Experimental Study On Laser Speckle Suppression Based On Flexible Diffractive Optical Element(DOE)

Pico-projector is a new type of projection device that is small in size,light in weight,and easy to carry.Compared with the LED light source pico-projector,the laser light source pico-projector has a smaller size and can obtain higher quality images due to its higher brightness and larger color gamut.However,there is also the problem that the laser image is strongly modulated by speckle noise.Speckle is a granular random intensity pattern generated by the coherence of the laser.It reduces the quality of the image and limits the resolution of the imaging system.At present,laser speckle has become the most important obstacle for laser light source projection to obtain high-quality images.Speckle suppression methods proposed in previous studies mainly include the introduction of polarization diversity,wavelength diversity,and angular diversity of laser light sources.Among the currently effective speckle suppression methods,the most suitable for the use of pico-projectors is to add a dynamic diffractive optical element(DOE)to the optical system.The one-dimensional DOE produced on the transparent film by the hot pressing method is rolled end to end and looped together to perform a crawler-type movement,so that a dynamic two-dimensional DOE structure can be constructed in real time.In this paper,starting from the speckle suppression scheme based on the tracked flexible DOE loop,the principle of suppression of laser speckle by the Barker code and M-sequence two-dimensional DOE microstructure is introduced.Then,a series of researches are done on the key technologies required for the flexible DOE loop from theoretical simulation to practical application.The original flexible DOE loop manufacturing method was improved,the optical experiment scheme required fortesting various data was optimized,a new DOE loop motion control system was proposed,and the laser spot acquisition system and data analysis method were completed.At the same time,I put forward my own views on the technical obstacles in the practical application of the speckle suppression scheme based on the track-type flexible DOE loop,and made a focus on experimental research on the durability of the DOE loop.Finally,based on a large number of experimental studies,the tensile properties of the DOE loops of PVC and PP materials,and the effect of tensile deformation on the DOE loops' suppression of speckle were analyzed.The experimental results show that after several months of pre-use and one-month durability test experiments,the DOE loop of PVC material has produced about 10%deformation,and it can still effectively suppress the laser speckle contrast of the corresponding band to about 4%.And after 6 months of durability test,the DOE loop of PP material has less than 1% deformation,which can effectively suppress the laser speckle contrast of the corresponding band to about 3.5% for a long time.The work of this paper provides a theoretical and experimental basis for the speckle suppression scheme based on crawler-moving flexible DOE loop to be applied to pico-projectors.