Stimulated Brillouin Amplification And Phase Conjugation Properties Of Ince-Gaussian Beams

The Ince-Gaussian(IG)mode,as a representative of the generalized high-order global mode,is an exact solution of the paraxial wave equation in the elliptic coordinate system.It has a special elliptic parameter ,which can be adjusted to complete the transformation from Laguerre-Gaussian(LG)to Hermite-Gaussian(HG).At the same time,the IG beam has the characteristics of clear Gouy phase shift and unchanged propagation.It is widely used in optical communication,quantum storage,quantum key distribution and particle manipulation.Stimulated Brillouin Scattering(SBS)is a typical third-order nonlinear effect with the characteristics of laser amplification,phase conjugation and pulse compression.It has been used in weak signal amplification,distortion correction and high power laser.At present,there are many studies on the linear control of IG beams.The nonlinear control mainly focuses on the second-order nonlinear effects such as frequency up/down conversion,and there are few studies on the stimulated Brillouin scattering of IG beams.However,in the application research of IG beam long-distance optical communication,it is urgent to solve the key problems such as energy relay amplification and atmospheric turbulence distortion correction.Therefore,in view of these problems,this paper focuses on the stimulated Brillouin amplification(SBA)and phase conjugation characteristics of IG beams,which lays a foundation for the application of IG beams in long-distance transmission.Firstly,the SBA characteristics of IG beams are studied theoretically and experimentally.The SBA mathematical model of IG beam is constructed,and the influence of pump structure on the spot morphology of IG beam after Brillouin amplification under small signal condition is studied theoretically.The physical mechanism of the influence is analyzed from the perspective of IG mode decomposition.The research shows that the non-uniformity of the intensity distribution of the Gaussian pump beam can easily cause the lateral distribution distortion of the IG beam.It is necessary to continuously increase the waist ratio of the pump beam and the IG signal beam to alleviate the distortion problem,resulting in energy loss and conversion efficiency decline.To solve this problem,a scheme of using high-order super-Gaussian beam,namely flat-topped beam as pump beam,is proposed,which effectively solves the distortion problem of amplified spot.Secondly,the phase conjugation characteristics of IG beams are studied theoretically and experimentally.Phase conjugation refers to the process of obtaining the complex conjugate structure of the incident light wave after the interaction between the light wave and the nonlinear medium,which is often used to correct the distorted light wave.The method of generating phase conjugate waves by SBS has been applied to high power laser systems to correct the distorted wavefront caused by the thermal effect inside the active material.The study of the phase conjugation characteristics of the IG mode shows that the helical Ince-Gaussian(HIG)mode with chirality cannot produce its own phase conjugation,and can only degenerate into the corresponding single odd/even mode IG mode.The single odd/even mode IG mode can undergo a phase conjugation process,and the mode of the phase conjugated light is consistent with itself.The physical mechanism is theoretically analyzed and experimentally verified.