Fujitsu Achieves Terabit-WDM Transmission at
40 Gbps per Channel over Legacy Optical Fiber Cable

Tokyo, October 16, 2002 -- Fujitsu Laboratories Ltd. today announced that it has successfully transmitted a 1.76-terabit per second signal over 600 km of the most conventional type of optical fiber currently installed around the world (*1). The signal consisted of 44 separate single-wavelength signals, each with a data rate of 40 Gbps, multiplexed together. This achievement opens the way to cost-effective deployment of terabit-class transmission over existing optical fiber cables, obviating the expense of swapping them out for new cables.

Background
Growing demand for high-speed Internet and broadband network services in recent years has heightened requirements for future optical transmission systems that can cover longer distances and offer greater capacity. Responding to these needs, Fujitsu has been developing a next-generation 40 Gbps per channel wavelength-division multiplexing (WDM) system. To date, however, the company's 40 Gbps WDM system could only run on the very latest optical fiber with optimized chromatic dispersion management and low polarization-mode dispersion (PMD) (*2). But the most commonly used fiber in the world, which is relatively old, would not support these systems.

Supporting a 40 Gbps WDM transmission system over legacy fiber requires:

1. Improved technology to enhance the optical signal-to-noise ratio.

2. Technology to compensate for waveform degradation that results from variations in chromatic dispersion due to temperature changes in the installed cable (chromatic dispersion compensation).

3. Technology to compensate for waveform degradation due to slight fiber-core (PMD compensation).

About the technology
Fujitsu developed the following technologies to address each of these requirements.

1.

A three-stage design (distributed Raman amplifier (*3), concentrated Raman amplifier (*3), and erbium-doped fiber amplifier) for its hybrid optical inline node. This improves the signal-to-noise ration by roughly 80%.

2.

An automatic feedback-control function in the form of a virtually imaged phased array (VIPA) variable dispersion compensator (*4) that optimizes the compensation value while monitoring incoming signal characteristics.

3.

A PMD compensator consisting of a polarization controller and an adjustable retarder that optimizes the feedback control while monitoring the degree of polarization of the incoming optical signal, thereby automatically compensating for PMD alone, without dependence on transmission speed, modulation scheme, or fiber chromatic dispersion.

Bringing together all these technologies for the first time, Fujitsu developed a prototype system and tested it on a conventional single-mode fiber with high-PMD value of 8 picoseconds on average. In this test Fujitsu succeeded in the multiplexed transmission of 44 channels of 40-Gbps per channel modulation to achieve a WDM signal of 1.76 terabits per second over a distance of 600 km. In so doing, the company was able to verify the stability of its newly developed control system.

Because each of the three technologies that contributed to this breakthrough can be used independently, they can be combined as needed in other systems to suit the transmission speed, capacity, distance, and fiber type at hand, enabling greater flexibility in system design.

*1. Fiber-optic cable
The most conventional type of optical fiber installed around the world is a single-mode optical fiber with a zero-dispersion wavelength in the 1.3 micron band (the zero-dispersion wavelength is the wavelength at which the optical waveform degradation will be the smallest, where the dependency of the transmission speed on the wavelength is zero). However, when this fiber is used with the currently prevailing 1.55 micron band, optical losses are minimal, but chromatic dispersion is extremely high, making long-distance transmission difficult. This has created a need for chromatic dispersion compensation technology for transmission speeds faster than 10 Gbps.

*2. Polarization-mode dispersion (PMD)
Because the fiber-core contains slight asymmetries, a delay will arise between two orthogonal axis in the fiber, resulting in waveform degradation. Older optical fibers, especially overseas, are known for their high PMD values, thereby placing correspondingly serious limitation on transmission distance as transmission speeds get faster. Furthermore, PMD varies with random mode coupling along the fiber's length due to environment fluctuations like temperature and stress.

*3. Raman amplifier
An optical amplifier that takes advantage of the Raman scattering effect, a nonlinear effect in optical fibers. Incidence of a high-power pumping light into an optic fiber will amplify a signal power on a wavelength roughly 100 nm separated from the pump light. Typically, the amplification medium on transmission fiber is a distributed Raman amplifier, in a receiving terminal or in-line repeater, it is a concentrated Raman amplifier.

*4. Virtually imaged phased array (VIPA) variable-dispersion compensator
This bulk optical device consists of a VIPA plate-a wavelength diffractive grating, consisting of reflective coatings on both sides of a thin glass plate-and a three-dimensional mirror. The wavelength dependence of the transmittance and the group delay has periodical characteristics with 200-GHz spacing. Moving the three-dimensional mirror horizontally results in variable dispersion compensation with a range of -800 to +800 ps/nm over the entire C-band (1530-1560 nm) for a 40-Gbps NRZ signal.

About Fujitsu Laboratories Ltd.
Founded in 1968 as a wholly owned subsidiary of Fujitsu Limited, Fujitsu Laboratories Limited is one of the premier research centers in the world. With a global network of laboratories in Japan, China, the United States and Europe, the organization conducts a wide range of basic and applied research in the areas of IT Core Systems, IT Media, Networks, Peripherals, Advanced Materials and Electronic Devices.

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