| Pulsed fiber lasers have prominent advantages including low laser threshold,good output beam quality,high conversion efficiency,good heat dissipation effect and so on.They are widely used in the fields such as optical communication,optical fiber sensing,material processing,biomedicine,optical metrology,etc.By virtue of the saturation absorption effect of saturable absorbers(SA).Q-switching,mode-locking and harmonic mode-locking(HML)can be achieved,while multi-wavelength pulses can be generated based on their high non-linearity.Therefore,the application of saturated absorber in fiber laser is an effective way to generate multi-wavelength pulses,and this research hotspot has attracted widespread attention.Recently,various two-dimensional materials including carbon nanotubes,graphene,gold nanoparticles,topological insulators,layered metal dichalcogenides and black phosphorus have been employed as saturable absorbers to generate Q-switched.mode-locked and harmonic mode-locked pulses in fiber laser.Gold nanoparticles have a wide absorption band because the local surface plasmon resonance peaks can range from visible light to near infrared region by adjusting the size,shape and internal structure of molecules.The most significant characteristic of layered metal dichalcogenides is that monolayer has direct band gap,while the few-layer or bulk structures have indirect band gap,thus band gap can be adjusted by changing the layer number.Therefore,gold nanoparticles and transition metal dichalcogenides have been widely studied and applied recently.However,there is a lack of research on multi-wavelength and harmonic mode-locked fiber laser based on these saturable absorbers.In this thesis,we demonstrated a dual-wavelength passively Q-switched fiber laser based on gold nanocage(GNCs)saturable absorbers,and the generations of dual-wavelength mode-locking,dual-wavelength low-order HML and single-wavelength high-order HML in fiber lasers using tin diselenide(SnSe2)as saturable absorbers.In addition,the formation mechanism and evolution law of these laser operations have been carefully analyzed.The research work of this thesis mainly has the following aspects:1.The GNCs were successfully prepared by the seed-mediated method and integrated into a fiber-compatible saturable absorber with polyvinyl alcohol(PVA)matrix.A dual-wavelength passively Q-switched Yb-doped fiber laser was achieved for the first time,based on the GNCs-PVA saturable absorber with the modulation depth of 5.3%and the saturation intensity of 0.16 MW/cm2 measured by open Z-scan technique.The formation mechanism of dual-wavelength Q-switched pulses were analyzed and discussed.and the dual peak wavelengths were measured as 1059.9 nm and 1060.5 nm.respectively.A maximum average output power of 6.03 mW with minimum pulse width of 2.06 ?s and maximum repetition rate of 134.9 kHz is obtained at a pump power of 385 mW,corresponding to optical-to-optical conversion efficiency of 1.57%and slope efficiency of 2.75%.2.Tin diselenide was successfully prepared with liquid phase exfoliation method and embedded into PVA as a saturable absorber in a dual-wavelength passively mode-locked Yb-doped fiber laser.When pump power ranged from 54.5 to 241.7 mW,by properly adjusting polarization controllers,four kinds of dual-wavelength mode-locked pulses with peak wavelength intervals of 0.4,1.0.1.5 and 1.9 nm were obtained at fundamental frequency of 333 kHz.3.SnSe2-PVA saturable absorber was used as both a mode locker and a filter to demonstrate a dual-wavelength low-order harmonic mode-locked fiber laser.By properly adjusting polarization state in the fiber cavity,the low-order harmonics from the 2nd order to the 23rd order were observed.The mechanism of harmonic Generation,mode competition and some harmonic missing phenomenon were carefully discussed.4.By virtue of both the saturation absorption effect and high nonlinearity of SnSe2-PVA saturable absorbers,a single-wavelength high-order harmonic mode-locked Yb-doped fiber laser was achieved.By properly adjusting polarization state in the fiber cavity,four kinds of high-order HML with frequencies of 400,460.520 and 550 MHz were achieved at fundamental frequency of 333 kHz,corresponding to 1200th,1380th,1560th and 1650th order harmonics.For each kind of harmonics.their fine frequencies were analyzed and the phenomenon of fine spectrum peak clustering in the spectrum diagram was observed.Taking the slope efficiency as the entry point,the same values of these four kinds of harmonics revealed the whole consistency of the high-order harmonics in the whole pump power range. |
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