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1.53 Petabit per Second Transmission in 55-mode Fiber with Standard Cladding Diameter

- Technology demonstration for future high-capacity ICT infrastructure -

November 10, 2022
(Japanese version released on October 3, 2022)

National Institute of Information and Communications Technology
Nokia Bell Labs

Highlights

  • The world's first 55-mode transmission at a record data-rate of 1.53 petabits per second in any standard cladding diameter optical fiber was demonstrated.
  • A novel 55-mode fiber enables ultra-high spectral efficiency with high-data-rate achieved using only the most common optical communications (C-) band.
  • This demonstration shows the potential of multi-mode transmission for future high-capacity backbone networks
A group of researchers from the Network Research Institute of the National Institute of Information and Communications Technology (NICT, Japan) led by Georg Rademacher in collaboration with Nokia Bell Labs (USA), Prysmian Group (Prysmian, France, and the Netherlands), and the University of Queensland (Australia) succeeded in the world's first large-capacity transmission experiment with large mode multiplexing technology using 55 modes. The experiment also reports data-rate of 1.53 petabits per second, a record in any standard cladding diameter (0.125 mm) optical fiber to date. 55 modes were successfully multiplexed throughout the commercially adopted optical fiber transmission window (C-band), with a dramatic increase in spectral efficiency compared to conventional fibers and previous multi-mode transmission. A further expansion of transmission capacity can be anticipated by combining this technology with multi-band wavelength-division multiplexing technology. This demonstration serves as important step in the maturity of high mode-count multi-mode transmission technology and the development of Beyond 5G and subsequent information and communications infrastructure technology. 
The results of this experiment were accepted as a post-deadline paper presentation at the 48th European Conference on Optical Communication (ECOC 2022) and presented on Thursday, September 22, 2022.

Background

In recent years, transmission systems using advanced optical fibers with the same cladding diameter as standard single-mode fibers, but able to support multiple propagation paths have been investigated. NICT constructed transmission systems using either a standard cladding diameter 4-core optical fiber and a single-core 15-mode optical fiber, succeeding in transmission experiments at 1 petabit per second with both fibers. The number of cores in standard cladding diameter multi-core fibers is limited by inter-core crosstalk, however, further increasing the number of spatial channels in standard diameter optical fiber is still possible by enlarging the core of a multi-mode fiber. However, the propagation characteristics for each mode can vary something leading to degradation of the signal quality and an increase in signal processing complexity. Hence, increasing the mode-count requires precisely engineered components such as multiplexers and fiber as well as complex processing techniques to recover the signals after mode-mixing and enable large-capacity transmission beyond the 15-modes already demonstrated.

Figure 1
Figure 1 : Transmission System (Circled numbers correspond to the description in Figure 7)

Achievements

NICT has constructed a transmission system using Prysmian's single-core 55-mode optical fiber and a mode multiplexer / demultiplexer designed and manufactured by Nokia Bell Labs and the University of Queensland. Using these components, the group has successfully transmitted a total of 1.53 petabits per second over 25.9 km. To evaluate the 55-mode signal, we constructed a high-speed, parallelized receiver system. The 55- independent signal streams could then be separated by 110 x 110 MIMO processing to recover the transmitted data. We could successfully receive polarization multiplexed 16 QAM signals at 184 wavelengths in the C wavelength band. Compared with the previous 15-mode multiplexed transmission, the spectral efficiency has improved by more than three times (332 bits/s/Hz) with the increase in the number of modes. This demonstration showed that with such high spectral efficiency only use of the C-band was needed to transmit more than 1.5 Pb/s and so further expansion can be expected by adopting multi-band wavelength-division multiplexing in the future.
At present, the Beyond 5G (6G) information communications society is being promoted around the world. As the amount of network devices and data traffic continues to increase, study of information and communication infrastructure technology for the post Beyond 5G era must also be investigated. This achievement is an important technological step towards this goal.

Table 1: Comparison of the present results with past results of NICT


 		 Transmission capacity (bits/second) Number of modes or cores Number of wavelengths Frequency band (total) Spectral efficiency (bits/s/Hz)
S-band C-band L-band
December 2020  1.01 Peta  15    189  193  9.6 THz  105
May 2022 1.02 Peta 4 335 200 266 20 THz 51
This work 1.53 Peta 55   184   4.6 THz 332

Future Prospects

In the future, we will explore further transmission capacity by expanding the frequency band, as well as fundamental technologies necessary for long-distance transmission and network deployment.
The results of this experiment were accepted as a post-deadline paper presentation at the 48th European Conference on Optical Communication (ECOC 2022) and presented on Thursday, September 22, 2022 local time.

References

European Conference on Optical Communication (ECOC2022)
Title: 1.53 Peta-bit/s C-Band Transmission in a 55-Mode Fiber
Authors: Georg Rademacher, Ruben S. Luís, Benjamin J. Puttnam, Nicolas K. Fontaine, Mikael Mazur, Haoshuo Chen, Roland Ryf, David T. Neilson, Daniel Dahl, Joel Carpenter, Pierre Sillard, Frank Achten, Marianne Bigot, Jun Sakaguchi, and Hideaki Furukawa

Previous NICT press releases

Appendix

1. Newly developed transmission system

Figure 7
Figure 7 Schematic diagram of the newly developed transmission system (circled numbers refer to Figure 1).

Figure 7 shows a schematic diagram of the newly developed transmission system.

