Safwat W. Z. Mahmoud

Analysis of semiconductor laser dynamics under gigabit rate modulation

A theoretical study of the dynamics of semiconductor lasers subjected to pseudorandom digital modulation at gigabit rates is presented. The eye diagram, turn-on jitter (TOJ), and power fluctuations in the modulated laser wave form are analyzed. The study is based on numerical large-signal analysis of the laser rate equations. Influences of the biasing and modulation currents on the eye diagram and TOJ are examined. The degree of eye opening is measured in terms of a Q factor of the laser signal analogous to the Q factor determining the bit-error rate in transmission systems. Influence of optimizing both the sampling and decision times on the signal Q factor is modeled. We show that the most eye opening corresponds to shortening the sampling time associated with lengthening the decision time. We also assess the relative contributions of the laser intrinsic noise and pseudorandom bit pattern to the TOJ. The results show that the bit pattern is the major contributor to the TOJ when the setting time of the re...

Noise performance of high-speed radio over fiber links employing vertical-cavity surface-emitting lasers

This study investigates the intensity noise in high-speed vertical-cavity surface-emitting lasers (VCSELs) and its contribution to the noise performance of radio over fiber (RoF) links. We evaluate the sinusoidal modulation of VCSELs in terms of the second-order harmonic distortion (2HD) and third-order intermodulation distortion (IMD3) in additions to the relative intensity noise (RIN). The spurious-free dynamic range of the proposed VCSEL is estimated. The noise performance of the RoF link is assessed by the noise figure. The modulation characteristics of the VCSEL and the gain and noise factor (NF) of the fiber link are compared under conventional and high-speed modulations of VCSELs. Also, we present comparison of the NF between short (300 m) and relatively long (2 km) fibers.

Multimode modeling of digital modulation in nearly single-mode semiconductor lasers

We present multimode modeling of the digitalmodulation characteristics of nearly single-mode semiconductor lasers. The model takes into account the mechanisms of spectral suppression of modal gain; namely, symmetric and asymmetric gain suppressions. The digital modulation performance of the laser is quantitatively examined in terms of the turn-on jitter, on-off ratio and a Q-factor. We study the effect of the modulation parameters on the modulation characteristics of the laser. We also clarify the effect of the asymmetric gain suppression on the modulated signals of the oscillating modes. In addition, we examine the validity of modeling the digital modulation of the laser via a single-mode rate equation model. We show that the modulation characteristics improve with an increase in the bias and/or modulation current or with a decrease in the bit rate due to reduction in the bit-pattern effect. The asymmetric gain suppression does not affect the characteristics of the total laser signal, however it enhances the modulation of the nearest modes on the long-wavelength side of the gain spectrum.

Study of the optical feedback-induced noise in semiconductor lasers and applications to the optic disc system

The dynamics and noise of semiconductor lasers under optical feedback (OFB) have been simulated. The study is performed as applied to an optic-disc system in which laser radiation is reflected by the disc surface and re-injected into the laser cavity. We examine the possibility of suppressing OFB-induced noise in the optic-disc system by the technique of superposition of high-frequency current. The study is based on numerical integration of the time-delay rate equations of semiconductor lasers under OFB. The laser noise is evaluated in terms of the spectral profile of relative intensity noise (RIN). It is shown that RIN is enhanced when states of chaos are generated, and attains minimum levels under continuous-wave operation just before the laser starts the route to chaos. The suppression of RIN in the low-frequency regime is achieved when the superposition-current frequency exceeds the laser resonance frequency by factors of 0.8, 1.0, and 1.1 and when the modulation depth exceeds 0.4.