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Study On Soliton Characteristics Of Passively Mode-locked Fiber Laser

Posted on:2017-11-16Degree:DoctorType:Dissertation
Country:ChinaCandidate:J Y HuoFull Text:PDF
GTID:1318330512957946Subject:Communication and Information System
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High-repetition-rate and high-energy ultrashort pulses generated from mode-locked fiber lasers can become an ideal information carrier in an optical communication system. They have the advantages of high gain, high conversion efficiency, high stability, low threshold and low cost. Moreover, they have widely been applied in many fields such as precision machining, nonlinear frequency conversion, sensing, medical treatment and national defense. Based on the different methods of mode-locked implementation, the mode-locked fiber lasers can be divided into two types: actively and passively mode-locked. A passively mode-locked fiber laser has a simple structure, low cost, and high reliability, being a real all-fiber device. It can generate ultrashort optical pulses by using the nonlinear effect of the optical fiber. Based on the characteristics of dispersion distribution in fiber laser, the passively mode-locked fiber laser can produce conventional negative dispersion solitons, self-similar pulses, stretched pulses, and dissipative solitons. The dissipative soliton concept is a fundamental extension of the concept of solitons in conservative and integrable systems. Dissipative solitons exist in non-conservative systems and thus their dynamics is very different from that of conventional solitons. A fiber laser with purely normal GVD(group-velocity dispersion) or large normal GVD together with small anomalous GVD would presumably have to exploit dissipative processes in the mode-locked pulse shaping. Dissipative solitons have attracted great interest in the development of fiber lasers because they are able to significantly improve the deliverable energy of pulse and break through the energy limit of traditional solitons without wave breaking.In this thesis, the output pulse evolution characteristics of passively mode-locked fiber lasers are studied. The pulse characteristics influenced by cavity parameters are investigated. Passively mode-locked fiber lasers based on two-dimensional material saturable absorber are investigated experimentally and theoretically. The main contents of this dissertation are as follows:1. The basic principles of mode-locked fiber laser are thoroughly analyzed. The theories, mode-locked mechanism and classification of mode-locked fiber laser are introduced. The establishing process of the NLSE is elaborated. The classification of the simulation model of the mode locked fiber laser is introduced. The model of key components of passively mode-locked fiber laser is established respectively. The simulation model is constructed.2. Different kinds of solitons in a passively mode-locked fiber are analyzed. The formation mechanism and physical characteristics of conventional negative dispersion solitons, self-similar pulses and dissipative soliton pulses are introduced. The chirp of self-similar pulses is extracted using short-time Fourier transform and Wigner-Ville distribution. The advantages and disadvantages of these two time-frequency analysis methods are compared and analyzed.3. The influences of cavity parameters on output pulses are studied. Firstly, the influences of the net averaged group-velocity dispersion, small-signal gain, gain bandwidth and gain saturation energy on pulse width, pulse peak power and single pulse energy of the dissipative solitons are analyzed by simulation. Secondly, the characteristics of the bound dissipative soliton pulses pairs formed in a passively mode-locked fiber laser are studied. The numerical simulation experimental observation of bound dissipative soliton pulses pairs in certain system conditions is demonstrated. With increasing the net averaged group-velocity dispersion, small-signal gain and gain bandwidth, the pulse separation increases. With increasing the gain saturation energy, the pulse separation decreases. With increasing the net averaged group-velocity dispersion and gain bandwidth, the pulse peak power decreases. With increasing the small-signal gain and gain saturation energy, the pulse peak power increases. The total pulse energy increases with the increase of four parameters. The pulse width remains unchanged. It only increases with the increase of the small-signal gain. We find that there are unstable three dissipative solitons before the bound dissipative soliton pulses pairs. Three dissipative solitons generate with increasing the gain bandwidth from 39 nm to 44 nm. The gain bandwidth has a minimal impact on the characteristics of the dissipative solitons. Finally, the influence of pumping schemes on the characteristics of self-similar pulses in a passively mode-locked Yb-doped fiber laser is investigated by simulation. The structures of the forward-pumped and backward-pumped passively mode-locked laser are studied. We concentrate on the influence of gain bandwidth of gain fiber on pulse duration, peak power, and single pulse energy of self-similar pulses. The comparison and analysis of the results are performed based on the simulation.4. The structure of the passively mode-locked fiber laser based on the two-dimensional material as a new saturable absorber is studied by modeling and analysis. Firstly, the characteristics of two-dimensional materials are studied. The physical properties of graphene and black phosphorus are emphatically introduced. The two-dimensional material can be suitable for optoelectronics applications such as a saturable absorber in passively mode-locked fiber laser. Secondly, the structure of the passively mode-locked fiber laser based on black phosphorus is established. The research focuses on analyzing the influences of the gain bandwidth and gain saturation energy on the pulse duration, peak power, and single-pulse energy of the dissipative solitons. The results show that the single-pulse energy has relation with the gain saturation energy while independent with the gain bandwidth. Finally, a novel kind of soliton molecule in a graphene-based passively mode-locked fiber laser with anomalous dispersion is experimentally investigated. There are three pulses in the soliton molecule. The experimental observation shows that the peak separation changes randomly. A notable feature is that rule-less modulation occurs on the whole spectral profile, which is different from the regular modulation of the bound-state soliton. There are no reports about the soliton molecule, in which the pulses have unstable relative positions. Our experimental results are different from the observations reported by others. Therefore, the proposed soliton molecule is a very novel operation state for fiber lasers and can be regarded as the transitional state between the conventional soliton and bound-state soliton.
Keywords/Search Tags:passively mode-locked fiber laser, dissipative soliton, bound-state soliton, gain bandwidth, gain saturation energy, saturable absorber
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