# Qualitative Modeling and Simulation of Free Running Semiconductor Laser

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## Qualitative Modeling and Simulation of Free Running Semiconductor Laser

ABSTRACT

The thesis presents the modeling and simulation of free running semiconductor laser. The rate equations which were derived on the basis of the fact that there must be a balance between carriers that undergo transition and the photons generated and annihilated were simulated to examine what laser looks like when ran without positive optical feedback nor any optical injection. The result shows the photons get amplified due to stimulated emission but no oscillation of the output is obtained. Such a set up is therefore seen as an optical amplifier.

Title page i
Acknowledgement ii
Abstract iii
Table of Content iv

CHAPTER ONE
Background
1.1 Introduction 1
1.2 Aims of the Thesis 2
1.3 Overview of the Thesis 3

CHAPTER TWO
Physics of Semiconductor Laser
2.1 Spontaneous Emission 4
2.2 Stimulated Emission 6
2.3 Photon Absorption 7
2.4 Population Inversion 9
2.5 Maser and Laser Amplifier 11
2.6 Threshold Condition for Lassing 11
2.7 Pumping 13
2.8 Operation Principle of Semiconductor Laser 15
2.9 Homojunction Semiconductor Laser 17
2.10 Double-heterostructure Semiconductor Laser 20
2.11 Brief Review of Rate Equations and Semiconductor Lasers 22

CHAPTER THREE
Modeling and Simulation
3.1 The Rate Equations 24
3.2 Modeling of The Rate Equations 25
3.3 MATLAB Simulation of The Rate Equations 31

CHAPTER FOUR
Results and Discussions
4.1 The Plots of the Dynamics 34
4.2 The Carrier Density 35
4.3 The Photon Density 36
4.4 Discussions of Results 37
4.5 Conclusion 39
Reference 40

CHAPTER ONE

Background

1.1 Introduction

The word laser is an acronym for the most significant feature of laser action: Light Amplification by Stimulated Emission of Radiation. The basic component of laser is the material with ability to amplify radiation. Such materials are referred to as active medium. The amplification process results from the phenomenon known as the stimulated emission (this will be explained in chapter two) which was discovered by Albert Einstein in 1916. The first laser was constructed by T. H. Maiman in 1960 and different types have been invented ever since. Common among the different types of laser are the solid state lasers, gas lasers, dye lasers, chemical lasers and semiconductor lasers.

Semiconductor lasers were first demonstrated in 1962 by two US groups led by Rober N. Hall at the General Electrical Research Centre and by Marshall Nathan at the IBM T. J. Watson Research Center 1. It has applications in a number of areas.

It serves as sources as well as amplifiers and multiplexers for optical communications. Semiconductor lasers also serve as pump sources for solid state lasers and sources of beams for spectroscopy. Infrared and red laser diodes are commonly used in CD players, CD-ROMs and DVD technology. Diodes lasers are used in instruments such as laser scanners, pointers, barcode readers and printers.

The attributes of semiconductor lasers which have made them useful in several areas include their relatively small in size and low cost. They are also efficient and require low power current source and have been shown to operate at wide range of wavelengths; from the near ultraviolet to the far infrared. The operation of semiconductor lasers is similar to that of other lasers with semiconductor material serving as the active medium. The p-n junction is forward biased to produce the pumping process necessary for the stimulated emission or the pumping can be done using another laser, a process known as the optical pumping.

The more detail physics of the semiconductor laser is presented in chapter two.

1.2 Aims of the Thesis

The semiconductor lasers have found a wide range of applications as highlighted above. As such, they underwent an intensive research and development since the time of their first operational regime was achieved in 1962. This thesis aims at simulating the free running semiconductor laser using the Matlab as the computational tool. This is done using the rate equations which are represented two systems of coupled ordinary differential equations for carrier density and photons.

This gives the continuous wave and the dynamic behaviour of the laser from which the output power of the laser can also be obtained.

1.3 Overview of the Thesis

Chapter two reviews the basic processes involved in the laser action. The processes discussed include spontaneous emission, stimulated emission, optical absorption, population inversion, the threshold condition for lasing and pumping.

Homojunction and double-heterojunction lasers are also discussed. The chapter concludes with a brief overview of related previous work.

Chapter three presents a detailed derivation of the rate equations followed by a brief description of the Matlab code used in the simulation.

In chapter four, the results obtained from the simulation were presented and discussed. Conclusion is also given in chapter four.

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