Experimental multiplexing approach via code key rotations under a joint transform correlator scheme Edgar Rueda a, ⁎, Carlos Rios a , John F. Barrera a , Rodrigo Henao a , Roberto Torroba b a Grupo de Óptica y Fotónica, Instituto de Física, Universidad de Antioquia, A.A. 1226, Medellín, Colombia b Centro de Investigaciones Ópticas (CONICET-CIC) and UID OPTIMO, Facultad de Ingeniería, Universidad Nacional de La Plata, C.C.3 (1897), M. B. Gonnet, La Plata, Argentina abstract article info Article history: Received 18 November 2010 Received in revised form 17 January 2011 Accepted 19 January 2011 Available online 4 February 2011 Keywords: Encryption Optical security Optical information processing We introduce a novel experimental procedure to multiplex optical information by using the joint transform correlator architecture. The rotation of the input plane encoding mask admits the multiplexing capability, otherwise not feasible by a simple shifting due to the classical invariance to in plane mask translations. We realize the procedure by using an optodigital technique with a Mach–Zehnder interferometer. We apply a virtual optical implementation to remove unwanted information and then to reposition the encoded image, before data multiplexing. The end user employs a procedure with another virtual optics setup, having fewer steps than other known methods, and avoiding the usual background noise influence of non-decoded data. Actual experimental results are presented. © 2011 Elsevier B.V. All rights reserved. 1. Introduction As optical data encrypting systems become attractive, researchers have made an effort to find different optical architectures to protect the coded information. In addition to the basic 4f encrypting architecture [1], several variations were introduced, among them the joint transform correlator (JTC) encrypting scheme [2]. On the other hand, several other attempts were developed to strengthen security. We found that one important feature that reinforces optical encryption is the multiplexing concept. This procedure brings the chance for storing multiple messages in a single recording medium. As examples of applications in the classical 4f scheme, we may mention many multiplexing techniques to store multitude of images, for instance into a single crystal, without crosstalk, including angular [3], wavelength [4], shift [5], polarization [6], and multi-aperture multiplexing [7]. We also find an interesting encryption proposal using an in-plane rotation of random phase diffusers constituting a sandwich diffuser at the Fourier plane of a 4f encoding system [8]. Although computer simulations represent an important tool to design practical solutions, experimental results are in fact most significant to verify and to assert the realistic constrains. We find one experimental example in a multiplexing encryption–decryption approach via lateral shifting of a random phase mask using 4f architecture in reference [5]. In this sense, there are just a small number of experimental results presented in the JTC configuration. We propose the use of a holographic digital configuration, using the basic JTC setup in one arm of a Mach–Zehnder interferometric arrangement [9]. Besides, we identified and filtered the unwanted terms in order to prevent noise. We are particularly interested in using an approach similar to in plane shifting, as it brings experimental flexibilities to the imple- mentation of the technique. Unfortunately, the JTC scheme shows the well known property of being space invariant to in plane translations in the input plane. Therefore, such phase changes are not reflected in the intensity record of the joint power spectrum (JPS). Instead, we know that a simple in plane rotation of a ground glass is readily sensed, consequently, this feature could be used instead of a translation. In the frame of encryption applying phase shifting protocols, we now propose the use of successive rotations of the key encoding diffuser placed in one window of the JTC arrangement. Therefore, we sequentially change input objects as we introduce an adequate rotation to the encoding diffuser. This approach combined with the optodigital technique used in reference [9] allows us to experimen- tally multiplex and reconstruct encrypted data. Consequently, in the paper we propose what to our knowledge is a new multiplexing method for security systems in the JTC architecture. This method based on a diffuser rotation encoding scheme, provides a versatile approach that can be used to retrieve different target images from the encrypted record without changing the random phase mask, the angle between write-in beams, any polarizer arrangements or the wavelength. In summary, we want to present an experimental optical encrypting station based on a JTC architecture, implemented on a Mach–Zehnder interferometer. We have the aim of multiplexing data Optics Communications 284 (2011) 2500–2504 ⁎ Corresponding author. Tel.: +57 4 2196556. E-mail address: erueda@fisica.udea.edu.co (E. Rueda). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.01.052 Contents lists available at ScienceDirect Optics Communications journal homepage: www.elsevier.com/locate/optcom