Two-way DWDM uses beat-frequency locking
Korean engineers have come up with a way to allow bidirectional operation of fibers without experiencing problems from the noise normally associated with scattering. By keeping track of the interchannel spacing via the beat frequency, signals can go in both directions with tightly interleaved, but not overlapping, sets of wavelengths. This means that the bandwidth of the fiber is increased rather than split between the two directions as had been done in previous systems.
Engineers have been looking to send signals both ways through existing fibers, but Rayleigh scattering, stimulated Brillouin scattering, and various other reflections caused by a signal traveling at one wavelength in one direction represent a source of noise for that same wavelength traveling in the other. Options considered have included using completely separate wavelength bands for the two directions, and doubling the channel spacing so wavelengths are less likely to be too close to each other.
The problem is that the lasers are normally temperature-sensitive, changing wavelength during transmission. Therefore, to design the right channel spacing, the uncertainty in the precise wavelength emitted has to be considered in both directions, so the overall uncertainty is doubled and the bandwidth per channel is halved.
The new system was developed by researchers at Kwangwoon University in Seoul and ETRI in Taejon. If the sets of wavelengths can be controlled with respect to each other (if they can be kept relatively constant), then they are no longer independent. Thus, there is only one uncertainty that needs to be considered in the two-way scenario and the bandwidth per channel stays the same.
Keeping this frequency offset constant is achieved by detecting beats set up between adjacent counter-propagating channels. A feedback loop is used to maximize the amplitude of the beat-frequency signal after passing through an RF amplifier centered at around 7 GHz (calculated to allow low-noise transmission) by controlling the temperature—and therefore the wavelength—of the laser. The same procedure is carried out for all the other lasers emitting in that direction, beating with their counterparts. The results are twofold: the frequency offset is kept constant, and the signal going forward must have the same channel spacing as that going backward.
For more information, contact Yong-Sang Ahn at Kwangwoon University, Seoul 139-701, Korea.
- Y.-S. Ahn et al., IEEE Photon. Tech. Lett. 13 (8) (August 2001).