Companies pursue different ways to cut cross-talk in dense wavelength transmission systems.
By Neil Savage
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From lab to market
Laboratory experiments have successfully demonstrated data transmission in the range of 7 Tb/s, a development that the researchers say heralds continued progress in high-speed optical communications.
At post-deadline sessions during September's European Conference on Optical Communication (ECOC) in Munich, Germany, both NEC Corp. (Tokyo, Japan) and Siemens Information and Communications Networks (Boca Raton, FL) reported on their achievement of transmissions in the lab of 40 Gb/s per channel over a single fiber. NEC used 160 channels for an overall transmission of 6.4 Tb/s, while Siemens reached 7.04 Tb/s by using 176 channels.
"What we proved basically is that even with a 40 Gb bit rate and 50 GHz channel spacing we can achieve almost 200 km transmission," said Milorad Cvijetic, chief technical officer of NEC America's Public Networks Group.
The NEC system transmitted 80 channels in the L band and 80 channels in the C band, spaced 50 GHz apart. The multiplexer at the transmitting end set the polarization of neighboring channels in different modes, so that two channels with the same polarization were actually separated by 100 GHz.
"When you have so many channels and high bit rates, the main issue is crosstalk between the channels," Cvijetic explained. "Neighboring channels with different polarizations will cause crosstalk to be reduced."
Researchers were able to achieve a spectral efficiency of 0.8 bits per second per Hz of bandwidth. By using fiber fabricated with the most up-to-date technologies, they were able to keep polarization mode dispersion down to acceptable levels, below 0.8 ps. They used gallium arsenide heterobipolar transistors to create optical receivers with a 40 Gb/s capacity.
Though the total distant over which the researchers transmitted was only 186 km, their aim is to reach 400 km. Cvijetic said that should be possible using RZ carrier suppressed modulation techniques and forward error correction (FEC). And Cvijetic said it should be no problem to match Siemens' 7.04 Tb/s rate by adding a few more channels.
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While NEC improved channel spacing through polarization interleaving, Siemens achieved it's total through bi-directional transmission. The system, designed by Siemens subsidiary Optisphere Networks (Reston, VA), used four subbands, two in the C band and two in the L band. Each had 44 channels with 100 GHz spacing, with a C-band and an L-band group propagating in one direction while the other two subbands propagated in the opposite direction.
Christian Scheerer, Siemens' group leader for WDM systems, points out that, as it is necessary to transmit in two directions, two fibers using this system would provide high-speed transmission with back-up in case of failure in one. A protected system with one-directional transmission would need four fibers, two in each direction.
The Siemens system also provided spectral efficiency of 0.8 bit/s/Hz. The channels were modulated by two lithium niobate Mach-Zehnder modulators, and electronic time-division multiplexing generated a 40 Gb/s signal from four 10 Gb/s input data streams.
Although Siemens only demonstrated transmission over 50 km of fiber, Scheerer said the distance can easily be doubled. The experiment did not use and Raman amplification of FEC coding, which would also increase the distance.
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From lab to market
Both Scheerer and Cvijetic predicted their companies could be selling products based on this high-speed work within about two years. Siemens demonstrated 3.2 Tb/s transmission last year. "Our next product, which will come next year, will have this 3.2 Tb," he said. "This gap between laboratory demonstration and product is shrinking all the time."
Frank Briamonte, a spokesman for the optical networking group at Lucent Technologies (Murray Hill, NJ), agreed transmission capacity is increasing rapidly, and Lucent is also working on input speeds of 40 Gb/s. Lucent will have 800 Gb/s equipment on the market by the end of this year, and "1.6 [Gb/s] is definitely something that's reachable and attainable by next year."
Other companies, such as Alcatel (Paris, France) and Fujitsu (Tokyo, Japan), are also working to increase transmission capacity.
"It's highly competitive, and you can see that looking all around the market," Briamonte said. "Customers want more and more bandwidth so they can have their more advanced data services."
Cvijetic agreed the race toward higher transmission speeds is not likely to end soon.
"After this there will be the next target. It doesn't mean this bit rate is the final bit rate. We will try to go further on," he said.
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About the Author:
Neil Savage is a freelance writer based in Massachusetts.