| Vortex beams,including vector vortex beams and phase vortex beams,are light beams with special vortex structures.Vector vortex beams,also known as cylindrical vector(CV)beams,have annular intensity distributions and axisymmetric polarization distributions.They have been widely studied in super-resolution imaging,laser processing,optical tweezers and surface plasmon excitation due to their special properties under the tight focusing condition.Phase vortex beams,also known as orbital angular momentum(OAM)beams,have helical phase distributions related to topological charge l and carry OAM.They have shown potential applications in optical manipulation,optical communication,sensing and so on.In recent years,generating CV beams or OAM beams in the fiber laser has become one of the important research fields of vortex beams.In the early research,the combination of the offset-coupled fiber structure and the few-mode fiber Bragg grating was often used to generate CV beams.Due to the narrow spectral reflection characteristic of the few-mode fiber Bragg grating,these lasers can only output CV beams with single wavelength.Recently,few-mode long period fiber gratings and mode selective couplers were reported to be used for generating vortex beams.Because these devices can operate within a wide spectrum,people can obtain vortex beams with wide spectra by using of them.In some applications,such as optical communications and sensor systems,multiwavelength light sources are often desired.Multiwavelength vortex beams would be more attractive in practical applications.Until now,few reports have paid attention on the generation of multiwavelength vortex beams in an all fiber laser.Besides,the number of output wavelengths and tuning range were relatively limited in these reports.Thus,it is necessary to further study the multiwavelength fiber laser with vortex beams output.The main research works of this dissertation are summarized as follows:1.Two kinds of vortex mode converters,including the azimuthally polarization mode selector and the high-order OAM mode converter,are designed and studied.The azimuthally polarization mode selector is composed of the metal-clad fiber.Due to the absorption of the metal-clad,only azimuthally polarized beams are able to pass through the metal-clad fiber with low loss.The research results show that the device can operate within the wavelength range of 900-1300 nm.The high-order OAM mode converter consists of a mode selective coupler and a multi-mode elliptical cladding fiber.The high-order LPl1 mode is first obtained through the mode selective coupler.Then the elliptical cladding fiber is cascaded behind the mode selective coupler with a proper angle to introduce the appropriate phase difference between the/Pl1? mode and the LPl1-? mode through the stress birefringence.Finally,OAM beams with topological charge of ±1,±2 and ±3 are able to be obtained under the suitable conditions.2.We propose a method to generate multiwavelength OAM beams in an all fiber laser through the cascaded stimulated Brillouin scattering excited by the external narrow linewidth Brillouin pump and the mode conversion provided by the few mode long period fiber grating.By optimizing the primary pump power,Brillouin pump power and Brillouin pump wavelength,OAM±1 beams with up to 10 wavelength channels are obtained.By adjusting the tunable bandpass filter in the cavity,the central wavelength of multiwavelength OAM beams can be tuned between 1535 nm and 1570 nm.The purity of multiwavelength OAM beams generated in our experiment is measured to be higher than 96%.3.We design an all fiber laser for multiwavelength CV beams emission by combination of the nonlinear effect in the 6-km long single-mode fiber,the wavelength selection of the fiber Mach-Zehnder interferometer and the mode conversion of the few mode long period fiber grating.By adjusting the Mach-Zehnder comb filter,the multiwavelength output with wavelength spacing of 0.09 nm,0.11 nm,0.17 nm,0.24 nm and 0.3 nm is obtained.With sufficient pump power,the laser can operate with more than 10 wavelength channels when the wavelength spacing is tuned to?0.09 nm.By adjusting the flat-top tunable bandpass filter,the central wavelength of multiwavelength CV beams can be continuously tuned between 1534.37 nm and 1570.06 nm.4.A random fiber laser with CV beams output is constructed by using a fiber ring resonator to provide backscattering and a mode selective coupler for mode conversion.The output spectra exhibit the behavior of random multiwavelength lasing and also indicates the low temporal coherence of the laser output.To confirm the effectiveness of the designed laser in reducing laser speckle,we measure the speckle contrasts of the multiwavelength radially polarized beam generated in our laser,the single-wavelength linearly polarized fundamental mode beam and the single-wavelength radially polarized beam when they pass through the diffuser.The results show that speckle contrast decreases by about 21%when the single-wavelength radially polarized beam is used compared with when the single-wavelength linearly polarized beam is used.And the use of the multiwavelength radially polarized beam reduces the speckle contrast by a further 16%compared with the use of the single-wavelength radially polarized beam.The main innovations of this dissertation are summarized as follows:1.A multiwavelength OAM fiber laser with large tuning range and wavelength channel number is proposed.High purity OAM±1 beams with up to 10 stable wavelength channels with tuning range of 35 nm are achieved.It is the highest number of operating wavelengths and tuning range in an all fiber laser for orbital angular momentum beams emission to the best of our knowledge.2.A multi-wavelength CV fiber laser with tunable central wavelength and wavelength spacing is presented and experimentally investigated.The CV beams with more than 10 wavelength channels are obtained,and the central wavelength of the multiwavelength CV beams can be tuned continuously between 1534.37 nm and 1570.06 nm.In addition,multiwavelength CV beams with wavelength spacing of 0.09 nm,0.11 nm,0.17 nm,0.24 nm and 0.3 nm are also realized.3.A random fiber laser with CV beams output is proposed and experimentally investigated.Because one of the feedback of the laser is provided by the backscattering in the fiber ring resonator,the output CV beams show the behavior of random multiwavelength and low temporal coherence.The speckle contrast measurements show the effectiveness of the designed laser in reducing laser speckle. |
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