Semiconductor lasers are the most important light sources in optical communication systems. Further improvements of performance and characteristics of the devices contribute to further development of the communication systems. In this regard, two important physical properties affecting performance of semiconductor lasers have been analyzed; namely the quantum noise and optical feedback. The performance of semiconductor lasers, which is used in optical communication systems, is theoretically investigated. Effects of nonlinear gain and nonradiative recombination on characteristics of intensity and phase noises of lasers are investigated. The results showed that both intensity and frequency noises around the relaxation frequency are suppressed when counting the nonlinear gain in the rate equations [1]. An improved theoretical model to analyze dynamics, operation and noise of semiconductor lasers under arbitrary strength of optical feedback ranging from very weak to very strong optical feedback is presented [2]. A new set of modified rate equations of lasers operating under optical feedback are proposed [2]. The simulation results showed that the operations of semiconductor lasers are classified into continuous wave (CW), chaotic, and pulsing operations depending on the operating conditions. The optical feedback noise is found to be as low as the quantum noise level when the laser is injected well above its threshold level [3].