978, 63, pp. 396-404
0013-5194/82/010016-02 $150/0
InGaAsP/lnP DUAL WAVELENGTH LASERS
Indexing terms: Lasers, Semiconductor lasers
The first successful dual wavelength lasers emitting at 1-2 /im
and 1-3 fim wavelengths are described. The lasers operated up
to 0°C.
A multiwavelength light emitter would be one of the important
components in a simplified wavelength-division-multiplexed
optical communication system in which the use of separate
multiplexers and demultiplexers is avoided. 1 The dual wave-
7th January 1982 Vol. 18 No. 1
17
length InGaAsP/InP LED was reported in our previous
paper,2 but the dual wavelength laser diode has not yet been
reported. This letter reports the first successful laser operation
of dual wavelength InGaAsP/InP lasers which emit at 1-2 and
1-3 /jm wavelengths.
1
3
perature To defined by J,h oc exp (T/To) of LD32 and LD31 are
51 K and 44 K, respectively, in the temperature range 210
K < T < 270 K. Lasing operation up to 0°C is obtained. By
mounting the chips with junction down on the heat sink, room
temperature operation might be possible.
We are now preparing new stripe lasers which will make the
direct coupling to optical fibre easy and be able to operate at
room temperature. The results will be published in the near
future.
24th November 1981
S. SAKAI
T. AOKI*
M. UMENO
LD 32
Department of Engineering Science
Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya 466, Japan
* Present address: Musashino Electrical Communication Laboratory,
NTT, Midori-cho, Musashino 180, Japan
References
1
Fig. 1 Schematic illustration of dual wavelength laser
SAKAI, s., AOKI, A., TOBE, M., and UMENO, M.: 'Simplified dual channel
optical transmission using integrated light emitters and photodetectors', Jpn. J. Appl. Phys., 1981, 20, pp. L205-L207
Q l and Q2 mean the quaternary layers
The laser structure is essentially the same as in Reference 2
and shown in Fig. 1. Five layers, Sn-doped w-InP (7 /mi),
nondoped Ino.75Gao.25Aso.6oPo-4o (0-4 /mi), Zn-doped p-InP
(6 /mi), nondoped Ino^sGao^Aso^Po^s (0-4 /mi) and Sndoped n-InP (2 /mi) are grown sequentially on a (lOO)-oriented
n-InP substrate by the LPE method. After the growth, a part of
the wafer is etched in HC1:CH 3 COOH:H 2 O 2 = 1:1:1 at
25°C to expose the p-layer, and the three electrodes are positioned as indicated. The laser chips are mounted on copper
blocks with the junction-up configuration.
2
SAKAI, S., AOKI,
A., AMEMIYA, Y., a n d UMENO,
M.: ' A n e w
InGaAsP/InP dual-wavelength LED', Appl. Phys. Lett., 1979, 35,
pp. 588-589
0013-5194/82/010017-02S1.50/0
DUAL WAVELENGTH InGaAsP/InP TJS
LASERS
Indexing terms: Lasers, Semiconductor lasers
InGaAsP/InP dual wavelength TJS lasers emitting at 117 //m
and 1-3 pm wavelengths at room temperature are described.
The threshold currents for both diodes are the same. The
fabrication procedure and characteristics of the lasers are
presented.
The dual wavelength laser diode (LD) is a very attractive
device for simplified dual wavelength optical transmission.1
Wefirstdemonstrated a dual wavelength InGaAsP LED2 and
laser diode3 which include the two DH structures on one
substrate. But, because the device structure was asymmetric,
the characteristics such as threshold currents and series resistances of the two diodes were rather different. This letter
describes new dual wavelength TJS lasers whose two LD characteristics are the same except for the lasing wavelengths.
The structure of the dual wavelength TJS laser is shown in
Fig. 1. The fabrication procedure is as follows. First, five
layers: n-InP (8 /mi), In0.79Ga021As0.49P0.51 (0-2 /mi,
Xg = 117 /mi), n-InP (6 /an), Ino.75Gao.25Aso-6oPo-4o (02 /mi,
kg = 1-30 /mi) and n-InP (2 /mi) are grown sequentially on a
LD21
0
50
100
pulse current , mA
150
|980/2|
LD 32
Zn diffused P*
anodic
Au-Zn
Fig. 2 Injected current against output power relations
The current-output power relation of the laser under pulsed
condition is shown in Fig. 2. The emitting spot sizes of LD31
and LD32 are about 20 /mi and 70 /mi, respectively, without
any stripe geometry, and the distance between the two spots
are about 50 /mi. The cavity length is about 200 /mi. The lasing
wavelength of LD31 and LD32 are 113 /mi and 1-23 /mi, respectively, at 135 K, and 1-20 /mi and 1-30 /mi, respectively at
270 K. The high threshold current of LD31 might be caused by
the high series resistance (about 30 Q) of the diode because the
current flows parallel to the junction plane through a thin
p-layer. The shape of the emission spectrum is the same as that
of conventional broad contact lasers. The characteristic tem18
7777,
ULULU1L
n-InP
_
substrate
active regions
Au-Sn
Fig. 1 Schematic illustration of dual wavelength TJS laser structure
Q l and Q2 are the quaternary layers
ELECTRONICS LETTERS
7th January
1982 Vol. 18 No. 1
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