380 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 4, APRIL 2000 Effect of Source and Load Resistance on the Performance of Bistable Lasers M. G. Madhan, P. R. Vaya, and N. Gunasekaran, Member, IEEE Abstract—The effect of source and load resistances on the bistable behavior of multiquantum-well laser diodes is studied. Lower values of source resistance are found to affect the threshold current with a slight change in the hysterisis width. Also, the turn ON delay is found to decrease exponentially with the increase in source resistance. Lower values of load resistance leads to smaller hysterisis width, and for values less than 125 , for the device under study with an input resistance ( ) of 1 k , an astable behavior is observed when triggered by an electrical pulse in the gain section. Index Terms—Bistability, carrier transport, circuit model, mul- tiple quantum wells, semiconductor lasers. I. INTRODUCTION T HE STUDY of optical bistability in semiconductor lasers has received considerable attention in recent years owing to its application in optical signal processing and digital optical computing. It is well known that a saturable absorber introduced inside the laser cavity leads to bistability or self-pulsation de- pending upon the carrier lifetime and gain variation along the laser cavity. Multiple-quantum-well (MQW) bistable lasers are expected to have improved switching and hysterisis character- istics [1]. In this letter, we study the effect of absorber load re- sistance on the static and dynamic behavior of the bistable laser by equivalent circuit approach. Further, the influence of source resistance, which depends on the driver circuit, is also analyzed, since it significantly affects the frequency response of the device [2]. The analyses are carried out by simulating the equivalent circuit by circuit simulation program Pspice [2], [3]. II. LARGE SIGNAL MODEL The MQW laser rate equations incorporating carrier transport proposed by Nagarajan et al. [4] are used in the reduced form, as suggested by Lu et al. [3], to model the gain section of the bistable laser (1) (2) Manuscript received September 23, 1999; revised December 30, 1999. M. G. Madhan and N. Gunasekaran are with the School of Electronics and Communication Engineering, Anna University, Madras 600 025, India. P. R. Vaya is with the Department of Electrical Engineering, Indian Institute of Technology, Madras 600 036, India. Publisher Item Identifier S 1041-1135(00)02820-2. Carriers are injected into the gain section only, and the absorber is kept unpumped. Hence the following normal rate equations are employed for the absorber section: (3) and (4) where represent the electron, photon densities, gain constants, and carrier lifetimes, respectively, in the cavity. The subscripts denote the confinement layer, gain section, and absorber section, respectively. is the input current to the device, and is the velocity of light in the lasing medium. Other parameters and their values are given in Table I. Multiplying (1), (2), and (4) by and (3) by , and rearranging, one gets (5) (6) (7) and (8) where , , and are the spontaneous emission terms included as diode currents. The stimulated emission currents in the gain and absorber sections are repre- sented as and . denotes the transparent current in gain and absorption regions. In the optical side, , , and . The source resistance (parallel to the input current source ), lead inductance ( ), and parasitic capaci- tance ( ) are included in the device model. The voltage drop outside the active region is modeled through a resistance . The space charge capacitance is connected in parallel with the diode [3]. The voltage drop ( ) across the resistor 1041–1135/00$10.00 © 2000 IEEE