Motionless Decoherence Thin Film Based On Multi-Exposure Lithography And Its Application In Speckle Reduction

Laser display is regarded as the fourth-generation display technology after CRT display,LCD display,and LED display.Laser display has the advantages of high brightness,large color gamut,long lifetime and low power consumption.Due to the high coherence of laser,when lights are scattered from an object with rough surface,the interference and scattering of the lights produce granular bright and dark regions,which are named as speckles.Speckle seriously degrades the quality of the projected picture,and it must be suppressed.At present,the mainstream speckle suppression methods in laser display include vibration diffuser,scanning micromirror,and vibration screen.This type of method achieves speckle suppression by temporal superpositon of the multiple independent speckle patterns during the integration time of the human eyes.This type of method can effectively suppress speckle,but due to the introduction of vibration or rotating mechanical devices such as motors into the system,the system volume is increased while the stability of the optical system is reduced,and additional noise pollution is generated.In this paper,we study a static speckle reduction method based on destroying the temporal and spatial coherence of the laser.And we fabricate a 3D micro-optical structure by multi-exposure lithography to obtain motionless Decoherence Thin Film(DTF).The DTF can modulate the optical path of the laser,reducing the temporal coherence of the laser,and thereby destroying the spatial coherence of the laser.Therefore,speckle reduction can be achieved.Applying the DTF with a diffuser in the field of laser display can achieve the goal of no extermal power supply,and speckle reduction efficiency is high.The DTF can also be used in holographic imaging,laser lighting and other fieldsThe main content of this article includes the following two aspects:1.Fabrication of proof-of-concept DTF with 3D micro-optical structure based on multi-exposure lithography.SU-8 photoresist is used to fabricate the DTF with 3D micro-optical structure through multi-exposure lithography.The DTF has 2×2 duplicated micro-structured and multi-layer cell,and the height of the adjacent cells is different,the DTF can modulate the optical path of the laser,thereby destroying the temporal and spatial coherence of the laser.2.The experiment of speckle reduction based on the proof-of-concept DTF.A HeNe laser and multimode laser diode with different coherence properties are used as light sources to investigate the speckle reduction ability of the DTF.Because the laser diode has a shorter coherence length than HeNe laser,when laser diode is light source,the efficiency of speckle reduction of laser diode based on the DTF is better than HeNe laser.The corresponding objective speckle contrast is reduced from 0.68 to 0.42.When the light source is HeNe laser,the corresponding objective speckle contrast is reduced from 0.71 to 0.66.