Europe ponders 1300-nm transmission

By STEPHEN HARDY

Conventional wisdom says that Europe is at least a couple of years behind the United States when it comes to high-speed optical networking, particularly the use of dense wavelength-division multiplexing (DWDM). If this is the case, then European carriers now face a dilemma their U.S. counterparts are only beginning to answer: How will the embedded base of standard singlemode fiber accommodate multichannel, multi-gigabit transmission in the 1550-nm window?

Having anticipated this question a few years ago, the European Commission`s Advanced Communications Technologies & Services (ACTS) program funded investigation of high-speed optical transmission within the 1300-nm window where standard singlemode performance is already proven. Research results from the ACTS efforts and other inquiries were reported recently at the European Conference on Optical Communication. While conference attendees heard that progress has been made, the question of how often optical transmission streams will pass through the 1300-nm window in real-world applications remained unanswered.

ACTS conducted much of its 1300-nm transmission research within the context of the upgrade program, an effort to find a way to enhance the capacity of European fiber backbones without ripping standard singlemode fiber out of the ground. The researchers chose soliton transmission at 1300 nm as the most likely candidate for this task, as the solitons would require a low power budget and would avoid the high group-velocity dispersion experienced at 1550 nm within standard singlemode fiber. upgrade team members included Uniphase Netherlands NV and Eindhoven University of Technology, both of the Netherlands; Telefonica I&D of Spain; Fondazione Ugo Bordoni and IT-Aviero of Italy; Portugal Telecom; Lucent Technologies, Deutsche Telekom, and the universities of Dusseldorf and Kasierslautern, all of Germany; Imperial College of the United Kingdom; and Rensselaer Polytechnic of the United States.

According to the project`s leader, Dr. John Reid of Uniphase, the team entered the program with a great deal of optimism, based on laboratory experiments that demonstrated soliton-based transmission at 20 Gbits/sec over standard singlemode fiber using semiconductor optical amplifiers (SOAs). But bringing these results to the field proved impossible, Reid says, because of the differing amounts of dispersion found in the variety of cable typically encountered in deployed networks. Nevertheless, the upgrade team demonstrated 10-Gbit/sec transmission over a 210-km link between the German cities of Kassel and Hanover at last year`s CeBIT show.

This demonstration served as a dress rehearsal for the project`s main field trial, an 800-km link from Madrid to Lisbon via San Pedro de Merida. While the team initially hoped to transmit at 10 Gbits/sec over the entire span, financial constraints prevented the placement of an optical regenerator that they had demonstrated in the lab. Thus, the run from Merida to Lisbon transmitted at 2.5 Gbits/sec.

Network details

In all, the network used 25 optical repeater units (ORUs) based on SOA technology. Each ORU was designed to accommodate a pair of polarization-insensitive SOAs for bidirectional operation. (However, the Madrid-to-Lisbon link operated in only one direction, says Reid--again because of cost constraints.) The ORUs were developed with amplification factors ranging from 12 to 29 dB to compensate for the varying lengths of the individual cable runs. But the saturation output powers were kept at a constant figure of approximately 13 dBm, as a constant power was discovered to be important in providing optimal gain from the SOAs. Telefonica was responsible for building the ORUs.

Meanwhile, Uniphase provided the SOAs as well as the RZ/soliton transmitters. The company adapted an NRZ transmitter board for the latter purpose. The short pulse generation (a pulsewidth of 40 psec, according to Reid) resulted from a distributed feedback laser with an integrated electro-absorption modulator (EAM). In practice, the laser operates in CW mode and the EAM is sinusoidally modulated via a 10-GHz clock signal. The data is applied to a Mach-Zender modulator, which results in a dynamic extinction ratio of approximately 16 dB. A semiconductor booster amplifier then amplifies the signal to 1 to 3 dBm. The receiver consists of a PIN photodiode and a transimpedance amplifier; the unit uses the same components as those for non-return to zero (NRZ) applications. A reshaping filter completes the conversion of the RZ signal to NRZ.

Reid says the network operated successfully, and that it was being used for a telemedicine demonstration. Meanwhile, the components were being offered in sample quantities for evaluation by European carriers--which leads to the question of when, if ever, such technology will be applied commercially. In the United States, 1300-nm technology is usually discussed in conjunction with erbium-doped fiber amplifiers to open a second DWDM window (see Lightwave, August 1998, page 74) or in partnership with 1550-nm techniques to provide two-way transmission capabilities in cable-television networks. The role of 1300-nm transmission as a high-speed option in its own right appears to be a matter of debate.

Pros and cons

On the pro side is Stephen Montgomery, president of ElectroniCast Corp. (San Mateo, CA), a market-analysis firm. In a conversation at the conference site, Montgomery said he believes there is a market in Europe for this kind of technology in light of the predominance of standard singlemode fiber on the continent and the advances made in SOAs.

On the other hand, Deutsche Telekom has not jumped at the opportunity to expand its experience with the technology beyond the CeBIT field trial. Conrad Burke, optoelectronics marketing director for Lucent Technologies in the United Kingdom, says the future of 1300-nm transmission in Europe remains very much in doubt, particularly in the context of a potential market for amplifiers at that wavelength. Burke, who will head Lucent`s new European amplifier design center, said the new facility will concentrate on more-conventional 1550-nm amplifiers for European DWDM applications as it begins its ramp up to full staff. He noted, however, that the population densities typical of Europe naturally lead to shorter cable runs, which in turn make amplifiers less of a factor in designing fiber-optic networks.

Thus, the determination of whether the upgrade program represents the near-term future of high-speed networks in Europe or merely an interesting research exercise remains to be seen. As the accompanying sidebar indicates, however, the amount of research into 1300-nm technology suggests there is a home for this "second window" somewhere in Europe`s fiber future. q