① Optical comb source: 184 carriers are generated in a single optical comb source.
② Signal modulation: Carriers are modulated with 16 QAM signals and polarization multiplexed.
③ Parallel signal generation: Signals are branched for each mode, and path delays are applied to emulate independent data streams.
④ Mode multiplexer: Each signal is transformed into a different spatial mode and launched into a 55-mode optical fiber.
⑤ 55-mode optical fiber: Signals propagate through a 25.9 km long 55-mode optical fiber.
⑥ Mode demultiplexer: Signal for each spatial mode at the receiver side is separated and transformed into a fundamental mode.
⑦ High-speed parallel receivers: The mode-demultiplexed signals are wavelength-demultiplexed by filters and converted into electrical signals by parallel coherent receivers.
⑧ Off-line signal processing: MIMO processing to eliminate signal interference during fiber propagation.

2. Results of this experiment

In the experimental system shown in Figure 7, the transmission capacity (data-rate) of the system was estimated by directly applying error coding on the received bits. The blue dot in Figure 8 shows the data rate after applying error corrections with flexible overheads to maximize the throughput for each wavelength channel. Although the data rate slightly decreased at the long wavelength end of the C-band (near 1,565 nm), almost uniform and stable data rates were obtained in other wavelength regions, and a total of 1.53 petabits per second was realized after error corrections.

Figure 8
Figure 8 Summary of achieved data-rate measurement

Glossary

Figure 2
Figure 2 Cross section of the 55-mode optical fiber and
conceptual image of multi-mode propagation

Mode multiplexing

Each mode of a multimode fiber can be used as an independent transmission channel. The multi-mode optical fiber used in this experiment is designed to support up to 55 propagation modes and to minimize the delay difference between the modes. The core diameter is 0.050 mm.

Terabit, petabit

One petabit is 1,000 trillion bits, one terabit is 1 trillion bits, and one gigabit is 1 billion bits. 1 petabit per second is equivalent to 10 million channels of 8K broadcasting.

Figure 3 Profile of standard single-mode optical fiber

Standard cladding diameter optical fiber

The international standard specifies that the outer diameter of the glass (cladding) of the optical fiber is 0.125 ± 0.0007 mm and the outer diameter of the coating layer is 0.235 to 0.265 mm. The optical fiber widely used in the present optical communication systems is a single-core single-mode fiber with a cladding diameter of 0.125 mm, and the transmission capacity is limited to approximately 250 terabits per second. Research and development of new standard cladding diameter optical fibers is actively conducted.

Optical fiber transmission windows

There are C-band (wavelength 1,530 to 1,565 nm) and L-band (1,565 to 1,625 nm), which are mainly used for commerical communication applications, and O-band (1,260 to 1,360 nm), E-band (1,360 to 1,460 nm), S-band (1,460 to 1,530 nm), and U-band (1,625 to 1,675 nm). Only the C band was used this time.

Figure 4
Figure 4 Optical communication wavelength band.
Figure 5
Figure 5 Comparison of spectral efficiencies in recent large-capacity transmission results achieved with standard cladding diameter optical fibers.

Spectral efficiency

An indicator for the amount of information that is carried in a certain spectrum, indicating efficient transmission. In this experiment, the spectral efficiency was more than three times higher compared to the result of the previous 15-mode high-capacity transmission.

Wavelength-division multiplexing (WDM) technology

WDM is a method of transmitting optical signals of different wavelengths within a single optical fiber. WDM is a widely used technology to increase the transmission capacity in proportion to the number of wavelengths. However, the available bandwidth suitable for efficient optical transmission is limited and the current number of wavelengths used in current long-distance optical transmission systems is typically around 90 in the C-band. The number of wavelengths can be expanded by using L-band and S-band together (multiband).


4-core optical fiber

The capacity limit of the standard single-core, single-mode optical fiber currently used is about 250 terabits per second. To solve this problem, multi-core optical fibers with increased number of cores (signal paths) and multi-mode optical fibers have been investigated. The 4-core optical fiber with standard cladding diameter is attractive for early adoption of new space-division multiplexing fibers in high-throughput and long-distance links since it is compatible with conventional cable infrastructure, expected to have mechanical reliability comparable to standard single-mode fibers, and does not require the complex signal processing needed for unscrambling signals in multi-mode fibers.

Figure 6
Figure 6 Mode-dependent propagation characteristics.

Propagation characteristics for each mode

In a multimode optical fiber, the propagation loss and delay are generally different among modes. When the mode dependent loss is large, the quality of the signal deteriorates. When the difference in the propagation delay is large, the scale of the circuit required for the MIMO processing increases.

Mode multiplexer / demultiplexer

Transmitter and receiver sub-systems are largely based on single-mode fiber technology. Therefore, a novel device is necessary that can convert the signal from 55 input signals to be compatible with the 55 modes, guided by the multi-mode fiber. In this experiment, a novel wideband mode multiplexer was employed that was based on reflecting the light from 55 input fibers multiple times on a phase plate to generate signals that are compatible with the 55-mode fiber.


MIMO (Multiple-input-multiple-output) digital signal processing

MIMO is a signal processing technology that was originally used in wireless communications to eliminate multi-path interference. In optical communications, MIMO is a signal processing technology to remove interference between signals that propagate in the same fiber core. The complexity of MIMO processing increases with the number of fiber modes and if the range of propagation speed between modes increases.


Polarization multiplexed 16 QAM

QAM is a multi-level modulation format with high spectral information density. 16 QAM uses 16 different signal symbols and can therefore encode 4 bits of information. The spectral density of 16 QAM is therefore 4 times higher than for simple modulation formats such as on-off keying. The data rate can be doubled by polarization multiplexing in which different data signals are transmitted in two orthogonal polarization states.

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Photonic ICT Research Center
Network Research Institute

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