Downloaded 24 May 2006 to 131.215.240.9. Redistribution subject to AIP license or copyright, see http://rsi.aip.org/rsi/copyright.jsp Operating injection lasers by fast square current pulses of variable amplitude D. R. Armstrong, A. Katzir, and A. Yariv California Institute of Technology, Pasadena, California 91125 (Received 12 March 1975) A simple solid state circuit was used to drive GaAs injection lasers by fast ("' 100 nsec) square pulses of variable amplitude (0--25 A). The amplitudes of the current pulses and the corresponding emitted light pulses were measured by a dual peak detector circuit. Using these circuits we were able to plot automatically the current vs light curve and determine the threshold current of the laser diodes. INTRODUCTION Gallium arsenide injection lasers driven by fast current pulses may well serve as optical pulse generators for optical communication systems. In the course of studying GaAs- GaAlAs distributed feedback injection lasers 1 at low tem- peratures, we tried to transfer fast square current pulses along a transmission line to lasers mounted in a Dewar and , to measure accurately the amplitudes of the current pulse and the light pulse emitted by the laser. In the testing of experimental injection lasers, a great deal of time can be saved if the plotting of light output versus current, or current versus voltage, can be done automatically on an X- Y recorder, rather than by point-by-point pro- cedures. Since these measurements must be made using short pulse techniques, the requirements for an automatic plotting system include a source of square current pulses of program- mable amplitude and circuits that detect the peak values of the diode current and photodetector output and convert them to de analogs for operating the recorder. The circuits described recently in the literature 2 - 4 were inadequate for this application, and we report here some circuits that have given satisfactory results. PULSE GENERATOR The preferred characteristics of current pulses for testing injection lasers include the following: (1) A short risetime to minimize preheating, preferably less than 10 nsec. (2) A flat top, so that the amplitude is precisely known. (3) A sufficiently narrow width to avoid excessive power dissipation, usually less than 200 nsec. (4) Variable amplitude over a range of approximately 0.1-20 A. (S) Adequate repetition rate to meet the requirements of the rest of the system. (6) Freedom from amplitude or time delay jitter. Needless to say, perfection on all counts lies in the realm of wishful thinking, but an acceptable level of performance is possible with some compromises. Of the various discrete devices used to pulse injection lasers, the avalanche transistor appears to have the best combination of characteristics. 5 However, since a transistor will avalanche (if at all) only over a narrow range of collector voltage, thus providing a correspondingly narrow range of current into a fixed load, there is a problem of how to in- crease the current range. Furthermore, the simplest method of obtaining a square pulse with an avalanche transistor is to discharge a charged transmission line through it, but in order to get the necessary current from a line, while staying within the voltage limita- tions of the transistor, the line impedance must be very low. For example, if the maximum collector voltage is 120 V (a representative value), a current of 20 A into a matched load requires an impedance of about 3 Q. Higher currents may, of course, be obtained by operating the line into a load of less than its characteristic impedance, but the resulting voltage reversal at the end of the pulse is hazardous to GaAs diodes, which have a low reverse voltage tolerance. Ordinary silicon rectifiers are not fast enough to bypass these reversals satisfactorily; however, the recently available high speed Schottky barrier rectifiers are useful for this purpose. Since there is little available information regarding the avalanche or pulse characteristics of commercial transistor types, the development of practical circuits of this kind must be done largely by empirical means. Distributed-constant transmission lines with an impedance of a few ohms are difficult to prepare in the necessary lengths (approximately 9.5 m/100 nsec), but at the sacrifice of some ideality of pulse shape, a lumped-constant network may be used. A form that works quite well consists simply of a ladder of ceramic disk capacitors soldered to a pair of parallel wires. The values and spacing of the capacitors can be ad- justed experimentally to give a reasonably square pulse into a desired load resistance. The general circuit of the pulse generator is shown in Fig. 1. The approach used is to charge a transmission line (TL2) to the voltage necessary to give the desired current into the load, and to discharge it through a fast transistor (Q2) that is rapidly driven into saturation by avalanching anoaer transistor (Ql) into its base. Because of the low impedance level, the impedances of the transistors and con- nections are a not insignificant fraction of the load on the transmission line. Consequently, t.lte pulse shape is influenced 1646 Rev. Sci. lnstrum., Vol. 46, No. 12, December 1975 Copyright © 1975 by the American Institute of Physics 1646