R&D Information
14-Tbit/s over a Single Optical Fiber—
Demonstration of World’s Largest Capacity

NTT has successfully demonstrated ultrahighcapacity optical transmission of 14 Tbit/s over a single 160-km-long optical fiber. As this greatly exceeds
the current record of about 10 Tbit/s, NTT is claiming
to have set a new world record for transmission
capacity. Composed of 140 channels each carrying
111 Gbit/s, it is equivalent to transmitting 140 digital
high-definition movies in one second.
The present core optical network is an optical transport network (OTN) with a capacity of about 1 Tbit/s.
It is based on wavelength division multiplexing(WDM) of signals with a channel capacity of 10Gbit/s and uses optical amplifiers with a bandwidthof about 4 THz. To increase the transmission capacity, two goals had to be achieved simultaneously:
WDM transmission with high spectral efficiency andoptical amplifiers with greatly enlarged bandwidth.
NTT used the carrier suppressed return-to-zero differential quadrature phase shift keying (CSRZDQPSK) format and ultrawide-bandwidth amplifiers
for the experiment. 70 wavelengths with a spacing of
100 GHz were modulated at 111 Gbit/s using theCSRZ-DQPSK format and then multiplexed andamplified in a bandwidth of 7 THz. In addition, each
111-Gbit/s signal was polarization division multiplexed, so the number of channels was doubled to140. This yielded a total capacity of 14 Tbit/s. The
160-km transmission was successfully achieved byamplifying these signals in newly developed opticalamplifiers.
In addition, this experiment demonstrated, for thefirst time, that it is possible to transmit a 100-Gbit/ssignal with the forward error correction bytes andmanagement overhead bytes of the OTN frame overlong distances. This will allow the construction ofoptical networks with capacities of 10 Tbit/s or more.
■ Key technologies(1) CSRZ-DQPSK modulation format and highspeed optoelectronic device technologies: DQPSK isa phase modulation format with four phase states. Its
benefits include its high spectral efficiency and excelVol. 4 No. 11 Nov. 2006

lent receiver sensitivity; both are superior to thoseoffered by the conventional binary intensity modulation (ON-OFF-keying) format. The combination ofthis format with pulse modulation (CSRZ), developed by NTT, enhances the sensitivity and enables
dense WDM long-distance transmission. NTT newly
developed a hybrid integration technology for silicabased planar lightwave circuits and lithium niobate
lightwave circuits. Both devices simplify the configuration and support the fast modulation speed of 111
Gbit/s for the CSRZ-DQPSK format. It also
improved the structure of the photodetector with theresult that the new balanced receiver operates at over
50 GHz and has high sensitivity for 111 Gbit/s
CSRZ-DQPSK demodulaton. InP integrated circuits,
which can be operated at over 50 GHz, were used in
the multiplexing and demultiplexing circuits and thewaveform shaping part to generate high-quality 111Gbit/s DQPSK signals. These technologies make itpossible to generate dense WDM signals with bit
rates of 100 Gbit/s and beyond per channel and transmit them over long distances.
(2) Ultrawideband inline optical amplification
technology: NTT succeeded in extending the bandwidth of an L-band amplifier to 7 THz, which is about
1.75 times that of conventional amplifiers. Low noisecharacteristics were achieved by improving the amplification medium and the amplifier’s configuration.
NTT aims to construct a 10 Tbit/s-class core optical
network that excels in terms of economy and quality.
This will lead to a long-distance transmission systemthat supports 100-Gbit/s channels.
For further information, contact:
E. Tamechika
Planning department
NTT Science and Core Technology LaboratoryGroup
Tel: +81-46-240-5152
http://www.ntt.co.jp/sclab/contact_e/53