Fastest optical transmission at 2Tbits/s per wavelength

Fastest optical transmission at 2Tbits/s per wavelength

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By Nick Flaherty

NTT has shown the world’s fastest optical transmission experiment of digital coherent optical signals over 2Tbits/s per wavelength.

The company developed an ultra-wideband baseband amplifier module and digital signal processing technology that can compensate for distortion in the optical transceiver circuit with extremely high accuracy. This then demonstrated the transmission and reception of digital coherent optical signals at 2.02Tbits/s per wavelength over 240 km.

This follows a similar demonstration by Nokia Bell Labs over 2Tbit/s.

Reaching these speeds requires a boost in the performance of the CMOS devices. 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 gigabaud.

However, to achieve multi-terabit optical transmission speeds it is necessary to achieve both a wider bandwidth and higher output of the driver amplifier for driving the optical modulator. This also needs compensation for differences in signal path length and loss variations due to signal paths  with extremely high accuracy.

The baseband amplifier is based on an InP-based heterojunction bipolar transistor technology and equipped with a 1mm coaxial connector that supports frequencies up to 110 GHz. The module is mounted in a package and has ultra-wideband performance and sufficient gain and output power as the driver for an optical modulator, operating in a high-power output range.

This means the nonlinearity of the driver amplifier output  becomes a problem and the optical signal band noise ratio deteriorates. In addition, with ultra-high-speed signals, degradation of signal quality becomes noticeable due to deviation from the ideal inside the optical transceiver.

The digital signal processing technology compensated for non-linear distortion generated in the modulator driver and the deviation from the ideal inside the optical transceiver with ultra-high precision.

Expanding the operating range of the module boosted the optical signal quality to create an optical amplification repeater using PCS-144QAM5. The PCS (Probabilistic Constellation Shaping) reduces the signal-to-noise ratio requirements for signal transmission by optimizing the distribution and arrangement of signal points based on information theory. By applying PCS technology to the 144 pint QAM system, it becomes possible to optimize the signal quality according to the transmission path conditions.

This optimizes the distribution of signal points and was applied to an ultra-high-speed optical signal of 176 gigabaud to generate an optical signal of up to 2.11 Tbits/s.

This demonstration result suggests that further scalability of digital coherent optical transmission technology can achieve both a large capacity per wavelength, and a long transmission distance. This core technology is expected to lead the development of the All-Photonics Network of the IOWN and 6G initiatives.

The All-Photonics Network, which is IOWN’s backbone optical communication network, must cost-efficiently achieve even greater capacity. To transmit ultra-high-speed Ethernet signals of 1.6 Tbit/s or more over long distances, NTT says it hopes to achieve long-distance optical transmission by expanding the transmission capacity per optical signal wavelength and the signal symbol rate, optimizing the amount of information per symbol.

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