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NASA Tech Briefs, Nov 2001

As the demand for bandwidth grows, the industry races to find better and faster switching and other technologies.

It is a peculiar moment in the life of fiber optic technologies related to communications. Potential customers face a glut of bandwidth for the current traffic, but no one doubts that demand will someday catch up with supply, as Internet telephony and web use, computerto-computer communications, streaming digital video and audio, and other services grow in volume. According to sources at Coming Inc., demand for bandwidth is expected to grow at an average rate of more than 100 percent per year for the next 10 to 15 years.

But with so much existing fiber optic cabling, new instrumentation is required to get that cabling to yield increased bandwidth. Most of the existing system uses optical fiber for long-distance transmission, but the light signals must be converted to electrical signals for switching and amplification. This process is costly, and limits the bandwidth the systems can carry. So the industry is searching for technologies that can do away with the electrical repeaters and meet bandwidth demands with all-optical systems.

One approach to the problem is wavelength division multiplexing (WDM), by which multiple optical signals are transmitted on a single fiber strand. This process has increased the amount of information carried on a fiber from between I and 2.5 gigabits per second (Gb/s) to between 10 and 40 Gb/s, the latter capacity just now reaching the newest installations. Researchers are also working on 400 Gb/s carriers.

But carriers cannot realize these gains with the existing electrical equipment that routes signals. Existing switches cannot deal with throughput above 2.5 Gb/s. So many companies in the industry are striving to be the first to develop an all-optical-core switch that can route signals without the assistance of electrical switches.

One source of funds for such development is the Ballistic Missile Defense Office (BMDO) and its Small Business Innovation Research department (SBIR). This agency's interest is in battlefield communications, where data is collected, interpreted, and communicated to various military systems, enabling complex battle management decision-making. Synchronization between multiple and sometimes distant ground and air subsystems is required, and response times must be the fastest possible.

Several companies are building solidstate optical-core photonic switches that do not rely on an electrical impulse, changing a signal's direction based on its wavelength or polarization. Radiant Photonics of Austin, TX, has developed a model switch that is insensitive to polarization differences or slight variations of wavelength in incoming signals, thereby eliminating costly correcting equipment. It is based on polymers used in thermo- or electro-optic prisms. These prisms can vary the index of refraction in response to temperature or an input voltage. The device determines where a signal hits a diffraction grating, and thus controls which output fibers the signal will reach. Doping the gelatin used in the prisms gives them their refractive characteristics. Radiant's switch can operate equally well in the C, L, and S communications bands, and provides response speeds of just 1 nanosecond (compared with 10-15 milliseconds for competing technologies), and has an insertion loss of less than 1 decibel.

Other companies are developing polymer switches, too, but most rely on complicated phase delays, and are thus sensitive to polarization differences and wavelength variations. Radiant's switch would do without more expensive lasers and correcting equipment. It also would nave taster response tunes than micromechanical switches, also under development. It could have as many as 50 output channels. BMDO funded the work because it needed a polymer for high-speed fiber optic components that could withstand the temperatures of airborne and spaceborne applications.

Meanwhile, SpectraSwitch Inc. of Santa Rosa, CA, has patent-pending technology for using liquid crystal as the switching medium. Its WaveWalker(TM)' components include a low-port-count photonic switch called the WaveWalker(TM) 1 x 2, used in single-mode transmission in an all-optical network. The company's switch uses liquid crystal and birefringence -- the ability to refract unpolarized light into two separate orthogonally polarized rays - with the material's response to an electric field. A liquid crystal cell rotates the polarization of incoming light when a voltage is applied. SpectraSwitch's device is one of the fastest-switching under development, reducing the current optomechanical industry standard from 10-15 milliseconds to less than 4. Because the material is rugged and immune to vibration degradation, the company claims it will have a billion-cycle durability. SpectraSwitch expects to develop a full line of WaveWalker(TM) components, including variable optical attenuators, optical add/ drop multiplexers, polarization mode dispersion compensators, and multifunctional modules. The technology was funded in part by BMDO.