**1. Introduction**

152 Selected Topics on Optical Amplifiers in Present Scenario

Uh-Chan Ryu, K. Oh, W. Shin, U. C. Paek, IEEE Journal of Quantum Electronic vol. 38, pp.

In 1961, it is suggested that stimulated emission can occur in semiconductors by recombination of carriers that are injected across a p-n junction. This is the backbone in the concept of semiconductor lasers and amplifiers [1]. This concept is connected with the "one dimensional electron in a box", a problem discussed by quantum mechanics text books, to generate a new field of confined semiconductor structures known thereafter as quantumwell (QW), quantum-wire (QWi) or quantum dot (QD) structures where the carriers are confined in semiconductor in one, two, or three directions, respectively [2]. Semiconductor optical amplifiers (SOAs) and lasers performance may be substantially improved by using the QD-SOAs characterized by a low threshold current density, high saturation power, broad gain bandwidth and week temperature dependence as compared to bulk and multiquantum well devices [3]. QD active region results from reducing the size of conventional (bulk) crystal to a nanometer size scale in all the crystal directions. This results in a complete quantization of energy states. The density of states becomes comparable to a delta-function. In order to cover losses of the waveguide of the SOA, QDs are then grown by a large number of dots grown on a WL which is already in the form of QW layer, see Fig. 1.

In this chapter, we examine the effect of changing composition of the layers constructing the Antimony (Sb)- based quantum dot semiconductor optical amplifiers (QD-SOAs). We startoff in sections 2-3 a general description of QD-SOA and the importance of SOAs, especially Sb-based SOAs, in solving problems. Manufacture imperfection in the shape of QDs are taken into account in the gain calculations in section 4, where they are represented by an inhomogeneous function. Along with the gain description in this section, a global Fermifunction is used where the states in the barrier layer, wetting layer (WL) and ground and excited states of the dots are included in the calculations of Fermi-energy. This mathematical formulation coincides with our subject of study since we would like to see the effect of all the QD-SOA layers. QD-SOA is modeled in section 5 using rate equations (REs) model for the barrier layer which is assumed to be in the form of separate confinement heterostructure layer (SCH), wetting layer (WL), ground state (GS) and excited state (ES) in the QD region of Sb-based structures and then solved numerically. A 3 REs model is discussed first where the

The Composition Effect on the Dynamics of Electrons in Sb-Based QD-SOAs 155

electronic components can be replaced by ultrafast all-optical signal processing components. SOAs are among the most promising candidates for all-optical signal processing devices due to their high speed capability, low switching energy, compactness, and optical integration compatibility. In optical fibers, as the in-line amplifier has only to carry out one function (amplification of the input signal) compared to full regeneration, it is intrinsically a more reliable and less expensive device. Optical amplifiers can also be a useful as power boosters, for example to compensate for splitting losses in optical distribution networks. It can be

A semiconductor optical amplifier (SOA) is basically a semiconductor laser (gain medium) with a low feedback mechanism and whose excited carrier amplifies an incident signal but do not generate their own coherent signal. Thus, it operated as a broadband single-pass device for amplification. Each SOA requires some form of external power (a current or optical source) to provide the energy for amplification. An electrical current inverts the medium, by transferring electrons from the valence to the conduction band, thereby producing spontaneous emission and the potential for stimulated emission yields the signal gain. The first studies of SOAs were carried out around the time of the invention of the semiconductor laser in the 1960's. In the 1970's Zeidler and Personick carried out early work on SOAs. Research on SOA device design and modeling gets a lot of importance in 1980's especially for AlGaAs SOAs operating at (830 nm) wavelength and InGaAsP/InP SOAs operating in the (1300-1550 nm) region. In 1989 SOAs designed as devices uses a symmetrical waveguide structures giving much reduced polarization sensitivities [5].

Mid-to-far-infrared wavelength range (2000-30000 nm) has a variety of applications including remote sensing, medical diagnostic, free-space optical communications, atmospheric pollution monitoring, chemical sensing, thermal imaging, high power and midinfrared light sources [6]. Although HgCdTe material is already used to build devices working at these ranges, it suffers from slow response, high-power dissipation and undesired noise [7]. III-Sb based devices appear as a counterpart, since it is offering several advantages such as wide spectral range, low electron effective mass and high mobility at room temperature [8]. Although its research begins since the 70s of the past century, Sbbased devices still need a lot of length of study especially when the active region is in the form of QD nanostructures. Using QD active region in the Sb-based devices adds the advantages resulting from QD nanostructures to that expected from the Sb-based infrared

QD-SOAs can be used in the building blocks of many photonic devices. Its response is controlled by their dynamic behavior which can be studied by the use of a three-level system of REs to describe QD-SOA states for ground state (GS), excited state (ES) and WL state, which is considered as a QW and then can be approximated by a single state. While quasi-Fermi levels in the conduction and valence subbands are typically calculated from the surface carrier density per QD layer, now in some recent researches, both WL and barrier layer are included [9]. The barrier layer considered here is assumed to be in the form of a separate confinement heterostructure layer (SCH). This layer used in semiconductor structures to assure carrier confinement, where the SCH layer must have a high bandgap compared to other layers constructing the semiconductor device. Accordingly, the barrier layer becomes included in the gain calculations. Due to this, we begin in our laboratory

used as optical preamplifiers to improve receiver sensitivity [3,4].

**3. Sb-based QD-SOA** 

devices.

barrier SCH layer is neglected in this model, as done in many literatures, then a 4 REs model is described, where the SCH layer is included. Sb-based QD-SOA structures, the matter of study, are described in detail both in the shape and composition in section 6. Results of the calculations from these models are described in detail in section 7 where the material confinement is examined through the changing of Sb-composition in the QD, WL and SCH layers and it is shown to affects QD-SOA gain and dynamics. Changing WL composition affects the dynamics of ES and GS. InSb dots are shown to be more appropriate for inline static amplification. Results from 3 REs model overestimates the carrier dynamics in comparison with 4 REs calculations. Thus, the SCH barrier layer must be included in the RE models for convenient description of the processes in the QD-SOA. Our results supports the importance of inclusion global quasi-Fermi energy in explaining the results. Finally, section 8 concludes the main finding from this chapter.

Fig 1. Schematic illustration of the QD active layer which consists of an arrays of Quantum Disks grown on an wetting layer covered by SCH barrier.
