Investigation On The Chaotic Dynamic Of A Weak-Resonant Cavity Fabry-Perot Laser Diode Under External Disturbance And Its Application In Chaotic Optical Communication

Chaos is suitable for secure communication,chaotic cryptography,image encryption,multimedia copyright protection and other fields due to its ergodicity,sensitivity to initial conditions,irregular motion in phase space and other iconic features.Semiconductor laser(SL)is a kind of non-linear device,under different external disturbance,it can generate chaotic signal with the advantages of high randomness and large bandwidth,which is favored by people and is widely used in chaotic radar,physical random number acquisition,and chaotic optical communication,etc.The SL-based chaotic optical communication technology aims to securely transmit data from the physical layer of the communication network,with higher security,broadband,high speed,low attenuation and being compatible with current commercial optical communication systems.For this reason,a lot of manpower and material resources have been invested in research at domestic and overseas.However,it is still in its infancy,even if certain results have been achieved,there are still many issues worth discussing.For example,how to improve the capacity of information transmission.However,it is still in its infancy,even if certain results have been achieved,there are still many issues worth discussing.For example,how to improve the capacity of information transmission.Weak resonant-cavity Fabry-Perot laser(WRC-FPLD)is a new type of laser developed on the basis of the traditional Fabry-Perot laser(FP-LD).WRC-FPLDs possess wider gain spectrum and smaller mode spacing due to its lengthened cavitiy and lower front-end surface reflectivity.As a result,WRC-FPLDs can lasing more modes within a certain wavelength range.It has broad application prospects in the acquisition of multi-channel physical random numbers,multi-channel chaotic radar,multi-channel chaotic optical communication and other related applications.Therefore,it is of great significance to study the chaotic dynamic characteristics of WRC-FPLD under external disturbance and its application.At present,although the preliminary research work of generating chaotic signal based on WRC-FPLD has been reported,there are still some key problems to be solved when applying the chaotic signal to secure communication.It includes generating high-quality chaotic signals suitable for secure communication based on a WRC-FPLD under different external disturbance,reasonably constructing chaotic synchronization system based on WRC-FPLDs,mastering the influence of synchronization quality on communication performance,and the influence of other key parameters on transmission capabilities in secure communication.Based on the above analysis,taking WRC-FPLD as the laser source,this paper focuses on the acquisition of chaotic signal based on the WRC-FPLD under external disturbance,the construction of chaotic optical communication systems based on chaotic synchronization,systematically studies the synchronization quality and the performance of chaotic optical communication system based on WRC-FPLDs.The main work and results of our studies are summarized as follows:1.Based on the multi-mode rate equation model of SL,we numerically study the chaotic dynamic characteristic of the WRC-FPLD subject to fiber Bragg grating(FBG)feedback.According to the Lang-Kobayashi model combined with the coupled mode theory of FBG,a rate equation describing the dynamic characteristics of the WRC-FPLD under FBG filtering feedback is established.By adjusting the Bragg frequency of FBG and feedback factor k,different longitudinal mode of the WRC-FPLD can become the lasing mode.And the selected lasing mode presents a chaotic state by further adjusting the feedback factor k.After fixing the Bragg frequency of FBG,via changing the feedback factor k,we systematically study the influence of feedback factor k on the chaotic bandwidth.The results show that with the increase of k,the chaotic bandwidth first increases and then decreases.Such a variation trend is consistent with our previous experimentally reported.Moreover,the effects of the 3 d B reflection bandwidth of FBG and the frequency detuning?f between the central wavelength of FBG and the lasing longitudinal modes of the free-running WRC-FPLD on the chaotic bandwidth are further studied systematically.The results show that by adjusting the appropriate feedback parameters,the