Generation Of Radially Polarized Beam By Use Of Thermal Effect Of Laser Crystal

Radially polarized beam which was discovered several decades ago, is a kind of beam with spatial axial symmetry polarization. Since found in recent years, it has been used generally in High-resolution Microscopy, Electron Acceleration, High-density Storage and Metal cutting field, because of its unique mode and polarization. It attracts more and more attention from international laser technology field, and how to generate this special laser beam becomes a hot pot.Radially polarized beams can be used in experiment and industry, if only it has good beam quality, pure radial polarization and higher power. Here are several kinds of methods to produce radially polarized beam, some of which are through adding special optical components, or shaping beam mode out of cavity, or making use of birefringence and selectivity oscillation of radially and azimuthally polarized laser. Every method has its advantage, and the third one has simplest system structure, purest polarization, and highest output power.This paper generally introduces the principle of producing radially polarized beam by thermal birefringence theory at the beginning. And then we analyze the thermal effect of Nd:YAG rod in lamp pumped system, the refraction rate change caused by temperature gradient, and stress and strain. We found that the change of indicatrix is the dominating reason why normal polarized beam become aberrated, and the radially and azimuthally beam depart from each other. Then we got the thermal foci of two different beams, depend on this we derived stability diagram of laser cavity containing a thermal lens and birefringence, and proof it by finite element transmission matrix method. Through experiment, we got 20.6W radilly polarized beam output.