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Demonstration of World Record: 319 Tb/s Transmission over 3,001 km with 4-core optical fiber

- >120 nm signal bandwidth comprising 552 WDM channels and using both-doped fiber and Raman amplification -

July 12, 2021

(Japanese version released on June 21, 2021)


National Institute of Information and Communications Technology

Points

  • 319 Tb/s long-haul transmission of wideband (>120 nm) S, C and L-bands signal using 552 PDM-16QAM, wavelength-division multiplexed channels in a 4-core optical fiber
  • Long-distance transmission over 3,001 km enabled by adoption of both erbium and thulium doped-fiber amplifiers and distributed Raman amplification
  • Demonstration shows potential of SDM fibers with standard-cladding diameter and compatibility with existing cabling technologies for near-term adoption of high-throughput SDM fiber systems
Researchers from the National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.), Network Research Institute, succeeded the first S, C and L-bands transmission over long-haul distances in a 4-core optical fiber with standard outer diameter (0.125 mm). The researchers, lead by Benjamin J. Puttnam, constructed a transmission system that makes full use of wavelength division multiplexing technology by combining different amplifier technologies, to achieve a transmission demonstration with date-rate of 319 terabits per second, over a distance of 3,001 km. Using a common comparison metric of optical fiber transmission the data-rate and distance produce of 957 petabits per second x km, is a world record for optical fibers with standard outer diameter.
In this demonstration, in addition to the C and L-bands, typically used for high-data-rate, long-haul transmission, we utilize the transmission bandwidth of the S-band, which has not yet been used for further than single span transmission. The combined >120nm transmission bandwidth allowed 552 wavelength-division multiplexed channels by adopting 2 kinds of doped-fiber amplifier together with distributed Raman amplification, to enable recirculating transmission of the wideband signal. The standard cladding diameter, 4-core optical fiber can be cabled with existing equipment, and it is hoped that such fibers can enable practical high data-rate transmission in the near-term, contributing to the realization of the backbone communications system, necessary for the spread of new communication services Beyond 5G.
The results of this experiment were accepted as a post-deadline paper presentation at the International Conference on Optical Fiber Communications (OFC 2021).

Background

Over the past decade, intensive research has been carried out worldwide to increase the data rates in optical transmission systems using space-division multiplexing as a means to meet the exponentially increasing demand for optical data transmission. More recently, interest in fibers with the same 125 µm cladding diameter as standard single-mode fibers has grown due to their compatibility with conventional cabling infrastructure and concerns over the mechanical reliability of larger fibers. Particularly with multi-core fibers (MCFs), reducing the cladding diameter limits the number of spatial channels, leading to increasing interest in combining such fibers with wider transmission bandwidths in order to meet the expected growth in transmission capacity expected in SDM fibers. Until now, NICT has built various transmission systems that make use of wavelength division multiplexing across the C and L-bands together with state-of-the-art modulation technology to explore high data-rate transmission in a range of new optical fibers. Recently, NICT and research groups around the world have begun to explore S-band transmission, leading to several new records for transmission capacity in optical fibers, but transmission distance has been limited to only a few tens of kilometers. 

Achievements

Fig. 1
Fig. 1 Transmission demonstrations using 125 µm diameter fibers
[Click picture to enlarge]
NICT has built a long-distance transmission system around a 4-core optical fiber with a standard cladding diameter to exploit wider transmission bandwidth of >120nm across S, C and L-bands. The system exploits wavelength division multiplexing (WDM) and a combination of optical amplification technology to enable long-haul transmission of 552 WDM channels from 1487.8 nm to 1608.33 nm. The system was used to measure achievable transmission throughput with each channel modulated with PDM-16QAM modulation at distances up to 3,001 km, where a data-rate of 319 terabits per second was achieved. This result may be compared to achievements in other SDM fibers and transmission regimes by calculating the product of transmission capacity and distance, often used as a comparison metric. The data-rate x distance product becomes 957 petabits per second x km, which is over 2.7 times larger than previous demonstrations in SDM fibers with standard outer diameter.  
Long distance transmission, not previously demonstrated with S-band signals, was enabled by constructing a recirculating transmission loop experimental set-up that combined 2 kinds of rare-earth doped fiber amplifiers  with Raman amplification distributed along the transmission fiber itself.
The 4-core MCF with standard cladding diameter is attractive for early adoption of SDM fibers in high-throughput, long-distance links, since it is compatible with conventional cable infrastructure and expected to have mechanical reliability comparable to single-mode fibers. Beyond 5G, an explosive increase from new data services is expected and it is therefore crucial to demonstrate how new fibers can meet this demand. Hence, it is hoped that this result will help the realization of new communication systems that can support new bandwidth hungry services.

