| The short pulse fiber laser with advantage of simple, compact construction and low cost has very important applications in all optical communication, industrial, medicine and laser remote sensing. In this dissertation, passively Q-switched and mode-locked fiber lasers by using a SESAM as the saturable absorber are proposed and experimentally demonstrated, and the application of a dual-wavelength passively Q-switched fiber laser in generation of tunable microwave/millimeter waves is presented. The main contents are outlined as follows:1、Based on the Maxwell’s equations, the non-linear Schrodinger equation(NLSE) in time domain is derived. Pulse propagation in fiber is simulated with the split-step Fourier method. The influence of loss, dispersion and nonlinear effect is researched, and the evolution pulse shapes are obtained. The propagating equation of transverse lightwave is identical in form to that of equation describing the temporal evolution of an optical pulse in a dispersive medium. A general analogy of spatial diffraction and temporal dispersion is presented in details. By using the transformation pairs of spatial location and time, wave number and dispersion parameter, some more general expressions, such as spot size/wavefront curvature and pulse-width/chirps, space-angle spectrum product and time-bandwidth product, spatial Fresnel number and temporal Fresnel number, focal length of lens and focusing time, are derived.2、A passively Q-switched Er doped fiber laser using a semiconductor saturable absorber mirror (SESAM) is demonstrated. Q-switching is observed with the output power produced at a slope efficiency of 29.4% with respect to the absorbed pump power. The maximum average output power of 8.37mW is achieved. The pulse repetition frequency obtained can be turned from 1.72 kHz to 7.95 kHz. The pulse energy of 17.2nJ has been obtained at the pump power of 46.75mW, and the pulse width is 30μs. By adjusting the polarization controller carefully, the dual-wavelength Q-switched pulse with wavelength of 1557.76nm and 1560.16nm, pulse duration of～56μ.s are observed. After placing a fiber Bragg grating (FBG), served as dispersion compensation element in the cavity, a ring cavity passively mode-locked fiber laser using a SESAM has been demonstrated, and its output performance is discussed. Stable soliton mode locked spectrum with 3dB bandwidth of 3.2nm, center wavelength of 1555.8nm and average output power of 316.1μW is observed at the pump power of 110mW. The pulse repetition rate is 25 MHz, as determined by the cavity length of 8m and in case of the output pulses are sech2 transform-limited pulses, the output pulses duration of 0.79ps, single pulse energy of 12.64pJ can be obtained.3、A passively Q-switched dual-wavelength Er-doped fiber laser based on a semiconductor saturable absorber mirror is proposed and experimentally demonstrated. The proof of concept device consists of two cascaded fiber Bragg gratings and a semiconductor saturable absorber mirror. By changing the operation temperature of the fiber Bragg grating, stable dual-wavelength Q-switched pulses are obtained. Results demonstrate that dual-wavelength simultaneous oscillation at 1549.69nm and 1550.5nm (1548.2nm and 1549.69nm) is achieved with the pump threshold power of～110mW, pulse repetition rate is a function of the operation temperature of fiber Bragg grating. The typical output pulse trains with pulse energy of 7.2nJ, pulse duration of 80μs and pulse repetition frequency of 8.50 kHz are achieved at a pump power of 140mW. Based on the dual-wavelength Q-switched Er-doped fiber laser, a novel photonic tunable microwave generation is proposed and experimental demonstrated. By changing the operation temperature of the fiber Bragg grating, stable dual-wavelength Q-switched pulses with tunable wavelength spacing are achieved, and wavelength spacing of 0.24-0.66nm, corresponding to millimeter wave signals of 30-82.5GHz can be obtained at the photodetector. |
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