Cylindrical Vector Beam Fiber Laser And Application Research

Cylindrical-vector beam have a symmetric polarization and a doughnut shape intensity distribution,which are different from traditional Gauss beam.Because of the unique properties under high numerical aperture focusing,CVBs have attracted significant attention recently and have many potential applications,such as laser machining,surface plasmon resonance optical trapping and so on.Many methods of generating of CVBs have been reported,including active methods and passive methods in fiber or space.LP11 mode is the second-order scalar approximate mode in fiber,which consist four engine CVB modes-TE01?TM01?HE21even and HE21odd mode.TE01 mode and TM01 mode are the azimuthally and radially modes,respectively.LP11 mode can be generated by long period Bragg gratings or offset coupling in fiber.But it is difficult to obtain high purity second-order mode,so that a few mode fiber Bragg grating(FMFBG)is utilized to filter out purity LP11 mode and reflect the fundamental-mode back to the cavity,and two polarizations is used to get single CVB mode.In this dissertation,we introduced the mathematical expression,optical properties and the generation methods of CVBs.Then we discussed the mode coupling theory of single mode fiber Bragg grating(SMFBG)and FMFBG.Based on FMFBG and offset coupling,we demonstrated all-fiber Q-switched CVB laser and flat-top flattop laser.The main work and research results of this dissertation are as follows:1.We fabricating the composite film of polyvinyl alcohol(PVA)with topological insulator(TI)Bi2Te3 and carbon nanotubes(CNT),respectively.Based on that as saturable absorber(SA)and FMFBG as mode selector,we demonstrated an all-fiber Q-switched CVB laser,and obtained pure azimuthally and radially beam.The repetition rate of the laser based on Bi2Te3 increases from 31.54 kHz to 49.4 kHz.The corresponding pulse duration decreases from 5.15?S to 3.71 ?S,and the single pulse energy increases from 102 nJ to 124 nJ.The repetition rate of the laser based on CNT increases from 30.88 kHz to 92.75 kHz.The corresponding pulse duration decreases from 3.7?S to 1.2?S,and the max pulse energy is 90.14 nJ.2.Based on a FMFBG and two SMFBGs,we established a double cavity fiber laser.One of the SMFBGs corresponds to the fundamental(LP01)to the fundamental mode reflection peak of the FMFBG,while the other one corresponds to the second-order mode to the second-order mode reflection peak of the FMFBG Thus the laser can output both fundamental mode and second-order mode beam.By tuning the pump of two cavity,we can eventually acquire high quality flat-top beam.The normalized Root-Mean-Square of the flat-top beam is 0.048 and the edge steepness is 0.872.3.We designed a ring core fiber with metal film coated out of the cladding.Due to the surface plasmon effect between light and metal,the loss of each polarization mode light is different,so that mode selection of fiber is realized.After calculation and simulation,a single TE01 mode of low loss output can be achieved in a short fiber,while all other modes are depleted.The innovation of this dissertation are as follows:1.Using two kinds of saturable absorbers with high damage threshold and polarization insensitivity,the Q-switched laser is obtained respectively.And combined with offset fusion and FMFBG,the Q-switched output of high purity azimuthally polarized light and radial polarized light is achieved in all fiber.2.A double cavity laser is established by using one FMFBG and two SMFBG.The fundamental mode light and CV beams can be obtained simultaneously.The power of the two kinds of light can be adjusted separately with no interference between each other.Finally,high quality flattop beam is realized.3.We proposed a new type of large mode fiber of which the core is ring like.And the low loss transmission to the TE01 mode is realized by the plasma effect of metal surface,while the loss of other modes is very high,resulting in the formation of single TE01 mode transmission.