| In recent years,high power passive mode-locked fiber lasers with advantages of simplicity,reliability,low cost,high stability,have become more widely used in industrial processing.One of the ways to generate a high power ultrafast fiber laser is to use a stable mode-locked pulsed fiber laser as a seed source,combined with a chirped pulse amplification method.In this thesis,the characteristics of the Raman-assisted mode-locked fiber lasers is investigated,and slight changes of this laser is used as a seed source for high power ultrafast fiber laser systems.Finally,an all-fiber high-power ultrafast laser system based on master-oscillator power amplifier(MOPA)structure is experimentally obtained.The major results in the thesis are obtained as follows.1.Based on the method proposed by Liangjia Zong et al.,the D value of the dispersion parameter at 1550 nm is approximately 17.23 ps/(nm · km),which is consistent with the actual value.The dispersion slope dD(λ)/dλ is 0.0591ps/(nm2*km)and the error is 5.72%,which is acceptable.The dispersion value of the light with a wavelength of 1.0 μm propagated in the SMF28e is also measured.2.The Raman-assisted mode-locked fiber laser is experimentally studied.A 7.5m highly nonlinear fiber is introduced into the ytterbium-doped NPR mode-locked laser cavity.The stable broad-spectrum mode-locked pulses are obtained in the presence of Raman at a cavity length of 10.5 m.From the experimental results,the spectrum has a distinct double-peak structure,the central wavelength of the main peak and the Raman peak are 1040.16 nm and 1086.31 nm,respectively.The wavelength interval of the peak is 46.15 nm.The 20 dB bandwidth of the spectrum exceeds 64.04 nm.The signal-to-noise ratio is as high as 77 dB.It is proved by the extra-cavity filtering results that the Raman part is not ’noise’ and participates in the nonlinear polarization rotation(NPR)mode-locked operation.The Raman gain in the cavity is equivalent to expand the mode-locked spectral range and the Yb gain bandwidth.3.Based on MOPA structure,the all-fiber high-power ultrafast laser system is experimentally studied.The slight changes are made in the laser mentioned in Chapter 3,the Raman effect is limited by shortening the cavity length.The repetition frequency of this laser is 27.55 MHz,the 3 dB bandwidth of the spectrum is approximately 20.56 nm,the pulse width is 16.993 ps,and the time bandwidth product is 93.28.Using this laser as a seed source for a high-power ultrafast laser system,a laser output with an average power of 40 W has been successfully obtained.The initial pulse width of the seed source is 16.993 ps,it is broadened by long fiber to 1.13 ns and then compressed by the grating pair to 26.611 ps.The compression ratio reaches 42.5 times.The mental and glass materials can be burned by this laser system,and under the microscope,the clear signs of burning can be observed on the surface of the materials.The main innovations work and results of this thesis are as follows.1.By introducing 7.5 m highly nonlinear fiber in a 10.5 m ytterbium-doped NPR cavity,a stable broad-spectrum mode-locked fiber laser in the presence of Raman is obtained.It is also demonstrated that the Raman is not ’noise’ and participates in the NPR mode-locked operation.The Raman gain in the cavity is equivalent to expand the mode-locked spectral range and the Yb gain bandwidth.2.Byy shortening the cavity length to limit the Raman effect,a stable ultrashort mode-locked pulse in the absence of Raman is obtained in an approximately 7.3 m ytterbium-doped NPR mode-locked fiber laser.Using this ultrshort pulse as seed source for a high-power ultrafast laser system based on MOPA structure,the laser output with an average power of 40 W is obtained.The 16.993 ps pulse width of seed source is broadened to 1.13 ns by a long fiber and then compressed to 26.611 ps by a grating pair.The compression ratio is 42.5 times.The system is in a linked state. |
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