Laser speckle reduction using a dynamic polymer-based diffraction grating spatial phase modulator

Abstract

Speckle is the constructive and destructive interference pattern observed from an optically rough surface when a highly coherent laser light is used as the illumination source. In this article, we present some methods to reduce the undesired laser speckle in laser display projector and show measurement results indicating reduction in speckle contrast obtained by using a dynamical flexible polymer-based diffraction grating phase modulator. Light is diffracted into multiple orders by the dynamic polymer diffraction grating. This diffracted light, after collection and homogenization, is used as the illumination source for displaying picture information. Due to the time-varying phase of the diffraction grating, the diffraction pattern changes thus creating time-varying speckle patterns on the rough screen. The time-varying speckle is captured by a CCD camera with appropriate integration time to mimic the averaging action of the human eye. Thus the time-integrated speckle is minimized by the intensity averaging of many independent speckle patterns. Different configurations of the diffraction grating along with random phase plate and beam combining lens were explored to ascertain the corresponding reduction in speckle contrast and its dependence on the time-period of the driving voltage of the diffraction grating.