الفهرس 
Chapter 1: INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
A comprehensive study on semiconductor laser (SL) characteristics under gigabit-per-second digital modulation has been presented. Comparison of the modulation characteristics under both fonnats of the return to zero (RZ) and non-return to zero (NRZ) of pseudorandom modulation bits has been introduced in order to explore the better code for achieving higher digital modulation performance. The modulation characteristics included the eye diagram of the modulated laser signal, the eye-diagram opening, the turn-on jitter (TOl), and the associated frequency chirp. Variations of these characteristics with the modulation conditions; namely, bias current (lb), modulation current (1m) and modulation bit rate (B) were examined. A primary objective of the study was to elucidate how the digital modulation performance of SLs changes with variation of the modulation bit rate relative to the maximum modulation bit rate and the so-called ”setting bit rate” of the relaxation oscillations, which are decided by the laser transients. The relative contributions of the intrinsic noise of the laser and the pseudorandom bit-pattern effect to the modulation characteristics, especially the eye diagram and TOl, were differentiated. The study was based on solving the rate equation model of SLs, which describes the time evolution of the number of the emitted photons and the number of injected electrons in the active layer in addition to the phase of the optical field. The study was focused on 1.55-Jlm InGaAsP single-mode lasers as the popular light sources in optical fiber communications.
The rate equations were first solved numerically by assuming a rectangular electrical pulse and neglecting the sources of the laser intrinsic noise in order to obtain rough estimates of the transient parameters of the SL and their variation with the bias and modulation currents. These parameters include (I) the turn-on delay time whose
fluctuations under pseudorandom bit modulation decides TOl, (2) occurrence time of the first overshot of the laser intensity whose reciprocal defines the maximum modulation bit rate at a given modulation condition, and (3) the setting time of the relaxation oscillations whose reciprocal defines the setting bit rate above which the modulation characteristics are influenced by the pseudorandom pattern of the modulation bits. The rate equations were also solved analytically in the frequency domain by means of the approximated small-signal analysis in order to estimate the modulation bandwidth frequency and its variation with the bias current. The modulation bandwidth frequency was calculated as the modulation frequency at which the small-signal modulation response DROPs to its half max. (3dB) value. Correlation between this maximum modulation bandwidth frequency and the maximum modulation bit rate was then investigated.
The obtained results showed that the turn-on delay time decreases with the increase of the modulation current when the bias current was set at the threshold level. At higher bias currents, the turn-on delay time vanished. Both the maximum modulation bit rate and the setting bit rate were found to increase with the increase of the bias and/or modulation currents. The former increases from 5 to 11.1 Gpbs, while the latter increases from 490Mbps to 1.17Gbps with the increase of 1m from 0.2 to 41th when h is set to 1.51th. The modulation bandwidth frequency increased with the increase of the bias current. The predicted value of the upper limit of the modulation bandwidth for the considered SL was 11.1 GHz when the bias current is 6 times the threshold current, which was comparable to the predicted maximum bit rate of 12.5Gbps.
The rate equations were then augmented by Langevin nOise sources to account for the laser intrinsic noise as well as by a source of pseudorandom electrical bits. These stochastic rate equations were solved