Study On Frequency-domain Dynamics And Evolution Characteristics Of Erbium-Doped Fiber Ring Laser

Aiming at the phenomena of complex multimode dynamics occurring in practical applications of fiber ring laser(FRLs), this thesis has put forward an improved real-time multichannel frequency-domain monitoring method, which breaks up frequency-domain limitations of traditional measuring tools for laser dynamics. This breakthrough promotes the understanding and analysis on nonlinear dynamics of FRLs from a low dimension to higher, also revealing the complicated correlation between the individual behavior and the collective behavior of dense longitudinal modes, and corresponding inherent physics.The frequency-domain dynamics of laser is a hard problem in the field of optical complex systems. Actually, FRLs belong to a type of optical complex system with large degree of freedom(hundreds and thousands of longitudinal modes can typically coexist), exhibiting such nonlinear mode dynamics as complex mode hopping, high-dimensional chaos. However, most of the theoretical models and experimental tools for analysing FRLs dynamics only focus on the collective behavior of dense longitudinal modes, omitting the dynamics individual modes. Generally, those menthods describe FRLs by very low-dimensional systems, inducing that such nonlinear dynamical problems as mode hopping suppression, generation and control of high-dimensional chaos in frequency domain, cannot be resolved effectively. To handle these issues in effect, not only theoretical models containing mutual actions and coupling effects among dense modes are needed, but also an experimental tool for observing frequency-domain multimode dynamics in detail is indispensable.This thesis adopts erbium doped fiber ring laser(EDFRL) as the research object. Firstly, frequency-domain dynamic models for dual-mode and multimode fiber lasers are established espectively, in which the quenching effect in ion pairs and the cross coupling mechanism among modes are considered. The laser models could effectively describe the individual behavior and clustering behavior of a large number of modes, and reproduce theoretically the multimode dynamics of high-dimensional dynamical system in frequency domain. Simulation results show that all modes simultaneously enter chaos via such routes as intermittency, period-doubling, quasi-period, when the ion-pair concentration or pumping rate is increased; the current multimode laser is found to be a high-dimensional hyperchaos system with its correlation dimension as high as 5.1.Based on both optical heterodyne and joint time-frequency analysis(OH-JTFA), a novel frequency-domain method for monitoring multimode dynamics of fiber lasers is proposed. This method has a frequency resolution of k Hz-magnitude, and can be used to extract simultaneously the nonlinear time series of multi parameters, i. e., frequency and intensity for dense modes of EDFRL. Experimentally, the frequency-domain dynamics of three fiber laser systems, including EDFRL with a FBG selector, self-organized feedback EDFRL and modulated chaotic EDFRL, are measured and analysed espectively by this method, which reveals the complex interaction and evolutional law between the individual behavior and the clustering behavior of modes.The EDFRL with a FBG as wavelength selector is usually considered as a typical single-wavelength laser. However, hundreds of intrinsic modes coexist within the reflective band of FBG and present unsteady multi-longitdinal-mode(MLM) oscillations under autonomous conditions. With the help of OH-JTFA method, the fruitful local dynamical phenomenon of the dense modes generated by this kind of EDFRLs are clearly obtained for the first time, which demonstrates that the individual mode shows a typical chaotic behavior whereas the total modes clustering behaves steadily.By introducing a saturable absorber in the laser cavity to form ultra-narrow bandwidth adaptive grating, self-organized feedback EDFRL could maintain stable single-longitudinal-mode(SLM) operation in theory. However, there are various mode hops occurring frequently in real lasers. Unfortunately, the acquisition of key information of the whole process of mode hopping is limited by the conventional mode-hopping detection method. With the OH-HTFA method, we have successfully achieved real-time monitoring of the whole mode-hopping process, which is illustrated directly by the time-frequency waterfall graph, and thus all physical parameters of the transient mode-hopping process are acquired exactly.A modulated chaotic EDFRL is typically a low-dimensional dynamical system, into which an additional freedom is introduced to realize chaos output. Similarly, this system contains a large number of dense longitudinal modes. Moreover, the dynamics and evolution of these modes in frequency domain are still unclear when the total output of the system is chaotic. By improving the frequency resolution of OH-JTFA, the temporal evolution of the frequency, spectrum and intensity of a single mode in chaotic EDFRL are extracted respectively. It is found that when the total intensity exhibits low-dimensional chaos, the frequency modulation and spectral broadening phenomena occur for a single mode in frequency domain, and the mode intensity is characterized by high-dimensional chaos or random fluctuation.The main results and innovations in this thesis are expounded by the following three aspects:1、By using OH-JTFA method,we have found a class of dynamic phenomena of nonlinear dynamic systems with very large degree of freedom in the MLM-EDFRL with a single FBG. The results show that the local modes demonstrate the dynamic behavior in frequency domain which is unable to present in total intensity, including mode self-pulsing, mode antiphase dynamics, chaotic MLM oscillation and random MLM oscillation.2、The time-frequency waterfall graphs of the typical mode hopping process and the instantaneous MLM oscillation evolution of the self-organized feedback EDFRL are obtained by the OH-JTFA method. At present, this method exhibits the only way to synchronously extract parameters in frequency domain such as mode-hop duration, mode-hop frequency spacing and direction, number of modes participating in the mode competition, evolution of mode intensity. It provides a more comprehensive observation method for further understanding of the mode instability and dynamic mechanism, and has important reference value.3、By improving the frequency resolution of OH-JTFA, the frequency-domain dynamics of a pump-modulated chaotic fiber laser system is investigated. It is found that the dense longitudinal modes in frequency domain show complex and high-dimensional dynamics. The nonlinear time series analysis of each mode show that chaotic modes and random modes are coexisting under the total output behaving low-dimensional chaos. Moreover, we have explored the feasibility of utilizing the vast chaotic or random modes in a single low-dimensional chaotic fiber laser as the photonic entropy source to generate multi-channel physical random code.