NTT Corporation (President and CEO: Akira Shimada, “NTT”) has succeeded in the world’s fastest optical transmission experiment of digital coherent optical signals exceeding 2 Tbits/s per wavelength.
In this experiment, NTT developed an ultra-wideband baseband amplifier IC module and digital signal processing technology that can compensate for distortion in the optical transceiver circuit with extremely high accuracy. We 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.
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 IOWN and 6G initiatives.
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, we 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 rate, optimizing the amount of information per symbol.
To expand the transmission capacity per wavelength, it is necessary to overcome the speed limit of silicon CMOS 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 gigabaud. 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.
An Ultra-wideband Baseband Amplifier IC Module
NTT has been researching and developing an ultra-wideband baseband amplifier IC based on InP-based heterojunction bipolar transistor (InP HBT) technology and equipped with a 1 mm coaxial connector that supports frequencies up to 110 GHz. We have succeeded in creating a module that is mounted in a package and has ultra-wideband performance (Figure 2, left) and sufficient gain and output power (Figure 2, right). Presently, we have applied this baseband amplifier IC module as a driver amplifier for driving an optical modulator.
An Ultra-high-precision Optical Transceiver Circuit Distortion Compensation Technology Based on Digital Signal Processing Technology
NTT has developed an ultra-wideband baseband amplifier IC module based on InP HBT technology enabling us to generate ultra-high-speed signals. However, when it is used as a driver amplifier for driving an optical modulator, it must operate in a high-power output range, so the nonlinearity of the driver amplifier output (where the output power is not proportional to the input power) becomes a problem and the optical signal quality (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.
In this experiment, NTT's world-leading 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. We have expanded the operating range of the IC module and succeeded in improving the optical signal quality (Fig. 3). Using this high-quality ultra-high-speed optical signal, we conducted an optical amplification repeater transmission experiment. The PCS-144QAM method, which optimizes the distribution of signal points, was applied to an ultra-high-speed optical signal of 176 gigabaud to generate an optical signal of up to 2.11 Tbits/s. Furthermore, we succeeded in transmitting an optical signal of 2.02 Tbits/s over 240 km using technology that allocates the optimal amount of information according to the transmission distance (Fig. 4).
This technology is expected to enable highly reliable transmission of large-capacity signals by multiplexing optical signals exceeding 2 Tbits/s per wavelength. In particular, technology for increasing the modulation speed of optical signals not only contributes to increasing the capacity per wavelength, but also, as shown in Fig. 5, can generate large-capacity signals when combined with wavelength resource expansion technology. Our technology is also expected enable long-distance transmission. NTT 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.