The armed services and civilian space programs share a growing need for increased communication bandwidth. The fighting forces are becoming highly networked in order to provide significant advances in situation awareness, system health management, real-time targeting, and command and control. Civilian satellite networks and autonomous deep-space spacecraft share these needs. Problems in space can lead to catastrophic failures and loss of life. Spaceborne platforms share the risks found in the industrial environment, for example, but without the advantage of readily available emergency facilities. Aerospace network designs must thus be robust against failures and attacks.
High-speed network architectures such as Gigabit Ethernet and Fibre Channel are readily available in the commercial marketplace. Some have been adapted for the more rugged aerospace applications, such as the Fibre Channel network being developed for the F/A-18E/F. This 1-Gb/s network uses an electronic switch fabric and fiber-optic links to route information among the mission computers, sensors, and displays. The ability of these electrically based switched networks to grow in capacity becomes difficult as the bandwidth increases beyond 1 Gb/s.
Wavelength-division-multiplexed all-optical networks (AON) using array waveguides provide an approach to scale these networks to tens of gigabits per second. The AON is used as a passive transport layer to allow different network architecture protocols such as Fibre Channel to run concurrently within the AON (see figure below). This permits each protocol to be optimized for its specific use. AON bandwidth growth can permit transmission of more than 40 Gb/s over each channel with a potential of 200 channels that can be switched in approximately 30 ns. This system is based off of commercial off-the-shelf (COTS) technology that needs to be repackaged for use in flight hardware. It is also possible to provide a robust self-healing optical network in both hard-lined and free-space systems using phase-conjugate techniques.Designing for Space
Aerospace companies have been evaluating gigabit network protocols and photonic components for transmission of very- high-frequency radio-frequency waveforms. To gain insight for fielding the all-optical technology, they are performing medium-speed AON copper-versus-fiber trade studies, including fiber-optic cable plant characterization, transceiver evaluations, optical connector and contact evaluation, network market assessment, and link budget methodology.
The extreme environmental requirements in space present design challenges. There are significant differences between space and commercial terrestrial applications. The space environment can be extreme, from rapid thermal cycling to high radiation doses from solar flares. The most important parameters are thermal cycle, thermal shock, vibration, mechanical shock, fluid immersion, humidity, electromagnetic interference, radiation, contamination, and, especially, handling. Of these, radiation, thermal cycling, and vibration are the most problematic for the newer wavelength-division multiplexing (WDM) designs. Most of the cable-plant solutions of today can tolerate the space environment. Singlemode connectors, however, still require further test and development for space applications.1
Electronic controlled photonic network designs are sensitive to the total ionizing dose (TID) effects on Si microelectronics, memory, and processor devices. For photonic semiconductor components (LED lasers and optocouplers, etc.) supporting passive WDM designs, photon-induced displacement (electron) damage effects are more important. In both designs, the cable plant is TID sensitive. Protons in space are a key factor in determining the radiation tolerance for these devices. Characterization and qualification testing methods used for Si microelectronics need to be evaluated prior to using them for photonic component testing. Photonic components can exhibit different failure mechanisms (for example, ionization damage for LEDs with internal lenses, affecting light transmission through the lens) and caution must be exercised when using the more established Si microelectronics qualification techniques.
A great amount of photonics reliability and radiation tolerance work has been performed under the NASA Electronic Parts & Packaging (NEPP) Program.2 Rapid changes in technology and techniques create new failure mechanisms that need to be understood for space applications. To keep up with radiation hardness assurance (RHA) requirements and technology development, the aerospace and COTS industries need to jointly review RHA standards and update them regularly.3
The aerospace industry has demonstrated innovative adaptations of recent telecommunications WDM designs and has increased the number of usable wavelengths for aircraft and space applications. However, it has not been able to provide enough market to justify specific photonics development from the COTS telecommunications component manufacturers. More collaboration with the COTS manufacturers for component technology modification is thus needed. This opens a business opportunity for suppliers to modify commercial pieceparts in COTS-derived systems to meet aerospace needs. oeReferences
1. T. Weaver, R. Smith, "Photonic Vehicle Management," IEEE 20th Digital Avionics Systems Conference, Daytona, FL (2001).
3. C. Barnes, et al., "Recent Photonics Activities Under the NASA Electronics Parts and Packaging (NEPP) Program," NASA NEPP website.
Gerry Nissen is a Phantom Works engineer specializing in Integrated Vehicle Health Management technology development at the Boeing Corp., Huntington Beach, CA.