The Measurement And Control Of The Nonlinear Effect Of Rubidium Vapor With Femtosecond Laser Field

Nonlinear optics is a contemporary branch of optics that focuses on studying the nonlinear phenomena resulting from the interaction between intense laser and medium.The study of nonlinear optics holds immense significance in various fields such as nonlinear spectroscopy,laser cooling of atoms and molecules,phase conjugation and adaptive optics,and ultrafast optics.Ultrafast lasers offer several advantages such as a wide frequency range,short pulse time,and high peak power.The interaction between ultrafast lasers and medium has resulted in various new phenomena such as spectral broadening,higher harmonic generation,conical radiation,modulation up-conversion amplification,and nonlinear interaction enhancement.These developments have injected new energy and vitality into the field of nonlinear optics.The alkali atomic medium is characterized by a simple structure and abundant energy levels that can be effectively isolated from the surrounding environment.As an excellent nonlinear optical medium,it is widely used in the fields of all-optical device,quantum sensing and quantum communication network.The nonlinear effects of atomic media,including the Kerr effect,four-wave mixing,and phase modulation,have always been a focal point of research.When the laser field interacts with an atomic medium,it causes distortion of the electron cloud within the medium.This nonlinear effect will lead to the change of the medium susceptibility,which has important scientific and practical value in the research and practical application of nonlinear optics.In this thesis,the nonlinear characteristics of rubidium vapor have been systematically measured and adjusted through the interaction between femtosecond laser and rubidium atomic medium.The main contents and innovations are as follows:1.Study on the hyperfine interaction of rubidium atoms.A 778 nm/760 nm laser is used to excite rubidium atoms from the 5S_1/2 state to the 5D_5/2/7S_1/2 state.The high resolution monochromatic two-photon transition spectrum is obtained by improving the signal-to-noise ratio of the spectrum with intensity modulation method.The effects of experimental parameters such as laser scanning speed,laser power,laser polarization,and atomic density on the two-photon transition intensity are systematically studied.The magnetic dipole interaction constant A=-7.57（0.06）MHz and the electrical quadrupole interaction constant B=1.26（0.18）MHz of the ⁸⁷Rb 5D_5/2 state are accurately measured by 5S_1/2-5D_5/2 high-resolution monochromatic two-photon transition spectroscopy.The⁸⁵Rb 5S_1/2（F=3）-（F=2）hyperfine energy level interval of 2752（3）MHz and the ⁸⁷Rb5S_1/2（F=2）-（F=1）energy level interval of 6196（5）MHz are accurately measured by the 5S_1/2-7S_1/2 two-photon transition spectroscopy.2.Measurement of the rubidium atomic vapor thickness using multi-path optical interferometer.A high-precision,multi-path optical interferometer based on femtosecond optical frequency comb is constructed experimentally.The high-resolution multi-path interference spectra are obtained by placing atomic vapor cell in the interference path.The corresponding amplitude spectra are obtained by Fourier transform of the interference spectra.The phase information related to the optical path differences of different interference paths is extracted by the inverse Fourier transform of the amplitude spectrum,and the rubidium atomic vapor thickness is characterized.The atomic vapor thicknesses at different locations are accurately measured by scanning the rubidium vapor point by point,in which the measurement uncertainty of～6μm.The validity of rubidium atomic vapor thickness measurement using multipath interferometry is effectively verified by the rubidium atoms 5S_1/2-5P_3/2 transition reflection spectra at different locations.3.The measurement and control of the rubidium vapor nonlinear refractive index based on an optical frequency comb using the high sensitivity z-scan method.The self-defocusing nonlinear effect of atomic vapor is observed when the frequency of the femtosecond optical frequency comb is red detuning to the ⁸⁷Rb 5S_1/2（F=2）-5P_3/2（F’=3）transition.The self-focusing nonlinear effect is observed when the laser frequency is blue detuning to the ⁸⁵Rb 5S_1/2（F=3）-5P_3/2（F’=4）transition.The corresponding z scan dispersion curves are obtained through the above nonlinear effects.The third-order nonlinear refractive index ₂n≈10^-6 cm²/W of rubidium vapor is obtained,which has an order of magnitude enhancement compared with the results of continuous-wave lasers.The continuous control of atomic vapor nonlinear refractive index is realized from 0.58×10^-6 to 13.92×10^-6 cm²/W by adjusting the average laser intensity and frequency of the optical frequency comb.