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14 Tbit/s transmitted over single fiber

19 Oct 2006

NTT demonstrates "world's highest" optical transmission capacity - equivalent to 140 digital high definition movies delivered in one second.

Nippon Telegraph and Telephone (NTT) has demonstrated optical transmission of 14 Tbit/s over a single 160 km long optical fiber. This achievement, based on 140 channels of 111 Gbit/s, exceeds the previous record of about 10 Tbit/s. The result was reported in a post deadline paper at the European Conference on Optical Communication 2006, Cannes, France between 24-28 September.

The current core optical network typically has a capcity of 1 Tbit/s. Based on the wavelength-division-multiplexing (WDM) of signals with the channel capacity of 10 Gbit/s, the network uses optical amplifiers with a bandwidth of about 4 THz. Data traffic continues to double every year due to the rapid spread of broadband access. Consequently there is pressure to lower the cost and raise the capacity of the network while maintaining its reliability.

The 10 Tbit/s transmission over a single optical fiber has been achieved in the [NTT] laboratory. To do this it was necessary to use linear amplifiers that covered two or three amplification bands due to the limited range of existing amplifiers. Also, such a multi-band configuration is not cost-effective. To increase transmission capacity, NTT needed to achieve two goals simultaneously: WDM transmission with high spectral efficiency and optical amplifiers with greatly enlarged bandwidth.

The experiment

The experiment employed a technique known as the carrier suppressed return-to-zero differential quadrature phase shift keying (CSRZ-DQPSK)*1 format in conjunction with ultra-wide-bandwidth amplifiers. A total of 70 wavelengths with 100 GHz spacing were modulated at 111 Gbit/s using the CSRZ-DQPSK format and then multiplexed and amplified in the bandwidth of 7 THz.

Each 111 Gbit/s signal was polarization-division-multiplexed so the number of channels was doubled to 140. This yielded the total capacity of 14 Tbit/s. The 160-km transmission was achieved by amplifying these signals in NTT's newly-developed optical amplifiers.

Core technologies

The CSRZ-DQPSK modulation format and high-speed optoelectronic device technologies make it possible to generate dense WDM signals with bit rates above 100 Gbit/s 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 format.

The combination of this format with pulse modulation (CSRZ), developed by NTT, enhances sensitivity, and enables dense WDM long-distance transmission. To achieve a CSRZ-DQPSK signals at 100 Gbit/s or above, NTT 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 is commonly 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 Gbit/s.

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

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.

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 Gbit/s DQPSK signals.

Amplification

It is necessary to expand the bandwidths of the optical amplifiers in order to amplify the 10 Tbit/s 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), amplify each band separately, and then remultiplex the bands.

Future schedule

NTT is aiming to construct a 10 Tbit/s-class large capacity core optical network that excels in terms of its economy and quality; it will promote the realization of a long-distance transmission system that supports 100 Gbit/s high-speed channels.

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