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

154 Selected Topics on Optical Amplifiers in Present Scenario

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

Fig 1. Schematic illustration of the QD active layer which consists of an arrays of Quantum

Transmitters, receivers, electronic switches and routers limits the capacity of optical communication systems that can exceed 10 Tb/s by the speed of their electronic components [3]. Optical fibers suffers from attenuation (which limits transmission distance) and dispersion (leading to an increase in the system bit error rate, BER). 3R method (reshapingretiming-retransmission) is then used to regenerate the optical signal in optical fibers. It has a number of disadvantages. This involves low optical and electrical transparencies in addition to network unreliability. These limitations may be overcome by using SOAs. The

Disks grown on an wetting layer covered by SCH barrier.

**2. Semiconductor optical amplifier** 

8 concludes the main finding from this chapter.

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 devices.

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

The Composition Effect on the Dynamics of Electrons in Sb-Based QD-SOAs 157

distribution of the *th i* optical transition. The terms *fc* and *fv* are the respective quasi-Fermi level distribution functions for the conduction and valence bands, respectively. Recent researches [8] uses global states to describe the global quasi-Fermi levels *Fc* and *Fv* in the conduction and valence bands where the contributions to the Fermi-levels from the barrier layer and WL are included in addition to that from QDs. They are determined from the

<sup>2</sup> <sup>2</sup> , <sup>2</sup> <sup>2</sup> <sup>2</sup>

3/2

*D 2 <sup>i</sup> EE 2 <sup>s</sup> h hj h <sup>p</sup> N e f E,F dE 2D D v hv h <sup>2</sup> <sup>i</sup> <sup>2</sup> <sup>h</sup>*

 

*<sup>w</sup> <sup>w</sup> m KT F E KT h B v hm B ln 1 e <sup>2</sup> <sup>m</sup>*

 

*Wm KT e B F E KT <sup>c</sup> <sup>B</sup> <sup>e</sup>*

ln 1 <sup>2</sup>

 

*<sup>B</sup> me <sup>B</sup> H Ec cE <sup>b</sup>*

*i*

1 2 2 2 <sup>2</sup>

and valence bands.

barrier layer.

*<sup>e</sup>* and

**5. Rate equations of Sb-based QD-SOAs** 

**5.1 Three rate equations model** 

distributions, respectively. The term *Ec*

*l*

,

*c cc c f E F dE*

*<sup>h</sup>* are the spectral variance of the QD electron and heavy-hole

is the energy in the conduction band and *Eh*

(4)

(5)

is the

*W cl*

*<sup>s</sup> E E c e n N D D <sup>e</sup> f E F dE c cc c <sup>i</sup> <sup>e</sup>*

*D ci*

*3/2 <sup>B</sup> <sup>1</sup> 2mh <sup>B</sup> H E <sup>E</sup> <sup>f</sup> <sup>E</sup> ,F dE b h 2 2 <sup>h</sup> <sup>v</sup> <sup>h</sup> <sup>v</sup> <sup>h</sup> <sup>2</sup>*

where *n*2*<sup>D</sup>* and *p*2*<sup>D</sup>* are the surface densities of electrons and holes per QD layer,

respectively. *DEci* and *DEhi* represents the respective confined QD states in the conduction

energy in the valence band. Heavy hole subbands are included, only, in the calculations of valence subbands since light hole subbands are deep and can be neglected. *Bk* is the Boltzmann constant and *T* is the absolute temperature. The terms *<sup>W</sup> me* ( *<sup>W</sup> mh* ) and *WEel* ( *WEhm* ) are the effective electron (hole) mass and the subband edge energy of the conduction (valence) band of WL. The term *Hb* is the thickness of SCH barrier. The terms *<sup>B</sup> me* ( *<sup>B</sup> mh* ) and *<sup>B</sup> Ec* ( *<sup>B</sup> Eh* ) are the electron (hole) mass and the conduction (valence) band edge energy of

QD-SOA characteristics can be studied using REs system constructed from three equations which describe the dynamics in WL, GS and ES of QD layer. The QD inhomogeneity due to

surface carrier density per QD layer by the following relations [13]

(NNRL) a series of studies for some of the characteristics of Sb-based QD devices [10]. Here, the carrier dynamics in III-Sb based QD-SOAs are considered using four-level REs system. QD, WL and SCH barrier compositions are examined to specify their characteristics. The results then, compared with those of three-level REs show the importance of including the barrier layer in the QD SOAs calculations. Because of the much larger effective mass of holes and lower quantization energies of the QD levels in the valence band, electrons behavior limits the carrier dynamics while holes in the valence band are assumed to be in quasithermal equilibrium at all times [11]. Thus, we determine carrier dynamics here by the relaxation of electrons in the conduction band only.
