One Chirping Laser Sings 1022 Frequencies on a Fiber

Stephanie A. Weiss

In standard communications math, 40 lasers + 40 modulators + one fiber = 40 channels of dense wavelength division multiplexing. In Lucent Technologies' new math, one laser + one modulator + one fiber = 1022 channels of ultradense wavelength division multiplexing.

Scientists at Lucent Technologies' Bell Labs announced the record wavelength density in November to highlight the enormous potential of femtosecond laser technology in communications. The demonstration extends a 1997 experiment that transmitted 206 wavelengths on one fiber, and Wayne Knox, head of Bell Labs' Advanced Photonics Research Department, said the technology could stretch to produce 16,000 wavelengths from one laser on one fiber.

If it ever leaves the lab, a single-laser ultradense wavelength division multiplexing system would radically alter the economics of communications.

In the latest arrangement, a modelocked erbium-doped fiber ring laser from Clark-MXR of Ann Arbor, Mich., creates femtosecond pulses of broadband ("white") light. The light passes through a combination of Lucent's dispersion-compensating and dispersion-shifted optical fibers. The fiber effectively stretches the laser pulses while separating wavelengths, producing 10-GHz wavelength spacings within the traditional communications wavelength band. A time division multiplexed electroabsorption modulator encodes data at 37 Mb/s onto the resulting chain of wavelengths.

The technology offers tremendous economic prospects for communications firms that buy one transmitter per wavelength for today's state-of-the-art wavelength division multiplexing systems. A significant stumbling block for commercialization, however, will be producing femtosecond sources that can satisfy the typical communications lifetime, temperature and vibration requirements.