High Purity Cylindrical Vector Beam All-fiber Laser

Cylindrical Vector Beams(CVBs) including radially and azimuthally polarized beams, different from the traditional polarization beam, have a symmetric polarization and doughnut shape intensity distribution. With properties of symmetric electromagnetic field, CVBs have unique and attractive application in lots of areas, such as plasmon excitation, optical tweezers, particle acceleration, laser machining and so on. Therefore, the methods generating CVBs have drawn great attention, of which the method based on Few-mode fiber is much convenient and low-cost.In the few-mode fiber, the second order eigen mode has four degeneracy modes, of which radial mode TMoi and azimuthal mode TEoi included. So, CVBs are obtained by exciting the desired mode of fiber through offset or tilt injection. However, this method could not give consideration to high purity of mode and efficiency. To overcome it, we propose a few-mode fiber Bragg Grating (FMFBG) based method.There are multi-peak shown in the reflection spectrum of FMFBG, and each of which is corresponding to mode with different order. Based on that, we can reflect and filtering the fundamental mode generated in the mode coupling process to achieve CVBs with high purity. We have demonstrated all-fiber CW CVB laser and mode-locking CVB laser, besides a "transverse mode-wavelength" associated tunable fiber laser using FMFBGThe major researches as follows:1. we demonstrate a DBR structure all-fiber Yb-doped CVB laser, outputting radially polarized beam with purity of94.2%and azimuthally polarized beam95.1%. The laser operates at a single wavelength with narrow linewidth(0.2dB at40dB position) and has a quite low threshold of16mW. Meanwhile, we proposal a simple method to measure the purity of mode by fiber-end outputting.2. Using NALM as the mode-locking element, we demonstrate a FBG-cascading figure-8fiber laser to achieve square-wave mode-locking pulses under a narrow linewidth(0.35dB at50dB position), and the outputting radial mode has a high purity of96%.3. Based on the corresponding relationship between transverse mode and wavelength during the reflection of FMFBG, we achieve a king of "traverse mode-wavelength" associated tunable fiber laser. The laser could output fundamental mode, second order mode and hybrid mode when the laser operating at three different wavelength of FBG reflection.4. We design two kind of metallic core fiber based the loss distinction based on surface plasmon. By simulation, it is shown that the round metallic core fiber could achieve a pure TEoi mode within6cm and the gear-like metallic core fiber could get a pure TM01mode within1cm.Highlights of the dissertation are as follows,1. We proposal a FMFBG based method achieve high purity CVBs. Based on this mechanism, we demonstrate CW laser and mode-locking laser outputting CVBs. More importance, this method could be expanded and applied to other wavelength band by choose suitable fiber and grating.2. Based on the corresponding relationship between transverse mode and wavelength, we demonstrate a fiber laser outputting different order modes, including fundamental mode, second order mode and hybrid mode, by tuning the operating wavelength of laser easily.3. we design two kinds of metallic core fiber based on surface plasmon, the round and gear-like metallic core fiber could achieve high purity TE01mode and TM01mode by absorbing other mode.