Research On High-repetition-rate And High-pulse-energy Dye Laser Technology

Planar Laser Induced Fluorescence(PLIF)imaging technology with tunable lasers has been widely used in flow field and combustion diagnosis.In recent years,high-speed dynamic flow field PLIF diagnosis has been applied to the design of new engines and hypersonic aircraft,and has urgently demanded high-repetition-rate and high-pulse-energy tunable lasers.Information of flow field is important for the design of engines and aircraft,while PLIF imaging is an important method to obtain flow field information.With the rapid development of new engines and hypersonic aircraft in recent years,higher requirements have been placed on PLIF imaging.At present,PLIF imaging of 10 Hz repetition rate can no longer meet the needs of the flow field and combustion diagnosis in hypersonic engines and aircraft.As a result,there is an urgent need to develop high-speed PLIF imaging.Tunable laser is the core device of PLIF imaging.The repetition rate of tunable laser determines the frame rate of PLIF imaging,and the pulse energy determines the signal-to-noise ratio of PLIF images.This article will focus on the requirements of high-speed PLIF imaging,in order to carry out theoretical and experimental research on high-repetition-rate and high-pulse-energy tunable dye laser.Under high repetition rate and high pulse energy,the thermal effects in the laser dye medium will be extremely serious.Thermal effects will decrease the efficiency of dye laser,so it has become the core problem restricting the output of tunable laser.To solve the problem,this paper shows a two-dimensional pump spot shaping system.The shaping system increases the gain volume of the amplifier by improving the matching of the seed laser and the pump laser at the amplifier.The shaping system will suppress the thermal effect in the medium,and improve the efficiency of dye laser.By using the two-dimensional pump spot shaping system,the output pulse energy of dye laser is increased by 86.8%,and the efficiency is increased by 79.4%.The experimental results show that matching of the seed laser and the pump laser effectively improves the efficiency of dye laser.The thermal effect will cause the pulse energy of dye laser pulse to be unstable,and will cause the uneven intensity distribution of the dye laser.The instability of the pulse energy will change the signal-to-noise ratio of the PLIF images.The uneven intensity distribution will cause the distortion of PLIF images,which will affect the accuracy of dynamic flow field diagnosis results.In order to solve those problems,this paper simulates and analyzes the thermal effect distribution in the dye medium under high energy pumping,according to the energy level structure and rate equation theory of the laser dye.The results show that negative thermal lens effects are shown after pumping,and the focal length of the thermal lens decreases with the pump pulse energy increasing.At the same time,the thermal lens will also change the propagation direction of the output dye laser.In order to verify the simulation results,this paper uses pump detection method to measure the spots of probe laser passing through the dye medium.The distortion of the spots of probe laser is studied.The research results show that the establishment time of thermal effects in the working medium is 2.5?s.The focal length of the thermal lens decreases with pump pulse energy increasing.When the pump pulse energy is 30 m J,the equivalent focal length of the thermal lens is-0.62 m.To study the uneven intensity distribution of the dye laser,this paper uses sampling method to indirectly measure the dye laser spots in the pulse burst.This paper conducts an experimental study on the intensity distribution of pulse burst dye laser.The results show that under short pulse interval,the thermal effects deflect the propagation direction of the pulse burst dye laser.The deflection of the propagation direction reaches the steady state after 0.4ms,which shows the thermal effects accumulation in the dye medium under short pulse interval.The deflection angle of the dye laser propagation direction increases with pump pulse energy increasing,and increases with pulse interval decreasing.When the pump energy is 30 m J and the pulse interval is 0.1ms,the maximum deflection angle is 0.85 mrad.To study the instability of the pulse energy of the dye laser,this paper conducts an experimental study on the pulse energy characteristics of the pulse burst dye laser.When the repetition rates are fixed,adjustable spatial matching and spatial filtering technology are used to obtain tunable dye laser output with repetition rate of 500 Hz.Dye laser with pulse energy of 8.1m J at 566 nm,pulse energy of 1.7m J at 283 nm,ultraviolet conversion efficiency of 5.86%,and energy fluctuation of 4.3% is obtained.When the dye laser is working at burst mode,it adopts comprehensive utilization of dye cell at Brewster angle and with gain filtering technology.As a result,the shortest pulse interval is 0.1ms and the equivalent repetition rate is 10 k Hz.The pulse energy at 560 nm is 7.7m J,pulse energy at 280 nm is 1.45 m J,ultraviolet conversion efficiency is 4.47%,and energy fluctuation is 8.4%.Experimental results show that the efficiency and energy stability of pulse burst dye laser has been significantly improved.According to the research results shown above,this paper uses the ultraviolet pulse burst dye laser with high repetition rate and high pulse energy to carry out PLIF imaging experiments.This paper obtains the OH-PLIF image sequence of alcohol blowtorch,with a time interval of 0.1ms and up to 18 images.The intensity distortion of the OH-PLIF image is corrected according to the deflection of the propagation direction of the output dye laser.As a result,the consistency of the OH-PLIF image sequence is improved.