Research On Orbital-angular-momentum-and Wavelength-tunable 2 μm Vortex Laser

Vortex beams have the characteristics of unique transverse mode structure,helical phase distribution,and photons carrying orbital angular momentum（OAM）,which have aroused extensive and profound research in recent years.These studies even gave birth to the concept of structured light.With the development of research,the role of vortex beams in new fields such as free space optical communication,particle manipulation,and Nanotechnology has also been gradually discovered.In particular,the orbital angular momentum carried by the vortex beam can be used as a degree of freedom for the multiplexing of optical communication like the wavelength.Multiplexing of two dimensions at the same time can greatly improve the information carried by optical communication.Simultaneously achieving multiplexing in two dimensions requires wavelength and orbital angular momentum tunable laser,so it is of great significance to carry out research on orbital angular momentum and wavelength tunable vortex laser.This paper conducts theoretical analysis and experimental research on the two-dimensional tuning of Tm:YLF lasers.The theoretical and experimental research results are as follows:In theoretical research,firstly,the electric field of the two transverse modes are obtained by solving the wave equation,from which the optical field distribution of the two modes and the characteristics of the orbital angular momentum carried by the vortex beam are obtained.Based on the mutual conversion relationship between the two transverse modes,a mode converter is designed to enable the two modes to be converted to each other.Subsequently,the output of the laser is simulated by solving the quasi-three-level rate equations.The reason why off-axis pumping leads to transverse modes tuning is obtained through simulation,which provides a theoretical basis for OAM tuning based on off-axis pumping and mode converter.Based on the transmittance formula of the F-P（Fabry-Perot）etalon,the spectral response curve of the etalon is simulated,and finally the parameter index of the etalon as a wavelength tuning device is established.In the experimental research,the tuning of the HG_0,0（Hermitian-Gaussian mode）to the HG_17,0 mode is completed by off-axis pumping,and the beam quality of the HG mode is measured.The largest deviation value appears in the HG_17,0mode,and the beam quality in the Y and X directions are 1.61 and 36.48 respectively（theoretical values are respectively 1and 35）,so it can be considered that the beam quality is good.Laser thresholds are measured for off-axis processes and the threshold curves are found to match theoretical simulation results.Subsequently,the mode converter is used to realize the conversion from HG mode to LG（Laguerre-Gauss mode）mode,and finally realize the tuning of LG_1,0 to LG_17,0mode（OAM:1η-1 7η）,and also the beam quality of the LG mode measurements were taken.The highest deviation is in the Y direction of the LG_17,0 mode.The deviation between the measured value and the theoretical value is only 6.44%,it can be considered that The beam quality is good.The wavelength tuning of the Tm:YLF laser is realized by an etalon with thickness of 25μm in the resonator,and the wavelength tuning from 1921nm to 1937nm is finally realized without off-axis operation.Based on the two-dimensional tuning experiment,dual tuning of OAM and wavelength is carried out.The operation of off-axis pumping and rotating etalon was carried out at the same time,and finally the tuning of LG_1,0 to LG_16,0modes is realized（OAM:1η-1 6η）,and the corresponding wavelength tuning range is LG_1,0 mode:1921nm-1937nm to LG_16,0:1928nm-1937nm.Finally,the wavelength tuning method is optimized,and the wavelength tuning range became LG_1,0 mode:1898nm-1943nm to LG_16,0:1898nm-1937nm.