TRANSIENT ANALYSIS OF ASYMMETRIC MULTI QUANTUM WELL LASER USING EQUIVALENT CIRCUIT MODEL Abbas Zarifkar 1 , Sasan Seifollahpour 2 1. Optical Communications Group, Iran Telecom Research Center (ITRC), Tehran, Iran E-mail: azarifkar@itrc.ac.ir 2. Islamic Azad University, Tabriz Branch, Tabriz, Iran E-mail: seifollahpour@iaut.ac.ir ABSTRACT Analysis of asymmetric multi quantum well (AMQW) laser is presented using an equivalent circuit model based on carrier and photon rate equations. The circuit model has been implemented on SPICE circuit simulator. Also, the numerical solution of the rate equations is made by finite difference time domain (FDTD) method in order to compare the results obtained from two approaches. It is shown that the transient response characteristics, including the frequency and damping factor of the relaxation oscillations, the resonant peak, and the turn on delay, which are resulted from the circuit modeling, are in good agreement with the numerical simulation results. Index Terms— AMQW laser; Circuit modeling; Transient, FDTD 1. INTRODUCTION Asymmetric multiple quantum-well (AMQW) lasers have created great interest since the first demonstration of a dual AMQW laser by Ikeda in 1989 [1]. AMQW lasers have been developed to achieve a wide gain profile and an improved wavelength tuning using an external cavity. Due to their broad tuning ranges, AMQW lasers have applications in many areas such as wavelength-division multiplexing (WDM) systems, multi-wavelength interferometry, laser spectroscopy, and optical data processing. A single AMQW laser with a wide gain spectrum can replace a laser array or a fiber amplifier array to obtain multiple wavelengths [2]. Application of an AMQW laser as a broadly tunable source has also been demonstrated [3]. These devices consist of several quantum wells (QWs) of different widths or compositions where the sum of the gain contributions from each QW results in a broad net gain curve [4]. AMQW lasers generally have wider tuning ranges than conventional MQW lasers since the different QWs operate at different wavelengths so that the gain spectra can be extended. The gain spectra of AMQW lasers have been simulated using the free carrier gain theory [4]-[5]. In an AMQW laser with two lasing wavelengths, the long and short lasing wavelengths are close to the transition wavelengths of the wide (deep) and narrow (shallow) wells, respectively. More recently, the rate equation approach has been extended to simulate the steady state and transient response of AMQW lasers. A number of techniques have been used to study the modulation response of semiconductor lasers. Some of the common methods include the analytical or the numerical solution of the rate equations [6]. The alternate method used here is to transform the rate equations into a circuit model, which can then be solved using circuit analysis techniques. The circuit model gives an intuitive idea of the physics of the device. Also it can be easily interfaced to the parasitic network, enabling the terminal impedance and device circuit interactions to be determined. In this paper, an AMQW laser with two wide wells and two narrow wells is investigated and its transient response are obtained from the circuit simulation and also from the numerical simulation of the rate equations using FDTD method. In section 2, a detailed rate equation model of the device is introduced. In section 3, the equivalent circuit representation of the three-level rate equations is implemented and the behavior of the AMQW laser is investigated using the model. The results are analyzed and compared with the numerical simulation results. 2. RATE EQUATION MODEL A schematic of the potential energy profile for a two-well device with 5 and 10nm wells separated by a 10nm barrier is shown in Fig. 1. Although a two-well structure is considered here for simplicity, the model can be extended to include any number of QWs. The carrier and photon densities being considered are labeled in this figure for which the arrows indicate the various time dependent carrier transport mechanisms and the energy transitions that occur. A rate equation model has been developed to simulate the steady state and transient response characteristics of AMQW lasers. The model involves differential equations for the carrier densities in the different layers and the photon densities in the laser cavity. As is often the case