Watermarking is a process of embedding copyright information into an image to verify the ownership without any kind of corruption. In optical applications, the usage of diffusers is required to obtain a homogeneously distributed spectrum. However, the use of a diffuser in the input plane and the finite size of the sensor leave the recovered watermark with noise speckles. This noise affects negatively the verification process. We propose the usage of pseudo-random band-limited diffusers to place watermarks on optical images using a method based on Fourier-transform and random phase encoding algorithm. Such diffusers can help to recover a free-speckle watermark. The robustness to typical attacks is analyzed. Numerical results have shown its effectiveness.