Research On Square-Wave Pulse In Passively Mode-Locked Fiber Laser

Fiber lasers have been extensively investigated for their excellent gain property, high efficiency, low threshold and the compact, economic, and stable performance. And they have a wide range of applications in science and industry, such as optical frequency comb, micro-machining, optical sensors and so on. The traditional dispersion soliton, formed by the balance of anomalous dispersion and nonlinear effect in fiber laser, is stable and easy to be achieved in laser cavity with anomalous dispersion. However, its pulse energy is limited to about 0.1nJ according to the soliton area theorem, and a larger energy will lead to soliton breaking. In recent years, dissipative soliton fiber laser working in normal dispersion cavity has attracted great interest of researchers. The formation of dissipative soliton relies on the dynamic balance between the gain and loss in the nonlinear laser system. Due to the different physical characteristics, the dissipative soliton can break the energy limit of traditional dispersion soliton. Thus researches about dissipative soliton have important practical and potential applications. In this dissertation, a new kind of dissipative soliton fiber laser, the square-wave passively mode-locked fiber laser is investigated experimentally and theoretically. This new kink of fiber laser generates square-shape pulse with flat top and sharp edges. When increasing pump power to the laser cavity, the pulse keeps its square shape and its energy increases linearly. In this thesis, the forming mechanism of square-wave pulse, the relationship between the peak power, pulse width, spectrum and the cavity parameters are investigated.The main contents of this thesis are as follows:1. The relationship between the peak power, pulse width and the cavity parameters are investigated experimentally. With a dual-pump passively mode-locked figure-8 fiber laser, we acquired width and amplitude tunable square-wave pulse. The pulse width can be tuned from 5.6ns to 30.4ns and the peak power range is between 4.5W and 8.3W. We analyzed the generation of the square-wave pulse with the nonlinear Schrodinger Equation and discussed this width and amplitude tunable square-wave pulse from the point of gain saturation effect.2. The spectrum of chopped square-wave pulses in 1.5μm band are investigated experimentally. We analyzed the chirp of square-wave pulse based on the experimental results. It’s a supplement for the pre-existing square-wave pulse chopping experiment.3. The spectrum of square-wave pulse figure-8 fiber laser in 1.0μm band is investigated. We proposed two methods to generate wide-spectrum square-wave pulse in figure-8 fiber laser in 1.0μm band. By increasing the dispersion in laser cavity, square-wave pulses with pulse width in hundred nanoseconds order and 3dB bandwidth approximating to 1nm are generated. Through introducing high-nonlinearity fiber, we can obtain flat-top and sharp-edge square-wave pulse which has a 3dB bandwidth of about 2.4nm and pulse width in ten nanoseconds order.4. The coherence of laser in 1.0μm band is investigated with the experiment of two-beam interference. We found that the wide-spectrum square-wave pulse and narrow-spectrum square-wave pulse in 1.0μm band have different coherence. By using nonlinear Schrodinger Equation, we analyzed the forming of the two square-wave pulse and explained the difference in their coherence.5. To solve the problem of self-starting and automatically retrieving mode-locked state of passively mode-locked fiber laser, we proposed and demonstrated a feedback control laser system which can find and get into the mode-locked state automatically.The innovative work and results in this thesis are as follows:1. A new dual-pump passively mode-locked figure-8 fiber laser is proposed and demonstrated. The dual-pump figure-8 fiber laser can generate width and amplitude tunable square-wave pulse. We numerically analyzed the generation of the square-wave pulse and discussed this width and amplitude tunable square-wave pulse from the point of gain saturation effect.2. The spectrum of square-wave pulse figure-8 fiber laser in 1.0μm band is investigated experimentally and two methods to generate wide-spectrum square-wave pulse in figure-8 fiber laser in 1.0μm band are proposed.3. The coherence of laser in 1.0μm band is investigated with the experiment of two-beam interference. The different coherence between wide-spectrum square-wave pulse and narrow-spectrum square-wave pulse in 1.0μm band are found and analyzed.