Modeling And Characteristics Of Two-photon Excitation Of Alkali Vapor Laser

Diode-pumped alkali vapor laser?DPAL?has been vigorously developed in recent years due to the advantages of high quantum efficiency,good beam quality and high-power output potential.Meanwhile,two-photon excitation of alkali vapor laser?TPEAL?has also attracted more and more researchers'attention because of its advantages of blue-and mid-infrared dichromatic coaxial output and superior physical characteristics.The implementation process of the blue-and mid-infrared laser output for TPEAL is that the alkali metal atom in ground state transmits to the high energy state(nS_1/2?n,n+1D)by two-photon absorption with the help of intermediate energy level?nP?or virtual energy level,and then laser outputs are realized by parametric four-wave mixing effect triggered by amplifying spontaneous emission.The parametric four-wave mixing in TPEAL process has been applied in photon storage,quantum memory and quantum computation.And it is expected to be applied in infrared guidance,infrared detection and Underwater Laser Communication in the future.To date,a large number of exploratory experimental researches on TPEAL process have been carried out at home and abroad.However,the systematic studies on its pumping threshold,absorption cross-section and other characteristics have been few reported theoretically because of the particularity of TPEAL process compared with ordinary four wave mixing,which greatly restricted the development of TPEAL.Therefore,this paper adopts a combination of theoretical modeling,numerical simulation,experimental data validation and analysis to conduct systematical research on the working characteristics of TPEAL process.The main contents are as follows:1.The research status of TPEAL at home and abroad is detailed,and the characteristics of two kinds of alkali metal vapor Rb and Cs are compared and analyzed,also with the existing problems in the research of TPEAL pointed out.Based on the introduction of the physical properties of alkali metal atoms,the pumping mode of TPEAL is expounded.And the key parameters needed in the construction of the theoretical model and the analysis of the characteristics are solved,which could provide parameters support for the theoretical modeling of TPEAL process.2.During the establishment of the TPEAL model,the coupled-wave equation is used to describe the parametric four-wave mixing process,and the small signal solution of the coupled wave equation is carried out.In the solution process,the propagation equation of electric field intensity of the mid-infrared and blue light is obtained by combination with the amplified spontaneous emission theory,by which can be given the boundary conditions and initial terms of the equation.And then the variation law of that is analyzed,which provides a theoretical basis for the systematic analysis of the output characteristics of TPEAL process.3.The parameter of two-photon absorption cross-section are proposed for better describing the two-photon absorption process of TPEAL and quantitatively analyzing the characteristics of two-photon absorption.Based on the coupled-wave equation,the absorption cross sections of rubidium atom and cesium atom in single-wavelength and dual-wavelength pumping mode are modeled and analyzed respectively.The simulation results are in good agreement with the experimental data,which verifies the validity of the model.Then the influence of temperature and frequency detuning of pumping laser are analyzed when the hyperfine structures of atoms are considered.It is found that the frequency detuning of pumping laser is the most important factor affecting two-photon absorption cross-section.The proposal of two-photon absorption cross-section is of great significance to the quantitative description and analysis of two-photon absorption process of TPEAL.4.The physical model of pumping threshold for TPEAL process is established and the characteristics of this model are systematically analyzed.The threshold model under single-wavelength pumping mode is established by combining the coupling-wave equation with the rate equation.The validity of this model is verified though comparison with the experimental data.Then a subsequent analysis on the pumping threshold confirms that the frequency detuning of pumping laser and the temperature of vapor cell have a vital influence on the pumping threshold.Simultaneously,we build the threshold model for dual-wavelength pumping mode on the basis of the model for single-wavelength pumping mode.By comparing with the results of pumping threshold for single-wavelength mode,it is found that the value of pumping threshold for TPEAL process in the dual-wavelength pumping method is much smaller than that of the single-wavelength pumping case,and there are different results in the influence of the frequency detuning of the pumping lasers and the working temperature on two-way pumping mode.5.Aiming at the phase matching of parametric four-wave mixing in TPEAL,the studies can be divided into non-collinear phase matching and collinear phase matching.The non-collinear phase matching is analyzed by graphic method,which could clearly show the relationship between the optical vector of pumping laser and the maximum phase matching angle.It is found that the non-collinear phase matching can be achieved in a small angle range of the pumping laser.A nonlinear term of refractive index is introduced based on the linear refractive index in order to more accurately describe the phase matching process of TPEAL and analyze the influence factors that affect the collinear phase matching.The results show that the power intensity and frequency detuning of pumping laser have a crucial impact on the nonlinear phase matching,and the studies on characteristics of phase matching is helpful to realize efficient TPEAL process.In this paper,two-photon excitation of alkali vapor laser has been carried out in depth,which solves the problem of quantitative description and analysis for the whole process and characteristics,and the researches on TPEAL also provide theoretical guidance for experimental researches on characteristics of TPEAL process and the realization of efficient laser-output.