**7. Conclusions**

This chapter, as I intend to, gave a brief description and theory of strong optical injection and its influence on the characteristics of the slave laser. Secure optical communications can be verified by such operation of the laser. Modulation bandwidth enhancement has been found by numerical simulation of the rate equations. The enhancement was found to be dependent on the injection parameters, frequency detuning and injection strength. Period doubling and relaxation oscillation frequency are the physical mechanisms of bandwidth enhancement by injection locking. Substantial reduction in frequency chirping, and a suppression of 14 dB/Hz in the RIN were observed in the direct modulation injection-locked laser. These characteristics of such a laser are the demands for high bit rate and longer optical communication network. Chaotic dynamics of the slave laser is the challenging candidate for secure optical systems.
