1. Home>
  2. Publicity>
  3. Press Release>
  4. 2024>
  5. 336 Tb/s Transmission with a Single Light Source
日本語
Print

336 Tb/s Transmission with a Single Light Source

- A simple, wide-band optical communication system realized with optical comb and frequency reference distribution -

October 4, 2024
(Japanese version released on July 24, 2024)

National Institute of Information and Communications Technology

Highlights

  • Achieves high-capacity transmission of 336 Tb/s with a single light source
  • Eliminates the need for hundreds of built-in light sources in conventional transponder modules
  • Uses optical comb and frequency reference distribution technologies to generate 650 high-quality, frequency-synchronized carrier/local oscillator pairs over most of the S, C, and L transmission bands
  • Expected to accelerate commercialization and lower the costs of wide-band optical communication systems, including the S band
An international research team led by the Photonic Network Laboratory of the National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.) demonstrated a coherent optical fiber communication system with a total transmission capacity of 336 Tb/s. The system uses a single light source combined with optical comb generation and frequency reference distribution, eliminating the need for hundreds of built-in light sources within transponder modules.
In this research, 650 sets of high-quality, frequency-synchronized pairs of carriers and local oscillators were generated over most of the S, C, and L transmission bands, through comb generation on the transmitter and receiver sides. Each comb line complies with the 25 GHz frequency standard of the International Telecommunication Union (ITU) and is qualified for dual-polarization 16-quadrature amplitude modulation (QAM) coherent optical communications. Optical frequency reference distribution was used to synchronize the two comb units.
This work will accelerate the commercialization of S-, C-, and L-band optical communication systems without the need for commercially available compact S-band light sources and will help to reduce the cost by simplifying the systems.
The results of this experiment were accepted as a post-deadline paper presentation at the 47th Optical Fiber Communication Conference (OFC 2024) presented by Ben Puttnam on Thursday, March 28, 2024.

Background

Figure 1: Conceptual image of an optical network with optical comb generation and frequency reference distribution which enables automatic frequency synchronization.
[Click picture to enlarge]

To cope with increasing data traffic demands, wavelength division multiplexing (WDM) and space division multiplexing have been investigated for high-data-rate optical fiber communications. NICT has demonstrated multiband WDM transmission with a total bandwidth of 37 THz by using all the major transmission bands of standard optical fibers. However, multiband WDM in a conventional optical communication system requires hundreds of compact, frequency-stabilized light sources within transponder modules. These light sources are currently not available for the S, O, E, and U bands.

Achievements

In this work, 650 sets of carrier/local oscillator pairs were generated over most of the S, C, and L bands (16 THz frequency band) through optical comb generation on the transmitter and receiver sides (see Figure 1). Each comb line complied with the 25 GHz frequency standard of the ITU and possessed sufficiently high quality (noise characteristics) for dual-polarization 16-QAM multimode fiber coherent communications. We also distributed an optical frequency reference to synchronize the two separate comb units on the transmitter and receiver sides. Consequently, each carrier and corresponding local oscillator automatically had the same oscillation frequency without needing independent frequency stabilization, as is the case for conventional coherent communication systems (see Figure 2).

Figure2: Comparison of 320 Tb/s-class optical communication systems based on the conventional and proposed schemes. The conventional system requires 200 state-of-the-art commercial transponder modules with built-in light sources, ranging from the O to U band (total 40 THz), whereas the proposed system requires only one light source.

We used a 39-core multicore fiber with 38 cores supporting three-mode propagation and 1 core supporting single-mode propagation. One of the three-mode cores was used for data transmission and the single-mode core was used for distributing the optical frequency reference. The total transmission capacity was 336 Tb/s, which was almost 200 times greater than the data rate of the state-of-the-art commercial optical transponder module (1.6 Tb/s). If we were to deploy a commercial optical communication system with the same transmission capacity using conventional methods, we would need 200 transponder modules, including independent built-in light sources over the O, E, S, C, L, and U bands (40 THz frequency band). In this demonstration, however, we only needed a single light source instead.
The results of this experiment were accepted as a post-deadline paper presentation at the 47th Optical Fiber Communication Conference (OFC 2024, 24 to 28 March 2024) and were presented on Thursday, March 28, 2024.

