Eneration And Detection Of Laguerre-Gaussian Beams

Laguerre-Gaussian mode is the eigen solution of the paraxial wave equation in the cylindrical coordinates.The solution contains the azimuthal index l(l chooses any integer)and the radial index p(p chooses zero or positive integer),which can distinguish different LGip modes.All the LGip modes form a complete and orthonormal basis,so that any scalar transverse modes can be expressed as the superposition of a sub group of LGlp modes.The origin of LG modes depends on the developments of lasers in the 1960s,because the laser cavity made up of spherical mirrors has the LG mode solutions for cavity modes.However,the LG modes with opposite handedness,like LG-ip and LGip modes,degenerate in the laser cavity,which hampers the generation of an ideal LGip mode.Therefore,the researches on LG modes stayed at a low level until the discovery that the phase term eil? in a LG mode carried orbital angular momentum of light(OAM)in 1992.The index l,also called topological charge,is used to evaluate the OAM.The passed 25-years research on OAM-carrying LG modes have covered the generations,applications and detections of OAM.The applications include the fields of optical tweezers,optical communications,quantum information,nonlinear optics,optical metrology,etc.Specially,the unlimited bandwidth of OAM has been used to increase the capacity of optical communication,so it is important to efficiently detect the OAM.Besides,as the development of LG modes in precision measurement and the increasing attention of p index in optical communication and quantum information,high-purity LGlp beams have been proposed to reduce the uncertainty.However,the generation of high-purity LGlp modes still faces great challenges.Aiming at the effective detection of OAM and the generation of high-purity LGlp beams,we carry out the following researches:1.A subwavelength metal hole is designed to detect OAM.Miniature OAM emitters fabricated by micromachining technology have been reported in recent years,which facilitates the development of OAM in photonic integrated circuit.However,the on-chip OAM detectors is still challenging to match the newly-developed compact OAM emitter and OAM transmission fiber.Here,we design an ultra-compact and integratable device,a subwavelength hole in a thin Au film,to detect OAM based on self-referenced interference.The size of the device is reduced down to a few hundreds of nanometers,which can be easily fabricated and installed in the current integrated OAM devices.Good fault-tolerance in OAM detection presenting by experimental results makes it adjustable to uncertainty induced by environment and fabrication process.Besides,the self-referenced interference device is a flexible and robust way for monitoring in-situ OAM and detecting other transverse modes.2.A spirally-poled nonlinear photonic crystal is designed for near-infrared OAM detection.Applying OAM to near-infrared band can enhance the near-infrared detecting ability like rotating object detection,and near-infrared information capacity in optical communication.But it needs suitable near-infrared OAM detection device to complete the processes.Based on the nonlinear holography,we design a spirally-poled LiNbO3 nonlinear photonic crystal to detect near-infrared OAM beams using the frequency conversion and phase modulation of the crystal.In this crystal,the generated second-harmonic(SH)waves from positive and negative domains have a ? phase difference,which is used to compose a nonlinear Fresnel zone plate for an experimental demonstration of generating high-quality SH OAM field at the designed focusing spot.The OAM loaded by the crystal can be used to counteract the OAM of fundamental beam,so that the focusing OAM field turns into a bright SH spot.The frequency conversion and OAM counteraction can be used to detect near-infrared OAM mode in a nearly-nondestructive way.In addition,the fundamental OAM beam can be used in next stage since the uniform refractive index of the crystal doesn't disturb fundamental wave fronts.3.A solid-state laser is designed to directly output high-purity LGl0 beams.LG beams generated using passive method normally has poor mode purity.The active method is potential to increase the mode purity,which is restricted by the degeneracy of opposite-handedness LG modes.We propose an intra-cavity mode conversion method,utilizing the passive and positive methods simultaneously to output high-purity LGl0 modes through a compact YVO4 solid-state laser.Spin-orbital angular momentum conversion of light is used to reversibly convert the transverse modes inside cavity,which overcomes the degeneracy of opposite-handedness LG modes.The topological charge can be controlled by changing the vortex half-wave plate and the quarter-wave plate inside the laser cavity.The generated LG10 and LG20 laser modes have purities of?97%and?93%and slope efficiencies of?11%and?5.1%,respectively.Moreover,our cavity design can be easily extended to produce higher-order Laguerre-Gaussian beams and cylindrical vector beams.Such compact laser configuration features flexible control,low threshold,and robustness,making it a practical tool for applications in super-resolution imaging,high-precision interferometer,high-capacity optical communication,and quantum correlation.4.The intra-cavity mode conversion method is applied to an optical parametric oscillator,by which we can generate high-purity LG10 and LG20 laser beams with wavelengths from of 880 nm to 920 nm.High-quality cylindrical vector beam can be also generated in the wavelength range of 900 ±50 nm by slightly modifying the cavity design.The broadband high-quality LG beams and cylindrical vector beams will facilitate more applications.