NTT Announces Successful Demonstration of World's Largest Capabcity 14 Tbps Transmission Over Single Optical Fiber

JCN Newswires, Sept 29, 2006

Tokyo, Japan, Sept 29, 2006 - (JCN Newswire) - Nippon Telegraph and Telephone Corporation has successfully demonstrated the ultra-large capacity optical transmission of 14 Tera bits per second (Tera is one trillion) over a single 160 km long optical fiber. The value of 14 Tbps (111 Gbps x 140 ch) greatly exceeds the current record of about 10 Tbps and so claims the record of the world's largest transmission capacity.

This result was reported as a post deadline paper in the European conference on optical communication (ECOC) that was held in Cannes, France from September 24 to 28.

The present core optical network is an optical transport network with about 1 Tbps capacity. Based on the wavelength-division-multiplexing (WDM) of signals with the channel capacity of 10 Gbps, it uses optical amplifiers with the bandwidth of about 4THz. The data traffic has been doubling every year due to the rapid spread of broadband access. We must lower the cost and raise the capacity of the core network while maintaining its reliability as the dominant communication infrastructure.

10 Tbps transmission over a single optical fiber has been achieved in the laboratory. However, it was necessary to use linear amplifiers that covered two or three amplification bands because of the limited range of existing amplifiers, and this multi-band configuration is not cost-effective. To increase the transmission capacity, we had to achieve two goals simultaneously: WDM transmission with high spectral efficiency and optical amplifiers with greatly enlarged bandwidth.

Outline of Experiment

Our experiment used the carrier suppressed return-to-zero differential quadrature phase shift keying (CSRZ-DQPSK)*1 format and ultra-wide-bandwidth amplifiers. 70 wavelengths with 100-GHz spacing were modulated at 111 Gbps using the CSRZ-DQPSK format and then multiplexed and amplified in the bandwidth of 7 THz. In addition, each 111 Gbps signal was polarization-division-multiplexed so the number of channels was doubled to 140. This yielded the total capacity of 14 Tbps. 160-km transmission was successfully achieved by amplifying these signals in newly developed optical amplifiers.

NTT demonstrated in this experiment, for the first time, that it is possible to transmit 100 Gbps signal with forward error correction*2 bytes and management overhead bytes of the OTN*3 frame over long distances allowing the construction of large capacity optical networks that offer 10 Tbps or more.

Core Technologies

(1) CSRZ-DQPSK modulation format and high-speed optoelectronic device technologies

These technologies make it possible to generate dense WDM signals with bit rates of 100 Gbps and beyond per channel and transmit them over long distances. DQPSK is a phase modulation format with four phase states. Its benefits include its high spectral efficiency and excellent receiver sensitivity; both superior to those offered by the conventional binary intensity modulation (ON-OFF-keying) format. The combination of this format with pulse modulation (CSRZ), developed by NTT, enhances the sensitivity, and enables dense WDM long-distance transmission. To realize a CSRZ-DQPSK signals at 100 Gbps or above, we had to overcome the problems of the complicated configuration of the transmitter block and the difficulty of raising the modulation speed. The Mach-Zehnder interference type, lithium niobate (LN) modulator has been used as a binary intensity or phase modulator in high-speed transmitters, but there is a trade-off between driving voltage and bandwidth and it was considered to be virtually impossible to raise the operation speed to at least 100 Gbps.

To overcome these problems, NTT newly developed a hybrid integration technology that yields silica-based planar lightwave circuits and LN lightwave circuits*4. Both devices simplify the configuration and support the fast modulation speed of 111 Gbps.

While the conventional binary intensity modulation format uses a photodiode in the receiver, the DQPSK receiver needs a pair of balanced photodetectors, usually realized by integrating two high-speed photodiodes, making it difficult to achieve high-speed operation, high sensitivity, and uniform conversion efficiency, simultaneously. NTT improved the structure of the photodetector with the result that the new balanced receiver offers high-speed operation at over 50 GHz as well as high sensitivity.

InP ICs, which can be operated at over 50 GHz were used in multiplex and demultiplex circuits and the waveform shaping part to generate high-quality 111 Gbps DQPSK signals.

(2) Ultra-wide-band inline optical amplification technology

It is necessary to expand the bandwidths of the optical amplifiers in order to amplify the 10 Tbps or more signal in one optical fiber. While most fibers have bandwidths in excess of 10 THz, conventional amplifiers have bandwidths of approximately 4 THz. This means that it was necessary to divide the channels into two bands (C and L band) or three bands (S, C, and L band) *5, amplify each band separately, and then remultiplex the bands.