Study On The Generation Of Cylindrical Vector Beams From Ytterbium-doped Fiber Lasers

Cylindrical vector beam(CVB)has an axisymmetric(or rotational symmetry)polarization mode distributed across the beam cross section,including radial,azimuthal and hybrid polarization,which has doughnut-shape intensity distribution.CVB has attracted significant attention recently,due to its unique high numerical aperture focusing and diverse applications,such as surface plasmon resonance,optical trapping,optical sensing,and material processing and so on.A series of methods to generate CVB have been proposed because of its application value.Compared with the method of generating CVB by space light modulation,the all-fiber method to generate CVB has the advantages of compact structure,high flexibility and high efficiency.In this thesis,few-mode fiber grating and mode selection coupler have been utilized to generate radially and azimuthally polarized light from CW,Q-switched,and mode-locked ytterbium-doped fiber lasers,respectively.The main work and results are as follows:1.Continuous-wave(CW)cylindrical vector beam ytterbium-doped fiber laser based on few-mode fiber Bragg grating(FM-FBG).The characteristics of the FM-FBG are firstly investigated with coupled mode theory.Then,a linear-cavity ytterbium-doped fiber laser based on FM-FBG for higher order mode selection has been proposed and demonstrated,where splicing technique with lateral misalignment is used to excite higher order mode.The CW cylindrical vector beam operated at the wavelength of 1053.95 nm,with signal-to-background ratio of more than 50 dB,maximum output power reaching 75 mW,and purity of >95.5% has been obtained.2.Q-switched cylindrical vector beam ytterbium-doped fiber laser based on few-mode fiber Bragg grating.An all-fiber Q-switched CVB fiber laser based on acoustic optical modulator(AOM)for Q-switching,and FM-FBG for higher order mode selection has been proposed and demonstrated.With the aim to obtain a stable pulse train,the factors which would lead to pulse splitting and multi-pulse generation,such as pump power,modulation frequency and rising time of the AOM,have been investigated.The Q-switched CVB with repetition rate in the range of 10~60 kHz and pulse width less than 180 ns have been obtained.By adjusting the polarization controllers,the polarization purity of both radially and azimuthally polarized beams is more than 94.5%.3.Mode-locked cylindrical vector beam ytterbium-doped fiber laser based on mode selection coupler.A special mode selection coupler was firstly made by using the method of weak coupling to obtain the output of higher order mode directly with the principle of phase matching.Then,a mode-locked all-fiber laser with high-purity CVB generation,using a polarization-dependent isolator(PD-ISO)to generate mode-locked pulse train and a mode-selective coupler(MSC)as transverse mode converter and splitter,has been experimentally proposed and investigated.When the pump power is 371.2~630 mW,the pulse width is 453~733 ps,and the pulse energy is 0.246~0.439 nJ.The peak power of the pulse has a maximum of 0.6 W.The purity of the cylindrical vector beam is measured to be >94.5%.