Research On Yb-Doped Mode-Locked Fiber Laser Based On The Graded-Index Multimode Fiber As Saturable Absorber

Graded-index multimode fiber(GIMF)has the characteristics of small mode dispersion and short self-focusing position,exhibiting great potential in applications of linear and nonlinear fields.In the field of nonlinear,GIMF-based saturable absorber(SA)demonstrates the advantages of high damage threshold,simple fabrication,and the operating wavelength is tunable,which makes up for the inadequacy of the traditional SA in mode-locked fiber lasers and promotes the development of all-fiber mode-locked lasers.In this thesis,a series of studies mainly focused on Yb-doped fiber lasers based on GIMF-SA are carried out:1.An all-fiber mode-locked Yb-doped laser based on GIMF-SA with ring-shaped cavity is constructed.Benefiting from the intracavity birefringence-induced filtering effect and the filtering effect introduced by multimode interference of the structure of SMF-MMF-SMF(SMS),the output of wavelength-tunable DS and dual-wavelength DSs are implemented.Firstly,the characteristic of periodical bandpass filtering of the structure of SMS is theoretically analyzed.Secondly,the mode-locking operation is realized by employing the GIMF-based SA.By adjusting the polarization controller in the cavity to optimize the intracavity birefringence-induced filtering effect,tunable singlewavelength DS and switchable dual-wavelength DSs are obtained.The tuning range of single-wavelength DS is about 46 nm,and the central wavelengths of dualwavelength DSs can be switched from 1032.3/1054.1 nm to 1049.3/1072.6 nm.Futhermore,the spectral filtering of the structure of SMS is enhanced by adding a short piece of no core fiber(NCF)between SMF and GIMF.By stretching this hybrid element,switchable noise-like pulses(NLPs)and tunable single-wavelength and dualwavelength DSs are realized.Among them,the tuning range of single-wavelength DS is about 54 nm and the tuning range of dual-wavelenth asynchronous DSs with a spectral interval of about 22 nm is about 13 nm.The experimental results provide a new perspective for the study of wavelength-tunable and multi-wavelength modelocked fiber lasers.2.An all-fiber mode-locked Yb-doped laser based on GIMF-SA with linear cavity is constructed.Benefiting from the intracavity stimulated Raman scattering(SRS)effect,the output of the fundamental frequency-Raman composite soliton is implemented.Firstly,the output of the “fundamental frequency + Raman” dual-wavelength NLPs is obtained in a linear cavity composed of two reflection mirrors and shows high coherence bewteen the fundamental frequency pulse and the Raman pulse.To further study the coherence properties of the fundamental frequency-Raman composite soliton,one end of the linear cavity is replaced by an optical loop mirror,which excites stronger intracavity nonlinear effects and finally achieves the output of DS or the“fundamental frequency + Raman” multi-wavelength NLPs.For the DS,the pulse width is about 74.2 ps and the pulse energy is about 21.2 n J.For multi-wavelength NLPs,the pulse energy is about 29.5 n J.The autocorrelation trace of the composite soliton shows the characteristic of quasi-periodic beating coherent peak,confirming the coherence between the fundamental frequency pulse and the Raman pulse.The experimental results deepen the understanding of Raman pulses and have reference value for understanding the nonlinear dynamic process and pulse evolution behavior in the cavity of the mode-locked fiber laser.