**1. Introduction**

Strong optical injection locking of semiconductor lasers can give an output of chaotic behavior which makes them an attractive transmitter for high speed secure optical communication. Data secure and high transmission rates are the most demanded issues in communication network. Optical injection, chaos synchronization of semiconductor lasers had been studied theoretically and experimentally for a decade [1,2], due their durability and very vast progressing technology. The effects of optical injection locking mainly have two aspects: one is to improve the characteristics of the slave and the other is to synchronize the master and the slave. In the former, the locking is able to improve the properties of the slave laser such as a single wavelength emission, high side mode suppression ratio (SMSR), and narrow linewidth. While in the latter, the synchronization of the master and slave in a wavelength, phase and chaos state had led the injection locking to broad applications in the coherent communications. Other improvements of the semiconductor laser by optical injection locking had been reported, these improvements include, increasing modulation bandwidth and reducing chirp [3,4], a high gain of 20-dB with small signal modulation below resonance frequency [5]. Bistability had been reported when a two color Fabry-Perot laser was subjected to optical injection in both modes and could be the basis for an all optical memory element with switching times below 500 ps [6]. The generalized synchronization of chaos based on phenomenon of injection locking characteristics of semiconductor laser and signal amplification in nonlinear systems is an application for secure data transmissions and communications [6,8].

In order to determine in which conditions (optical power, wavelength of the injected signal) the Fabry-Perot laser diode (FP-LD) is locked, it is essential to map the operating regimes on a chart defined by the two parameters, injected power and detuning which corresponds to the difference between the wavelengths of the injected signal and the one of a specific mode

© 2012 Azawe, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Azawe, licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of FP-LD that is submitted to optical injection, it is so-called injection map, which is well known for a single-mode laser [9,10].

The maximum available modulation frequency of the laser is in the vicinity of the relaxation oscillation frequency. Optical injection can enhance the relaxation oscillation frequency of the slave laser, and hence the bandwidth. So we would expect higher-speed transmitter for optical communication. On the other hand, a laser with controlled chaos could be obtained. The bandwidth-enhancement of the semiconductor laser by optical injection as well as a chaotic transmitter is the major objective.

This chapter will focus on the improvements of semiconductor lasers by the optical injection locking regimes and its applications for secure optical communication networks. The injection locked semiconductor laser, utilizing such applications, noise properties are of vital importance especially, the relative intensity noise (RIN). The aspects of noise influence on the dynamical operation of the laser with injection locking will be emphasized. The deployment of such a high bandwidth and chaotic carrier transmitter will be feasible without extra protection afforded by other means.
