Development of a Free Space Optical Classical Communications System with Application in Quantum Communications

Abstract

Currently the free-space optical links have different applications (some of them are already commercialized and other are in the stage of development) such as: a) horizontal communication links within the same layer of Earth's atmosphere (FSO links, i.e. "free space optics"), among which are permanent or temporary links (used in emergency situations) for short-distance communication between buildings (Fsona, 2014), and high-speed communication in personal wireless optical LANs (Wang, K., et al, 2011), b ) vertical links (LASERCOM systems ) for communication between aircraft and satellites , optical links between satellites in different orbits (Chan , V., 2003), links from satellite to earth stations and vice versa (operating in various layers of the Earth's atmosphere and in deep space (Hemmati , H., 2006 )), c) optical-wireless highspeed connection between printed circuit boards (for distributing clock signals in a satellite and/or to interconnect microprocessors without EMI (Savage, N., 2002)). The development of optical communication links (bidirectional) towards the Earth and from the space has been and is of great interest for various space agencies around the world (JAXA, 2009); the European satellite system SILEX is a practical example of such technology (ESA, 2014); in addition, NASA launched on September 2013 the mission "Lunar Laser Communication Demonstration" which main objective is to demonstrate the reliability of the optical communication between a station in lunar orbit and earth stations within our planet (NASA, 2014). Usually the links abovementioned are "classical" (i.e. they operate with a relatively high number of photons per observation period), however, recently there has been a growing interest in the development of “quantum” communication systems (using low number of photons per observation period) (Hemmati, H., et al, 2012). For these systems different types of “quantum states” may be employed (whose properties have been and are investigated by scientists around the world (Becerra, F.E., et al, 2013)), among others, the “entangled states” (Ma, X., et al, 2012), or “weak coherent states" (WCS). We chose the WCS because its generation is possible with optical sources in the telecom band (Lopez, J., et al, 2013, Lopez, J., et al, 2012). In addition, the use of WCS for communications through free space allows the establishment of links unconditionally secure for quantum key distribution (QKD), satellite links (where the "original" information signal is classical but may arrive as a WCS signal in the receiver front due to the strong attenuation of the atmosphere), among others (currently these topics are of great interest worldwide). It is convenient to mention that we have already implemented some WCS-based Arvizu-Mondragón, A. et al. Revista Electrónica Nova Scientia, Nº 12 Vol. 6 (2), 2014. ISSN 2007 - 0705. pp: 248 – 271 - 252 - subsystems employing optical fiber (Lopez, J., et al, 2013), however, we want to develop WCSfree-space systems for satellite applications (Gutiérrez, C., et al, 2013). Prior to the practical implementation of a FSO- WCS system we consider very convenient the development of a classical FSO link whose subsystems are also used in free-space quantum links. We present the methodology for the design of free-space optical links and a computational tool developed for this purpose. We designed a spatial synchronization scheme (ATP) based on a four-quadrant APD and a magnetic levitation subsystem to monitor a classical optical beacon signal at 532 nm used to establish the communications link and to maintain it operating adequately. It is convenient to mention that although there is a significant amount of papers dedicated to the development of ATP systems with classical optical transmission, to the best of our knowledge has not been reported yet an ATP system based on magnetic levitation for quantum links employing WCS; from our point of view this could be an important contribution of this work. On the other hand, although here we focus on a classic FSO link, the results will be used to the development of links type WCS – FSO for satellite applications