الفهرس 
Chapter 1: INTRODUCTIONOnly 14 pages are availabe for public view
 |  | from | 157 |  |  |
| | | from | 157 | | |
Abstract
Semiconductor lasers are used as light sources in many modern technologies, such as optical disc systems. In such systems, laser radiation is reflected from the disc surface and re-injected into the laser cavity. This optical feedback (OFB) may cause chaotic dynamics, which enhance the noise level of both the laser and the system. A typical solution to control such chaos and stabilize the laser operation is to modulate the current exciting the laser with a sinusoidal signal. However, OFB and the modulation itself are nonlinear phenomena, and may increase the complexity and degree of nonlinearity of the laser. Therefore, optimizing the modulation parameters; namely, modulation frequency and depth, so as to reduce the OFBBinduced noise to the fundamental quantum noise level of the laser is not an easy task because it requires intensive numerical simulations of laser dynamics over wide ranges of the modulation conditions.
In this thesis, comprehensive simulations of laser dynamics and the associated intensity noise of semiconductor lasers under each/both of sinusoidal current modulation and OFB have been introduced. The studies were based on the rate equation model of semiconductor lasers, which describes the temporal evolution of the number of photons and carriers inside the laser cavity. The noise effect was included in the rate equations by adding Langevin noise sources to the equations. The analog modulation was taken into account in the rate equations by adding a sinusoidal component to the bias current. The OFB phenomenon was described by generalizing the rate equations by adding time-delay components of the complex electric field to the rate equations to describe the re-injected light into the laser cavity. The rate equations were solved numerically by the fourth-order Runge-Kutta algorithm. The relative intensity noise (RIN) was calculated by applying the fast Fourier transform (FFT) to the fluctuations of number of photons. The study was applied to AIGaAs lasers due to their importance in the optical disc system.
The thesis consists of 6 chapters. Chapter 1 introduces the present work its motivations. Chapter 2 is concerned with reviewing the fundamental principles of laser operation in semiconductor lasers, including description of the rate equation model, an introduction to the direct intensity modulation, and the general characteristics under external
Chapter 3 is devoted to present large-signal analysis of the direct analog modulation of semiconductor lasers. The deterministic modulation dynamics (without noise) were classified into eight dynamic types. Three types have uniform regular signals, namely; sinusoidal signals (SS), continuous periodic signal (CPS), and periodic pulse (PP). Other four types have
non-uniform regular signals; namely, CPS superposed by sub-peaks from relaxation oscillations (CPSRO), PP superposed by sub-peaks from relaxation oscillations (PPRO), CPS with period doubling (CPSPD), and PP with period doubling (PPPD). The last type is chaos. The characteristics of each of these types were explored in terms of the temporal trajectories of the photon and electron numbers and their phase portrait as well as the corresponding FFT power spectrum. The operating region of each type was mapped over a diagram of the modulation index versus modulation frequency. The noise content of the investigated dynamic types was evaluated in terms of the frequency spectrum of RIN. Influence of modulation parameters on the average value of the low frequency component of RIN, LFFRIN, was also investigated. The obtained results showed that under low-frequency modulation, LF-RIN is lowest under both SS and CPS types, increases when the CPSRO type is excited, and is enhanced under the PPRO type in which the pulse exhibits the first overshot (spike) of the relaxation oscillations accompanied by smaller discontinuous shots. When the modulation frequency is comparable to the resonance frequency, LF-RIN is higher under the PP type than under the SS and CPS types. Under higher modulation frequencies, the LF-RIN levels in the regimes of PPPD and chaos are higher than those in the regimes of the SS and CPS types.
In chapter 4, the laser dynamics and noise under OFB only were analyzed. The timeedelay rate equations were solved without considering the intensity and phase fluctuations in order to make clear the deterministic influence of OFB on laser dynamics. Procedures of such numerical integration were illustrated. An external cavity with length of 6 cm was assumed. The laser dynamics were classified based on the bifurcation diagram and the temporal trajectory of the laser intensity. The OFB-induced noise in the laser dynamics was then characterized. Analysis of the bifurcation diagram showed that the laser follows a quasiiperiodic rout-to-chaos operating, in respective, under continuous wave (CW), periodic oscillation (PO), quasi-periodicity (QP) and then chaos with the increase of the OFB level. The OFB level of each OFB-induced state was decided and the characteristics of the corresponding signal, phase diagram and power spectrum were determined. The noise content of these states were characterized by the frequency spectrum ofRIN as well as by the average value LF-RIN. The obtained results showed that under low OFB, LF-RIN is lowest under both CW operation and PO, increases under the QP, and is then pronounced in the chaos region.
Chapter 5 introduces a study on the control of chaos through current modulation, which is main topic of interest of the present work. This study was based on large-signal analysis of an improved time-delay model of rate equations with the injection current being