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Study On Output Characteristics And Chaos Of Mode-locked Fiber Laser

Posted on:2013-01-05Degree:DoctorType:Dissertation
Country:ChinaCandidate:G WuFull Text:PDF
GTID:1118330371482990Subject:Circuits and Systems
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Since the late1990s, with the rapid development of data services based oncommunications networks such as video on demand (VOD), E-commerce, dataretrieval, teleconferencing and distance education, the demand for communicationbandwidth of individual end-users, enterprises, universities and research institutionsgrows continuously, as a consequence, the current optical fiber communicationsystems can not meet the growing demand for communication capacity. At present,the methods used to improve the communication capability of optical fibercommunication systems, including the usage of optical time division multiplexing(OTDM), orthogonal frequency division multiplexing (OFDM) and code divisionmultiple access (CDMA) technologies to improve the communication capacity andthe usage of high-power laser light source and optical fiber amplifier to increase therelay distance, but the cost of this scheme is too expensive.In the current optical fiber communication systems, the main factors which limitthe transmission capacity and the transmission distance are the effects of lost anddispersion in optical fibers. With the development of optical fiber manufacturingtechnology, the fiber loss has been reduced to close to the theoretical limit value,therefore, fiber dispersion has become a major obstacle to achieve large capacity andultra-long haul optical fiber communication. Dispersion induced pulse broadening canbe detrimental for optical communication systems. Another effect in fiber namednonlinear self-phase modulation will lead to pulse compression. The fundamentalsech~2-shaped soliton which keeps its shape unchanged as evolving in optical fibers can be obtained though balancing the effects of group-velocity dispersion andnonlinear self-phase modulation. The use of soliton in optical fiber communicationhas the advantages of high transfer rate, long repeaterless distance and lowconstruction and maintenance cost, therefore it's recognized as the most promisingnext-generation optical fiber communication system. This thesis mainly research theideal light source of next-generation optical fiber communication system which hasthe ability to produce optical soliton—mode-locked fiber laser.The thesis mainly focus on the study of the output pulse evolution characteristicsand the nonlinear dynamics of mode-locked fiber laser and makes several creativeresearch work including the chirp extraction of self-similar pulse, the pulse evolutioncharacteristics in the transient region between self-similar pulses and boundself-similar pulse pairs and the chaos of mode-locked fiber laser. Combining with theexisting conditions of our laboratory, the following specific works have beenaccomplished:1. The working mechanism of mode-locked fiber laser is thoroughly analyzed.The mode-locked fiber laser is classified as either actively mode locking or passivelymode-locking according to the design of the laser cavity. The advantages anddisadvantages of different mode-locking technologies are investigated and the latestresearch progress of the each kinds of mode-locked fiber laser is introduced. Theestablishing process of the generalized nonlinear Schr dinger equation whichdescribes the pulse evolution in mode-locked fiber laser is elaborated. The model ofpassively mode-locked fiber laser based on saturable absorber, passively mode-lockedfiber laser using nonlinear polarization rotation effect and actively mode-locked fiberlaser is established respectively.2. Based on the models above, the soliton evolution in mode-locked fiber laserare numerically investigated with the split-step Fourier method (SSFM) and thesplit-step finite difference method (SSFD). The simulation is accomplished by thesplit-step Fourier method generally, the split-step finite difference method is used inthe special case of the model with strong nonlinear term. For researching characteristics of self-similar pulses, chirp extraction of self-similar pulses areaccomplished using several time-frequency analysis methods including short-timeFourier transform (STFT), Wigner-Ville distribution (WVD) and continuous wavelettransform (CWT). We compare and analyze the advantages and disadvantages of thesetime-frequency analysis methods using several statistics methods in this particularapplication.3. Characteristics of output solitons of the passively mode-locked Yb-doped fiberlaser.are investigated. The impact of the net averaged group-velocity dispersion onpulse width, pulse peak power and single pulse energy of the output solitons isanalyzed by simulation. Characteristics of output self-similar pulses of the passivelymode-locked Yb-doped fiber laser.are investigated: Firstly, the theoretical formulasof the shape and chirp of the self-similar pulse are derived, and the theory isconfirmed through simulation. Secondly, the variations of pulse width, pulse peakpower and single pulse energy of the self-similar pulses with the net averagedgroup-velocity dispersion are studied. Finally, the steady bound self-similar pulsepairs are obtained with β2net=1.35ps2/km. We observe, for the first time to ourknowledge, an interesting phenomenon that for fixed system parameters linearlychirped parabolic self-similar pulse and bound self-similar pulse pairs are obtainedrandomly under different initial conditions in the transient region between these twokinds of output pulses. We consider that the different chirps generated by differentinitial white noises might be a cause of this phenomenon, but the evolution of pulsesin the cavity of mode-locked fiber laser is so complex that obviously, a more completediscussion need to be taken in the future research.4. The nonlinear dynamic characteristics of passively and actively mode-lockedfiber laser are investigated respectively. At first, the concept and nature of chaos areintroduced, and the theorem of the period doubling route to chaos indicates that thenonlinear systems can produce chaotic motion is highlighted. Then, by appropriatelyadjusting the small signal gain of the optical fiber amplifier, the nonlinear dynamicmotions of the passively mode-locked fiber laser based on nonlinear polarization rotation effect are observed by numerical simulation. At last, chaos in activelymode-locked fiber laser system are obtained by adjusting the elliptic modulus k of theactive modulator and the intra-cavity length appropriately in the actively mode-lockedfiber laser model based on the modulation characteristics of the Jacobian ellipticcosine function.
Keywords/Search Tags:mode-locked fiber laser, soliton, self-similar pulse, chirp, bound-state self-similarpulse pairs, chaos
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