Theory Research Of A Passively Mode-locked Fiber Laser Using Nonlinear Polarization Rotation Technique

The ultra-short pulse generated by a passively mode-locked fiber laser is not only the top edge research, but also the key technology to the OTDM/WDM systems. Compared to the semiconductor laser, the fiber laser with erbium-doped fiber as gain medium hold good gain features, high conversion efficiency, low threshold and compatibility with fiber -optic communication system; due to high stability, compact structure, and simple volume, the passive mode-locked laser using the fiber laser have been the best choices for the fiber laser to generate ultra-short pulses. Owing to its simple structure, much higher damage threshold and extremely fast relaxation time based on the fast response time of nonlinear fiber Kerr effect, the nonlinear polarization rotation (NPR) technique has been the preferred option to achieve mode-locked in a fiber laser. With this mind, the study is focused on a passively mode-locked fiber laser using the nonlinear polarization rotation technique based on the coupled complex nonlinear Schrodinger equations which includes:(1)The physical mechanism of a mode-locked fiber laser is investigated, and the advantages of different mode-locked technologies have been analyzed. As this paper is focused on the passively mode-locked Erbium-doped fiber laser, several models that can achieve mode-locked in a fiber laser have been summarized and discussed.(2)The theory model of a passively mode-locked fiber laser using the nonlinear polarization rotation technique based on the coupled complex nonlinear Schrodinger equations is established. From this theory model, we find that once the fiber laser is mode-locked, it emits uniform soliton pulses at first. With the polarization controllers fixed, when increasing the pump power, we obtain the soliton pulse period-doubling route to chaos pulse. In addition, we find that with the pump power fixed, rotating the polarization controllers, soliton pulse period-doubling also can route to chaos pulse. The numerical results show that the mechanism of the periodic intensity variations is caused by the interaction of the nonlinear polarization rotation and the passive polarizer in the cavity.(3)The phenomena of the multiple-soliton in a passively mode-locked fiber using the nonlinear polarization rotation technique have been intensively studied with numerical simulation. When with the polarization controllers fixed, we focus on the evolvement of the multiple-soliton pulse and soliton energy quantization by increasing the pump power. It is found theoretically that the formation of the multiple-soliton is the result of the peak-power-clamping effect such as the dispersive waves and pulse-splitting. We also observe the soliton energy quantization which is the final consequence of the dynamic balance between the gain competition and the loss caused by the nonlinear polarization.(4)We sum up our work and give expectation and some advice on the ultra-short pulse generated by a passively mode-locked fiber laser using the nonlinear polarization rotation technique.