Cylindrical Vector Beam Fiber Lasers With Mode-locked Output And Efficiency Enhancement

Cylindrical vector beams(CVBs)refers to the beams with an axially symmetric distribution in the cross section,which are the symmetrical beam solution to the full vector electromagnetic wave equation.Two common CVBs show the electrical distribution of radially and azimuthally polarization beam.These beams can be generated by active and passive methods.Techniques of CVB manipulations with the polarization maintainment have also been developed.Their special symmetry gives rise to unique high numerical aperture focusing properties that find potential applications in the areas of high accuracy imaging,metal processing and particle manipulation.The LP11 mode consists of four engine CVB modes(TMo1,TEo1,HE21even and odd),TM01 and TE01 are the radially and azimuthally polarization beams,respectively.The LP11 mode can be excited with an offset coupling or long period Bragg gratings,however it is hard to realize a high purity mode emitting with a high excitation ratio.Thus the scheme of using few mode fiber Bragg grating(FMFBG)as a mode filter have been proposed for high purity CVB generations.In this dissertation,we introduced the mathematical properties and generation methods of CVBs,and clarified the reflection and mode filter theory of FMFBG based on the fiber mode theory.Our research is funded by the National Nature and Science Foundation of Chin,realized the mode-locked CVBs output and enhanced resonance efficiency for CVBs.The primary results of this dissertation are as follows:1.We realized a stable actively mode-locking through the cascaded usage of an electro-optical modulator in a fiber laser,the pulsed CVBs output was obtained by cascade connection of offset coupling and FMFBG.The generated pulsing CVBs had a repletion rate of 12.059MHz with the pulse duration of 2ns,the fiber laser operated within a narrow spectrum of 0.2 nm at 30 dB depth under mode-locking state,it give rise to a high mode purity of the output CVBs.2.We realized a passively mode-locked CVB output with ultrafast pulses based on the saturable absorption of Carbon Nano-tube(CNT),the CNT integrated the properties of high damage threshold,wide operation waveband and polarization independence was chosen as the intra-cavity saturable absorber to achieve self-starting passively mode-locking.The pulsed CVBs with a repetition of 10.61 MHz and duration of 22.7 ps was obtained through adjusting the intra-cavity polarization controller.The output spectrum of the fiber laser was well limited within the fundamental mode reflection peak of the FMFBG with a spectra width of 0.3 nm at 30 dB depth,the mode purity of the generated radially polarization beam was calculated to be 98.03%with a mode integration method.3.We introduced an optically induced long period fiber grating(OLPG)into a DBR fiber laser which was caused by the gain saturation and beat frequency between different transversal modes,the OLPG realized a strong enhancement of the resonance efficiency for CVB output.The slope efficiency of the fiber laser had a sudden change from 13.36%arrives 32.48%when the pump power reach a threshold.We contributed the physical origin of this threshold property of the OLPG due to the experimental conditions and the exhaustive investigation.Meanwhile,we clarified the influence on laser resonance through numerical simulation.The simulation predicted the same phenomenon with the experimental results,the operation of LP01 and LP11 modes in the cavity under different pump power was also realized.4.To get rid of the narrow reflection peak of the FMFBG,we designed two kind of mode response elements which operated within a wide spectrum.Mode selective coupler was contributed based on the waveguide mode coupling theory,the feasibility has been demonstrated by the method of tapered fiber melting.In addition,we designed a scheme based on the planer lightwave coupler(PLC)and explained the coupled theory by mode superposition along with the coherent propagation.An azimuthally polarized selective element within wide spectrum was also designed by metal coated fiber taper,the influences on the loss of different modes of the tapered diameter,metal thickness and operation wavelength were demonstrated through numerical simulation.The innovative works and highlight results in this thesis are as follows:1.The actively mode-locked fiber laser with CVB output was realized by the cascaded usage of electro-optical modulator,offset coupling and FMFBG.2.The CNT with high damage threshold,wide operation waveband and polarization independence was chosen as the intra-cavity SA for the self-starting passively mode-locked fiber laser,ultrafast pulsed CVB was also obtained through the usage of FMFBG.3.The OLPG was introduced into a DBR fiber laser which was caused by the gain saturation and beat frequency between different transversal modes,which realized a remarkable efficiency enhancement for the CVB output.The numerical simulation combined of rate equations and OLPG clarified the influence on the laser resonance and predicted the same phenomenon with the experimental results.