Generation Of Coherent Laser Base On Four Wave Mixing In Cesium Vapor

Laser is a kind of monochromatic,collimated and bright photon queue.Thanks to the excellent characteristics of laser,laser technology has widely used in modern production,life and scientific research.The beam quality and spectrum diversity of laser are the common standards to measure laser light source.It is very important to obtain excellent coherent light source.One of the traditional ways to obtain a laser source is to use different methods to excite a nonlinear crystal to achieve optical parametric oscillation and amplification effects to generate a coherent light source of a new wavelength.Many scholars have also used two-photon excitation of alkali metal atoms and four-wave mixing effects to produce laser sources with good coherence and multiple wavelengths selectivity,and it has certain application value in the fields of quantum communication,quantum information storage,and quantum entanglement.In this paper,based on the ladder-type energy level system of cesium atoms,two continuous and low power infrared lasers are used to pump and excite atoms,and coherent lasers are generated by parametric four-wave mixing process.The main innovations are as follows:（1）Experimental study on coherent blue light enhancement via repumping in cesium vapor.Based on the diamond energy level 6S_1/2（F=4）→6P_3/2（F’=5）→6D_5/2（F”=6）→7P_3/2（F’=5）→6S_1/2（F=4）system of cesium ¹³³Cs atom,two pump lights with wavelengths of 852 nm and 917 nm are cascaded to excite cesium atoms,and 455 nm continuous coherent blue light is generated by four-wave mixing effect.It is observed by two-dimensional grating that the generated 455 nm light has good spatial coherence.Although an optimized and closed energy level system has been selected experimentally,due to the collisions between atoms and with the inner wall of the ¹³³Cs vapor cell,a part of the atoms are accumulated in the ground state 6S_1/2（F=3）that does not participate in four-wave mixing.Therefore,we repopulate these atoms to the ground state6S_1/2（F=4）that participates in four-wave mixing by adding an repumping light at 894 nm,so that the generated coherent blue light is significantly enhanced.The blue light band is a transparent window of seawater,and has certain application value in the field of underwater free space optical communication.（2）Investigation on the generation of multi-color coherent lights in multi-diamond energy level system.Based on the three-diamond energy levels6S_1/2→6P_3/2→8S_1/2→7P_3/2、7P_1/2、6P_1/2→6S_1/2system of cesium ¹³³Cs atom,the generation of 455 nm,459 nm and 761 nm three-color coherent lights can be observed simultaneously by using 852 nm and 795 nm as the pump light to pump the atom from the ground state 6S_1/2to the excited state 8S_1/2and injecting a seed light of 894 nm.The competition relationship of these three four-wave mixing processes is preliminarily studied.The above work is of great significance for the subsequent research of narrow bandwidth and multi-color entangled light.