Alcatel, world leader in intelligent optical networking, has successfully demonstrated in its laboratory another first in optical networks: transmission of 125 DWDM channels of data at an effective rate of 40Gbit/s per channel, representing a total capacity of 5Tbit/s, over 1,500 kilometers of its TeraLightTM Ultra optical fiber. This record is added to the many set by Alcatel in the past year (see charts for more information on Alcatel`s 2001 optical transmission breakthroughs), as the Company continues efforts to maximize available bandwidth and transmission distance over a single fiber while minimizing costs with its DWDM (dense wavelength division multiplexing) technology.
The breakthrough was achieved with Alcatel`s dual-stage hybrid Erbium/Raman amplifiers, 40Gbit/s DWDM systems, Forward Error Correction (FEC) technology, and TeraLightTM Ultra optical fiber specifically optimized for high-speed, long-haul and ultra long haul transmission. Additionally, this means that total throughput would allow the simultaneous transport of 80 million voice calls or more than 500,000 high bit rate ADSL Internet lines per second over a single optical fiber.
TeraLightTM Ultra, Alcatel`s non-zero dispersion shifted fiber (NZDSF) offers several features which make it ideal for successful operation over high data rates and long distances. Enhanced performance of polarization mode dispersion (PMD), chromatic dispersion slope and reduced attenuation at key Raman amplification wavelengths, all combined for record-setting efficiency that network operators need for optimized long-haul network economy with advanced technologies.
A formula used by technologists to describe these results, called the `capacity times distance product`, combines the key variables of the number of channels, data rate and distance carried. It is an important indicator of not only a fiber`s ability to carry huge amounts of data on a single fiber, but also the distance required between regeneration points. Maximizing this distance helps telecom carriers reduce their equipment costs. Alcatel`s latest result yields 7.5Pb/s*km (5Tbit/s over 1,500 km), a record in terrestrial transmission based upon 40Gbit/s per channel.
"The series of terrestrial and submarine optical transmission technology breakthroughs performed by Alcatel over the year 2001 is critical to the development of our next-generation products", said Bernard Le Mou"el, chief technology officer of Alcatel`s Optics Group. "Indeed, these very high capacity and ultra long-haul experiments allow us to significantly extend the performance and lower the costs of the new products we put on the market. Such improvements require a variety of advanced technologies which must be developed with a system view in order to reach the best trade-off between cost and performance. Moreover, the result we are announcing today demonstrates the TeraLightTM product family`s readiness for the evolution of systems."
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The laboratory trials which achieved these results consisted of 125 WDM channels at 40Gbit/s effective rate. The channels were separated alternatively by 50 and 75GHz and demultiplexed by optical Vestigial-side band (VBS) filtering. The transmission was carried over fifteen spans of TeraLightTM Ultra fiber 100 km each, separated by Raman-assisted erbium-doped fiber amplifiers operated in the C and L bands.
Raman amplification
Raman amplification is a technique that takes advantage of an inherent property in optical fiber that shifts power from lower to higher wavelengths. The Raman "pump" sends a lower wavelength in the opposing direction of the transmission signal, which in turn absorbs power and is amplified. Raman pumps operate at 1450nm, so TeraLightTM Ultra`s low attenuation in this same region makes the amplification more efficient and effective.
Polarization Mode Dispersion (PMD)
PMD is the naturally occurring effect of light`s two polarizations separating because they travel at different speeds, eventually degrading the quality of transmission. Factors which influence PMD are the attributes of the optical fiber in the system, compensation devices, and even amplifiers. Cable structures, thermal fluctuation, and other mechanical stresses are also influential.
Chromatic Dispersion and Chromatic Dispersion Slope
Chromatic dispersion is a property found in all fibers that causes light pulses to spread. These dispersion effects accumulate over a distance, eventually degrading the quality of the transmission if not corrected(through dispersion compensation). Furthermore, wavelengths disperse differently as they travel. The end-to-end accumulation of the differences in dispersion is referred to as chromatic dispersion slope. High dispersion slope increases the accumulated dispersion differences between channels, requiring channel by channel dispersion compensation, which can add complexity and cost to networks. By optimizing dispersion and dispersion slope, TeraLightTM keeps the accumulated dispersion of all wavelengths at an optimum level within an amplifier band.
TeraLightTM Ultra fiber
TeraLightTM Ultra is Alcatel`s Non Zero Dispersion Shifted Fiber (NZ-DSF) optimized for transmission distances greater than 200km. As long-haul networks continue to evolve to higher bit rates and longer distances, the advancements found in TeraLight TM Ultra support the newest and emerging technologies with the best overall network economy. Greater capacity per fiber, low attenuation, and improved polarization mode dispersion (PMD) performance enable TeraLightTM Ultra to offer superior transmission quality at the lowest cost per available bit. TeraLightTM Ultra is fully compliant with the ITU industry standard G655b.
Forward Error Correction (FEC)
The FEC is a technique by means of which redundancy (i.e. extra information) is transmitted together with transported data, using a pre-determined algorithm. The receiving device has the capability of detecting and correcting multiple bit errors that could occur during transmission thanks to the redundancy (i.e. extra information). The signal transmitted with FEC is more `robust` thus allowing operators to build up longer distance connections without the deployment of many repeater stations.
WDM (wavelength division multiplexing)
In WDM, several signals (or channels) are transported simultaneously over one fiber but at different wavelengths. Each channel is usually Time Division Multiplexed (TDM). The capacity of a WDM system is thus given by the number of wavelengths x the bit rate of the TDM channel.
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