Generation And Characteristics Of Hollow Laser Beams By A Multimode Liquid-core Optical Fiber

A hollow laser beam (HLB) has wide applications in atom trapping and guiding,laser processing, biomedical, and so on. The generate methods and characteristics ofHLBs have been the research focus in the field. Many techniques have been used togenerate HLBs, such as geometrical optical method, mode conversion and opticalholography. However, the experimental arrangements of these methods are usuallymore complicated. Afterward, some simpler techniques were introduced by using asmall hollow fiber, multimode fiber and photonic crystal fiber (PCF). However, theloss of hollow fibers is very large; the DSS span of the HLB is confined owing to asmall numerical aperture (NA, about0.3) of multimode fibers; special PCFs are moreexpensive and the charging technology is rather difficult. To solve the problemsabove, we propose a method to generate a HLB by a liquid-core optical fiber (LCOF),and investigate theoretically and experimentally the characteristics of the HLB.Based on the light transmission and wave theory, the reason why multimodeoptical fiber with different core diameters could generate the HLB is investigated,and then the phase and polarization characteristics of the corresponding HLB areanalyzed. The dependences of the dark spot size (DSS), ring beam width (RBW) andthe quality of the HLB generated by large core multimode fiber on the incident angleof laser beam, the length and core diameter of the LCOF are analyzed theoreticallyby using Zemax simulation. And then, an experimental system of generating HLB byLCOF contained with CS2is set up, the experiments have been carried out to verifythe theoretical results. The results show that high-quality and controllable HLB canbe generated by LCOF and the DSS of HLB is proportional to the incident angle ofthe laser, while inversely proportional to the core diameter of LCOF, but hardlyvaries with the length of LCOF. The RBW of HLB only get related to the corediameter of LCOF. In addition, the RBW of HLB is only related to the core diameterof LCOF, and the quality of HLB is connected with the numbers of total reflectionsof the skew rays in the fiber. Therefore, we can improve the quality of HLB effectively by elongating LCOF and decreasing the core diameter of LCOF.Experimentally, a high quality and thin ring HLB is obtained by a1meter-longLCOF with the core diameter of100μm.The phase and polarization characteristics of HLB generated by multimodeoptical fiber and the distribution of HLB affected by fiber bending are investigated inexperiment. The experiments have demonstrated that the HLB generated by amultimode LCOF with core diameter of10μm do be vector vortex beam with singletopological charge number. Furthermore, vector vortex beam with the polarizationstate hybrid polarized, azimuthally polarized or radically polarized can be acquiredby varying the incident condition. As to the HLB generated by multimode LCOFwith large core diameter larger than100μm, the phase singularity characteristic is notobvious. Compared with multimode silica fiber, the HLB generated by a multimodeLCOF has better stability and higher efficiency, and is not sensitive to externalinfluences such as optical fiber bending.