Abstract: In this paper, we present a novel and simple optical watermarking system aiming at overriding some practical problems when the state-of-the-art digital watermarking techniques are applied to authenticate the printed documents. We name our technique as optical watermarking differing from traditional digital watermarking in a sense that the watermark extraction is done by some optical and visual means like photocopier while no any digitization is required. The system security is guaranteed by adopting content-based key share scheme originated from visual cryptography. The non-obtrusiveness effect of watermarked document is achieved by modulating the watermark into a higher resolution gratings level. Experimental results demonstrate the availability and practibility of the proposed approach.