| Continuous faceplate discrete actuator deformable mirrors(CFDA-DMs)are the most widely-used aberration correctors in high power laser systems.Working in such systems,to compensate the distorted wavefront effectively,the DM needs to reflect a large fraction of the laser radiation without significant heating.It is also important that any heating that occurs does not significantly impact the DM's performance.However,when the output power of the system is increased to a very high level,the faceplate will still absorb a large amount of energy though it is coated by film with very high reflectivity.This will engender a significant change in the surface shape of the DM,which,in turn,would cause a considerable change in the influence function and lower the aberration correction ability and effectiveness.In this paper,we will investigate the characteristic of laser-induced surface shape of the DM and the influence factors of it.The influence of thermoelastic deformation on the DM's aberration correction ability will be analysed as well.To begin with,the deformation of a 39-element DM was calculated using the finite element method and the surface shape was fitted with Zernike Polynomials.The simulation results showed that laser-induced surface shape is mainly constituted by low order components,especially defocus.However,after all the low order components were removed,the PV value of the remaining high order residual was still nearly half of the total PV value of the surface.Then,influence function change of the DM caused by thermoelastic deformation was studied.The influence functions of the DM under different irradiation conditions were calcualted using ANSYS and compared with the one without laser irradiation.It was found that the coupling of the functions would decrease whereas the Gausss Coefficients of the functions grew larger when the DM was irradiated by high power laser.At the same time,amplitudes of the influence functions for the actuators near the center of the faceplate experienced a dip while the corresponding ones for the actuators close to the edge of the faceplate climbed.Besides,the influence factors of thermoelastic deformation was analysed.Here,we mainly concerned about the material of the faceplate and the actuator layout.The results showed that materials with high thermal conductivity and low thermal expansion coefficient should be applied on the faceplate to lower the thermoelastic deformation.As far as actuator layout was concerned,compared with the one with a hexagonal layout,the DM with a square layout would suffer from a surface shape with a lower PV value.However,the maximum displacement of it was larger and there were more high order components in the surface shape of it.In addition,simulations and corresponding experiments were conducted to analyse the DM's ability to compensate its own deformation and to what extend the DM's aberration correcting ability would decline because of influence function change.It could be concluded that it had little impact on the DM's ability to compensate its own deformation whether considering the influence function change or not.However,if this change was not taken into account,the strokes of the actuator would sacrifice more for the sake of compensating the DM's deformation,thus leading to a larger deterioration in the DM's aberration correction ability.Furthermore,the DM would show a worse performance when correcting the low-order aberrations because of deformation,which could be demonstrated by the fact that the PV value of the residuals became bigger when the DM was heated.Apart from that,if the static influence functions were still utilized to calculate the driving voltages of the actuators,the compensating results would grow worse.Finally,the characteristic of the residuals after the wavefront was compensated using a CFDA-DM was studied.The simulations and experiments all revealed that no matter the DM was irradiated by high power laser or not,the residuals all showed a high order characteristic. |
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