NTT achieves breakthrough optical transmission of over 2 Tbits/sec/wavelength

Oct. 17, 2022
NTT Corporation announced that it has succeeded in the world’s fastest optical transmission experiment of digital coherent optical signals exceeding 2 Tbits/s per wavelength.

NTT Corporation announced has succeeded in the world’s fastest1 optical transmission experiment of digital coherent2 optical signals exceeding 2 Tbits/s per wavelength.

In its experiment demonstrating the breakthrough transmission speed, as recounted in a footnoted press release, NTT developed an ultra-wideband baseband amplifier IC3 module and digital signal processing technology that can compensate for distortion in the optical transceiver circuit with extremely high accuracy.

The company then demonstrated the transmission and reception of digital coherent optical signals exceeding 2 Tbits/s per wavelength and succeeded in a 240 km optical amplification repeater transmission experiment of an optical signal of 2.02 Tbits/s.

The company contends this result suggests that further scalability of digital coherent optical transmission technology can achieve both a large capacity per wavelength―which is more than double the conventional level―and a long transmission distance.

This core technology is expected to lead the development of the All-Photonics Network of the IOWN4 and 6G initiatives, stated NTT.

Communication traffic is predicted to increase in the future due to the proliferation of 5G services that will address various social issues and the development of IOWN and 6G services. The All-Photonics Network, which is IOWN's backbone optical communication network, must cost-efficiently achieve even greater capacity.

In the future, to economically transmit ultra-high-speed Ethernet signals of 1.6 terabits per second or more over long distances, NTT's experts said they hope to achieve long-distance optical transmission of more than 2 Tbits/s per wavelength by expanding the transmission capacity per optical signal wavelength and the signal symbol rate6, optimizing the amount of information per symbol.

As further explained by NTT's statement:

To expand the transmission capacity per wavelength, it is necessary to overcome the speed limit of silicon CMOS7 semiconductor circuits. To date, NTT has been researching and developing optical transmission systems and integrated devices using band doubler technology that overcomes the speed limit of silicon CMOS using AMUX and has succeeded in generating optical signals with a symbol rate exceeding 100 gigabaud8. However, to realize optical transmission of multi-terabits per second or more, it is necessary to achieve both a wider bandwidth and higher output of the electrical amplifier (driver amplifier for driving the optical modulator) in the optical transceiver. In addition, as speeds continue to increase, there is a demand for technology that can compensate for deviations from the ideal optical transmission/reception circuit (differences in signal path length, loss variations due to signal paths, etc.) with extremely high accuracy.

Now, for the first time in the world, we have demonstrated the transmission and reception of a digital coherent optical signal exceeding 2 Tbits/s per wavelength (Fig. 1, left) and successfully conducted an optical amplification repeater transmission experiment of 2.02 Tbits/s over 240 km (Fig. 1, right). Our team achieved this feat through the advanced fusion of NTT's original ultra-wideband baseband amplifier IC module and ultra-high-precision digital signal processing technology.

Ultra-wideband baseband amplifier IC module

NTT stated that it has been researching and developing an ultra-wideband baseband amplifier IC3 based on InP-based heterojunction bipolar transistor (InP HBT) technology9 and equipped with a 1mm coaxial connector that supports frequencies up to 110 GHz.

The new technology is expected to enable highly reliable transmission of large-capacity signals by multiplexing optical signals exceeding 2 Tbits/s per wavelength. In particular, the company noted that its technology for increasing the modulation speed of optical signals not only contributes to increasing the capacity per wavelength, but also can generate large-capacity signals when combined with wavelength resource expansion technology 10.

The company said the demonstrated technology is also expected enable long-distance transmission. NTT said it will promote research and development by continuing the integration of its own device technology, digital signal processing technology, and optical transmission technology toward the realization of an All-Photonics Network of the IOWN and 6G initiatives.

1 According to NTT’s research as of Sept. 2022.

2 Digital coherent technology is a transmission method that combines digital signal processing and coherent reception. Coherent reception is a technology that makes it possible to receive the amplitude and phase of light by causing interference between a light source placed on the receiving side and the received optical signal. Modulation methods such as polarization multiplexing and phase modulation improve frequency utilization efficiency, and high-precision optical signal compensation using digital signal processing and coherent reception achieve a significant improvement in reception sensitivity.

3 An ultra-wideband baseband amplifier IC (Integrated Circuit) developed by NTT that has the widest bandwidth in the world. InP-HBT realizes an amplifier IC that applies our unique high-precision circuit design technology and new circuit architecture technology that enables broadband. NTT News Release: “Achievement of Amplifier IC with World's Widest 241 GHz Bandwidth: Expected as General-Purpose Ultra High-Speed ​​Device Technology for Next-Generation Data Centers and Beyond 5G”

4 NTT Technology Report for Smart World: What’s IOWN?:

5 PCS (Probabilistic Constellation Shaping) is a technology that reduces the signal-to-noise ratio requirements for signal transmission by optimizing the distribution and arrangement of signal points based on information theory. QAM (Quadrature Amplitude Modulation) is a modulation method that carries information on both the amplitude and phase of signal light, and 144QAM has 144 signal points. By applying PCS technology to the QAM system, it becomes possible to optimize the signal quality according to the transmission path conditions.

6 The number of times the optical waveform switches in one second. A 176 gigabaud optical signal transmits information by switching the optical waveform 176 billion times per second.

7 Complementary metal oxide semiconductors are used to realize large-scale functions such as CPU as a structure to realize a semiconductor integrated circuit. This type of circuit is often used for transmission and reception of large-capacity optical transmission because the amount of signal is large. Although the speed is increasing due to miniaturization, compound semiconductors are superior in terms of high speed.

8 NTT News Release: “World's First Successful Wavelength Multiplexing Optical Transmission Experiment for Long-distance Transmission of 1 Tbit/s per Wavelength: A Future Large-Capacity Communication Network Technology Supporting the Spread of IoT and 5G Services”

9 A heterojunction bipolar transistor using indium phosphide, a group III-V semiconductor. It is a transistor with excellent speed and withstand voltage.

10 NTT News Release: “World's First Success in Broadband Optical Amplification Relay Transmission Using Optical Parametric Amplifier: Capable of More than Twice the Capacity of Conventional Optical Amplifiers”

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