Fujitsu succeeds in world's first 1 Terabit/sec WDM transmission over distance of 10,000 km

WDM is an advanced technology that enables the transmission of multiple optical wavelengths over a single fiber. Data transmission capacity increases in proportion to the number of wavelengths transmitted. However, it is technically difficult to simultaneously enable both high capacity and long-haul transmission. Previously, the world's record for high-capacity, long-haul data transmission (as measured by capacity times distance) was 640Gb/s (64WDM) over a distance of 7,200km. The record transmission capacity for distances over 10,000km was 170Gbps over the distance of 10,850km. Fujitsu's achievement of 1Tb/s over 10,000km more than doubles the previous capacity times distance record.

Fujitsu currently supplies a commercial 320Gb/s optical submarine cable system that multiplexes 32 channels of 10Gb/s transmission. Its latest experimental achievement will be applied to producing a higher-capacity and longer-distance transmission system to meet the demanding requirements of the Internet era.

Technology Overview

Transmission capacity can be expanded by increasing bit rate per channel and/or the number of wavelengths. But when signal bit rate becomes higher and the channel spacing shorter, the signal-to-noise ratio required for long-haul transmission becomes difficult to achieve due to the larger nonlinear effect(*1) on the signal. To overcome this obstacle, Fujitsu used not only a 1550nm (nanometer) band but also a 1580nm band, and it developed several new technologies to suppress the nonlinear effect. The Fujitsu technologies applied in the experimental system included the following:

1. WDM Optical Fiber Amplifier
   A parallel combination of an already-developed 1550 nm (nanometer) band optical fiber amplifier and a new 1580 nm band amplifier achieves higher-capacity bandwidth characteristics to simultaneously amplify 104 channels of multiple wavelengths with 50 GHz (0.4 nm) channel spacing (44 channels at 1550 nm band and 60 channels at 1580 nm band).
2. Application of low loss and nonlinear effect suppressing fiber
   Fujitsu used a larger diameter optical fiber (which has lower loss) for the first half of the span and a different fiber to compensate the dispersion of each channel and induce waveform distortion in the latter half. By optimizing the length ratio of the two types of fibers, Fujitsu was able to suppress the nonliniear effect and achieve a higher signal-to-noise ratio.
3. To suppress the waveform distortion caused by nonlinear effect, phase modulation is necessary. This requires the optical signal transmitter/receiver unit to have intensity and phase modulators that make the size of the unit very large. To counter this, Fujitsu developed a new optical modulator that integrates intensity and phase modulation to achieve lower loss of signal and a smaller size. It also implemented forward error correction technology to compensate for the deficiency in signal-to-noise ratio per each wavelength during transmission.

Nonlinear effects in optical fibers originate from vibrational excitation of the fiber's silica material and nonlinear refraction related to the intensity dependence of the refractive index. Particularly for long-haul transmission systems over 10,000km and more than 100 WDM channels, accumulated nonlinear effects can lead to waveform distortion and crosstalk between WDM signals. The ratio of nonlinear effect generation and the penalties arising from it closely relate to chromatic dispersion in optical fibers. Therefore, dispersion management is very important.