Study On Intra-cavity Generation Techniques And Applications Of Optical Vortex

Optical vortex is a type of beam that possesses a spiral wave front phase,and has a phase factor of e^il?（l is the topological charge,and?is the azimuth angle）.There exists a phase singularity at the center of the vortex beam,thus optical vortex has a node at the center and it generally presents a donut-shaped beam.In 1992,Allen et al demonstrated that every photon of a vortex beam carries an orbital angular momentum（OAM）of l?,thus,optical vortex beam is also named as OAM beam.Due to these unique features,optical vortex has shown great potentials in the applications of optical communications,optical micro-manipulation,super-resolution microscopy,high-precision displacement measurement,quantum entanglement,strong-field physics and so on.Up to now,optical vortex is generated mainly through modulating a Gaussian beam using spatial phase elements such as spiral phase plate（SPP）,fork-shaped grating,spatial light modulator,q-plate,metasurface,etc.The phase element-based method is easy and feasible,but there exists obvious limitations in some aspects.Due to the processing defects and phase discontinuity of phase elements,the generated optical vortex does not have a perfect spiral wave front phase,and will lose the vortex feature and even collapse with transmission.In addition,phase elements can only be designed for a specific wavelength,thus they are not suitable for generating broadband femtosecond optical vortices in principle.Phase elements are mainly used to generate continuous wave（CW）optical vortices with narrow spectral bandwidth.High-order transverse modes are eigen-modes of laser cavity,therefore,Laguerre-Gaussian（LG）vortex beams can be generated from intra-cavity,or converted from Hermite-Gaussian（HG）beams by cylindrical lens mode converters.Because high-order transverse modes are eigen-modes of laser cavity,the generated optical vortices possess perfect spiral phases and support broad bandwidth,thus they are stable with transmission,avoiding the limitations of the phase element-based method.This dissertation mainly focuses on developing intra-cavity generation techniques of optical vortex,and we also studied the application of optical vortex in optical communications.The main works of this dissertation are as follows:Firstly,we invented the technique of generating ultraclean femtosecond optical vortex based on high-order transverse mode mode-locked lasers.We proposed noncollinear pumping method for generating high-order transverse mode beams（HG beams）.Using this method,we realized high-order transverse mode femtosecond mode locking for the first time,and directly generated femtosecond HG beams directly from intra-cavity.Then we converted the femtosecond HG beams into femtosecond vortex beams by a cylindrical lens mode converter.By measuring the spatial intensity contrast（SIC）of the generated femtosecond optical vortex,we found that the ring-to-center SIC reached 36dB,which is close to the theoretical limit of SIC of an ideal optical vortex,indicating that the generated optical vortex from laser is ultraclean.This work took an important step for chirped pulse amplification（CPA）of femtosecond vortices and the application of strong-field physics.Secondly,we developed the intra-cavity generation technique of optical vortex based on inscribing of cavity mirrors.Through inscribing a series of round patterns with various radii on the laser cavity mirror,a vortex beam with a specific topological charge is selected to oscillate.By numerical simulation and theoretical analysis,we acquired the quantitative relationship between the topological charge of optical vortex and the radius of round pattern,which provides a design criterion for generation of optical vortex in the cavity.With the method,we generated topological charge-controllable optical vortices directly from laser,and the maximum topological charge reached 288^th order.Up to now,the topological charges of optical vortices are relatively low(less than 10^th order)with traditional intra-cavity techniques,but with our methods,high-charge optical vortex over 100^th order was generated from intra-cavity for the first time.By studying the temporal characteristics of the generated optical vortex,we found that the vortex laser can operate in self-mode locking with high repetition rate at GHz.The developed picosecond vortex laser has important application prospects in time division multiplexing of optical communications.Thirdly,we further developed the intra-cavity generation technique of multiple vortex beams based on inscribing of cavity mirrors.By inscribing multiple ring structures on the output coupler,the laser generated multiple optical vortices simultaneously.We also studied the temporal characteristics of the generated multiple vortex beams,and found that they could simultaneously operate in self-mode locking.The generated picosecond multi-vortex beams have important application prospects in time-space division multiplexing of optical vortex communications.Fourthly,we studied the application of the generated multi-vortex beams in optical communications.We proposed a new concept on spatial encoding of multi-vortex beams,and demonstrated the spatial encoding of multi-vortex beams and free-space transmission of information in the experiment.Meanwhile,with the high-repetition-rate picosecond multi-vortex laser,we demonstrated time division multiplexing and time-space division multiplexing,respectively.The picosecond multi-vortex laser presents great application potentials in optical communications.In this dissertation,we systematically studied and elaborated the intra-cavity generation techniques of optical vortices.We also studied the applications of optical vortex in optical communications.The research works of this dissertation laid a foundation for optical vortex generation in the cavity,and drastically extended the application prospects of optical vortex in optical communications,super-resolution spatial measurement,and high-angular momentum quantum entanglement,etc.