Study On The Performance Of Mode Locking And Q Switching Of All-normal-dispersion Fiber Laser

Ultra-short laser pulses have the advantages of high peak power, narrow pulse width, broad spectrum and so on. They have been widely applied in such fields as probing the microworld, huge volume communication, micro-fabrication, biomedicine and so on. Passively mode-locking and Q-switching are the two effective methods to obtain ultra-short laser pulses.Mode-locked fiber lasers have several obvious advantages over their solid-state counterparts, such as minimal alignment, effectively thermal management, low cost and so on. However, the accumulation of excessive nonlinear phase shift presents a fundamental limitation to the increase of pulse energy in passively mode-locked fiber lasers. One effective way to increase the pulse energy is to make the fiber lasers operate in positive dispersion regime. Positive dispersion can broaden the pulse, thus effectively reduce the accumulated nonlinear phase shift. The broadened chirp pulse can be dechirped to near their transform-limited duration outside the cavity, thus femtosecond pulse is achieved.Except for pursuing high pulse energy, the researchers of ultrashort pulse technology also pay much attention to new saturable absorbers. Graphene has emerged as a new kind of potential saturable absorber with the advantages of broadband saturable absorption, ultrafast recovery time and so on. After the first report on graphene mode-locked fiber lasers in2009, its applications in field of photoelectric have attracted wide attention of researchers worldwide.In this thesis, the operating characteristics of an all-normal-dispersion fiber laser mode-locked by nonlinear polarization evolution (NPE) were numerically studied. A wavelength tunable, passively mode-locked Yb-doped double-clad fiber laser with a birefringent filter was demonstrated. The pulse splitting behaviour of a dissipative soliton fiber laser was investigated. We also studied the mode-locking and Q-switching characteristics of the fiber lasers with graphene as saturable absorber. The main contents of this thesis are as follows: 1. Based on the coupled Ginzburg-Landau equations and Jones matrix, a numerical model of an all-normal-dispersion fiber laser mode-locked by NPE was proposed. The polarization states of different points across the pulse were calculated along the cavity. It was found that when the linear birefringence of the fibers was strong, the evolution of polarization state in the fiber had round a period of one beat length. Over one beat length, the state of polarization changed from right-handed elliptic to linear, left-handed elliptic, linear, and then back to right-handed elliptic. Different from a common saturable absorber, the modulation depth of the NPE equivalent saturable absorber varied with the wave plates angles. The dependence of modulation depth on wave plates angles was calculated. The results show that compared with the half wave plate and quarter wave plate before polarization beam splitter, the one after polarization beam splitter had a more obvious effect on modulation depth.2. The dependence of the stability domain on filter bandwidth, small signal gain coefficient and the orientation of wave plates was calculated. The proposed all-normal-dispersion fiber laser could deliver dome-shaped spectrum,"cat-ear" spectrum, splitting spectrum and parabolic self-similar pulse. Evolution of the intra-cavity pulse and spectrum was calculated under different states.3. A wavelength tunable, passively mode-locked Yb-doped double-clad fiber laser with a birefringent filter was demonstrated. By carefully rotating the filter, the central wavelength of the mode-locked fiber laser could be continuously tuned from1063.9nm to1091.1nm. The pulse duration ranged from13.9ps to16.8ps, and spectrum width varied between15.8nm and1091.1nm. The maximum average output power of212mW was obtained at the central wavelength of1063.9nm. The corresponding pulse energy was8.5nJ. A theoretical model was established based on the coupled Ginzburg-Landau equations and the transmission function of the filter. Simulation results agreed with the experimental results.4. Pulse splitting in all-normal-dispersion dissipative soliton fiber lasers was investigated. By appropriately setting the orientation of wave plates and pump power, output pulse from mode-locked fiber laser taking NPE rejection port as the output split into two parts. Both simulation and experiment results show, pulse splitting phenomenon described here was a result of intrinsic NPE-mode-locking mechanism. When the strength of NPE effect was strong enough, the central region of the pulse passed through the polarization beam splitter (PBS) with almost no loss. In this case, the rejected pulse from PBS consists almost entirely of the pulse wings, which could be regarded as two sub-pulses. As the pulse was chirped, the spectrum split into two parts simultaneously.5. The principle of dispersion compensation was analyzed, and the characteristics and limitations of grating pair were discussed. The chirped pulse from the all-normal-dispersion fiber laser was compressed with transmission grating pair. The pulse duration before and after compensation were6.73ps and161.5fs, respectively. The average power before and after compensation were2.36W and1.22W, with the compensation efficiency of51.7%. The peak power before and after compression were6.09kW and131.4kW, respectively.6. A passively graphene Q-switched Erbium doped fiber laser around1.5μm was demonstrated. The graphene saturable absorber was fabricated by sandwiching a thin graphene film produced via chemical vapor deposition between two FC fiber connectors. Stable pulse trains were obtained with the pulse repetition rate varying between34.72and53.2kHz, and the average output power ranging from0.504mW to0.926mW. The achieved shortest pulse duration and highest pulse energy were3.2μs and17.41nJ, respectively.7. A passively Q-switched tunable Yb-doped double-clad fiber laser was demonstrated with graphene epitaxially grown on SiC. The spectral tuning of the Q-switched fiber laser was implemented by rotating a quartz plate filter inside the cavity. The central wavelength of the fiber laser can be continuously tuned from1038.54nm to1056.22nm. The maximum pulse energy of0.65μJ was obtained at the pump power of4.08 W, and the corresponding pulse duration, average output power and peak power were1.60μs,35mW and406mW respectively.8. By transferring graphene film onto side-polished D-shaped fiber, a mode-locked Erbium-doped fiber laser based on the evanescent field interaction between the propagating laser light and graphene layer was established. The operating performance of the mode-locked fiber laser was investigated with different output couplers. Under the output coupling ratio of30%, the laser generated maximum pulse energy of8.34nJ at the repetition rate of8.4MHz with the pulse duration of13.8ns. The main innovations of this thesis are as follows:1. A numerical model of an all-normal-dispersion fiber laser mode-locked by NPE was proposed. The polarization states of different points across the pulse were calculated along the cavity. The dependence of the modulation depth of the NPE equivalent saturable absorber on wave plates angles was calculated for the first time. We also investigated the changing of the laser stability domain with the cavity parameters.2. A wavelength tunable, Yb-doped double-clad dissipative soliton fiber laser was demonstrated. The average output power and pulse energy were212mW and8.5nJ, respectively, which was the highest result in tunable dissipative soliton fiber laser to our knowledge. A theoretical model was established based on the coupled Ginzburg-Landau equations and the transmission function of the filter. Simulation results agreed with the experimental results.3. Pulse splitting in a dissipative soliton fiber laser taking NPE rejection port as the output was investigated for the first time. Pulse splitting was a result of intrinsic NPE-mode-locking mechanism. When the strength of NPE effect was strong enough, the central region of the pulse passes through the PBS with almost no loss, while the leading and trailing edges were rejected.4. A tunable Yb-doped double-clad fiber laser passively Q-switched by graphene epitaxially grown on SiC was demonstrated for the first time. The central wavelength of the fiber laser could be continuously tuned from1038.54nm to1056.22nm. The maximum pulse energy of0.65μJ was obtained at the pump power of4.08W, and the corresponding pulse duration, average output power and peak power were1.60μs,35mW and406mW respectively.