Theoretical Study Of Transverse Mode Selection In Laser Resonator With Volume Bragg Gratings

The quality of laser beam has direct impact on the the application of the laser.Due to the small diffraction loss,energy concentration and small beam divergence angle,the fundamental transverse mode is required in many laser applications.However,the laser usually operates in multimodes state,and will make the output laser beam with nonuniform distribution,large beam divergence and poor coherence.In order to improve the output beam quality,the transverse mode selection is required in the laser resonator.The traditional transverse mode selection method mainly depends on the resonator parameters or using a small aperture in the resonator,but there are still have some problem need to solve,such as large diffraction loss,small mode volume and small output power.The volume Bragg gratings(VBGs)recorded in a photo-thermo-refractive(PTR)glass has several unique optical properties such as high diffraction efficiency,excellent angular selectivity,high damage threshold,and has many applications in high power lasers.In this thesis,volume Bragg gratings used in a laser resonator as transverse mode selection elements is proposed.First,the diffraction models for different types of volume Bragg gratings are introduced by the couped wave theory,the influence of different grating structure parameters on the diffraction characteristics are analyzed and introducing the design of grating parameters for transverse mode selection,and the diffraction loss of the different high order transverse modes diffracted by volume Bragg gratings are discussed.Secondly,a solution to calculate output modes of a laser resonator with volume Bragg gratings is established by the Fast Fourier transform method and the couped wave theory.At last,using the transmitting Bragg grating and the reflecting Bragg grating in a plane-parallel resonator as volume Bragg resonator modal respectively,then simulated the output modes of the volume Bragg resonator model,and the characteristics of the transverse modes are analyzed.The influences of angular selectivity of the transmitting Bragg grating and the reflecting Bragg grating on the transverse modes both in the near field and in the far field are discussed.The results show that under the effect of volume Bragg gratings in laser resonator,the intensity distribution of transverse modes is optimized,and the loss differences between different transverse modes are getting large.By matching the parameters of laser resonator,gain and angular selectivity of volume Bragg gratings,the loss difference between different modes enlarged effectively,the performance of the laser resonator in transverse mode selection improved,and the fundamental transverse mode output can be obtained.