Diagnosis And Analysis Of Coherence Of Low Coherence Light Field

In the process of laser driven inertial confinement fusion,the instability of the laser plasma interaction is a key problem affecting the fusion ignition.In order to inhibit the growth of the plasma instability,a variety of smooth beam technologies are introduced to reduce the coherence of light.However,due to the bandwidth limitation of traditional laser driver,the smoothing effect is limited.A large number of experiments and simulation results show that low coherent light can solve this problem.Using a low coherence light with larger bandwidth,more spectral components and lower coherence will effectively suppressed the growth of laser plasma instability.Different from the traditional high coherent light,the high power laser driven by low coherent light needs investigations.In this thesis,a high-power laser system with low coherent light is used as the platform to study the coherence characteristics of the optical field.Firstly,aiming at the problem that the high-power laser driver usually runs for a single time,a new single-time coherence measurement method is proposed based on the principle of equal-thickness interference.The measuring range of the equipment is 157.3 μm.The maximum optical path difference is directly proportional to the beam diameter and inverse proportional to the CCD measuring distance,and it increases with the increase of the wedge angle of the wave surface.The error fluctuation of the measurement results caused by adjusting the angle is less than 2%.The influence of spatial coherence and noise on the measurement results is analyzed.The interference of spatial coherence is reduced by introducing pinholes into the beam expansion system.In the light intensity calculation,the background light intensity is removed and the data are averaged by multi-line light intensity to reduce the influence of noise on the measurement results.This method is used to measure the chirpped pulses,the superluminescent diode source and the amplified spontaneous emission source.On the one hand,the results verify the reliability of our scheme.On the other hand,the results show that the broadband low coherent light has the characteristics of instantaneous broadband and random phase after comparing the different broadband optical coherence characteristics,which is beneficial to the suppression of nonlinear and speckle effects.In this thesis,the variation of the coherence of the low-coherent light in the amplification process is studied.The coherence evolution of four rod amplifiers in the pre amplification stage is compared in a low coherence high power laser.Because of the inhomogeneity of the gain spectrum of the amplification medium,the incoherent spectral composition and intensity will change during the amplification process.The spectral narrowing and the coherence of the optical field will be improved.In order to ensure the low coherence of the optical field and improve the efficiency of frequency doubling,the optical field needs modulation by spectral shaping.The modulation curve obtained by simulation calculation provides a reference for actual spectral shaping.