Bandwidth Enhancement And Time-delay Signature Suppression Of Chaotic Signal From An Optical Feedback Semiconductor Laser By A Highly Nonlinear Fiber Loop Mirror

Chaos generated based on semiconductor lasers has attracted considerable attention for its extensive applications in secure communications,chaotic lidar,optical time domain reflectometer and fast random bit generator.In general,different external disturbances such as optical feedback,optoelectronic feedback,optical injection,current modulation are introduced to semiconductor lasers to obtain the chaotic signal.Among these methods,optical feedback method is the most widely adopted due to its simple configuration,low cost and feasible controllability.However,the bandwidth of chaos signal generated in a semiconductor laser with optical feedback is usually at a level of several GHz and obvious time-delay signature(TDS)can be observed,which limits the transmission rate and degrades the security of the communication system.As a result,it is significant to enhance the bandwidth and suppress the TDS of the chaotic signal from semiconductor lasers with optical feedback.In this work,a highly nonlinear fiber loop mirror(HNFLM)composed of a fiber coupler(FC)and a highly nonlinear fiber(HNLF),is proposed to enhance the bandwidth and suppress the TDS of chaotic signal from a distributed feedback semiconductor laser(DFB-SL)with optical feedback.After adopting standard chaotic bandwidth and the autocorrelation function(ACF),the influence of HNFLM on the bandwidth and the TDS of chaos signals from DFB-SL with optical feedback has been theoretically investigated.The results show that,after passing through a HNFLM composed of a FC with 7:3 power-splitting ratio(PSR)and a 35 m HNLF,the chaotic bandwidth increases from 12.3 GHz to 41.2 GHz and the corresponding maximum value ? of the ACF near the feedback delay time decreases from 0.22 to 0.05.The chaotic bandwidth and TDS of the output chaotic signal are influenced by the incident average power of the chaotic signal into the HNFLM,the PSR of FC and the length of the HNLF.Especially,TDS suppression is mainly determined by the PSR of FC.Moreover,for the case of 7:3 PSR of FC,the mappings of the bandwidth and TDS in the parameter space of the average power of the incident chaotic signal and the length of the HNLF have been given and the optimized parameter region to obtain chaotic signals with low TDS and enhanced bandwidth has been determined.