Modulation and Directionality Characteristics of Free-space Optical Transmission Links X Jin, C M Collier, J F Holzman, and J Cheng School of Engineering University of British Columbia Okanagan Kelowna, Canada Abstract—Transmission characteristics of free-space optical (FSO) data transmission systems are characterized. Issues of optical directionality, being related to the optical modulation depth, are investigated for a variety of retroreflecting structures. A composite solid retroreflector is introduced as a novel element for such optical wireless systems, and it is shown that this device has an omni-directional retroreflective angular response covering the full 4π steradians solid angle. Such a configuration is a particularly intriguing solution for point-to-point and line- of-sight (LOS) optical communication implementations, and these dynamic modulation characteristics are investigated. I. INTRODUCTION Optical devices have revolutionized communications over the past few decades. Light-based systems offer incredibly high data transmission rates by way of a wide variety of time- division multiplexing (TDM) and wavelength-division multiplexing (WDM) arrangements. Fibre optic data transmission networks are now standard in long-haul data communication networks requiring broad signal bandwidths. While the transmission characteristics of fibre-based transmission systems are unsurpassed in long distance communications, a challenge remains for optical systems in dense urban environments with minimal pre-existing infrastructure. Within these environments, optical wireless communication systems have been proposed for point-to-point and line-of-sight (LOS) implementations [1]-[5]. These free- space optical (FSO) networks relay optically encoded signals between multiple communication nodes through the use of centralized laser transmitters and remote receivers. Smart Dust systems are a prime example of such a technology [6]. The fundamental principle behind the operation of FSO systems is the bi-directional nature of the optical communication links. Typically, FSO systems are implemented in either an active optical relaying format (with communication nodes having independent lasers directing modulated laser signals to the detectors of other nodes) or a passive optical relaying format (with passive laser beams and modulated reflective devices positioned on each remote node). Of these two implementations, the passive FSO arrangement is particularly attractive for optical wireless communications, as it can be made to be less sensitive to the optical beam alignment. With a suitable retroreflector employed at each node, the incoming optical beam can be modulated with the local information and redirected directly back to the transmission source. Recent research topics have focused largely on the optical encoding mechanisms of this process, and a variety of modulation schemes have been proposed, including mechanical modulation of the reflective mirrors [7] and liquid crystal modulation of the node entrance [8]. In contrast to this, the work presented here investigates and optimizes the inherent retroreflection process for these optical wireless systems. The process of retroreflection is at the heart of most passive FSO retroreflection arrangements, and the corner-cube retroreflector (CCR) is an excellent candidate for implementing this retroreflection. CCRs redirect incoming light rays back to their transmission source by flipping each of the three orthogonal ray vector components with three mutually perpendicular mirrors [9]. The exiting beam leaves the CCR in a direction that is antiparallel to the incident beam. Unfortunately, the interior corner of a CCR is by its very nature only effective at retroreflection over π/2 steradians of the full 4π spherical solid angle (1/8 th of the full sphere).The work presented here analyzes the characteristics of optical retroreflection and proposes a structure that is capable of broadening the CCR angular acceptance cone and replicating it over the full spherical solid angle. The design makes use of elemental CCRs in a composite arrangement. The directionality of the system is analyzed in a static optical system, and switching characteristics are analyzed in various dynamic modulation configurations. II. STACTIC RETROREFLECTIVE RESPONSE A. Elemental retroreflectors The fundamental structure within a retroreflector is the corner-cube. The CCR is comprised of three mutually perpendicular reflective surfaces that form an interior corner. A ray of light entering this interior corner will have each of its xyz Cartesian coordinates successively reversed, such that it exits the corner in a direction that is antiparallel to the incident ray. Such a scenario is shown in the coordinate geometries of Figs. 1(a) and (b). Note here that the success of this retroreflection is highly dependent on the spherical coordinate 978-1-4244-3355-1/09/$25.00©2009 IEEE 3