Chaos bandwidth enhancement via optical injection and direct modulation of semiconductor lasers

A scheme for enhancing the chaos bandwidth by directly modulating semiconductor lasers subject to optical injection is proposed. A unidirectional master-slave configuration is constructed using two free-running semiconductor lasers. By introducing direct modulation to generate modulation sidebands in semiconductor lasers, the bandwidth of the original chaos signal can be enhanced through optimization of modulation depth, modulation frequency, frequency detuning and injection intensity. Experimental results show that when the current modulation frequencies applied to the master and slave lasers are 1.3 GHz and 1.6 GHz, respectively, the maximum bandwidth of the chaos signal of an optically injected distributed feedback semiconductor laser increases from 7.2 GHz to 14.1 GHz, corresponding to nearly a twofold enhancement. Further analysis of the modulation-induced spectral evolution reveals that the bandwidth enhancement originates from beat-frequency interactions between the modulation sidebands of the two semiconductor lasers, which effectively broaden the chaotic spectrum.