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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Modified coherence-modulation techniques for secure transmission

Bruno Wacogne, Henri Porte, P. Mollier, David A. Jackson

Published Nov 1, 1996• 1 citation
Publisher

Abstract

In this presentation, we discuss the security aspect of coherence modulation for telecommunications. Recently, we demonstrated a simple technique for decoding the signal without any decoder interferometer. We now demonstrate a secure alternative method based on the corruption of the interferometer's optical path difference. Experimental results at low bit rate using fiber interferometers as well as transmission at high bit rates using large-imbalanced integrated optic interferometers are presented. Considerations about the polarization state of the light are also discussed.

DOI
10.1117/12.256346

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In this presentation, we discuss the security aspect of coherence modulation for telecommunications. Recently, we demonstrated a simple technique for decoding the signal without any decoder interferometer. We now demonstrate a secure alternative method based on the corruption of the interferometer's optical path difference. Experimental results at low bit rate using fiber interferometers as well as transmission at high bit rates using large-imbalanced integrated optic interferometers are presented. Considerations about the polarization state of the light are also discussed.
We show that key-specified interferometer path-length difference modulation (often referred to as coherence modulation), operating in the photon-counting regime with a broadband source, can provide a quantifiably high level of physics-guaranteed security for binary signal transmission. Each signal bit is associated with many photocounts, perhaps numbering in the thousands. Of great importance, the presence of an eavesdropper can be quickly detected. We first review the operation of key-specified coherence modulation at high light levels, illustrating by means of an example its lack of security against attack. We then show, using the same example, that, through the reduction of light intensities to photon-counting levels, a high level of security can be attained. A particular attack on the system is analyzed to demonstrate the quantifiability of the scheme's security, and various remaining research issues are discussed. A potential weakness of the scheme lies in a possible vulnerability to light amplification by an attacker.
In this conference, we demonstrate a new communication system exhibiting a double security level. Information coding is based on differential phase shift keying of the light emitted from a highly coherent CW source. An imbalanced Mach-Zehnder interferometer whose time delay matches the bit duration is used to decode the data. Since the optical power is constant in the transmission link, security levels comparable with those obtained in coherence modulation may be achieved. To further enhance the security, a low coherent source is used to send false data down the transmission link. The discrimination between real and false signal is based on common mode rejection. A study of the signal to noise ratio achieved with this system is also discussed in this manuscript.
Nowadays, it is important to consider not only the possibility of information multiplexing, but also the security aspect of its transmission. This article presents a novel telecommunication system based on relative phase coding. A highly coherent source is used to transmit data, which consists of a sequence of time intervals of duration ?, each exhibiting a different phase. At the receiver, this sequence propagates through an imbalanced Mach-Zehnder Interferometer (MZI) exhibiting a differential time-delay matching the time-interval duration ?. At the MZI output, two consecutive intervals overlap, and interferences occur. If the phases of two consecutive intervals are identical, constructive interferences produce a binary '1' at the decoder output. Conversely, if the phases of these intervals differ by ?, destructive interferences produce a binary '0'.
The growing awareness of the vulnerability of information transmitted on communication systems within the government, military and commercial sectors, has stimulated a number of areas of research within the optical community to design optical hardware encryption systems providing inherent immunity to espionage techniques. This paper describes a hardware optical encryption technique that utilises off the shelf telecommunication equipment and negates the necessity for an independent key distribution system with respect to the data transmission system, as is common with alternative encryption system implementations. This method also lends itself easily to fiber optic or free space communication and is applicable within any optical waveband. The encryption-decryption of the optical signal is achieved through low coherence optical interferometry. This requires the instantaneous processing and analysis of the signal, optically, to retrieve the relevant optical phase information hidden in the transmitted optical noise. This technology allows an authorised user to transmit encrypted information at a high data rate securely, while maintaining opaqueness to an unauthorised observer that data transmission is occurring. As the instantaneous optical field properties of the signals present in the system are essential to the optical encryption - decryption process, the system is inherently protected against electronic recording and advances in computational decryption algorithms. For organisations wishing to protect sensitive data and levels of communication activity these are highly desirable features.