Future prospects

This technology will eliminate the need for developing and implementing S-band built-in light sources and so will accelerate the commercialization of multiband WDM communication. The simple configuration (one light source) and automatic frequency locking between carriers and local oscillators will contribute to cost saving. Although we used only one of the three-mode cores in the 39-core fiber, full use of spatial channels (cores) will introduce further cost saving in optical communication systems.

References

International Conference: Optical Fiber Communications Conference (OFC) 2024, Post Deadline Session
Title: Wideband S, C,+ L-Band Comb Regeneration in Large-Scale Few-Mode MCF Link with Single-Mode Seed Channel
Authors: B. J. Puttnam, D. Orsuti, R. S. Luis, M. S. Neves, M. van den Hout, G. Di Sciullo, G. Rademacher, J. Sakaguchi, C. Antonelli, C. Okonkwo, L. Palmieri, and H. Furukawa

Previous NICT Press Releases

Appendix

1. High-capacity optical communication system using optical comb generation and frequency reference distribution

Figure 5: Schematic of the optical transmission system in this study.
Figure 5 shows a schematic of the optical transmission system.
(1) Optical frequency reference with a wavelength of 1558.98 nm was generated and input into the optical comb generator on the transmitter side and the single-mode core of the 39-core fiber.
(2) An optical comb with 650 wavelength channels and 25 GHz frequency spacing was generated across the S, C, and L bands on the transmitter side.
(3) The wavelength channel being measured (test channel) and the remaining wavelength channels (dummy channels) were separated by wavelength division demultiplexing. 
(4) Dual-polarization 16-QAM modulation was applied to the demultiplexed comb lines.
(5) Test and dummy channels were multiplexed again, branched into three paths with temporal decorrelations, converted into three-mode signals by a mode multiplexer and input into the three-mode core of the 39-core fiber. 
(6) The three-mode signals and the optical frequency reference propagated through the 39-core fiber over 13 km. 
(7) The three-mode signals were converted into three conventional optical fiber outputs by a mode demultiplexer. 
(8) From the propagated optical frequency reference, an optical comb with 650 wavelength channels and 25 GHz frequency spacing was generated across the S, C, and L bands on the receiver side.
(9) The received three-mode signals and the receiver-side optical comb output were demultiplexed by wavelength division. The receiver-side optical comb output was used as the local oscillators of the three-mode coherent receivers. 
(10) Offline MIMO signal processing was applied to the stored data to recover the transmitted signals. Finally, the signal quality was evaluated to obtain the data rate.

2. Experimental results

In the experimental system shown in Figure 5, optimal error correction was applied to each wavelength channel to maximize the transmission capacity (data rate) of the system. Each symbol in the plot of Figure 6 represents the three-mode combined data rate after an implemeted error correction scheme, and the sum of all data rates amounts to 336 Tb/s.

Figure 6: Measured data rate, after error correction, for each wavelength channel.

Glossary

International research team

The NICT Photonic Network Laboratory developed the optical transmission system and conducted the transmission experiment. Students and researchers from the University of Padua (Italy), Eindhoven University of Technology (the Netherlands), the University of L'Aquila (Italy), and the University of Stuttgart (Germany) also participated in the transmission experiment.


Coherent optical fiber communication, transponder module

Coherent optical fiber communication is a type of optical fiber communication that uses coherent detection. On the transmitter side, data is encoded in a coherent carrier light wave and the modulated signal is transmitted to the receiver side. On the receiver side, a local oscillator light wave with the same oscillation frequency as the carrier is prepared and mixed with the received signal to convert it into baseband electric signals. Therefore, one light source on the transmitter side and another on the receiver side are required for each wavelength channel.
The state-of-the-art transponder module can transmit and receive data signals at a maximum data rate of 1.6 Tb/s per wavelength channel. The maximum modulation rate is 200 billion times per second and the output signal occupies a 200 GHz frequency slot.


