Study On The Mechanism Of Laser Jamming On Ultraviolet Imaging

Ultraviolet?UV?imager is an important part of the ultraviolet missile approach warning system of acombat platform,which plays significant roles in ensuring the safety of the combat platform.In this paper,we conduct,both theoretically and experimentally,researches on the mechanism of laser jamming of UV imagers,aiming to obtain the optimal parameters of jamming the UV imager and to formulate corresponding defensive approaches.The interference and damage mechanisms and effects on UV imager under the illumination of continuous laser and pulsed laser have been carried out.The contents are composed of the following three parts:1.The analysis of the jamming mechanism of continuous laser on microchannel plate of the UV imager based on transition line method?TLM?.1)The experiment of continuous laser jamming on the UV imager was carried out.With the increase of laser power,the UV imager appears point saturation and surface saturation in turn.Comparatively,no linear saturation and pixel crosstalk were observed.The input and output characteristic curve of the UV image intensifier indicated thatthe image intensifier will gain saturated when the laser power density is higher than 9?W/cm²2)The gain saturation mechanism of image intensifier under continuous laser irradiation was studied.Based on the transmission line method,a signal amplification model of image intensifier was established,and the perturbation method was applied to solve the model.The numerical simulation results show that the gain saturation of the image intensifier originates from the electric field distortion of the microchannel plate caused by the amplification of strong light.3)The experiment of mutual interference between channels of the UV imager was carried out.The plateform for jamming light with large area and target light with small area was established.The curve of the gray value of target light in image intensifier with the intensity of interference light was measured.The experimental results show that there is interference between channels when the UV imager is irradiated by continuous laser and their coupling effects will benefit to suppress the target signal by reducing the gain of adjacent channel and irradiated channel.4)The theoretical mechanism of interchannel interference in UV imager irradiated by continuous laser was studied.The electrical coupling model between channels was established,and the influence of transverse resistance between adjacent channels on the gain characteristics of adjacent channels was studied by nodal analysis.The numerical simulation results show that the transverse resistance between channels is the main cause of interference between channels,which will greatly influence the gain of the adjacent channels.2.Simulation and experimental researches of pulsed laser jamming on microchannel plate.1)The effect of pulse laser repetition rate on the jamming effect of UV imager was studied in this part.Based on the transmission line model,the transient solution method was applied to analyze the amplification process of the pulse signal.And the influence of the distributed capacitance on the channel gain was considered in the solution process.The microchannel plate has no inhibitory effect on the interference of the continuous laser,but it has an inhibitory effect on the pulse laser.The inhibitory effect is related to the laser repetition frequency.The higher the repetition frequency,the weaker the inhibition effect on the imager.When the repetition frequency of pulse laser is greater than 2 kHz,the inhibitory effect of microchannel plate can be neglected and the jamming effect of pulsed laser rises to the same level of continuous laser.2)The mechanism of resolution degradation of the UV imager irradiated by pulsed laser was studied.The electron multiplication process of the UV imager was simulated by Monte-Carlo method,and the electrons in the microchannel plate were tracked.Given to the electron position distribution of UV imager in saturated and unsaturated state,the resolution characteristic of theUV imager in both states was calculated.The results show that the spatial resolution of UV imagers will decrease significantly with the increase of incident electron number.3)The interference-free effect between channels was studied when the UV imager was irradiated by pulsed laser.The electric field distribution generated by accumulating positive charge at the end of the channel was studied by using the electrostatic field module in COMSOL Multiphysics software.The influence of cumulative positive charges on the electric field of the channel was analyzed.The results show that the electric field produced by positive charges in the channel has a great influence on the electric field of the channel,but the influence on the adjacent channel is really weak.Therefore,there is no interference between channels when the UV imager is irradiated bypulsed laser.3.The interference and damage mechanism of pulsed laser on photocathode of UV imager.1)The interference platform of the pulsed laser on the photocathode of UV imager was built,and the saturation threshold of the UV imager was measured by changing the energy density of the pupil laser.When the laser energy density rises to 2?J/cm²,the output of photocathode reaches the maximum.The improvement of laser energy can not further enhance the jamming effect of UV imager.2)The theoretical mechanism of the saturation of amulti-alkali photocathode was studied.Based on the three-step electron emission model of the photocathode,the electron emission process of a multi-alkali photocathode irradiated by strong light was analyzed by using the Monte-Carlo method.Both the influences of space charge effect and light pressure effect on the electron emission process were considered in the model.Numerical simulation results indicate that the saturation of multi-alkali photocathode is mainly due to the space charge effect in proximity region of the image intensifier,and the effect of light pressure is very weak.3)The damage experiment of pulsed laser on the UV imager was carried out.The damage of each component was determined by observing the change of the morphology of the component before and after irradiation.With the increase of laser energy density,each component in UV imager will be damaged in order of multi-alkali photocathode,microchannel plate and optical window.The corresponding laser damage thresholds are1.8mJ/cm²,3.3mJ/cm² and 17.6mJ/cm²,respectively.4)The laser damage mechanism of multi-alkali photocathode and optical window was studied.Based on the heat transfer module and the solid mechanic module of the COMSOL Multiphysics,a photocathode thermal analysis model and a thermal stress analysis model of the optical window were established.Both models take into account the strong absorption of the incident laser by the photocathode.The numerical simulation results show that the damage of the multi-alkali photocathode is caused by volatilization of Cs elements on the surface of the cathode from laser thermal effect.The significant decrease in the damage threshold of the optical window is caused by the effect of the internal high-intensity thermal stress resulted from the absorption of laser energy by the cathode film.5)Based on the photocathode thermal analysis model,the influence of the window material on the damage threshold of the cathode film was studied by changing the thermal conductivity of the substrate material.The results show that as a substitute for fused silica,CaF₂ can be used as window material because it has much higher damage threshold for laser irradiation.Benefited from its relatively higher thermal conductivity,the damage threshold of CaF2-made multi-alkali photocathode increases from 2.206mJ/cm² to3.485mJ/cm² compared with traditional fused silica one.