Study On Tunable Passively Mode-locking Fiber Laser With Long Cavity

Ultrashort pulse with low repetition and high energy is widely used in MicroMechanical Machining, medical treatment or laser radar etc. Tunable fiber lasers areimportant sources for the High speed large capacity Dense Wavelength DivisionMultiplexing(DWDM) and Optical Time Division Multiplexing(OTDM) system.Studies on tuning mechanisms of mode-locked fiber laser with long cavity both intime-domain and frequency-domain are performed in the following parts. A tunablepulse with low repetition, high energy has been reported in this paper, with majorcontents as follows:1. Introduce the major methods to achieve passively mode-locking fiber laser.Their advantages and disadvantages have been analyzed. A brief review of currentresearch situation in home and abroad is done in the following paragraphs.2. Study on the mechanism of passively mode-locked fiber based on nonlinearpolarization rotation (NPR). The propagation of light in birefringence fiber isanalyzed using the coupled nonlinear Schr dinger equation (CNLSE). With variousfactors of three-order dispersion and nonlinear coefficient, we have a simulation onoptical solitons propagated in the common fiber.3. The methods to generate square wave pulse are classified introduced. Thesquare wave pulse generated based on NPR and NALM is analyzed theoretically.We introduced a kind of passively mode-locked fiber laser that can generate squarewave pulse through the NPR effect in the8-figure cavity. When the coupling ratio is90:10, a stable square wave pulse with a repetition rate of31.25kHz and a tunablerange from27ns to2347ns is generated. Tuning the pump power from108mW to508mW, the single pulse energy would increase from16nJ to236.8nJ. Different kindsof fiber with various lengths are also applied in the mode-locked fiber laser based onNOLM to analyze the square wave pulse. The range is proportional to the nonlineareffect in the fiber.4. A study on super-continuum spectrum and amplification is implementedexperimentally. After being amplified, the single pulse energy increased from64nJ to222nJ.5. Brief review of current research situation in home and abroad and an introduceof method to achieve wavelength tunable fiber laser are performed in the thesis. Aresearch is conducted by incorporating a tunable filter in the cavity, obtaining a stablepulse with a tuning range from1535nm to1569nm and a3dB bandwidth around2.2nm.