1268 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 10, OCTOBER 2005 Acceleration of Gain Recovery and Dynamics of Electrons in QD-SOA Y. Ben-Ezra, B. I. Lembrikov, and M. Haridim Abstract—We propose a new method for reducing the patterning effect in quantum-dot semiconductor optical amplifier (QD-SOA) by using an additional light beam. A detailed theoretical analysis of the carrier dynamics in QD-SOA is presented. It is shown that the increase of the bias current only partially improves the QD-SOA temporal behavior. The additional light beam drastically reduces the patterning effect. Index Terms—Optical commuication, quantum dots (QDs), semiconductor optical amplifiers (SOAs). I. INTRODUCTION T HE ENORMOUS communication capacity of optical fibers in optical networks is limited by the relatively low switching speed of electronic devices which direct the traffic to its destination [1]. The switching bottleneck in next- generation optical networks can be solved by the development of all-optical methods [1]. In particular, semiconductor optical amplifiers (SOAs) can be used for all-optical switching [2], [3]. To that end, the SOA must have a fast gain recovery to avoid patterning effects [2]. The gain recovery is limited by the carriers lifetime which can be shortened by both the increase of the applied bias current and the light intensity in the active layer [2]. Recent experimental investigations show that in the case of the bulk SOA, bias currents in the region of (150 450) mA and the optical injection level of 80 mW can accel- erate the gain recovery to 27 ps maintaining a gain of 14 dB in the 1550-nm wavelength band [2]. In quantum-dot SOAs (QD-SOAs) possessing unique features such as low threshold current, high gain, and ultrafast dynamics, the optical power and the bias current are at least one order of magnitude lower than in bulk SOA [4]–[19]. A comprehensive investigation of the carrier dynamics under various conditions of bit-rate, input signal power and bias current in QD-SOA is required in order to obtain a deep understanding of the device characteristics important in com- munication applications. The general theory of QD-SOAs based on density matrix equations and optical pulse prop- agation equations has been recently developed in [4]. This theory describes adequately both linear and nonlinear effects in QD-SOAs and explains their unique physical properties such as the low power consumption, high saturation power, broad gain bandwidth, pattern-effect-free operation under gain satu- ration, high speed (40–160 Gb/s) wavelength conversion by the Manuscript received April 11, 2005; revised June 15, 2005. The authors are with the Department of Communication Engineering, Holon Academic Institute of Technology, Holon 58102, Israel (e-mail: benezra@ hait.ac.il; borisle@hait.ac.il; mharidim@hait.ac.il). Digital Object Identifier 10.1109/JQE.2005.854131 cross-gain modulation (XGM), and low frequency chirping [4], [5]. Analysis of QD-SOAs operation requires the knowledge of QD electronic structure. So far, most of the investigations are related to InAs self-assembled QDs grown on GaAs substrates which do not emit at wavelengths longer than 1350 nm [20]. Detailed quantum mechanical calculations of InAs–GaAs and free-standing InAs QDs electronic structure taking into account their shape (lens-shaped, pyramidal), size, composition profile, and production technique (Stranski–Krastanov, colloidal) have been carried out, and the results have been compared with the experimental data [21]–[26]. In InAs–GaAs QD systems, in the valence band there exist three different hole energy levels with very small energy separations of about 10 meV. In the conduction band, there are three electron energy levels with comparatively large energy separations of about 60–70 meV, and two additional electron energy levels with very small energy separations of only several millielectron volts from the main levels [23]. The electron wetting layer (WL) acting as a carrier reservoir [8] is situated 150 meV above the lowest electron energy level in the conduction band, i.e., the ground state (GS) and has a width of approximately 120 meV. The spacing between the hole WL and the hole GS is approximately 48 meV. The hole binding energy is about 194 meV [23]. QD-SOAs emitting in the 1550-nm wavelength range impor- tant for telecommunications are based on the InAs QDs grown on a InP substrate [20]. Recently, the strong influence of InAs–InP QDs size, strain and temperature on their optical properties and electronic structure has been studied experimentally [20], [27]. The electron and hole GSs are 1.232 and 0.428 eV with respect to the InP valence band maximum, respectively, and the elec- tron and hole binding energies are 0.192 and 0.428 eV, respec- tively [27]. Wavelength conversion based on four-wave mixing (FWM) and XGM in a 1550-nm InAs–InP QD-SOA has also been experimentally performed [28]. Recent publications based on the phenomenological ap- proach deal with the gain recovery time, gain saturation and nonlinear effects such as XGM and four-wave mixing (FWM) in QD-SOA [8]–[19]. In these works, the theoretical and the ex- perimental results are mostly analyzed using rate equations for both charge carriers and photons. The two-level rate equation model with only one QD carrier population and one population of the WL approximated as a narrow quantum well (QW) permits the description of the unique properties of QDs [8]. In such a model, a large population of carriers in WL is coupled to discrete QD populations on a picosecond scale [8]. This model appears to be appropriate for the electronic structure of InAs–InP QDs mentioned above. The dynamics of electrons in QD-SOA have been investigated in [12] for two particular 0018-9197/$20.00 © 2005 IEEE