High Efficiency Cylindrical Vector Beam Fiber Laser

Cylindrical vector beam(CVB)is the product of polarization modulation of the optical field and has a symmetric polarization in the cross-section,which is different from the uniform polarization distribution of the traditional linearly or circularly polarized beam.Owing to the existence of polarization singularity,CVB has doughnut intensity distribution and zero intensity at the beam center.Due to the unique focusing property and polarization distribution,CVB has huge application prospects in optical tweezers,high-resolution imaging,surface plasmon excitation,laser processing,and so on.Therefore,the generation methods of CVBs have been getting increased attention,especially generating methods in an all-fiber structure.However,mostly all-fiber CVB lasers essentially operate in fundamental mode,which require a mode convertor to generate CVB.Due to the high insertion loss and low conversion efficiency of the mode convertor,it is difficult to improve the output efficiency and power of these lasers.To overcome the limitation,we proposed the all-fiber CVB laser that oscillated in high-order mode directly.Furthermore,we designed and fabricated two novel active fibers to improve the efficiency of the all-fiber CVB laser.The major works and research results of this dissertation are as follows:1.We proposed and demonstrated the polarization dependence of the fiber Bragg grating(FBG).By utilizing this property,we demonstrated a fiber laser that can realize multi-wavelength oscillation and switchable transverse modes simultaneously.In this laser,single-,dual-and triple-wavelength oscillation can be realized by merely adjusting the polarization controller inside the cavity,and the output mode was tunable.Besides,we obtained the CVBs with mode purity over 97%.2.Based on the polarization dependence of FBG,we proposed and demonstrated an all-fiber CVB laser with high-order mode direct oscillation.Besides,we designed and fabricated a novel annular doping YDF(ADYDF)that has doughnut-shaped Yb ions doped distribution and matches the CVB mode better because of the doughnut-shaped intensity distribution of CVB.Therefore,the all-fiber CVB laser with ADYDF can improve the competition and the output efficiency of CVB mode,and reduce the laser threshold.The output CVB mode purity is over 93%,and the center wavelength is 1054.58nm with 30dB linewidth of 0.13nm.Besides,the slope efficiency of this all-fiber CVB laser is 53.3%.3.We designed and fabricated a ring-core YDF(RC-YDF),and demonstrated an all-fiber CVB laser by employing RC-YDF as active fiber.In this fiber laser,annular doping and ring-type beam pump were realized for the first time.What's more,the fundamental mode was restrained,and the competition of CVB mode was improved because of annular doping;on the other hand,the overlap between pump and signal beam was increased,which leads to the high absorption of pump beam.Due to the effect of RC-YDF,the fiber laser operated in high-order mode directly,and the output efficiency is highly improved.The purity of radially polarized mode was calculated as 93.2%.Besides,the central wavelength of the laser was 1055.46 nm with 30 dB linewidth of 0.14 nm,and the slope efficiency is as high as 64.5%under the appropriate length of RC-YDF.4.We proposed and demonstrated a Q-switched CVB fiber laser,which operated in high-order mode and avoided the high insertion loss of mode convertor.In this fiber,a pair of FMFBG was used to realize high-order mode oscillation,and WS2 was employed as saturable absorber.The center wavelength was 1055.98 nm with 30dB linewidth less than 0.19 nm,and the slope efficiency is as high as 39%.On the other hand,the repetition rate of the Q-switched pulse can be tuned from 44.18 kHz to 58.16 kHz.The maximum single pulse energy is 299nJ with a repetition rate of 44.18 kHz and a pulse duration of 2.67?s at pump power of 92.3mW.Furthermore,we proposed a mode filter based on FMFBG and obtained CVB pulse with high purity.5.We fabricated a long-period fiber grating(LPFG)with 12dB(93.7%)bandwidth of 125 nm.The ultrafast mode-locked CVB fiber lasers were realized at 1.0?m band based on this LPFG.In the intra-cavity conversion scheme,the gold-plated connecter with a broad reflection spectrum and LPFG were used as output coupler and mode convertor,respectively.The spectrum width of output CVB pulse is 5nm(at 3dB),and the pulse duration is 168ps with mode purity over 95%.On the other scheme,the LPFG is outside the cavity and used as mode convertor.In this fiber laser,we obtained the high-purity CVB mode-locked pulse with 3dB linewidth of 10nm and pulse duration of 420fs.Highlights of this dissertation are as follows:1.We demonstrated the polarization dependence of the fiber Bragg grating(FBG)for the first time.Furthermore,based on the polarization dependence of the FMFBG,we proposed and demonstrated an all few-mode fiber laser that can realize multi-wavelength oscillation and switchable transverse modes simultaneously.2.Based on the polarization dependence of the FMFBG,we proposed and demonstrated an all-fiber CVB laser with high-order mode oscillation.Besides,we designed and fabricated two novel active fiber(ADYDF and RC-YDF).Annular doping and ring-type beam pump can be realized in the all-fiber CVB lasers,which leads to high-order mode oscillation.What's more,the efficiency of the CVB lasers was highly improved.3.We proposed and demonstrated a Q-switched CVB fiber laser with high-order mode oscillation,and this fiber laser avoided the high insertion loss of mode convertor.Furthermore,we proposed a mode filter based on FMFBG,which can be used to improve the purity of CVB modes.4.Based on the gold-plated connecter and LPFG with a broad spectrum,we solved the contradiction between spectrum width and mode purity of the mode-locked CVB pulse,and obtained high-purity CVB pulse with a broad spectrum.On the other hand,we obtained the high-purity CVB femtosecond pulse at 1.0?m band by utilizing LPFG as a mode convertor.