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Study Of Mode-locking Fiber Laser Based On Nonlinear Polarization Rotation Technology

Posted on:2010-01-08Degree:MasterType:Thesis
Country:ChinaCandidate:T LeiFull Text:PDF
GTID:2178360302957614Subject:Optics
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As an attractive topic in research, high power ultrashot pulses have various applications in laser micro-fabrication, ultrafast optics, optical communication and so on. It is possible to realize high power ulrashot optical pulses output in passive mode-locking fiber lasers. Comparing with solid laser, passive mode-locking fiber lasers have some advantages such as: small cavity, compact structure, water-cooling free. However, excess nonlinear phase shifts accumulated in optical fibers causes wave-breaking and limits the single pulse energy. The single pulse energy in fiber laser is much lower than that in solid lasers. Base on nonlinear polarization rotation (NPR) technology, the pulses energy are greatly increased in self-similar mode-locking fiber laser and all-normal dispersion mode-locking fiber laser. The pulses in these lasers can tolerant much more nonlinear effects, which help the pulse energy increase to 100 times of steady soliton. Yb3+ doped fiber with high gain coefficient and large spectral bandwidth is suitable for high power ultrashot pulse fiber lasers. According to recent developments in optical fiber lasers, in this thesis, we focus on the theoretical and experimental research on Yb3+ doped mode-locking fiber lasers, which operate in self-similar and all-normal dispersion regions. The main parts are as follows:1. Introduce the basic theory of mode-locking fiber laser. The pulses propagations in optical fibers are studied by solving Nonlinear Schrodinger Equation (NLSE) and the self similar evolution process of ultra-short pulse is obtained. The influences of the gain, dispersion as well as nonlinear coefficients on the pulse self similar evolution results are studied. In temporal shape, we find that the wings of the pulses are covered by the central part gradually. A theoretical explanation for self-similar pulses evolution is given which is proved by the numerical simulation.2. Describe the polarization components by stokes vectors and Mueller matrixes. Analyze the effects of the wave plates and polarizer in the cavity. Choose a simple combination of wave plates as polarization controller which can realize the mode-locking function in NPR mode-locking fiber lasers.A theoretical model is established to study the self-similar pulses in NPR mode-locked fiber lasers. The propagations of pulse in single mode fibers and gain fibers are described by coupled Ginzburg-Landau equation (GLE). Two wave plates and a polarizer are considered to realize the NPR mechanism in simulation. The polarization states of different parts across the pulse are simulated along the laser cavity. It is found that polarization states across the pulse are modulated from elliptical to almost circular before passing through the polarizer, which is proved a typical character of self-similar pulses. By modulating the parameters in the cavity, the region of steady self-similar pulses operation is found. In our simulations the laser generates high quality self-similar pulses.3. Conclude the theoretical study on all-normal dispersion fiber laser. There are three important parameters: nonlinear phase shift, GVD and spectral bandwidth which determine the pulses evolution in the laser cavity. We analyze the effects of these parameters and build an all-normal dispersion fiber laser. With the pump power fixed at 400mW, we investigate the four types of laser operations. And the influences of length of cavity on pulse characters are studied. The power of output pulses in this laser oscillator is very low and the mode-locking status is not stable. To overcome these disadvantages, we use a filter to sharp the pulses in the cavity. In addition, we design a fiber laser system with a mode-locking seed source, a primary amplifier and a secondary amplifier. It can generate stable mode-locking pulses with a repeat frequency 33.3MHz, whose energy reach 30nJ. The width of pulse auto-correlation curve is 14ps, so the peak power should be over 2kW. The extremely high power of the pulse give itself nonlinear chirp, which can not be compensated by pairs of gratings, thus the pulse compression outside the cavity is inefficient.4. Illustrate the theories and technologies of ultrashot pulses measurements. An SHG auto-correlator is built in our laboratory, which can measure the pulses ranging from 100fs to 100ps width. The parameters and process of measuring the auto-correlation curve for ultrashot pulses are recorded in a user guide. We also measure the output pulses of the all-normal dispersion fiber laser in our laboratory, in order to demonstrate how to calculate the data and get the auto-correlation curve. The results from our auto-correlator are almost the same as that from a commercial one.
Keywords/Search Tags:mode-locking fiber laser, nonlinear polarization rotation, self-similar, all-normal dispersion, auto-correlation
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