Grating images of a multi-phase grating optical low-pass filter

The grating optical low-pass filter (GOLF) has lots of advantages over traditional birefringent low-pass filter. But, one disadvantage of it is that the shadow image of the grating is visible, especially when the numerical aperture of a camera is large. We found that we can reduce the visibility of a grating image, if we adopted a multi-phase grating instead of a two-phase grating. First, from the simulation of the Fresnel diffraction pattern of a one-dimensional multi-phase grating, we found the optimum width and phase of each grating step that minimizes the modulation of the Fresnel diffraction. And then, we designed a two-dimensional multi-phase GOLF by extrapolating one dimension structure. Four types of GOLF, four-phase, seven-phase, eight-phase, and also two-phase GOLF were manufactured with a semiconductor process. Finished GOLF samples were installed to a digital camera to take pictures of a test target and compare them with each other. Experiment results show that as the number of phase step increases, the grating image becomes more indistinctive.