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Optoelectronic switching in a stub transmission lineJ. A. Buck, K. K. Li, and J. R. Whinnery Citation: Journal of Applied Physics 51, 769 (1980); doi: 10.1063/1.327290 View online: http://dx.doi.org/10.1063/1.327290 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/51/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Controlling the transmission line shape of molecular t-stubs and potential thermoelectric applications J. Chem. Phys. 135, 154109 (2011); 10.1063/1.3653790 Transmission filtering of a waveguide coupled to a stub microresonator Appl. Phys. Lett. 89, 101113 (2006); 10.1063/1.2345251 Direct optoelectronic generation and detection of sub-ps-electrical pulses on sub-mm-coaxial transmission lines Appl. Phys. Lett. 85, 6092 (2004); 10.1063/1.1839645 Acoustics spectral gaps and transmission in periodic stub tuners J. Acoust. Soc. Am. 114, 2356 (2003); 10.1121/1.4776789 Subpicosecond optoelectronic study of resistive and superconductive transmission lines Appl. Phys. Lett. 50, 350 (1987); 10.1063/1.98197
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Optoelectronic switching in a stub transmission line J. A. Buck, K. K. Li, and J. R. Whinnery Electronics Research Laboratory, University of California, Berkeley, California 94720
(Received 3 July 1979; accepted for publication 23 August 1979)
We have demonstrated optoelectronic switching in a pulse-forming microstrip line, which utilizes the energy in a laser pulse for triggering. Electrical pulses 1 ns in width and of 1-2 V in amplitude were produced using a lO-ns dye laser pulse of l-kW amplitude. The possibilities of further pulse shortening appear very promising.
PACS numbers: 85.60.Jb, 84.40. Wv
We have shown that narrow width electrical pulses can be produced by triggering the discharge of a stub transmission line by light pulses of significantly longer duration. The technique is closely related to earlier work in optoelectronic switching on micros trip lines. 1.2 The idea of a stub section for this purpose was first suggested by Auston3 for use with picosecond light pulses. Its principal advantage is that only one laser pulse is required per electrical pulse. Additionally, the laser pulse need not be short in duration, but must only have a short rise time.
The stub line switch is shown in Fig. 1. It consists of a GaAs wafer, onto which is evaporated gold to form a ground plane, and a top strip whose width is chosen for a characteristic impedance of 50 fl. Two gaps exist in the top strip, the first of which (A) serves as a passive resistor, while the second gap (B) is exposed to the laser pulse. The length ofline I between the two gaps is the stub section. Upon being fully charged, the section is essentially isolated, since the resistance of gap A is typically over 1 Mfl. The resistance of gap B is made higher than that of A to assure containment of charge within the section. A light pulse is then caused to impinge upon gap B, generating electron-hole pairs on and just beneath the semiconductor surface, which effectively lowers the gap resistance. The stub then discharges through the gap to form an electrical pulse.
The pulse forming mechanism in such a situation is well known. Ideally, an electrical pulse can be formed whose width is equal to twice the length of the stub section divided by the velocity of wave propagation in the line. Iflaser pulses are used whose rise times are long compared to the roundtrip time of the section, however, this ideal situation cannot be achieved. Carriers in the gap cannot be excited quickly enough to provide impedance matching between the stub section and the output line, and what is seen instead is a discharge process consisting of numerous reflections of the voltage wave in the section. The resulting electrical pulse can nevertheless still be quite short.
The discharge analysis is similar to that carried out in Ref. (4). At time t = 0, the leading edge of the laser pulse impinges on gap B, and begins to decrease its resistance R (t). Current then begins to flow across the gap, and a voltage wave begins to propagate toward the output of the device. The resulting depletion of charge in the stub section is manifest as a decrement voltage wave V- which propagates back toward gap A. At this end, V- is almost totally reflect-
ed, and it propagates back toward gap B, where it is then partially reflected, and the process repeats. The amount reflected is determined by the reflection coefficient associated with R (I). Actually, decrement waves are constantly being created, since R (t) is continually being lowered. Analysis is simpler though, if the situation can be handled discretely. This we have accomplished by subdividing the stub roundtrip time into small intervalsLlt. From t = 0, a new V- wave is initiated at each Llt. What is then seen is a train of decrement waves of separation Llt propagating in the section. Each wave, when it returns to gap B, is partially reflected, and an additional decrement is superimposed on it to produce a new wave. The values of the additional decrements are determined by the magnitude of R (I) at the times of their creation. Immediately before each reflection from gap B, a reading is taken of the current flowing through R (I), which enables the calculation of the instantaneous voltage which propagates toward the output. The output pulse shape is then determined by displaying these voltage contributions together on the appropriate time scale. This process is continued until the section is completely discharged. The accuracy of this method is reasonably good ifLltis much less than the rise time of the laser pulse.
The resistance R (I) of gap B is inversely proportional to the integral of the laser pulse intensity up to time I, i.e.,5
R(I)=(n+l)2 w 4n e
xC 1 )d2J11' I(I')dl', Vte + Jlh / Jo
(1)
where I (I) is the laser pulse intensity with time, d is the gap length, n is the substrate refractive index, and Jle and Jlh are
Applied Slub vollaQe
r- Laser pulse
•
Gold microstrip line
Go As substrate Ground plane
FIG. I. The stub line switch.
