The Study Of Dual-wavelength Passive Q-switched Cylindrical Vector Beam Fiber Laser Based On Few Mode Fiber

Cylindrical vector beam?CVB?refers to the beam with different polarization states corresponding to different positions on the wave front.Its light field intensity is an axisymmetrically distributed donut.Cylindrical vector beam mainly includes three types:radially polarized beam,azimuthally polarized beam,and mixed-polarized beam.Radially polarized beams are focused by the large-aperture lens and produce a longitudinal electric field component in the direction of the optical axis and a smaller lateral spot compared with the spot focused by the uniformly polarized beam.Based on these special properties,cylindrical vector beams have broad application prospects in many fields such as surface plasmon excitation,high-resolution imaging,laser processing,and optical particle capture.And so,researchers have proposed a number of methods to generate cylindrical vector beams.At present,the methods of generating cylindrical vector beam are divided into active methods and passive methods according to whether it is in a cavity or not.It is a convenient and effective method to generate cylindrical vector beam by using a few-mode fiber.By solving the eigensolution of the wave equation in ordinary step fibers,it can be known that the radially polarized beam and the azimuthally polarized beam correspond to the TE₀₁ mode and the TM₀₁ mode in the LP₁₁ mode,respectively.Therefore,we can use a few-mode fiber to excite the beam from the fundamental mode LP₀₁ mode to the high-order degenerate mode LP₁₁ mode,and then using a series of polarization control devices to select the modes.Finally,radially polarized and azimuthally polarized beam can be obtained.In this paper,we first analyze the uneven polarization distribution of the cylindrical vector beam and its tight focus characteristics at large numerical apertures,and derive its mathematical expression.This article also describes the application of cylindrical vector beam in the fields of surface plasma excitation,optical capture,laser processing,and several methods to generate it.Then the Q-switched theory of fiber lasers and several methods to make saturable absorbers and measure their characteristics are introduced.The coupling mode equation in the fiber mode coupler is deduced,and the energy conversion process in the mode conversion coupler is simulated.This paper designs and builds a dual-wavelength passive Q-switched cylindrical vector beam fiber laser.The laser is implemented by an integrated device based on saturable absorption and mode selection.By adjusting the polarization controller in the laser resonator,the output of Q-switched pulsed laser beam with radially and azimuthally polarization are achieved,respectively.The main contents and innovations of this article are divided into the following points:1.Nano-sized MWCNT saturable absorbers were prepared by liquid phase stripping,and their characteristics were characterized by Raman scattering and transmission electron microscopy.The linear absorption spectrum of MWCNT was measured in the experiment,and the transmittances around 1?m and 1.5?m were 31%and 36%,respectively.Then a P-scan device was built to measure the saturable absorption characteristics of MWCNT at 1?m and 1.5?m.The modulation depth and saturation intensity at the two wavelengths are1.34%,8.8 MW/cm² and 2.94%,7.5 MW/cm²,respectively.2.By controlling the core radius of the tapered fiber,the phase matching of the fundamental mode and the higher-order mode in different fibers and the excitation of a specific mode light field can be achieved.Then,a layer of MWCNT film is used as a saturable absorber to cover the tapered region of the tapered fiber to form the saturable absorber mode selective coupler.3.A dual-wavelength passive Q-switched cylindrical vector beam fiber laser was designed and constructed experimentally,which achieved the output of 1?m and 1.5?m Q-switched vector lasers.The laser threshold of the Yb³⁺resonator is 160 mw,the center wavelength of the obtained spectrum is 1036 nm,and the 3 d B width of spectral is 0.9 nm.The laser output pulse width range is 2.46?s-1.21?s,and the corresponding repetition frequency varies from35 k Hz to 120 k Hz.The maximum single pulse energy and peak power of the output laser can be 0.56 n J and 0.5 m W,respectively.The threshold of the Er³⁺resonator is 100 mw,the center wavelength of spectrum is 1560 nm,and the 3 d B width of spectral is 2.2 nm.The laser output Q-switched pulse width ranges from 8.9?s to 2.1?s,and the repetition frequency ranges from 5 k Hz to 30 k Hz.The maximum single pulse energy and peak power of the output laser are 13 n J and 6 m W,respectively.