WRC-FPLD under filtering feedback can output wideband chaotic signals with tunable center wavelength and conthrollable bandwith.2.A scheme for simultaneously generating multi-channel broadband chaotic signals is proposed and demonstrated experimentally,based on two unidirectionally coupled WRC-FPLDs with almost the same mode interval.In the system,one WRC-FPLD is utilized as master WRC-FPLD(M-WRC-FPLD)and the other one is used as slave WRC-FPLD(S-WRC-FPLD).The output of free running M-WRC-FPLD is injected into the S-WRC-FPLD.The frequency detuning of the corresponding longitudinal modes of the two lasers is the same,due to the identical mode spacings of the two WRC-FPLDs.Under the proper injection intensity and frequency detuning,multiple longitudinal modes of the S-WRC-FPLD can be simultaneously driven into chaotic states,which can provide multi-channel chaotic signals.Via an optical filter with center wavelength tunable,the performances of 9 channels chaotic signals were inspected.The experiment results show that,under the injection conditions(?f=-5 GHz and P_in=1.08 m W,?f=1 GHz and P_in=276.04?W,?f=5 GHz and P_in=1.27 m W),the chaotic bandwidths of the 9 channels chaotic signals are slightly different and located within the range of 8¹5 GHz.Moreover,the energy of each channel chaotic signal located at low-frequency region is stronger than that of the total modes chaotic output.The influence of injection parameters on the bandwidth of a single channel chaotic signal is further studied systematically.First,the frequency detuning?f is fixed and injection power P_in is changed.The experiment results show that,for a given?f,with the increase of injection power P_in,the chaotic bandwidth for a determined channel chaotic signal shows a trend of increasing firstly and then decreasing.For a given injection power P_in,the variation of the chaotic bandwidth presents an approximately M-shaped symmetrical distribution for?f varied within-30GHz to 30 GHz.Finally,based on the modified Lang-Kobayashi equation,the rate equation describing the dynamic characteristics of the two unidirectional coupled WRC-FPLDs is given.And it is soveled by Fourth-order Runge-Kutta algorithm.The numerical results are consistent with that in the experiment.By calculating the cross-correlation function between the intensity outputs of different mode.The physical essence of the low frequency energy of full mode chaotic signal lower than that in the power spectrum of the single mode chaotic signal is the mode competition inside the multimode laser.3.A chaotic optical communication system with switchable chaotic carrier wavelength is proposed and demonstrated experimentally,based on two WRC-FPLDs with almost identical mode interval.In such a scheme,a WRC-FPLD(named as T-WRC-FPLD)subject to filtered optical feedback provided by a tunable optical filter(TOF)is taken as the transmitter,and the other WRC-FPLD(named as R-WRC-FPLD)under optical injection from the T-WRC-FPLD is taken as the receiver.Through setting the central wavelength of the TOF and selecting the matched feedback strength,a desired mode in T-WRC-FPLD can be chosen to stimulate and output a chaotic signal,whose central wavelength is determined by the central wavelength of the TOF.Thus,the T-WRC-FPLD under filtered feedback can provide a chaotic carrier with a switchable central wavelength.The chaotic signal output from the T-WRC-FPLD is sent to the R-WRC-FPLD,and drive the R-WRC-FPLD for generating a chaotic signal.Under proper injection strength,high-quality chaotic synchronization between T-WRC-FPLD and R-WRC-FPLD can be realized,and therefore the chaotic communication with switchable carrier wavelength can be implemented in this system.We adopt three different chaotic signals provided by three different modes in T-WRC-FPLD as the chaotic carriers,respectively,and inspect the performances of the system.The experimental results show that,for the three cases,all the cross-correlation functions between T-WRC-FPLD and R-WRC-FPLD can be larger than 0.94 under optimized operating parameters.Via the high-quality chaotic synchronization between T-WRC-FPLD and R-WRC-FPLD,a 5Gb/s message encrypted in each chaotic carrier can be successfully decrypted at the receiver for the three cases,and the bit error ratios(BERs)are all lower than 3.8×10^-3,which is the hard-decision forward error correction(HD-FEC)threshold.This scheme provides a potential method for dense WDM(DWDM)chaotic optical communication networks.