Femtosecond Laser-induced Air Lasing And Grating Scattering

The physics research and application exploration of ultra-short and ultra-fast laser is always a hot issue in strong field physics.In recent years,air lasing has been widely studied because of its prospect in remote sensing;As a coherent optical detection technology,the application of laser induced grating scattering(LIGS)is being broadened.In this thesis,lasing effect from₂ and₂⁺are studied on the interaction between gas medium and strong field;The application of femtosecond laser-induced grating scattering(fs-LIGS)in gas pressure measurement is explored.We conducted a series of experiments around air lasing.In view of the lasing effect from₂,we mainly study the rapid inelastic collision caused in the process of population inversion may exist coherence between the molecules:pump-double probe experiment results show that the two beams is very close to,but an independent probe pulse does not affect the result of the respective lasing,showed that the nitrogen molecules gain medium does not establish macroscopic polarization;The amplification of lasing at 337nm,Raman scattering induced by lasing at 337nm and fine spectra of lasing at 337nm are also obtained;In addition,a thin plate with small holes is used to divide the cavity and control the length of the filament,and in this way the gain accumulation of the lasing in the light filament is studied,and the relationship between the lasing intensity and the length of the filament is obtained.In view of the lasing effect from₂⁺,we mainly studied the influence of polarization state of pump pulse on lasing intensity:We noticed that lasing intensity-polarization curve has an abnormal‘dip'structure,when the polarization of pump pulse changed from circular polarization to linear polarization,laser intensity increased gradually,close to linear polarization lasing intensity began to drop suddenly.Lasing spectrum,the pressure dependence and gain dynamics studies have shown that the abnormal‘dip'are widespread in the lasing effect from₂⁺.We hope to explain this phenomenon by means of population changes caused by changes in₂⁺yields of each excited state.More experiments and simulations are needed to improve the physical picture.On the other hand,we use the polarization gating(PG)to modulate the polarization state of the pumped pulse.The results show that the linear polarization gating(LPG)can effectively enhance the intensity of the P rotational branch of the lasing effect from₂⁺at 391nm,while the spectrum of the R branch is not sensitive to this.The enhancement can be well explained by the multipe-state coupling model.However,the selective enhancement of the rotational branch cannot be explained.In consideration of the experimental results of the elliptical polarization gating(EPG),we believe that there are other physical mechanisms that contribute significantly to the lasing at R-branch which need further study.In the study of laser-induced grating scattering technology,we used femtosecond laser as the pump pulse to generate grating to develop fs-LIGS technology,and used it to accurately measure the local pressure.We use two 800nm,35fs femtosecond lasers and make them time-space overlap to generate refractive index grating and acoustic grating structure.Then we use 532nm continuous laser as probe and incident grating at the first order Bragg diffraction angle,so as to generate coherent fs-LIGS signal light at the phase matching angle due to the four-wave mixing process.The attenuation characteristic time of fs-LIGS signal light has a specific relationship with the pressure in the grating area,which lays a physical foundation for fs-LIGS technology to measure the pressure.By changing the pressure and measuring the corresponding time-resolved fs-LIGS signal in the range of 0.2-3.0 bar,and then obtaining the measured value of pressure through theoretical fitting,we find that the measured value is linearly dependent on the true value,and the slope of the dependence curve is 0.96,which is very close to 1,indicating the feasibility of fs-LIGS technology for measuring pressure.