GUEST
Roberta A. Ewart
Michael Enoch
he purpose of the Free Space Laser Communications feature topic is to inform the communications community of the current state of technology and potential application of free space laser communications to address needs and requirements that cannot easily be addressed by other means. Most of the Communications Magazine readership will be familiar with free space RF communications or guided wave (fiber) optical communications. Each of these technologies operates under different conditions, with its characteristic benefits and challenges. R F communications offers the advantage of wireless connectivity and the ability to broadcast over a wide area, while fiber optics offers extremely high bandwidth potentials between connected points. Conversely, the availability of bandwidths for high-data-rate interference-free R F communications becomes more challenging as spectrum usage of all types continues to grow at unprecedented rates; and installation of new infrastructure for fiber optics can be a slow and expensive process, and will never be able to address the need for mobile and wireless connectivity. In many respects, free space laser communications can be the offspring of a happy marriage of free space R F and guided wave fiber optical approaches. It can provide many of the benefits of fiber optical communications for usage in wireless applications. In particular, free space laser communications is well suited for providing high bandwidth point-to-point wireless links. It can also present challenges that are characteristic of both parent technologies. Since the use of fiber optic and RF communications in a single system rarely overlap, practitioners in one discipline may be unaware of unique challenges and system advantages inherent in the other. We hope that the articles in this topic help address this. The concept of free space laser communications systems has been around as long as the laser itself, but has had very limited implementation in fielded systems. There have been several reasons for this, the principal being the lack of a demonstrable need for extremely high-bandwidth wireless systems; cost, complexity, and reliability of the technology involved; and the lack of success in premature attempts to field operational systems. However, a number of recent occurrences and events have made the application of free space laser communications a much more attractive proposition. With the unprecedented growth of the Internet and related data services, there has been a growing appetite for immediateEditorial Liaison: J. Elmiighani.
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ly available information and bandwidth. This trend is expected to continue for the foreseeable future. In many situations, the need for bandwidth is growing faster than the ability of the local infrastructure to serve it, and also faster than the rate at which the infrastructure can be augmented. As a result, a number of companies have recently started marketing free space optical systems to interconnect systems within campus and metropolitan environments. These systems have demonstrated high utility for temporary connectivity while wire or fiber infrastructure is being installed. This situation occurs when the user facility is temporary and does not merit investment in longterm infrastructure, when the connectivity is needed immediately, or when the right of way for the installation of fiberkable is unavailable. A recent demonstration system of this type provided an error-free 40 Gb/s link over a 4 km range in an urbanhuburban operating environment [l]. The technology to accomplish this high-capacity free space link was largely the same as that used in fiber optic WDM systems. As a result of the growth in the need for communications, there has been a tremendous industry investment in developing technology for terrestrial fiber optics and principally in the 1550 nm wavelength regime. This has resulted in very economical high performance, high reliability, compact optical components that can be used in free space optical systems. This 1550 nm wavelength technology has the added advantage of being inherently eye-safe at the power levels used in the free space systems, alleviating the health and safety concerns often raised with using lasers in an open environment where human exposure is possible. An added benefit from the use of free space laser communications is relief from the regulatory, licensing, and frequency management and coordination issues encountered in implementing R F systems. Other than the safety issues and regulations involving human exposure, which largely disappear with the use of inherently eye-safe wavelengths, free space systems are not currently subject to regulation or control by any frequency management organization, like the FCC or ITU. And since free space laser communications is typically implemented as a narrowbeam point-to-point connection, there is little likelihood of interference. These characteristics make laser communications well suited to applications such as intersatellite crosslinks. Much of the early work in free space laser communications was U.S. Government funded research directed at intersatellite links between communication satellites at geosynchronous orbit. However, the fast growing installed base of fiber optic connec-
IEEE Communications Magazine August 2000
tivity has largely eliminated the need for these links for most domestic uses in industrialized nations. However, the recent development of the large LEO mobile communication satellite constellations, the Teledesic concept in particular, with the need for flexible high-bandwidth crosslinks for message routing within the constellation, gave free space laser communications a market niche. Commercial companies in North America, Japan, and Europe have invested large amounts in internal development for product development and risk reduction in pursuit of an anticipated Teledesic requirement for hundreds of crosslink terminals. With the move from government funded development to competitive commercial development and the need to address performance and packaging requirements, huge progress has been made in the design and implementation of robust, manufacturable optical communication terminals. Optical terminals arc now more cost effective in several point-to-point systems applications. Due to market-related problems encountered by the Iridium system and the restructuring of the Teledesic architecture, in the near term the likelihood of seeing a large commercial constellation with high-bandwidth crosslinks is low. We can expect to see the technology applied in other niches. The first article, by Dr. David Begley of Ball Aerospace, provides a tutorial on free space laser communication for intersatellite links. It addresses the system architecture requirements and performance of satellite communication networks employing laser communications for intersatellite links in LEO (Teledesic), GEO (INTELSAT), and hybrid systems. Although this article docs not address thc effects of the atmosphere on the free space laser communication information signal, the fundamcntals it presents are also applicablc to terrestrial applications of laser communications. A different type of space communications is addresscd in thc second article, by Dr. Keith Wilson of the NASA Jet Propulsion Laboratory and Michael Enoch of Emergent Information Technologies. It describes the work being performed at JPL to apply free space laser communications to future deep space planetary exploration missions.
Laboratory (AFRL). He is a member of the IEEE Communications Society Space & Satellite Communications Technical Committee, Aerospace & Electronic Systems Society, Lasers & Electro-optics Society, and Signal Processing Society. He is also a member of the AlAA Satellite Communications Technical Committee. He holds a Bachelor of Science in engineering science from RensselaerPolytechnic in Troy, New York,
REFERENCES [ I ] G. Nykolak e t al., "A 40 Gb/s DWDM Free Space Optical Transmission Link over 4.4 km.", Proc. SPIE, vol. 3932 Free-Space Laser Commun. Tech. XII, Jan. 24, 2000, pp. 16-20.
BIOGRAPHIES LT. Cot. ROBERTA EWART [MI (
[email protected]) is an active duty Air Force officer currently assigned as a senior developmental engineer at the Directed Energy Directorate (DE) at the Air Force Research Laboratory (AFRL) at Kirtland Air Force Base, Albuquerque, New Mexico. She is currently leading efforts to field laser communications satellite crosslinks and satellite to aircraft laser links for the Department of Defense. Her experience ranges from mission control operations for the NASA Space Shuttle, t o satellite launch and early orbit for the Global Positioning System (GPS), to laser integration engineering for ground-based laser systems and radiation testing of laser components. She is a member of OSA and SPIE. She holds an M.S.E.E. from the University of Colorado, an M.A. in theoretical physics and philosophy from Oxford University, United Kingdom, and a D.E. in electrical engineering from Stanford University. MICHAEL ENOCH [MI (
[email protected]) is a Senior Member of Technical Staff of the Advanced Programs Division of Emergent Information Technologies, Inc. He is currently involved in development of systems and operations support for small spacecraft and providing system engineeri n g support t o the U.S. Air Force Research Laboratory. He has been involved in the system engineering of military and commercial satellite and terrestrial communication systems for over 1 2 years. His experience ranges from the small scale, such as testing of M A T systems and equipment, to the very large scale, development of frequency planning and management software for INTELSAT. His involvement with laser communications is relatively recent, a result of his involvement in development of a concept for laser communications t o an airborne platform performed as part of the Lasers and Space Optical Systems (LASSOS) study for the Air Force Research
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