Single laser emits record 206 signals
As a means of markedly increasing the number of channels used in fiber-optic wavelength-division-multiplexed (wdm) networks, researchers at Lucent Technologies` Bell Laboratories have demonstrated a single-laser wdm transmitter system that can generate an unprecedented 206 wavelengths or channels with independent data transmissions on each channel. Sixteen-channel wdm systems are just now coming into use in commercial applications. Potential areas in which wdm applications can be used include communications, medicine, environment, materials processing, metrology, counter-terrorism, and industrial process control.
The larger the number of wavelength channels, the lower the speed at which the related electronics have to operate, thereby decreasing system cost. More channels also add flexibility when allocating network capacity. In addition, their performance is less affected by fiber nonlinearities.
The transmitter system inventors and developers are Bell Laboratories researchers Martin Nuss, Wayne Knox, Luc Boivin, and Steve Cundiff at the advanced photonics research department in Holmdel, NJ, and Jason Stark at the optical physics research department in Murray Hill, NJ. According to the researchers, 206 wavelengths is the largest number of communications channels transmitted to date.
A patent has been granted to Knox and Nuss, who invented the "chirped-pulse" multiwavelength telecommunications system. The chirped-pulse-wdm technique defines and encodes data on a large number of wdm channels using a spectrally broadband source and a single modulator.
In this approach, laser pulses become chirped as they propagate through a dispersive optical fiber that introduces a time delay between their frequency components. Frequency bands that are useful as wdm channels are then selected from the pulses` continuous spectra by means of a modulator operating at a multiple of the laser repetition rate.
"To record data on all 206 channels, we took advantage of the fact that different wavelengths in a light pulse travel through optical fiber at different speeds; red light propagates faster than blue, for example," explains Nuss. "In most communications systems, this characteristic spreading of the light signal, called dispersion, presents a problem that must be overcome. Here, it`s essential to the operation."
Each wavelength arrives at the end of an optical fiber at a slightly different time so that the data can be encoded sequentially onto the wdm channels using a single data modulator. "We end up with a series of rainbows, each about 20 nsec long, flying along in the optical fiber," comments Stark.
For wdm transmission systems, the conventional multiwavelength technique uses many single-frequency lasers to launch multiple lightwave signals. In contrast, the new transmitter design uses a single femtosecond laser, a single dispersive optical fiber, and a single time-division multiplexed (tdm) electroabsorption modulator (eam). The transmitter provides a channel spacing of about 37 GHz (0.3 nm), a bit rate in each channel of 36.7 Mbits/sec, and a wavelength range of 1535.3 to 1596.3 nm.
The femtosecond laser is needed because conventional wdm systems are limited to dozens of channels rather than hundreds, and combing and stabilizing a large number of single-frequency lasers becomes complicated and expensive.
In the system design, a mode-locked erbium-doped fiber-ring laser is used as a source of sub-picosecond pulses with a bandwidth exceeding 70 nm and a repetition rate of 36.7 MHz (see figure). The spectrum of each pulse is mapped onto the time axis as it propagates through a singlemode fiber that has a total dispersion of -340 psec/nm. This propagation stretches out the pulses to a duration of about 24.2 nsec and provides a nearly linear relationship between wavelength and time delay within each pulse.
A tdm electroabsorption modulator that has a 12-GHz bandwidth is installed at the output of the chirping fiber to define and encode data onto each channel in a time- sequential manner. It uses a tdm-multiplexed pattern generator synchronized to the 271st harmonic, or 9.942 GHz, of the laser repetition rate.
The short-wavelength absorption edge of the modulator results in the spectral narrowing of the pulses to about 28 nm. To partially restore the original bandwidth and to equalize the transmitted spectrum, the modulator bias is adjusted dynamically using feed-forward equalization as each pulse passes through.
In addition, when the frequency components of a short-pulse laser are separated temporarily by propagation in a dispersive medium, a single tdm modulator can be used to encode data on all channels. Accordingly, a 271-bit tdm "word" is used for every pulse to encode data onto each of the 271 frequency slots defined by the modulator. Because the same word is used for each pulse, a stable optical spectrum is obtained.
The chirping fiber converts the tdm pattern into a wdm modulation with states "1" and "0" corresponding to high- and low- intensity frequency bands, respectively. Out of the 271 possible wavelength slots defined by the modulator, 206 can be identified that have less than 3-dB channel-to-channel power variations. By encoding different 271-bit tdm words onto every chirped pulse, independent messages can be sent through each channel at a bit rate equal to the laser repetition rate.
"[This technology] allows us to envision networks and systems more powerful than those based on existing technologies. While it is still in its infancy and not ready for immediate deployment, we are evaluating this technology as a platform for next-generation systems," says N. Anders Olsson, director, advanced lightwave systems technologies, network systems, Lucent Technologies. q