| Sub-nanosecond pulsed lasers with high-energy and high-repetition-rate operation are widely used in Doppler lidar,space debris lidar detection,Thomson scattering diagnosis,medical laser cosmetology and other fields.Stimulated Brillouin Scattering(SBS)is a simple and efficient pulse compression technology,which can compress long nanosecond pulses with large energy to sub-nanosecond pulses.Combining SBS technology and the main oscillation power amplification(MOPA)technology,the pulse lasers with four parameters of high repetition rate,high energy,sub-nanosecond pulse and high efficiency can be obtained at the same time.However,the current repetition rate of SBS pulse compression is limited to 200 Hz.In order to improve the repetition rate of SBS pulse compression to 1 k Hz,the key problems of high-repetition-rate SBS pulse compression are systematically studied.The research situation of high repetition rate lasers with high energy sub-nanosecond pulses is reviewed.The research status of the SBS medium,the compression structure and repetition rate operation are described in detail.The problems and reasons which limits the improvement of repetition rate operation of SBS pulse compression are analyzed.Firstly,the basic theory of SBS pulse compression technology is expounded.Considering the influence of the transverse spatial distribution of the pump,a transient theoretical numerical model describing the compression process of two-dimensional spatial SBS pulse compression is established.Considering the electrostrictive effect and the thermal effect caused by optical absorption,the theoretical model of stimulated thermal Brillouin scattering(STBS)is described.In order to obtain the temperature field distribution in SBS medium,a three-dimensional temperature field model of solid medium heated by repetitive short pulse laser is established.Secondly,the main reason for the large difference of the pulse width between the beam center and the edge of SBS pulse compression is the Gaussian cross-sectional intensity distribution of the pump beam.In order to obtain high energy efficiency SBS pulse compressions with a uniform spatial pulse width distribution,two new methods are proposed: blocking beam edge method and parameter optimization method,which are carried out and compared experimentally.The energy efficiency of the latter one is obviously higher than that of the former one.Based on the numerical calculation of the output parameters of SBS pulse compression with different media and structure parameters,the variation of the spatial pulse width distribution of SBS compressed pulse is investigated.The dependence of SBS pulse compression on different structures and media are studied experimentally.Based on the optimized parameters,sub-nanosecond SBS compressed pulses with a uniform spatial pulse width distribution are obtained based on a single-cell structure and compact two-cell structure,and the SBS energy efficiencies are both over 80%.Then,in order to solve the thermal problem of high-repetition-rate SBS pulse compression in the generator cell,two methods of rotating wedge and rotating offcentered lens are proposed to alleviate continuous heat accumulation near the focal spot.The influence of thermal effect on SBS output parameters is analyzed through the numerical calculation of three-dimensional temperature field characteristics and experimental study of the dependence of SBS pulse compression on laser repetition rate.The beam intensity distributions of focal spot for the two methods are simulated and compared by ray tracing method.The results show that the coma aberration distortion of focal spot in the rotating off-centered lens method is significantly smaller than the case of rotating wedge method.At the repetition rate of 1 k Hz,a rotating off-centered lens is used to compensate the beam spot distortion caused by thermal effect in SBS focusing structure,and improve the beam quality and energy reflectivity.Both theoretical and experimental results show that the rotating off-centered lens is a new effective method to alleviate the thermal effects of high-repetition-rate SBS pulse compression.On the basis of the rotating off-centered lens method,by measuring the beam intensity distribution and beam spot interference fringes of high-repetition-rate pulse laser propagating through the amplifier cell,it is analyzed that the cause of the quality deterioration of high-repetition-rate SBS compressed beam spot is the existence of selfdefocusing,thermal blooming,hot spot effect and thermal convection in the amplifier cell.Using HT270 medium with a high viscosity coefficient and a high boiling point,not only the heat convection and hot spot process in the amplifier cell are alleviated,but also the spot jitter caused by the highest temperature in the generator cell exceeding the boiling point is suppressed.Based on the medium purification method,the absorption heat of the medium and the optical breakdown frequency within the generator cell are reduced.Subsequently,the dependences of SBS pulse compression on viscosity coefficient,focal spot rotation,lens focal length and repetition rate are investigated experimentally.The results show that the liquid medium HT270 with a high viscosity coefficient is suitable for SBS pulse compression with repetition rate up to 1 k Hz,while the medium HT110 with a low viscosity coefficient is only suitable for SBS pulse compression with repetition rate of less than 200 Hz.Based on HT270 medium,a SBS compressed pulse with a pulse width of 820 ps and energy efficiency 52.2% is obtained under the pump enrgy of 50 m J at 1 k Hz.At last,diode-pumped solid-state laser systems has the most potential application because of its high stability.The stability of liquid-based SBS output is bad because of the viscosity coefficient of liquid medium is sensitive to temperature change.Fused silica solid medium with a high thermal conductivity is employed to improve the stability of SBS output in this paper.The stabilities of reflected energy and compressed pulse width of high-repetition-rate SBS pulse compression in liquid and solid media are compared experimentally.The energy relative standard deviation of fused silica-based SBS pulse compression reduces by approximately 55% compared with that of HT270 at 50 m J for 30 min.Based on fused silica,a 1 k Hz highly-stable SBS pulse compression with energy efficiency 37% and pulse width of 970 ps is achieved at a pump energy of 50 m J. |
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