Stimulated Brillouin Scattering Phase Conjugation Mirror Based On Rotating Wedge With High Pump Power

Current industrial and scientific applications such as material processing,high energy density physics,basic research of inertial fusion,and etcetera require lasers delivering high energy and high-quality optical beams whose divergence is within diffraction limit during beam propagation.Stimulated Brillouin scattering phase conjugation mirror(SBS-PCM)has the unique ability to restore an aberrated beam to the original undistorted state,control the phase conjugation-fidelity of lasers,produce high beam quality lasers in applications like space communication,improve the performance of high-power lasers,and effectively reduce laser system scale and cost.However,due to the continuous increase in heat accumulation at high pump power operation,other nonlinear effects like optical breakdown,thermal defocusing,and focal waist broadening are generated,which lead to beam quality deterioration.Thus,this thesis develops an SBS-PCM based on rotating wedge operating at high pump power.This thesis applies the phase conjugation property based on SBS theory to develop an SBSPCM based on rotating wedge operating under high pump power.It identifies optical breakdown and thermal effects as the major factors limiting the performance of SBS-PCM working at high-load conditions.There are three ways the optical breakdown could be reduced: by selecting a suitable SBS medium like FC-770 that has a low absorption coefficient and high optical breakdown,filtering the medium to minimize the microimpurities to the lowest possible level,and using a long focal length to reduce the power density at the focus.The following approaches will reduce the thermal effects: using a low absorption medium,highly purified medium,less light-medium exposure time,and a rotating wedge.The design of the wedge plate shows that the laser emergence angle,which determines the radius of focus rotation,depends on the laser light incidence angle,the wedge inclination angle,and the material refractive index.The customized SBS-PCM allows the laser focus to rotate 360° through a radius of 4 cm,distributing the accumulated heat across a large region,thereby alleviating the thermal effect.To effectively manage the thermal effect,this thesis also simulates the heat accumulated and probes the temperature gradient of the material,and caped the laser-medium exposure time to 300 s.For 0.65 W and 6.5 W input power,the average temperature distribution curves increase linearly until maximum.At 65 W input power,the temperature increases to an extremely high value which affects the SBS process,leading to a decrease of the reflected energy and poor phase fidelity of the reflected beam.The experiment of the SBS-PCM with a rotating wedge operating at high pump power is divided into two parts via the beam-quality of SBS-PCM and the energy reflectivity experiment.During the SBS generation process,the refractive index of the medium changes continuously due to laser radiation absorption of the medium impurities.For the unfiltered FC-770 medium,optical breakdown occurs at the input energy of 0.5 m J,50 Hz repetition frequency,and 25 degrees per second wedge rotation speed.After removing the microimpurities larger than 100 nm through filtration,the reflected energy and the beam quality of the SBS-PCM were much better.At 6.5 m J input energy and below 100 Hz pulse repetition rates,there is no observable optical breakdown phenomena or obvious thermal effects.Thus,both fidelity and stability were maintained by the SBS-PCM.