| With the maturity of special optical fibers and double-clad high gain fibers, passively mode-locked fiber laser has found wide applications in femtosecond laser source, fiber communication and sensing, biomedical imaging, laser gyroscope, optical microwave, and supercontinuum et al, due to the advantages of free alignment, excellent heat radiation, and compactness. Various nonlinear nanomaterials, such as carbon nanotube, graphene, topological insulator, have been proposed to modulate periodically the transmission of fiber resonator, to generate stable laser pulses when the phases of various orders of longitudinal modes are locked. However, limited by the accumulated heat effect, the present saturable absorbers suffer the drawbacks of low damage threshold, and the intense evanescent field interaction with the materials enhances the effective nonlinearity, leading to harmonic mode-locking. Those disadvantages restrict the operation of stable mode-locked laser with high power and high energy. Besides, up to now, the tunability of mode-locked fiber laser is mainly based on mechanics, through which the fundamental laser characters, such as pulse width and center wavelength, are tuned. The controllable parameters of the laser are rare, and the tunability methods are simple.As a coherent optical source with mutual longitudinal modes, more intentions on the tunability of the mode-locking characteristics, polarization characteristics, and coherence of such a mode-locked laser shall be paid due to the important applications in all optical signal processing, and special quantum source. In this thesis, with aim to realize ultrafast fiber laser with high power, high energy, and tunability, we analyze the limitations of the present passively mode-locked fiber laser, and propose two methods to fabricate saturable absorbers based on fiber taper and photonic crystal fiber, respectively. On this basis, we propose three ultrafast fiber laser systems with high power, and two mode-locked fiber laser systems with tunability of polarization states and wavelength spacing based on pump power variation. Finally, we propose and analyze a partially mode-locked fiber laser system, and the all-optical tunability of such a laser has been carried out based on optical analogy of electromagnetically induced transparency of graphene.The core contents of this thesis include:(1) We have analyzed the main problems in passively mode-locked fiber laser, namely, the low average power, rare controllable parameters and few tunability methods, and introduced two methods to fabricate saturable absorber. We have deposited graphene nanoparticles onto the fiber taper based on light pressure effect, and produced saturable absorber with high nonlinearity. Then, a saturable absorber with high stability and high damage threshold has been fabricated based on filling the cladding holes of photonic crystal fiber with graphene solution. The nonlinear responses of the two kinds of saturable absorbers have been tested with a home-made balanced two-detector system.(2) We have proposed three kinds of ultrafast fiber laser systems based on saturable absorber made from photonic crystal fiber. Conventional soliton laser with pulse width of 908 fs has been obtained in fiber laser system with anomalous dispersion, and a pulse width of 851 fs was obtained based on harmonic mode-locking. Dissipative soliton laser with an average power of 75 mW was obtained in laser system with normal dispersion, and the pulse width was compressed from 7.564 ps to 245 fs with anomalous dispersion of single mode fiber. Together with high efficient Er-Yb-codoped double-clad fiber and high damage threshold of the saturable absorber, we have obtained ultrafast fiber laser with average power of 1.14 W.(3) We have proposed two kinds of high coherent mode-locked fiber laser systems with tunabiliy of laser parameters through pump power, based on the saturable absorption and high nonlinearity. Vector dual-wavelength rectangular fiber laser was obtained in anomalous dispersion cavity, and polarization analysis has shown that periodically polarization switching existing in the two orthogonal polarization axis, and a nonlinear relation with respect to the pump power was obtained. Then, we have observed the bound states of Q-switched mode-locked laser through dispersion management and decreasing the nonlinearity of the saturable absorber. We have found that the bound state was ultrasensitive to the pump strength that the wavelength spacing varied from 0 to 0.15 nm for a pump power variation less than 10 mW.(4) We have proposed a partially mode-locked fiber laser system, and the evolution process under polarization tuning and pump tuning were analyzed. Primarily, the parametric instability of fiber ring cavity with periodic dispersion was analyzed theoretically. Secondly, we have built the partially mode-locked fiber laser system, and the single-shot spectrum was obtained through disperse Fourier transformation technique. In together with the average optical spectrum and temporal pulses train, the evolution of the partially mode-locked fiber laser was demonstrated. Finally, the evolution from high coherence to low coherence in forming partially mode-locked laser was proved through the analysis of autocorrelation, filtering, and polarization variation. Those results provide more information for analyzing the dynamics of non-soliton laser.(5) We have proposed the tunability on partially mode-locked fiber laser based on all-optical modulation of coherent population oscillation. Primarily, we have introduced the optical analogy of electromagnetically induced transparency in graphene, and analyzed the dispersion and absorption characteristics of graphene when illuminated by external laser with mutual wave-mixing. Then, we have demonstrated the optical analogy of electromagnetically induced transparency of graphene, based on experimental results about tunability of polarization, wavelength, and power of the external laser on the parametric instability gain sidebands. Finally, we have performed all-optical modulation of the partially mode-locked fiber laser with external laser, and found that significant depression of the mode-locking required much higher power, while the requirement of the polarization state of the external laser was comparatively relaxed.The main innovations of the thesis are as follow:(1) We have proposed three kinds of passively mode-locked fiber laser systems based on saturable absorber made from photonic crystal fiber. Ultrafast lasers with high power and high energy were obtained based on the dissipative soliton mode-locking, double-clad fiber pumped with high power, and high damage threshold due to ultraweak evanescent of photonic crystal fiber.(2) We have proposed a dual wavelength rectangular fiber laser based on saturable absorber fabricated by fiber taper. Due to the saturable absorption and high nonlinearity of such a device, we have obtained a rectangular laser pulses with tunability of polarization switching through pump power.(3) We have introduced the concept of bound state of Q-switched mode-locking, and demonstrated it with experiments. Multiwavelength laser was produced in such a state, and its wavelength spacing was ultrasensitive to the pump power. The wavelength spacing varied from 0 to 0.15 nm for a pump power variation less than 10 mW.(4) We have proposed a kind of low coherent partially mode-locked laser with random intensity, wavelength, and polarization based on cascaded vector four-wave-mixing among parametric instability gain sidebands, and the evolution of such a laser under polarization detuning and pump detuning was observed. A full analysis of autocorrelation of the single-shot spectrum, filtering, and polarization evolution exhibited the coherence loss in forming partially mode-locked laser, providing new insights for analyzing the dynamics of non-soliton laser.(5) We have proposed the optical analogy of electromagnetically induced transparency in graphene, namely the coherent population oscillation, and built the fiber ring cavity with external laser injection. The optical analogy of electromagnetically induced transparency in graphene was demonstrated based on experiments of tunability of the polarization, wavelength, and power of the external laser on the parametric instability gain sidebands. The coherent population oscillation of graphene was utilized to realize the all-optical modulation of partially mode-locked fiber laser. |
PDF Full Text Request