769 J. Appl. Phys. 51 (1), January 1980 0021-8979/80/010769-03$01.10 © 1980 American Institute of Physics 769
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r-o
Time, Nanoseconds
FI G. 2. Computer plot of electrical and laser pulses for a laser peak power of J 00 W. The electrical pulse precedes the laser pulse. The two pulses have no veri tical scale relationship.
the electron and hole mobilities. This formula assumes uniform illumination of the gap, and no carrier recombination. In practice, we have found that nonuniform illumination, i.e., tight focusing, produces irregularities in the device responses, which depend on the position in the gap where the light is focused. This has happened particularly in devices made from silicon, and work is curently in progress to more fully understand this. In the present work, however, uniform illumination was used.
Carrier recombination was important since the time scales that were worked with were of the same order as the carrier lifetime in GaAs. In this analysis, recombination was accounted for by redefining Eq. (1) in terms of carrier population, and then adjusting the population by the appropriate exponential decrements at each voltage wave reflection. This technique proved to be the most suitable for computer implementation.
With R (t) modeled in this fashion, computer analysis was carried out for the discharge process, and a typical result is shown in Fig. 2. This example shows the case where 1= 0.5 cm and the laser peak power is 100 W in a 5-ns pulse. It is seen that the electrical pulse forms very early in the rise of the laser pulse, and approaches zero again before the latter reaches its peak. The width of the electrical pulse is 2.3 ns, or about half that of the laser pulse. The model predicts that increasing the laser power will result in still shorter electrical pulses, which are formed progressively earlier in the rise of the light pulse. The opposite effect is predicted for a power decrease. The model also predicted significant decrease in electrical pulse width with a shortening of the device length. All of these effects were observed experimentally.
Several stub line switches of varied lengths were constructed for experimentation. The GaAs was chromium doped to a resistivity of 10 7 n cm, and typical dimensions of gaps A and B were 10 and 20 pm, respectively. The limit on the shortest feasible stub length, while being assured of transmission line behavior, is determined by the substrate thickness. The criterion was set for the ratio of stub length to substrate thickness as being at least five. For a wafer thickness of 0.3 mm, this yields a minimum device length of 1.5 mm.
770 J. Appl. Phys., Vol. 51, No.1, January 1980
A dc bias was applied to the input end of the switch. The output end was connnected to a Tektronix 7904 oscilloscope, as was the output of a PIN photodiode which detected a portion of the laser signal. The path lengths of both were adjusted so that the scope displayed the switch response and the light pulse in the proper time relationship. The laser source was a nitrogen laser-pumped dye laser (rhodamine 6G) which produced 580-nm pulses of 8-10 ns in width. The peak power was adjustable from 100 W to about 10 kW.
The shortest electrical pulses obtained were 1 ns in duration, from the 1.5-mm device, and these were triggered with the highest available power from the laser. The transferred voltage was far from satisfactory, being only a few volts for the 40 V applied to the input. This is mostly due to the finite resistance of gap B, which served as a voltage drop. The transfer should be considerably improved if a light pulse of faster rise time is used, thus increasing the gap conductance more quickly. Figure 3 shows the output of a 0.5-cm device under the influence of laser pulses of two different peak powers. It is seen that a relatively small percentage of the laser energy is used to trigger the switch. A shorter rise time light pulse would improve the efficiency here too, since more energy could be concentrated in the leading portion of the pulse with less overall energy.
Our experiments have shown that electrical pulses can be produced whose widths are less than the triggering laser pulses by as much as a factor of 10: These widths should be decreased significantly with increased laser power, or by the use of laser pulses with faster rise times. The width of the laser pulse is of no consequence. The limits on this device, in terms of the shortest electrical pulse obtainable, are not defined as yet. The creation of shorter pulses requires substan-
(a)
(b)
FIG. 3. Typical experimental results for aO.5-cm device at two approximate laser peak power levels, as indicated. In each oscillograph, the electrical pulse precedes the laser pulse, and they were scaled to have roughly the same amplitude. The horizontal scale is 2 ns/div and the vertical scale is arbitrary. 3(a)-JOO W. 3(b)-J kW.
Buck, Li, and Whinnery 770
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tial reduction in the switch's dimensions, which would then require a more sophisticated fabrication technology. These possibilities are now being looked into.
ACKNOWLEDGMENTS
The authors wish to acknowlege numerous helpful dis-
771 J. Appl. Phys., Vol. 51, No.1, January 1980
cussions with A. Dienes. Research was sponsored by National Science Foundation Grant ENG78-10233.
'D.H. Auston, Appl. Phys. Lett. 26,101 (1975) 'Chi H. Lee. Appl. Phys. Lett. 30, 84 (1977). 'D.H. Auston (private communication). ·S. Ramo, J.R. Whinnery, and T. Van Duzer, Fields and Waves in Communication Electronics (Wiley, New York, 1965), pp. 29-31.
'A.M. Johnson and D.H. Auston, IEEE 1. Quantum Electron. QE-ll, 283 (1975).
Buck, li, and Whinnery 771
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