Study On Nonlinear Dynamics Of Fiber Bragg Grating External Cavity Semiconductor Lasers

Semiconductor lasers(SLs)have gained much attention since its inception in 1960 s.With the development of semiconductor fabrication technology,semiconductor lasers have been developed into reliable and efficient coherent light sources.They possess some advantages such as small size,low cost,wide optical spectrum,high efficiency,easy to be directly modulated etc.and have been applied to optical communication,optical storage,optical interconnection and so on.As the relaxation time of the material polarization in SLs are much smaller than both photon and population relaxation times,SLs are classified into Class B lasers.Without external perturbations,SLs behave continuous-wave emission,however,as nonlinear devices,SLs can also exhibit various nonlinear dynamical characteristics including stable locking,period-one oscillation,period-two oscillation,quasi-periodic oscillation,and chaotic oscillation etc.under different perturbation schemes.These rich nonlinear dynamics of SLs have important application value.For example,when SLs operate at stable locking state,the linewidth of output shall be narrowed and the modulation bandwidth can be enhanced.When SLs operate at period-one oscillation state,they can generate high-frequency photonic microwave signals.Besides,when the SLs operate at chaos state,the noise-like waveform can be applied to optical chaos based secure communications,optical chaotic radar and fast physical random number generation.The common perturbation schemes include external cavity optical feedback,optical injection,optoelectronic feedback and so on.Among which,external cavity optical feedback SLs are widely adopted due to the relatively simple structure and easy to obtain high dimension chaotic output.In recent years,with the development of fiber grating writing technology,high performance fiber gratings have become essential devices in optical communication,fiber sensing and optical precision measurement etc.Fiber Bragg grating(FBG),a reflective fiber grating,can also constitute the external cavity of SLs and provide distributed optical feedback.At present,the investigation of nonlinear dynamics of FBG feedback SLs is relatively insufficient and mainly focuses on the edge-emitting distributed feedback semiconductor laser(DFB-SL).Compared with DFB-SL,vertical-cavity surface-emitting laser(VCSEL)has many beneficial features such as low threshold current,high bandwidth modulation,single longitudinal mode operation,and easily large-scale integration into two-dimensional arrays etc.Especially,due to cylindrical symmetry structure and weak anisotropy of the VCSEL cavity,the VCSEL output generally includes two orthogonal polarization components with different emission wavelength,defined as x polarization component(x-PC)and y polarization component(y-PC).When a FBG is introduced as the external cavity of VCSEL,it can be expected that the nonlinear dynamics of VCSEL subject to FBG feedback must exist difference between that of mirror feedback VCSEL because of the wavelength selective reflection of FBG.Moreover,as the bias current changes,VCSEL with FBG external cavity will present interesting polarization dynamics such as polarization switching and polarization bistability.These polarization dynamics have important application value in optical switch,optical storage and optical computing etc.Besides,the group velocity dispersion of FBG feedback can weaken the time delay signature of the chaotic output from the VCSEL with FBG external cavity and high quality optical chaos signals can be obtained.Aiming at the issues mentioned above,in this thesis,we theoretically and experimentally investigate the nonlinear dynamics,polarization switching characteristic,and time delay signature of chaos output of VCSEL with FBG external cavity.Furthermore,utilizing a weak-resonant-cavity Fabry-Perot laser diode(WRC-FPLD)with FBG external cavity,we investigate the ways for achieving wavelength-tunable broadband optical chaos signals.The main work and results are summarized as follows:1.Based on the spin-flip model of VCSEL and the coupled-mode theory of FBG,the rate equation of VCSEL with FBG external cavity has been derived,which can suitably describe the nonlinear dynamics and polarization switching characteristic.With the help of fourth-order Runge-Kutta method,we numerically simulate the time series,power spectrum,phase portrait of the output from FBG feedback VCSEL,determine the dynamical states,and explore the polarization switching between two polarization components.The results show that,under the same feedback strength ?,the two polarization components of FBG external cavity VCSEL can behave different dynamical states.With the increase of feedback strength ?,the output of FBG external cavity VCSEL undergoes stable state,period-one state,quasi-period state,and finally enters chaos state.By mapping the dynamical states in the parameter space of feedback strength ? and frequency detuning ?f between the central frequency of FBG and the central frequency of VCSEL,the parameter ranges of various dynamical states have been determined.By calculating the normalized output intensities of two polarization components,the polarization switching characteristic has been investigated and two-mode coexistence has been observed.2.Based on above theoretical investigations,we construct the experimental system of FBG external cavity VCSEL and record the time series,optical spectrum and power spectrum via a digital oscilloscope,an optical spectrum analyzer and an electrical spectrum analyzer,respectively.By analyzing the corresponding data,the dynamical route and polarization switching characteristic have been experimentally studied.Also,the physical mechanism of polarization switching in FBG external cavity VCSEL has been revealed.The results demonstrate that,with the increase of feedback strength ?,the total output of FBG external cavity VCSEL has a quasi-periodic route to chaos,which is in agreement with the theoretical work.When fixing the bias current I,three typical polarization switching routes have been observed by adjusting the attenuation rate of the variable attenuator.Moreover,when the attenuation rate of the variable attenuator is fixed,multiple times switching of dominant polarization mode and two-mode coexistence,which have been predicted in the theoretical work,can be observed via continuously varying the bias current.Further exploration reveals that both two polaration modes operate at chaos state when two-mode coexistence is achieved.3.We adjust the relative work parameters to drive the FBG external cavity VCSEL into chaos and then adopt the correlation function,the mutual information and the permutation entropy to correctly extract and quantitatively evaluate the time delay signature of chaos output.The effect of external parameters on the time delay signature of chaos output has been theoretically and experimentally investigated.In the theoretical work,we analyze the influence of feedback strength ?,the frequency detuning ?f between the central frequency of FBG and the central frequency of VCSEL and the coupling coefficient ?B of the FBG on the time delay signature and the effective bandwidth of chaos output from the VCSEL subject to FBG feedback.Compared with mirror external cavity VCSEL,FBG external cavity VCSEL performs better time delay signature suppression performance.By suitably selecting operation parameters,the time delay signature of chaos output from FBG external cavity VCSEL can be suppressed below 0.1.In experimental studies,we mainly discuss the influence of the bias current I and feedback strength ? on the time delay signature.Combing with the theoretical studies,the physical mechanism for suppression of time delay signature in FBG external cavity VCSEL has been explained.By mapping the time delay signature in the parameter space of feedback strength ? and bias current I,the optimal parameter regions in which the high quality chaotic signals without time delay signature can be obtained.4.To meet the demands of large capacity and high transmission rate in optical chaos based secure communication systems,we combine the characteristics of a weak-resonant-cavity Fabry-Perot laser diode(WRC-FPLD),which possesses a broader gain spectrum and smaller longitudinal mode spacing,and propose a WRC-FPLD with tunable FBG external cavity to generate wavelength tunable broadband chaotic signals.The experimental system of tunable FBG external cavity WRC-FPLD has been set up and the influence of feedback power on every longitudinal of WRC-FPLD within the wavelength range of 12 nm has been investigated.Also,the physical mechanism of chaotic bandwidth enhanced due to the FBG feedback has been presented.The further analysis shows that,through adjusting the central wavelength of the FBG and selecting suitable feedback power,twenty longitudinal modes of the WRC-FPLD can be individually driven into chaos with a flat power spectrum and the corresponding chaotic bandwidth can reach about 30 GHz.