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

In recent years,thulium-doped fiber laser,which is capable of generating lasing at 2 ?m wavelength band,has attracted more and more attention from many research teams.This wavelength is located in the safe band of human eyes.Also the 2 ?m spectral band is located in a transparent window of the atmosphere called "molecular fingerprint region",which has a strong penetrating ability to the atmosphere.Compared with short-wavelength fiber lasers,thulium-doped fiber lasers play an irreplaceable role in many fields such as eye surgery,air pollution emission,lidar and space communication.On this basis,it is important to further study the vector light beam generation at the wavelength band of 2 ?m.On the one hand,there are many types of vector modes,which have potential to boost the transmission capacity of space communication.On the other hand,due to the uniqueness of energy distribution and polarization sensitivity of non-uniform polarized vector beam,the precision and sensitivity of laser surgery and sensing could be greatly improved.As a representative vector beam,cylindrical vector beam has attracted wide attention in recent years.The electric field distribution of the cylindrical vector beam is symmetrical in the cross section,and the intensity distribution is a circular ring with a zero center.It is owing to this distinctive structure that the cylindrical vector beams are widely used in laser processing,particle manipulation and other fields.This thesis studies the generation of cylindrical vector beams based on a coupler-structure mode conversion device,and by combining the cylindrical vector beams with thulium-doped fiber laser,two kinds of all-fiber cylindrical vector beam thulium-doped fiber lasers are proposed and demonstrated,including the generation of multi-wavelength continuous cylindrical vector beams and ultra-fast pulse cylindrical vector beams.The main research results are as follows:1?Theoretical analysis of optical fiber mode at the wavelength band of 2 ?m.Based on Maxwell equations,the mode field theory and single mode condition in optical fiber are both analyzed,and the number of modes supported by an optical fiber at different wavelengths is also analyzed from the perspective of normalized cut-off frequency.The finite element optical method is introduced for simulation verification so as to obtain the types of modes existing in the fibers at the wavelength band of 2 ?m.Then the single-mode and few-mode optical fibers that can be applied to the 2 ?m band are selected.The working principle of the mode converter is analyzed based on the coupled mode theory.The variation curves of effective refractive index of different modes with cladding diameter are calculated by using finite element method.Thus,the fiber diameter parameters meeting the phase matching condition can be obtained.Then,the diameter parameters are applied to the beam propagation method for simulation calculation,and the dynamic diagram of the mutual transformation between the fundamental mode in single mode fiber and the high-order mode in few mode fiber is obtained,which further verify the feasibility of the simulation results.Based on the simulation parameters,a high performance mode converter at the wavelength of 1900 nm is fabricated successfully by using fused biconical taper method.2?Continuous-wave cylindrical vector beam thulium-doped fiber laser with multi-wavelength outputs: By using the filtering function of Sagnac ring filter,the free spectral range of the filter is 4.99 nm,a multi-wavelength cylindrical vector beam thulium-doped fiber laser has been realized based on a mode-conversion coupler.The single-wavelength(1988.93 nm,1993.92 nm and 1998.91 nm),dual-wavelength(1988.93 nm and 1993.92 nm,1993.92 nm and 1998.91 nm)and even triple-wavelength(1988.93 nm,1993.92 nm,1998.91 nm)continuous-wave cylindrical vector beam outputs are achieved in the same thulium-doped fiber laser by adjusting the in-cavity polarization controllers.3?Mode-locked cylindrical vector beam thulium-doped fiber laser: By using ultra-high numerical aperture fiber,a ring thulium-doped fiber laser dispersion compensated to the positive dispersion region has been proposed and demonstrated with cylindrical vector beam dissipative soliton generation.The experiment results show that the central wavelength of the mode-locked spectrum is 1932.1 nm,the 3 d B bandwidth is 10.4 nm,the pulse repetition frequency is 15.4 MHz,and the pulse duration is 1.84 ps.The signal-to-noise ratio of the fundamental frequency at 1 Hz resolution bandwidth is 69.6 d B measured by the radio-frequency spectrum analyzer.High purity pulsed cylindrical vector beams are also obtained at the output of the mode converter.