Optical comb generation

In optical communications, a light wave with a static oscillation frequency (carrier frequency) and low phase noise (coherent light) is generally used for each wavelength channel. Optical comb generation refers to various technologies that generate many coherent lights with equally spaced carrier frequencies (optical comb) out of a single coherent light (seed). As the comb frequency spacing increases, it becomes more difficult to obtain a wide-band comb spectrum, and as the comb spectrum becomes wider, it becomes more difficult to maintain sufficiently high quality (noise characteristics) for each comb line.


Optical frequency reference distribution

Optical frequency references, which consist of a light wave with a stable oscillation frequency, have been used in astronomy observations, gravitational potential measurements, seismic observations, and space-time standards. Optical frequency reference distribution is a technology that transfers the reference light wave to remote sites through optical fibers or other media while maintaining the original quality.
One well-known application is the redefinition of the time standard, which requires precise comparison of remotely located optical clocks. This type of application mostly suffers from low-frequency phase noise caused by environmental changes in optical fibers.
In this study, optical frequency reference distribution was used to expand the transmission bandwidth and reduce the cost of high-speed optical communication systems. This type of application mostly suffers from high-frequency noise due to optical amplification and other effects. Narrow-band filtering was used to reduce the high-frequency noise.


Transmission bands, WDM

The transmission bands suitable for telecommunication are limited. The C band (wavelength of 1530-1565 nm) and L band (1565-1625 nm) are mainly used in current long-haul optical communication systems. The T band (1000-1260 nm), O band (1260-1360 nm), E band (1360-1460 nm), S band (1460-1530 nm), and U band (1625-1675 nm) have not been commercialized yet. Multiband WDM refers to WDM that uses these uncommercialized bands.
WDM is used to transmit optical signals of different wavelengths in a single optical path. The optical signals are assigned to carrier frequency slots within the wavelength band. The total frequency bandwidth is then determined by the spacing and number of wavelength channels, so the transmission capacity can be increased by increasing the number of wavelength channels.
The widest-frequency bandwidth used in the multiband WDM experiments was 37.4 THz using the O, E, S, C, L, and U bands.


ITU frequency standard

The ITU specifies four types of frequency intervals between WDM channels, namely 12.5, 25, 50, and 100 GHz or more.


Dual-polarization 16-QAM
Multilevel modulation encodes multiple bits in a light wave by precisely controlling its amplitude and/or phase. QAM is multilevel modulation in which the amplitude and phase are used simultaneously. Because 16-QAM uses 16 different points in the complex phase space, it can encode four bits of information (24 = 16) in each transmitted symbol. Thus, the spectral density of 16-QAM is four times higher than that of a simple modulation format, such as on-off keying. The data rate can be doubled again by polarization multiplexing, in which different data signals are transmitted in two orthogonal polarization states. As the number of points increases, the quality required of the coherent light sources increases.

Space division multiplexing, multimode fiber, multicore fiber

Figure 3: Schematic of optical fiber communication using a single-core, single-mode fiber.

Single-core, single-mode fibers (see Figure 3), which are widely used commercially, have a transmission capacity limit of several hundred terabits per second. To overcome this problem, optical fibers with an increased number of optical paths (spatial channels) using multiple cores or modes have been studied.
A multicore fiber has many cores (physical optical paths) in a common cladding region, and its total transmission capacity can be increased by transmitting different data through each core.
A multimode fiber has a large core diameter that can support multiple modes within the same core. Intermodal signal interference occurs at the fiber connections, inputs/outputs, and during multimode fiber propagation. Therefore, multiple input, multiple output (MIMO) receivers that undo the interference through MIMO digital signal processing are required to recover transmitted signals. Fiber-optic communication technology that uses such optical fibers is collectively called space-division multiplexing.
This study used an optical fiber (Sumitomo Electric Industries, Ltd.) with 39 cores, of which 38 supported three modes and 1 supported a single mode (see Figure 4). One of the three-mode cores was used for data transmission, and the single-mode core was used for the optical frequency reference distribution.

Figure 4: Schematic of space-division multiplexing (multicore multimode fiber), multiband WDM, and multilevel modulation.

Technical Contact

SAKAGUCHI Jun, FURUKAWA Hideaki
Photonic Network Laboratory
Photonic ICT Research Center
Network Research Institute

Media Contact

Press Office
Public Relations Department