Highly Efficient Yb:YAG Vortex Microchip Laser

Vector vortex lasers have been widely used in optical communications,optical tweezer,laser processing,quantum information processing and so on owing to their unique ring-shape intensity distribution,spiral wavefront,phase singularity,carrying orbital angular momentum(OAM)and different polarization states.It is a hotspot for developing highly efficient,high beam quality vector vortex lasers.Compared to generation of vortex lasers by converting a fundamental Gaussian laser beam with spatial light modulator,spiral phase plates and so on,direct generation of vortex lasers in resonators has the advantages such as high beam quality,coherence,high power and high efficiency.Although annular pump beam has been widely used to achieve vortices in solid-state lasers,annular pump beams are formed by converting Gaussian beams with various optical elements and the utilizing efficieney of pump power is low.Nd3+-ions doped laser materials such as Nd:YAG and Nd:YVO4 crystals are used as gain media in annular beam pumped solid-state laser for generating vortex lasers,the optical efficiency is low owing to the low quantum efficiency of Nd3+-ions doped laser materials.Compared to Nd3+-ion doped laser materials,Yb3+-ion doped laser materials,especially Yb:YAG crystals,are widely used as gain media of laser-diode pumped solid-state lasers due to their low quantum defects,broad absorption band,long fluorescence lifetime,high doping concentration,and excellent thermal properties.And highly efficient,high power lasers have been achieved in laser-diode pumped Yb:YAG lasers.Therefore,it is necessary to develop annular beam pumped Yb:YAG microchip laser for directly generating vortex lasers with high efficiency and high power.To overcome the shortcomings such as low optical efficiency,small topological charge,and low beam quality of the annular beam pumped solid-state lasers with Nd3+-ions doped laser materials as gain media,annular beam pumped Yb:YAG microchip lasers for generating vortex lasers with hIgh beam quality and hIgh efficiency have been investigated in this paper.The mechanism of generating vector vortex laser and vortex array laser in annular beam pumped Yb:YAG microchip lasers has been investigated systematically by adjusting the absorbed pump power distribution inside Yb:YAG crystal.And highly efficient and high bean quality vector vortex lasers and vortex array lasers have demonstrated in annular beam pumped Yb:YAG microchip lasers.Polarization tunable vector vortex lasers with radial polarization(RP),anti-radial polarization(ARP),and hybrid polarization(HP)combining RP and ARP together have been demonstrated,for the first time to our best knowledge,in focused annular beam pumped Yb:YAG microchip laser by adjusting the offset distance(?z)between focus spot and Yb:YAG crystal.RP vortex laser has been obtained at ?z<-0.3 mm;ARP vortex laser oscillates at ?z>-0.1 mm;HP vortex laser is achieved when ?z is within the range from-0.3 to-0.1 mm.And the degree of polarization for three polarization states is large than 90%.When the incident pump power is set at 6.5 W,output powers of over 1 W for the vector vortices with three polarization states have been achieved within a wide range of ?z(e.g.-0.85 mm<?z<-0.3 mm for RP;-0.1 mm<?z<0.4 mm for ARP;-03 mm<Az<-0.1 mm for HP).The maximum output power is 1.68 W for HP vortex laser at available incident pump power of 7.8 W.And the corresponding optical-to-optical efficiency is over 22%.The beam quality factor is measured to be less than 2.4 for LG0,1-mode vector vortex lasers with three different polarization states,which is close to the theoretical value of the LG0,1 mode laser.High beam quality vortex laser has been obtained in the annular beam pumped Yb:YAG microchip laser.Highly efficient and high beal quality radially polarized vortex lasers with tunable topological charges from 1 to 14 have been generated in an annular beam pumped Yb:YAG microchip laser by using a 1 mm thick Yb:YAG crystal and a output coupler with 98%reflectivity.The experimental resuilts show that spiral phase and polarization state are stable and reproducible for radially polarized vortex lasers with different polarization states.The pump power dependent wavelength tunable and dual?wavelength laser oscillation in radially polarized vortex lasers has been observed.Radially polarized vortex laser oscillates with topological charge from 1 to 7 oscillates at 1030 nm;vortex laser with topological charge from 8 to 10 oscillates at 1030/1050 nm dual-wavelength;vortex laser with topological charge large than 10 oscillates at 1050 nm.Output power of 1.36 Wforradillay polarized vortex laser with 14topological charges is achieved at the incident pump power of 7.8 W,and corresponding optical efficiency is 17.5%.Near-diffraction-limited beam quality with M2 close to l+1 has been achieved for LG0,l-mode radially polarized vortex beams with tunable topological charges.Based on multi-mode laser rate equations,the threshold pump powers have been theoretically calculated for vortex lasers with different topological charges generated in annular beam pumped Yb:YAG microchip laser,which are in good agreement with experimental results.The mechanism of generating vortex lasers with tunable topological charges in Yb:YAG microchip laser has been proposed.By using 1-mm thick Yb:YAG crystal as gain medium and a output coupler with 95%reflectivity,vortex array laser with phase singularity of 10 has been demonstrated in a tilted annular beam pumped Yb:YAG microchip laser.Vortex array lasers with tunable phase singularity tuning from 1 to 10 have obtained by increasing the incident pump power,and point symmetry is observed for vortex array lasers with different phase singularities.The pump power range is larger than 0.3 W for stable oscillation of vortex array lasers with different phase singularities.The vortex array lasers oscillate at 1030 nm.The output power of vortex array lasers increase linearly with the incident pump power and the slope efficiency is 37.4%.Maximum output power of over 2 W is obtained for vortex array laser with 10 phase singularities at the available incident pump power of 8.2 W.And the corresponding optical-to-optical efficiency is as high as 24.5%.The theoretically calculated saturated inversion population distribution inside Yb:YAG crystal is uniform for tilted annular beam pumped Yb:YAG microchip laser,which is favorable for multiple modes oscillation simultaneously.The mechanism of generating vortex array lasers with tunable phase singularities in tilted annular beam pumped Yb:YAG microchip laser is illustrated by taking account into the laser threshold of LG0,l mode lasers.