Study On All-fiber Mid-infrared Ultrafast Raman Soliton Laser

Ultrashort pulse lasers in the mid-infrared(MIR,2-20 ?m)region have attracted increasing attention due to various applications in gas sensing,chemical detection,spectroscopy,military and medical surgery.In particular,MIR fiber-based ultrafast lasers possess inherent advantages,compared with the conventional solid laser,such as compact structure,good beam quality,and environmental reliability.Mode-locking of rare-earth-doped fiber laser is generally recognized as an effective technique for high-quality ultrashort pulse generation,but they have been limited to a few wavelengths in the MIR,that is?2 ?m,?2.8?m,?3.1 ?m and 3.5 ?m.It should be especially noted that there exists a spectral gap(2.1-2.7 ?m)between?2 ?m mode-locked Tm-doped fiber lasers and?2.8 ?m mode-locked Er-or Ho-doped fiber lasers.Soliton self-frequency shift(SSFS)is an effective way for extending the wavelength of ultrashort laser to MIR by using specialty optical fibers,such as chalcogenide fibers,fluoride fibers,and germania-core fiber.Therein,germania-core fiber can be fusion-spliced with standard silica fiber with low loss and a higher optical nonlinearity.Therefore,it is desirable to perfectly fill the whole 2.1-2.7 ?m spectral gap by compact all-fiber Raman soliton sources with SSFS in germania-core fiber.In view of the above mentioned and equipment conditions,this thesis mainly aims at the research topic of the ultrafast tunable germanium-doped Raman fiber laser.The major contents and accomplishments are listed as follows:?)Solving the nonlinear Schrodinger equation numerically,and we build the SSFS simulation model with MATLAB to explore the influence of Raman fiber length and peak power of the input pulse on the effects of SSFS.The simulation indicate that the higher the peak power of the input pulse is,the greater the frequency shift of the Raman soliton is when the fiber length and other factors are constant without considering the loss of Raman fiber.When other factors such as the peak power of the input pulse remain unchanged,the longer the Raman fiber is,the greater the frequency shift of Raman solitons will be.?)With reference to the conclusions of theoretical simulation,we designed a homemade 1.96 ?m ultrafast pump sources and a cascaded Raman structure to generate 2.1-2.7 ?m tunable femtosecond Raman solitons from a compact all-fiber system.First of all,the seed signal was amplified in a thulium-doped double-clad fiber amplifier where the first-stage Raman SSFS generate femtosecond pulses in the tunable wavelength range of 2.036-2.180 ?m.The first-stage Raman solitons further pumped a 94 mol.%germania-core fiber to generate the second-stage Raman SSFS.The second-stage Raman solitons could be continuously tunable from 2.157 to 2.690?m.According to the experimental parameters,a simulation comparison is performed,and the simulation data is in agreement with the experimental one.?)A method proposed by our research group to measure the width of ultrafast pulse has realized the width measurement of MIR pulses.Based on the interference principle of light,the pulse to be measured is divided into two parts.The data is recorded by combining a photodetector and a data processin g system,and then the mid-infrared ultrashort pulse width is obtained by hyperbolic secant function fitting.A commercial autocorrelator is used to measure the same 1.06 ?m picosecond pulse for comparison,and the operability and accuracy of the measurement method are analyzed.The measurement method and the equipment are simple,compact with good environmental stability,and the wavelength and pulse width of the pulse laser to be measured can be varied within a wide range.