A new design of thin-film grating optical low-pass filter and its fabrication

When sampling a periodic image that includes high spatial frequencies with a digital image device, alias distortion is always produced. To reduce the distortion, an optical low-pass spatial filter (OLPF), which cuts off the high spatial frequency, can be applied. In this study thin-film phase grating optical low-pass filter (GOLF) with two-phase and three-phase of have been analyzed. By matching the period of the grating to the thickness of the thin film of two-phase GOLF, the cut-off bandwidth was twice of the pass bandwidth. By using the same technique, the cut-off bandwidth of three-phase GOLF was five times of the pass bandwidth. Two-phase GOLF was fabricated by using physical vapor deposition technology with a metal mesh as mask and showed advantages of being simple, cheap and good for mass-production as compared with the conventional methods. The surface profiles of the thin-film phase GOLF was measured by a white light interferometer. The optical property, the modulation transfer function (MTF) was measured by a MTF measurement system. Both results showed good agreement with the theoretical calculation. The image of a digital image device also showed the Moire pattern was suppressed successfully by the thin-film phase GOLF.