| Mode-locked fiber lasers eimtting ultrafast pulses have received tremendous attention owing to their potential applications in widespread fields, such as fiber-optic telecommunications, biomedical research, and spectroscopy. Recently, the emerging nanomaterials have revolutionized the field of mode-locked fiber laser because of their unprecedented advantages including ultra-short recovery time, suitable modulation depth and broad operation range. Consequently, widely tunable mode-locked fiber lasers can be achieved by using one saturable absorber. To realize wideband wavelength tuning, it is of much interest to incorporate fiber gratings in these lasers because of their advantages such as all-fiber structure, low insertion loss and potentially low cost.In this dissertation, we have investigated two kinds of fiber gratings (i.e. long-period grating (LPG) and chirped fiber Bragg grating (CFBG)) and carbon nanotube (CNT) absorbers to develop valuable tunable mode-locked fiber lasers. The performance of optical solitons has been experimentally and theoretically analyzed. This thesis includes the following aspects:First of all, the preparation method of the CNT absorber film has been discribed at length. Compact CNT mode lockers have been realized by inserting the absorber film between two fiber connectors, and their nonlinear saturable absorption properties have been measured.Additionally, LPG has been employed for a widely tunable mode-locked fiber laser. The fabrication technique and platform of LPG have been discussed detailedly. A W-shaped spectral filter has been proposed by sandwiching a phase-shifted LPG (PSLPG) between two LPGs of different periods. By adjusting the temperature of the W-shaped filter, the central wavelength of the mode-locked laser can be tuned in a range covering C and L bands. Because of its wide pass-band, the proposed technique of using LPG enables shorter pulses free of extra grating dispersion. Moreover, the spectral shaping effect of the PSLPG has been investigated to suppress the Kelly sidebands effectively.Apart from thermally adjusting the LPG, an optofluidic tunable mode-locked fiber laser has been demonstrated based on the refractive index (RI) sensitivity of LPG for the first time, to the best of our knowledge. Ultrafine wavelength tuning has been enabled by integrating a PSLPG into a home-made microfluidic chip. By accurately adjusting the fluid RI, the central wavelength of the mode-locked laser can be tuned continuously. Moreover, stable bound solitons with different pulse separations have been observed.Later, a high-energy soliton fiber laser has been demonstrated by using a CFBG pair. The inscription technique of CFBG has been dicussed. The CFBG pair with overall group velocity dispersion (GVD) of ～5 ps/nm/km has been used to increase the net anomalous dispersion. Stable high-energy mode-locked solitons have been obtained without soliton splitting, which own higher pulse energy than the limit of the soliton area theorem. Numerical simulations on the in-cavity soliton propagation have been conducted by using the split-step Fourier method (SSFM), revealing that the soliton splitting threshold can be improved by the CFBGs.Moreover, tunable vector and scalar solitons have been demonstrated based on CFBG in a non-polarization-maintaining (PM) and PM fiber laser cavities, respectively. To construct an all-fiber tunable filter, the CFBG has been firmly mounted on the central line of a cantilever. In the non-PM fiber laser, tunable vector solitons L band have been obtained by adjusting the cantilever, while the polarization-locked state can be maintained. Numerical simulations have been also conducted, revealing that the FWM sidebands are correlated with the cavity birefringence. While in the PM fiber oscillator, environmentally stable scalar solitons have been obtained, and the laser wavelength can be tuned continuously in the C band.
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