Research On Ultra-long Cavity Mode-locked YB-Doped Fiber Laser

The ultra-long resonant cavity of the mode-locked fiber laser can be simply obtained by extending the length of optical fiber. With the increasing of the cavity length, the repetition-rate can be effectively reduced,as well as the pulse energy rapidly improved. The pulse laser with low-repetition-rate and high-energy can reduce the heat parasitism of the interaction object, which has been extensively employed in eye surgery, biomedical diagnosis, micro-machining and so on.Nowadays, the research on the ultra-long cavity mode-locked fiber laser based on a nonlinear optical loop mirror, nonlinear polarization rotation effect, semiconductor saturable absorption mirror and single-walled carbon nanotubes saturable absorber has made some progress. The ultra-long cavity mode-locked fiber laser based on nonlinear polarization rotation effect is the hot topic of research. While this type of fiber laser is strict with polarization and easy to lose the characteristic of saturable absorption with high-power pulse. Single-walled carbon nanotubes saturable absorber is a novel mode-locking nano-material in recent years, including the direct interaction and evanescent field interaction mode-locking technology. The direct interaction mode-locking technology has been used in ultra-long cavity mode-locked fiber laser, but its low damage threshold restricts the achievement of high-energy pulse. By contrast, the evanescent field interaction mode-locking technology is more remarkable superiority due to carbon nanotubes don’t insert the main optical path. Not only can it avoid the damage of high-energy laser, but also increase the cooling area. It is an optimization scheme to improve damage threshold, keep all-fiber structure and achieve stabilized high-energy pulse.In this paper, the low-repetition-rate, high-energy pulse,ultra-long cavity mode-locked fiber laser has been realized by using single-walled carbon nanotubes saturable absorber and nonlinear polarization rotation structure. It would be suitable for practical applications in supercontinuum generation, probe-detecting, biomedical diagnostics and a seed of all-fiber amplification system. The main contents are as following:(1) We introduced the classification of ultra-long cavity mode-locked fiber laser, summarized and analyzed its research status.(2) We set up the pulse transmission theoretical model by using the nonlinear schr?dinger equation, simulated the impact of the group velocity dispersion, third-order dispersion and self-phase modulation on the optical pulse evolution. We further took the Haus master equation to analyze the influence of each parameter of the cavity on the chirp, width, phase shift, stability of the optical pulse.(3) We fabricated the D-shaped fiber by using the femtosecond laser-engraving technique, compounded the dispersed water solution of single-wall carbon nanotubes, manufactured the evanescent field interaction saturable absorber by depositing the well-dispersed carbon nanotubes on the D-shaped fiber. Its characteristic of saturable absorption was tested and the mechanism of the evanescent field interaction with carbon nanotubes was further analyzed.(4) We established the ultra-long cavity carbon nanotube mode-locked Yb-doped fiber laser. With the different output coupler and the different cavity length, the pulse characteristics of the mode-locked fiber laser were respectively researched and the time-domain features were analyzed, such as noise-like soliton, the second-order harmonic noise-like solition, soliton rain, higher harmonic soliton rain, rectangular soliton. The cause of generating multi-wavelength was further analyzed by designing the spectral-filtering detection device.(5) We built the ultra-long cavity nonlinear polarization rotation Yb-doped mode-locked fiber laser, discussed the characteristics of the single-soliton and multiple-soliton. Finally, the saturable absorption of these two mode-locking mechanism was compared and the different pulse behaviour of the ultra-long cavity mode-locked fiber laser with these two mode-locking mechanism was further analyzed.