Wavelength Tuning And Extension Of Mode-locked Fiber Laser In 3 ?m Waveband

Wavelength-tunable mode-locked fiber lasers in the 3?m waveband have been widely used in gas detection,spectroscopy,biomedicine and other fields,and its wavelength-tunable capability makes it more sensitive and convenient in applications.The wavelength-tunable method based on the soliton self-frequency shift effect has many advantages,such as high beam quality,easy all-optical fiber,low cost and wavelength-tunable range on the micron scale,so it has become the mainstream method for wavelength-tunable mode-locked pulse in 3?m band.At present,there are a lot of reports about the soliton self-frequency shift effect,but there are still some problems to be improved,such as high threshold to frequency shift and poor pulse performance.Therefore,in this thesis,the generation of ultra-short mode-locked fiber laser and the soliton self-frequency shift in fluorotellurate fiber are studied,and a wavelength-tunable mode-locked laser at 3?m is established.The main research contents are as follows:(1)The numerical model of mode-locked laser was established based on coupled Ginzburg-Landau equation,and the parameters of mode-locked laser were optimized.The soliton self-frequency shift phenomenon is simulated based on the generalized nonlinear Schrodinger equation,and the conclusion is that the shorter pulse duration,the higher peak power and the longer fiber length are beneficial to the soliton to expand to the longer wavelengths.(2)According to the theoretical guidance,a mode-locked fiber laser with a wavelength-tunable range of 2.8 to 3.2?m based on the soliton self-frequency shift effect of fluotellurate fiber was constructed.Firstly,the 2796.0 nm ultra-short pulse seed source with an average power of 60 m W and a spectral width of 19.6 nm was constructed based on the nonlinear polarization rotation effect.Then,the power of the seed source is amplified by the active Er³⁺ZBLAN fiber,and the maximum power can be amplified to773 m W.In the process of amplification,the wavelength of the soliton shifted to 2910nm,and the corresponding spectral width was stretched to 88.9 nm.Finally,the amplified seed source is coupled into the fluorotellurite fiber.Based on the soliton self-frequency shift effect in the fluorotellurite fiber,the mode-locked laser with a wavelength-tunable range of 2796.0 nm to 3165.6 nm is realized.The overall Raman conversion efficiency is28.2%.When the wavelength of soliton is shifted to 3165.6 nm,the average power and energy are 38.9 m W and 0.744 n J,respectively.The preliminary experimental results confirm the potential of fluotellurate fiber with high nonlinear coefficient to be used in wavelength-tunable mode-locked fiber lasers in mid-infrared region.(3)Experimental study on the wavelength-tunable passively mode-locked fiber laser based on Ti₃C₂T_x saturable absorber.Using Ti₃C₂T_x as saturable absorber,a mode-locked laser with a wavelength-tunable range of 2709.0 nm-2827.6 nm was generated by introducing diffraction grating,and the maximum output power of 89.3 m W was obtained at 2786 nm.The results of the experiment indicate that Ti₃C₂T_x has a wide band saturable absorption capacity at 3?m band and has a good application prospect.