STUDY OF WELL BARRIER HOLE BURNING IN QUANTUM WELL BISTABLE LASERS M.Ganesh Madhan, P.R.Vaya* and N.Gunasekaran School of Electronics and Communication Engineering, Anna University, Madras. 600 025. India, mganesh@annauniv.edu * Department of Electrical Engineering, Indian Institute of Technology, Madras. 600 036. India, vaya@mango.ee.iitm.ernet.in ABSTRACT Quantum carrier capture and release effects on optical bistability in multiple quantum well (MQW) lasers are studied using the well barrier hole burning model. The ratio of carrier capture to release time (η) is varied and the hysterisis width, transient behavior are analysed by simulating the equivalent circuit of MQW bistable laser using circuit simulation program Pspice. The hysterisis width is found to increase with an increase in η. From the time response, improved damping and increased turn on delay are observed in switching characteristics for higher value of η. Keywords: Multiple quantum wells, well barrier hole burning, semiconductor lasers, bistability, hysterisis, circuit modeling INTRODUCTION Optical bistability in semiconductor lasers has received much attention because of its potential application in optical switching and signal processing. It is well established that the presence of an unpumped absorber in the laser cavity can lead to bistability. Introducing quantum wells in the active regions of gain and absorber sections could significantly improve the switching speed and controllability of hysterisis characteristics [1]. It is also found that the conventional single mode rate equations fail to explain the resonance characteristics profoundly, because of non inclusion of factors such as spatial and spectral inhomogenities. Hence well barrier hole burning model is introduced to incorporate the effects contributing to non linear gain [2,3]. MQW bistable lasers are normally analysed by numerically solving the single mode rate equations [1]. In this paper, we have attempted to study the effect of well barrier hole burning in MQW bistable lasers using circuit simulation technique. Circuit simulation method is adopted because of its advantages, such as inclusion of parasitics and device- circuit interactions [4]. The equivalent circuit is also vital for the design of control and driver circuits for laser [5]. The equivalent circuit is simulated for dc sweep and transient conditions using circuit simulation program Pspice. LARGE SIGNAL MODEL The bistable laser has two regions viz. gain and absorber sections. For the simulation purpose, we have considered InGaAs - InAlAs MQW structure. This structure consists of 6 quantum wells in the active region having 5 ps capture time. The carrier lifetime in the barrier and in the well is 1ns. The carrier lifetime in the gain section is assumed to be proportional to the carrier density, whereas it is constant (2ns) in the absorber section. The non linear rate equations incorporating well barrier hole burning [2] are included in the gain section of the bistable laser. Since the absorber section is unpumped, the carrier transport and capture effects are neglected. The above modifications are made in the rate equations of ref [1] and given as dN b J N b ( N b - ηN w )  =  -  -  (1) dt ed τ sb τ c dN w ( N b - ηN w ) N w  =  - - - v g G g S (2) dt τ c τ sw dN a N a  = -  - v g G a S (3) dt τ a