AW3P.5.pdf CLEO:2014 © 2014 OSA Optical Encryption Based on Cancellation of Analog Noise Ben Wu, Matthew P. Chang, Zhenxing Wang, Bhavin J. Shastri, and Paul R. Prucnal Lightwave Communications Laboratory, Department of Electrical Engineering, Princeton University, Princeton, NJ 08544, USA benwu@princeton.edu Abstract: We propose an optical encryption technique where the data is encrypted with wideband analog noise. Matching both the phase and amplitude of the noise is required, providing a large key space for the encryption process. OCIS codes: (060.2330) Fiber optics communications; (060.4785) Optical security and encryption. 1. Introduction Optical encryption provides an effective way to secure data transmission without compromising the bandwidth [1,2]. Different optical encryption methods have been studied, including optical XOR logic encryption [3] and optical chaotic encryption [4]. Although optical XOR logic can reach 20Gb/s [3], the encrypted data is a digital signal. Compared with analog noise, the digital signal leaves the original data undamaged, and once the eavesdropper knows the code for the encryption, he can still recover the original data from the encrypted digital signal. Optical chaotic encryption encrypts the data with noise-like analog signals; however, it requires the laser at the transmitter and receiver to be synchronized to recover the data. The synchronization parameters cannot be directly used as key distribution for encryption process. A high speed encryption method with both analog noise signal carrier and easily tunable parameters for large key space is required. Optical interference cancellation techniques have been widely studied to remove self-interference in wireless communication systems [5,6]. The most challenging problem of optical interference cancellation is to satisfy the matching condition between the interference path and the cancellation path. The challenging problem in optical interference cancellation can be used as an advantage for the optical encryption, because the precise requirement of the matching condition provides a large key space to the encryption process. In this paper, we propose and experimentally demonstrate an optical encryption method based on interference cancellation. The digital signals are combined with stronger analog interference noise. The signal can only be recovered by matching the interference noise and the cancellation noise. Both the phase and amplitude of the noise can be controlled and used as key distribution parameters between the transmitter and the receiver. 2. Experimental setup and principle The noise encryption process includes two channels (Fig. 1). Each channel is carried by a different wavelength, λ 1 and λ 2 . Channel 1 consists of both a signal and interference noise, and channel 2 carried the cancellation noise. The interference noise has a wide bandwidth and an amplitude stronger than the signal to protect the signal from being detected. At the receiver, a WDM filter is used to separate the two channels. The receiver can only cancel the interference noise by matching the noise signal between channel 1 and 2. To satisfy the matching condition, both the phase and amplitude of the noise signals have to be matched and these parameters can be the key of the encryption process. The phase can be controlled by a pair of optical delay lines at the transmitter and receiver. The amplitude can be controlled by both the modulation depth of the intensity modulator and the optical attenuator (Fig. 1). In our experiment, the data rate of the transmitted data is 12Gb/s. The bandwidth for both the interference noise and cancellation noise covers from 4.5GHz to 5.5GHz. We design the frequency of the interference noise to be less than half of the signal data rate. This is to avoid enabling the eavesdropper to use a low pass radio frequency (RF) filter to remove the noise. Furthermore, the eavesdropper cannot use a band pass RF filter to remove the noise, because the noise spectrum overlaps with the zeroth-order lobe of the signal spectrum. If the eavesdropper removes the noise in the RF spectrum range of 4.5-5.5GHz, he also removes the signal information. Fig. 1. Experimental setup (a: tunable optical attenuator; IM: intensity modulator; t: tunable optical delay; EDFA: erbium doped fiber amplifier; WDM: wavelength division multiplexer)