The Study Of Passively Mode Locked Ytterbium Doped Fiber Lasers And Wavelength Tunability

Ultrashort pulse lasers have the characteristics of ultrashort pulse width,high peak power,strong pulse energy and wide output spectrum.This kind of lasers has great influence in the fields of controlled nuclear fusion technology,nonlinear frequency conversion,super-resolution imaging technology,new materials research,biomedical research and etc.which plays a significant role in various research and application fields.Compared with other lasers,ytterbium doped fiber lasers have the advantages of low heat load,high quantum conversion efficiency,wide output spectrum,compact structure and low cost.It is a very suitable kind of lasers to realize ultrashort pulse,which makes the research of ultrashort pulse ytterbium doped fiber laser become one of important research directions among scientists.In recent years,with the rapid development of the preparation technology of low dimensional nano-materials,it has been found that this kind of materials has good saturable absorption characteristics,low cost and very wide spectral response range.Moreover,mode-locked fiber lasers based on low-dimensional nanomaterials have been concerned by researchers and widely studied.How to achieve narrower pulse has become one of the important research objectives.On the other hand,the wavelength tunable ultrashort pulse laser based on low dimensional nanomaterials is also one of the research hotspots.This kind of lasers is widely used in the field of biomedical detection and wavelength division multiplexing communication technology.In this dissertation,single-walled carbon nanotube saturable absorbers and nonlinear polarization evolution technology have been used to study the ultrashort pulse ytterbium doped fiber lasers and the wavelength tunability.Firstly,we built a picosecond ultrashort pulse ytterbium doped fiber laser by using the single-walled carbon nanotubes saturable absorber to start a study of using single-walled carbon nanotubes mode-locked ytterbium doped fiber lasers to generate femtosecond laser pulses.In order to achieve a narrower pulse output,the net dispersion of the cavity was managed by adjusting with diffraction grating which caused the realization of the femtosecond pulses output.Then,after using the combination of a grating and a fiber collimator as the tunable filter,a wide-tunable-spectrum output is achieved,and the output spectrum range can be precisely tuned.Finally,the nonlinear polarization evolution mode-locked ytterbium doped fiber laser is studied.In order to improve the compactness and flexibility of the fiber laser and reduce the cost,the cavity of the laser has been redesigned.A polarized beam splitter prism has been used to control the polarization state of the pulse in the cavity,and the output of the mode-locked pulse with very high efficiency and stability is realized.The specific research contents and innovations of this dissertation are as follows: Firstly,A ps-level YDF mode locked fiber laser has been built by using the single-walled carbon nanotubes saturable absorber film.Its central wavelength is 1019.9 nm,spectral bandwidth is 1.8 nm,and pulse width is 2.19 ps.On this basis,a pair of diffraction gratings are added into the cavity to manage the net dispersion,and a stable ultrashort pulse output has been achieved near zero dispersion.Its central wavelength is 1025.5 nm,spectral bandwidth is 37.2 nm,and pulse width is 379 fs.The pulse width of 175 fs is obtained by using a grating pair outside the cavity to compress the pulse.This provides a feasible scheme to realize femtosecond laser output in Yb-doped fiber laser based on the low dimensional nano materials saturable absorbers.Secondly,a wavelength tunable Yb-doped mode-locked fiber laser based on reflective grating is designed.The wavelength tuner based on reflection grating has the advantages of wide tuning spectrum,high tuning accuracy,fast response speed,low cost,and the mode-locked state does not change in the continuous tuning process.The output pulse width of this laser is 2.43 ps,the central wavelength is 1030 nm,and the spectral bandwidth is 1.6 nm.By adjusting the angle of the reflection grating,the continuously tunable mode-locked laser output in the range of 1005 nm-1060 nm can be realized.The laser supports precise tuning with a minimum step size of 0.01 nm.In the whole process of continuous tuning,the spectral bandwidth and pulse width of laser output will not change with the shift of central wavelength,and the stability of laser output is very good.Thirdly,the polarization state of the pulse in the cavity is controlled by a single polarized beam splitter,and the stable nonlinear polarization evolution ytterbium doped mode-locked fiber laser output is realized.Firstly,the standard nonlinear polarization evolution technology is used to realize the ultrashort pulse ytterbium doped fiber laser output.On this basis,the ytterbium doped mode-locked fiber laser is realized by using a single polarized beam splitter.The center wavelength of the output is 1036.4 nm,the full width half maximum is 8.5 nm,the pulse width is 4.9 ps,and the laser slope efficiency is more than 50%.