Optical signals ride ever-wider waves
W. Conard Holton
Wavelength-division multiplexing--the term doesn`t exactly roll off the lips; its acronym--WDM--doesn`t form a pronounceable word. Yet wavelength-division multiplexing is on the lips of people from research laboratories to Wall Street. This esoteric technology of moving multiple wavelengths of information-bearing light over optical fiber provides a utility and profitability that are easily understood.
All this attention springs from the demand for higher-speed data transmission and higher-capacity video communication. An overall speed of 40 Gbit/s through a single fiber can result from transmitting 16 wavelengths at 2.5 Gbit/s each. This overall speed is rapidly heading higher as more channels at higher bit rates per channel are added--40 channels are now being installed, 80 and 96 are planned, and 128 are in the offing.
These advances meet any futurist`s dream for an information society. But in the more prosaic world of the component manufacturers working to keep up the pace, many new issues are being raised. In the first article in our supplement on fiberoptic components, Raymond Nering discusses the demand for speed and capacity requires new developments in optical fiber, amplifiers, sources, and switches.
The ability to accurately multiplex and demultiplex the increasing number of wavelengths is another issue, and, in the second article, J. J. Pan and Y. Shi argue that a hybrid device combining fiber Bragg gratings and coated filters provides advantages. Such a device allows channels to be spaced as close as 0.4 nm, while still meeting stringent demands for accuracy by isolating each channel from the next.
As optical networks become more transparent, the requirement that passive optical components efficiently transmit signals increases. To achieve this efficiency, write Kevin McCallion and Michael Shimazu, components such as attenuators, modulators, and switches could be based on side-polished fiber technology. After enough cladding has been removed to create a window into the fiber, the signal can then be affected through engineered overlays in contact with the core.
Characterizing performance of all these WDM components is an important task for which several options are available. In the final article, Bruce Nyman evaluates the advantages and trade-offs offered by systems based on a broadband noise source and an optical spectrum analyzer (OSA), a tunable laser and a power meter, a tunable laser and an OSA, and a sweeping tunable laser.
Wavelength-division-multiplexing technology is developing very quickly. And although the term itself may be difficult to pronounce, that fact has posed no barrier to its rapid implementation.