Tunable lasers route optical signals
Programmable widely tunable transmitters open up new horizons in optical communications. The European collaboration project ACTUAL (project AC329 within the ACTS programme), completed in March 2000, aimed to evaluate the control and performance of tunable lasers as well as their potential in network applications.
The project brought together three manufacturers of tunable lasers: Altitun (Sweden), GEC Marconi Materials Technology (UK), and NTT (Japan), along with the University of Gent (Belgium), University College Dublin (Ireland), and network operator Telenor R&D (Norway). Five programmable laser modules from the three manufacturers were evaluated in a demonstrator network at the Telenor research labs (see figure).
A reconfigurable all-optical mesh network proposed by Telenor was realized using exclusively passive optical wavelength routing elements at the node. Wavelength routing was achieved by altering the wavelength of the transmitter, which resulted in a specific path across the network that delivered the signal directly to its end destination without any intermediate processing, switching, or reconfiguration at intermediate nodes. The switching functionality was, in other words, realized by means of the tunable laser.
The network was centrally controlled by laboratory software that addressed the interface of the transmitters. This network concept allowed protection, restoration, and dynamic bandwidth allocation to be provided in a reliable, simple fashion. In doing so, it used inherently low-cost, commercially available technology to realize the wavelength routers.
The demonstrator comprised a mesh network with typical distances of 50 km between neighboring nodes (primarily laboratory fiber) situated at three different labs in the same campus. A hundred channels at 50-GHz channel spacing were available in total, and a mix of 2.5-Gbit/s and 10-Gbit/s signals were transmitted using external modulation. Stability tests were carried out on three adjacent channels, with signals transmitted over a total distance of 150 km, passing through two intermediate nodes in addition to the transmitting and receiving ones.
Error-free transmission was obtained from each of the lasers after uninterrupted operation over several days, and reliable and reproducible tuning of the lasers to the desired channels was verified throughout the testing period, which extended over several months. Better than 10-GHz absolute frequency accuracy was obtained, for a tuning range of approximately 30 nm. In addition, crosstalk measurements showed negligible effects for 2.5-Gbit/s channels, whereas measurable, albeit low, crosstalk was obtained for adjacent 10-Gbit/s channels at 50-GHz spacing.
This network principle showed that a high-performance all-optical mesh network can be built without the use of optical cross-connects (OXCs). Optical cross-connects are not thereby rendered superfluous. Instead, the demonstration showed that much simpler and smaller OXCs than anticipated may suffice if programmable laser transmitters are used. Moreover, this network has the potential to realize fast reconfigurability that is controlled exclusively from the transmitter. This enables new architectures that have significant advantages for the data-dominated networks that operators need to realize in the near future.
Evi Zouganeli is a senior researcher at Telenor R&D, POB 83, N-2027 Kjeller, Norway. Tel: +47 63 84 86 08; fax: +47 63 81 00 76; e-mail: firstname.lastname@example.org.
An all-optical 3 × 3 regular mesh network demonstrator was realized using programmable tunable transmitters and otherwise passive optical components at the node.