Research On The State Of Polarization Of Frequency Doubled And The Influence Of Residual Birefringence In Lithium Niobate Crystal

Thanks to the emergence of laser,second harmonic(SH)has been widely used in material characterization and biomedical imaging due to its characteristics of harmonic signal,no energy absorption process,high sensitivity,non-interference and bio friendliness.For a long time,the research on SH mainly focuses on the intensity and phase,but the research on its polarization characteristics is not enough.At the same time,there is inevitably residual birefringence in the process of crystal growth and processing,which reduces the electro-optical properties of the crystal.The main work of this paper is to study the polarization state(SOP)of SH signal and the influence of residual birefringence in lithium niobate(LN)crystal.The main contents of this paper are as follows:(1)It is calculated that when the length of the crystal is shorter(20 mm),the angle of separation of e-light and o-light in lithium niobate crystal is less than 3 °(@532nm),and their emergent light is still in the laser spot,so the polarization problem of the second harmonic light still exists.(2)In this paper,the coordinate system is established with the z-axis as the direction of light propagation,and the expression of the second harmonic electric field is derived under the arbitrary polarization state of the fundamental frequency light.Then,the Stokes vector representation of the second harmonic is given.Taking the o-polarization of the fundamental frequency light as an example,with the help of the Poincare Sphere,the three rotations of the crystal around the optical axis,the crystal around the o-polarization direction and the crystal around the wave vector direction are studied.The evolution of the polarization state of the second harmonic under different crystal rotation situations.(3)Based on the on-line polarization controller(PC),a short wavelength polarization state measurement system is designed.The influence of optical fiber coupler on the polarization state in the measurement system is fully considered.The influence of equilateral triangle 1x3 coupler on the polarization state is analyzed.The working index of the on-line polarization controller is analyzed and designed.The possible measurement delay and the power balance of each branch are discussed.(4)The residual birefringence of LN crystal is studied.Based on the theory of quaternion polarization optics,the residual birefringence of LN crystal is studied.A compensation method for residual birefringence is proposed.The results show that for the crystal with intrinsic birefringence,the principal axis of polarization is determined by both intrinsic birefringence and residual birefringence,but the intrinsic birefringence dominates the principal axis of polarization,while the influence of residual birefringence on the principal axis of polarization can be ignored.That is to say,the influence of residual birefringence can be ignored when using the qualified LN crystal for electro-optic modulation;because the residual birefringence vector of the bicrystal is the same and the direction is opposite,the influence of residual birefringence can be completely eliminated by using the bicrystal method in theory.