Focus-tunable experimental optical cryptosystem

•A fractional Fourier cryptosystem using an electrically focus-tunable lens (EFTL) is implemented.•The performance of the EFTL is tested both numerically and experimentally.•The encryption-decryption processes in a wide focal range of the EFTL is demonstrated.•We present a multi-user protocol based on the variations of the EFTL's focal length. We present a first implementation of an optical encryption setup in the fractional Fourier domain using an electrically focus-tunable lens. Fractional Fourier transform cryptosystems allow the use of the fractional order of the transform as an encryption parameter. In our proposed scheme, the tunable lens enables fast control of this parameter and replaces the use of mechanical alternatives, increasing the stability and reducing the alignment requirements compared to previous fractional cryptosystems. The active electrically focus-tunable lens was accurately calibrated to ensure proper implementation and control of the tunable optical cryptosystem. We tested the encryption-decryption range of the electro-optical lens by encrypting objects using different focal lengths. Then we analyzed the tolerance to decryption with keys registered with different focal lengths. In this way, we found an operation range that enables the implementation of an effective multi-user encryption protocol. All results presented in this work are supported by numerical simulations and experimental results, demonstrating the viability and versatility of our proposal.