Highly Efficient Generation And Detection Of Vortex Beams

Vortex beams or optical vortices have an isolated point singularity of the center of the optical field,i.e.polarization singularity or phase singularity,leading to null intensity at the beam center.So,vortex beams commonly contain two types:polarization vortex and phase vortex,respectively.Polarization vortex also known as cylindrical vector beams(CVBs),which mainly includes radially polarized beam(TM01 mode)and azimuthally polarized beam(TE01 mode)and has an intense and well-defined longitudinal electric field at the focus point.So,it has found interesting applications in super-resolution spots,plasmon excitation,single-molecule spectroscopy,lasing machining,particle acceleration,optical communication and optical trapping of particles,etc.Due to the possessing of orbital angular momentum(OAM),phase vortex also known as OAM beams,which has helical phase wave-front comprising an azimuthal phase term exp(il?),where l denotes topological charge value,? is azimuthal angle.OAM beam has an OAM of lh per photon and can take,in principle,theoretically unlimited orthogonal states,where h represents Plank's constant h divided by 2?.Based on the unique features,OAM beams have found potential applications in optical tweezers,optical manipulation,microscopy,quantum information processing,and classical and quantum communications,etc.In consideration of the potential application,highly efficient generation and detection of vortex beams become very meaningful.A lot of methods have been proposed to generate CVBs or OAM beams,such as,exciting the eigenmode of cylindrical symmetry waveguide or superposition of HG01 and HGio mode which are linear polarized mode.For the generation of CVBs,methods based on sub-wavelength grating,half wave plate group,liquid crystal rotation plate,and spatial light modulator,can directly convert the desired polarized states.While for the generation of OAM beams,OAM mode converter such as spiral phase plate,spatial light modulator,cylindrical lens pair,q-plate,can convert the Gauss beam to OAM beams.By contrast,fiber based methods possess the advantage of simpleness,easier integration,and low cost,etc.According to fiber mode theory,the LP11 mode is a linear superposition of four degenerate vector modes,i.e.,TM01(radially polarized),TE01(azimuthally polarized),and HE21even/obb modes.Thus,by adjusting the polarization state of the fiber laser generating LPin mode,the CVBs(i.e.TE01 and TM01 modes)and OAM beams(i.e.HE21even ± iHE21odd modes)can be selectively obtained.There are many fiber based methods have been presented,however,most of them adopted an offset coupling or a mode selective coupler(MSC)as mode converter to generate LP11 mode,which led to relatively high insertion loss,as a result,restricted the slope efficiency and lasing threshold.In order to overcome this disadvantage,we introduced a LPFG in the cavity with low insertion-loss and high conversion efficiency.One need to detect the states of vortex beams in the process of generation or application.For the detection of CVBs,the polarization states of the output beams can be evaluated by inserting and rotating a linear polarizer before the CCD.For the detection of OAM beams,interference-based and diffraction-based methods have been demonstrated and each has its advantages and disadvantages.So,it is worth further investigating about that.The major research results of this dissertation are listed as the following:1.We demonstrated an all-fiber C W optical vortex laser with high slope efficiency and low threshold by introducing a long-period fiber grating(LPFG)into the laser cavity.Highly efficient mode conversion and low insertion loss were realized.The lasing threshold and slope efficiency of the laser were 24.5 mW and 35.41%,respectively.The output power was 72 mW with an absorbed pump power of 225 mW.The mode purities of output CVB and OAM beams exceed 98%.2.We proposed and demonstrated an all-fiber actively mode-locked laser producing optical vortex pulses with high efficiency and tunable repetition rate.The LPFG was also used as mode converter.A LiNbO3 Mach-Zehnder intensity modulator(MZIM)was served as a mode-locker to achieve actively harmonic mode-locking.The slope efficiency increases from 10.24%to 12.61%with the repetition rate of pulses train flexibly tuned from 15.65 MHz at fundamental mode-locking(FML)to 626 MHz at 40th order harmonic mode-locking(HML)with superior stability.Both CVBs and OAM beams were selectively generated under both FML and HML states with high mode purity of greater than 97%.3.An all-fiber based mode converter composing by a mode selective coupler(MSC)and an inner elliptical cladding fiber(IECF)for generating OAM beams was proposed and numerically investigated.By adjusting the parameters and controlling the splicing angle of MSC and IECF appropriately,the higher-order OAM modes with topological charges(TCs)of l=±1,±2,±3 can be obtained with fundamental mode LP01 injection,featuring a mode conversion efficiency of almost 100%.4.We demonstrated a method to detect the OAM beams by a sectorial screen.When the sectorial screen was illuminated by OAM beams,the far-field diffraction patterns can be used to visually determine the modulus and sign of topological charges(TCs,i.e.l).Detection of TCs up to ±10 was demonstrated with this scheme.We also proved that center alignment was not strictly required.5.A method to detect the TCs of OAM beams using radial phase grating(RPG)is proposed and demonstrated.The modulus of TCs can be obtained by the number of dark stripes of far-field diffraction patterns and the sign of TCs was determined by the orientation of the patterns.The diffraction efficiency was nearly 100%.Detection of TCs up to l = ±120 was demonstrated with this scheme.In addition,we also showed that this detection scheme had excellent alignment tolerance and did not have stringent requirement on the parameters of the grating.The main innovations of this dissertation are listed as the following:1.We proposed a LPFG based method to obtain high slope efficiency and low threshold of optical vortex fiber laser.We firstly introduced the LPFG into the laser cavity benefiting from its high mode conversion efficiency and low insertion-loss.Based on this mechanism,we demonstrated CW and mode-locking fiber laser generating high purified CVB and OAM beams.Meanwhile,by combining the MZIM,we demonstrated the high efficiency,tunable harmonic mode-locking(HML)optical vortex fiber laser.2.We proposed and designed an OAM mode converter composing by a MSC and IECF.The higher-order OAM modes with l=±1,±2,±3 can be obtained with fundamental mode LP01 injection,featuring a mode conversion efficiency of almost 100%.3.Based on the far-field diffraction features of gratings,two schemes to detect the TCs of OAM beams using the sectorial screen and RPG are proposed and demonstrated.The schemes possesses features of high diffraction efficiency,simpleness,and excellent alignment tolerance.Detection of TCs up to l= ±120 was demonstrated with the latter scheme,which can be further scaled up without the limitation of effective areas and resolution of SLM and CCD.