Investigation On Generation Of Tunable Chaotic Signal With Controllable Bandwidth Based On Weak-resonant-cavity Fabry-perot Laser Diodes

Semiconductor lasers(SLs)subject to optical feedback,optical injection,or optoelectronic feedback would readily emit large-amplitude optical chaotic signals.Hence,SLs have become the most suitable chaotic signal sources for the chaotic secure communication.By the above prescribed external perturbations,the generated optical chaotic signal's bandwidth will usually reaches a few gigahertz,which will greatly limits the maximum transmission rate and communication capacity of chaotic communication system.The application of wavelength division multiplexing(WDM)makes possible to improve the communication capacity of the chaotic secure communication system.However,most of the research reported so far are only the multiplexing of two channels in WDM chaotic secure communication due to the limitation of the chaotic signal sources with tunable wavelengths and the system cost.Therefore,a central wavelength tunable and bandwidth controllable broadband chaotic carrier sources become necessary to construct WDM-based multi-channel optical chaotic secure communication system.In recent years,weak-resonant-cavity Fabry-Perot laser diode(WRC-FPLD)has been used in WDM system due to its unique features of wide gain spectrum range and small mode interval.Thus,by introducing suitable external perturbation,the WRC-FPLD output broadband chaotic signal with central wavelength tunable will be of great significance source to the construction of high-speed WDM-based multi-channel optical chaotic secure communication system.A scheme for generating tunable chaotic signals with controllable bandwidth is proposed and experimentally demonstrated based on two WRC-FPLD.In order to achieve the chaotic signal,the scheme consist of a WRC-FPLD laser diode(named as M-WRC-FPLD)with tunable fiber Bragg grating(FBG)optical feedback.The chaotic output of M-WRC-FPLD is then unidirectionally injected into another WRC-FPLD(named as S-WRC-FPLD).The effect of S-WRC-FPLD output chaotic signal bandwidth is investigated by controlling injection power and detuning frequency.The experimental results show that,through adjusting the central wavelength of tunable fiber Bragg grating and feedback power,M-WRC-FPLD can emit chaotic signals of three adjacent modes with the central wavelength at 1549.880 nm,1550.450 nm and 1551.040 nm,which are used as chaotic light to be unidirectionally injected into S-WRC-FPLD to make the central wavelength of S-WRC-FPLD output basically consistent with the central wavelength of injected chaotic light.For a fixed injection intensity and increasing the detuning frequency gradually from-25.0 GHz to 25.0 GHz,under the three injected conditions,the bandwidth of S-WRC-FPLD chaotic output signal initially gradually increases and further decreases gradually after reaching the maximum value.The detuning frequency required at the maximum chaotic bandwidth is slightly different for three different injected cases,the S-WRC-FPLD output chaotic signal bandwidth under positive detuning frequency is greater than the negative detuning frequency for the same frequency detuning value.For a fixed detuning frequency and increasing the injected power,the bandwidth of S-WRC-FPLD chaotic signal increases rapidly to a maximum value,and then shows a decreasing trend with accompanying by fluctuations in the process of decline.In conclusion,the system could emit chaotic signals whose central wavelength can be tuned in a wide range within the tunable range of FBG,and the bandwidth of the tunable chaotic signal could be adjusted within the range of 10.0 GHz-30.0 GHz.