1978 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 12, JUNE 15, 2009 Effect of Doping on the Optical Characteristics of Quantum-Dot Semiconductor Optical Ampliﬁers Omar Qasaimeh, Member, IEEE Abstract—The inﬂuence of p-type and n-type doping on the op- tical characteristics of a quantum-dot semiconductor optical am- pliﬁer (SOA) is studied using a rate equation model that takes into account the effect of the multidiscrete energy levels and the charge neutrality relation. Our calculations show that the ampliﬁer op- tical gain can be greatly enhanced through p-type doping where the doping concentration should not exceed the certain level. We ﬁnd that increasing the acceptor concentration increases the un- saturated optical gain but at the same time decreases the satura- tion density and the effective relaxation lifetime. Also our calcula- tion reveals that the use of p-type doping will be associated with an increase in the transparency current where the increase in the transparency current depends on the incident photon energy. On the other hand, we ﬁnd that it is possible to increase the satura- tion density and enhance the linearity of the SOA by using n-type doping. Index Terms—Doping, quantum dot, semiconductor optical am- pliﬁer. I. INTRODUCTION Q UANTUM-DOT (QD) semiconductor optical ampli- ﬁers (SOAs) have demonstrated great promise for use in coarse wavelength-division-multiplexing (WDM) systems since they are fast, cheap, and small, and also can be integrated with other optoelectronic-integrated circuits. QD optical ampliﬁers have many interesting features, which can be utilized to perform optical ampliﬁcation, optical switching, and wavelength conversion [1]–[8]. Compared to quantum well optical ampliﬁers, QD-SOAs are characterized by ultrafast gain response, high saturation power, low-temperature sensitivity, ultrawide operating wavelength range, and low-noise ﬁgure. Due to large hole effective mass and strong band mixing, the energy band of QD active layers exhibits large numbers of hole states and few electron states, where the hole states are more closely spaced in energy ( meV) [9]. Since the energy separation between the hole states is small, thermal effects be- come signiﬁcant where thermal broadening of the injected holes decreases the QD ground-state gain and increases the tempera- ture sensitivity of the device. It has been experimentally demon- strated that the use of p-type doping is very efﬁcient to provide excess hole concentration and to improve the gain properties and the room temperature modulation speed of QD lasers [9]–[16]. Manuscript received June 08, 2008; revised August 20, 2008. First published April 14, 2009; current version published June 24, 2009. The author is with the Department of Electrical Engineering, Jordan Univer- sity of Science and Technology, Irbid 22110, Jordan (e-mail: Qasaimeh@just. edu.jo). Color versions of one or more of the ﬁgures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identiﬁer 10.1109/JLT.2008.2005589 Recently, signiﬁcant improvement of the linewidth enhance- ment factor in QD lasers has been achieved by using p-type doping [10]. To extend the capacity and transmission distance of course wavelength division multiplexing (CWDM) networks, low-crosstalk multichannel semiconductor optical ampliﬁers are needed for signal ampliﬁcation [17]–[19]. QD SOAs are suitable for this application, because they exhibit broad gain spectrum. Unfortunately, using QD-SOA in multichannel WDM systems suffers from large signal crosstalk originated from gain saturation effects between different wavelength channels [20]. In this paper, we proposed using n-type doping to improve the linearity of the ampliﬁer and to reduce crosstalk. With n-type doping the QD states in the conduction band is ap- proximately occupied, and the saturation effects are determined by the hole states. Up to our knowledge, there is no work in the literature that comprehensively studies the effect of n-type and p-type doping on the optical characteristics of QD semiconductor optical am- pliﬁers. In this paper, we present detailed analysis of the effect of doping on the performance of QD optical ampliﬁer. Our anal- ysis is very important for many applications where high optical gain or large linearity is important. II. SOA MODEL The investigated SOA is a separate conﬁnement heterostruc- ture with the inner and outer cladding layers and a QD active layer. We assumed that is the input facet, and is the output facet of the SOA where is the length of the ampliﬁer. The typical energy band diagram of the QD active layer consists of multiple energy states in the conduction and valence bands. The dots were assumed to have three nonde- generate energy levels in the conduction band and eight non- degenerate energy levels in the valence band, and are accompa- nied by a two-dimensional wetting layer (WL) state. The sepa- rations of the electron and hole energy states are 60 meV and 10 meV, respectively. At relatively high-injection density, Auger- type carrier–carrier scattering is very important where electrons can relax to a lower state in the QD by losing their energy to an- other carrier in the higher continuum states. Since the hole states are dense and very close to each other, electron–hole scattering will be dominant. The rate equation for the electron ground state (GS) is given by [21] (1) 0733-8724/$25.00 © 2009 IEEE