We present a method of digital hologram watermarking using a Fresnel hologram of a real-world 3D object and the fractional Fourier transform. A watermark is encrypted using double random phase fractional Fourier domain encoding technique and then encoded into the digital hologram. The hologram is watermarked in a plane at some known distance from the object so that even if a new hologram is generated from the original hologram the watermark can always be traced by propagating the new hologram back to the object and then onto the watermark plane. The watermark is retrieved successfully using the correct encryption parameters. We consider both numerical (full complex field) and optoelectronic (phase-only) reconstruction methods. We obtain the watermark from different windows of the hologram corresponding to different reconstruction perspectives.
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