Study On Continuous-wave And Pulsed Vortex Nd:YAG Microchip Laser Pumped By An Annular Beam

In recent years,in order to fully exploit and utilize the potential of laser,the investigation of the structural laser beam has attracted more and more attentions,and the vortex beam is one of the study directions.The vortex beam with helical wavefront possesses orbital angular momentum along its propagation direction.Due to the distinct feature of vortex beam,it has been applied in various fields,such as the optical manipulation,material processing,quantum information,spin-orbit coupling,nonlinear optics and optical communication.Hence,the generation of vortex laser beam is the fundamental of all the other studies.The vortex beam can be produced outside the laser cavity(passive methods)or inside the cavity(active methods).The passive methods need specially designed optical elements to shape the fundamental Gaussian laser mode to the vortex laser mode.During the beam transformation,there always exists great power losses and the degradation of the beam quality.Meanwhile,the vortex beam cannot achieve high laser power due to the relatively low damage threshold of the optical elements.As a comparison,the active methods are to design a proper laser cavity so that the vortex beam can be directly generated.The advantages of active methods are that it can produce high power vortex laser beam with good beam quality,and the structure of the device is relatively simple.The direct generation of vortex beam in the solid-state lasers is one of the most popular investigation areas.In end-pumped solid-state lasers,one of the most efficient methods is to utilize the annular beam to act as the pump source.According to recent reports,all the methods of producing annular beam are firstly to use optical elements to get collimated annular beam,then focused by extra positive lens to obtain the beam spot with high power intensity for pumping.Moreover,the position between the optical elements and the lens need be carefully adjusted to get a good pump spot.These kind of methods for producing annular pump spot make the laser collimation and focus system more complicated and lack flexibilities.On the other hand,currently the main technique for controlling the vector polarization and vortex handedness of the vortex laser beam generated in the end-pumped solid-state lasers is to add mode-selecting elements in the laser cavity.The inserted extra optical elements makes the laser resonator longer,which is not conductive to the generation of vortex beam with short pulse width and high repetition rate in passively Q-switched lasers.This thesis puts forward the design of Hollow Focus Lens(HFL).This lens can be regarded as the central part(the cylindrical region around the lens axis)of a positive lens replaced by a negative meniscus lens.The collimated fundamental Gaussian beam can be directly converted to the focused hollow beam by using the HFL,and the obtained beam can be used for pumping the solid-state lasers to produce vortex beams.Based on the HFL,a Nd:YAG microchip laser is set up under annular beam pumping,which can output continuous wave(CW)vector vortex laser beam.The lasing threshold is 0.7 W.The output vortex beam keeps radial polarization when the incident pump power(Pin)is lower than 5.5 W,while the output vortex beam keeps azimuthal polarization when Pin is higher than 5.5 W.The degree of polarization(DOP)of the output beam remains about 91%independent of the pump power.The variation of the vector polarization is attributed to the thermal-induced bifocusing effect in the Nd:YAG crystal.The output vortex beam possesses the topological charge number of 1,and the vortex handedness remains stable with the increasing pump power.The slope efficiency of the output laser is 16.8%when Pin is lower than 6.5 W,and the maximum output laser power is 10 W achieved at Pin = 1.16 W.After producing continuous-wave vector vortex laser,vortex pulses are also achieved in an annular-beam pumped Cr,Nd:YAG self-Q-switched(SQS)microchip laser,and the annular beam is created by the HFL.The thesis proposes a method to control the handedness of the output vortex beam by adjusting the tilted angle of the output coupler(OC)in the end-pumped solid-state laser.A Cr,Nd:YAG SQS microchip laser is established under the pump of the annular beam created by the HFL,and by tilting the OC angle the handedness of the output vortex beam can be controlled.The mechanism of handedness control is due to the fact that the tilted OC has different reflections for left-and right-handed Laguerre-Gaussian(LG)beams.The lasing threshold for vortex pulses is 0.9 W.The pulse width of 8.5 ns and repetition rate of 113 kHz is obtained at the absorbed pump power(Pabs)of 1.8 W.The slope efficiency of the output laser is 46.6%under higher pump power and the maximum laser power of 1 W is achieved.The polarization of the output vortex beam is close to circular polarization,and no vector polarization phenomenon is observed.The results of this study relating to the direct generation of vector vortex beams and vortex pulses are of theoretical and practical importance to develop novel vortex microchip lasers that can be used to generate high-power and handedness-controllable vortex beams.