Future Prospects

Fig. 2
Fig. 2 Part of the transmission system this time (Raman amplification section)
NICT will continue to develop wide-band, long-distance transmission systems and explore how to further increase transmission capacity of low-core-count multi-core fibers and other novel SDM fibers. Further, we will work to extend the transmission range to trans-oceanic distances. 
The paper containing the results of this experiment was published at the International Conference on Optical Fiber Communication ((OFC2021, June 6 (Sun) to June 11 (Fri)), one of the largest international conferences related to optical fiber communication, this year held virtually due to Corona virus travel restrictions. It was highly evaluated and was presented in the Post Deadline session, known for release of latest important research achievements on June 11 (Fri) 2021 local time.

References

International Conference: International Conference on Optical Fiber Communications (OFC 2021) Post Deadline Paper
Title: 319 Tb/s Transmission over 3001 km with S, C and L band signals over >120nm bandwidth in 125 μm wide 4-core fiber
Authors: Benjamin J. Puttnam, Ruben S. Luís, Georg Rademacher, Yoshinari Awaji, and Hideaki Furukawa

Previous NICT Press Releases

Appendix

1.　 Description of the novel transmission system

Fig. 7
Fig. 7 Schematic diagram of the transmission system

Fig. 7 shows a schematic diagram of the transmission system developed.

① 552 optical carriers with different wavelengths are collectively generated in a frequency comb.
② Polarization multiplexed 16QAM modulation is performed on the output light of the optical frequency comb light source, and a delay added to create different signal sequences.
③ Each signal sequence is launched into one core of a 4-core optical fiber.
④ After propagating through a 69.8 km long 4-core optical fiber, transmission loss is compensated by optical amplifiers in the S, C, and L-bands, and the optical fiber is introduced into the 4-core optical fiber via a loop switch. By repeating this loop transmission, the final transmission distance reached was 3,001 km.
⑤ The signal of each core was received and the transmission error was measured.

2.　Experimental Results

Fig. 8
Fig. 8 Experimental results

In the experimental system shown in Fig. 7, the data-rate of the system was determined by applying error correction coding on the transmitted bit-stream.
The graph of the experimental results in Fig. 8 shows the data-rate (Throughput) after decoding, and although there are some variations, the average data rate per channel is around 145 gigabits per second for each core, and the average data-rate of combined spatial super-channel (4-cores) was over 580 gigabits per second. The data rate of 319 terabits per second was achieved across the 552 wavelength channels.

Glossary

Fig. 3
Fig. 3 Profile of standard single-mode optical fiber

Standard outer diameter optical fiber

According to international standards, the outer diameter of the glass (cladding) of optical fibers 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 current optical communication is a single-core single-mode fiber with an outer diameter of 0.125 mm, and the capacity limit is considered to be about 100 terabits per second in the conventional C and L-bands and 200-300 terabits per second if adopting additional bands.

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.

Optical amplification method

Optical fibers have a very small transmission loss compared to coaxial and other electrical cables, but since data is transmitted over long distances, it is necessary to compensate for attenuation periodically, typically after several tens of kilometers. This is usually done in an optical amplifier which may amplify many wavelength (WDM) channels simultaneously. Optical amplification methods include using rare-earth doped-fibers, Raman amplification, and semiconductor optical amplifiers.

Terabit, Petabit

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

Product of data-rate and distance

One advantage of optical fiber transmission is a large capacity, based on many wavelength channels using the wide transmission bandwidth of optical fiber, and the possibility to transmit signals over long-distance with little signal degradation. Therefore, the product of the data-rate and distance is often used as to quantify optical transmission systems.

Wavelength band

Various wavelength bands for optical fiber transmission are defined, distinguished by regions with different transmission characteristics arising from the physical properties of the fiber. The C-band (wavelength 1,530 to 1,565 nm) and L-band (1,565 to 1,625 nm) are most commonly used for long-distance transmission, with O-band (1,260-1,360 nm), E-band (1,360-1,460 nm), S-band (1,460-1,530 nm), currently used only for short-range communications and U-band (1,625-1,675 nm) rarely used due to lack of suitable amplification. In this experiment we extended the range for long distance transmission to include the S-band, together with C and L-bands.

Fig. 4
Fig. 4 Optical communication wavelength band

Amplifier using rare earth-added fiber
By adding a small amount of rare earth ions such as erbium (Er3+) and thulium (Tm3+) to the base material of an optical fiber, amplification can be achieved by exciting these ions with lower wavelength pump lasers and then amplifying signal photons through stimulated emission. Such amplifers have significantly increased the transmission range of optical fiber communication and allowed amplification of many wavelength channels simultaneously.

Raman amplification

Raman amplification is based on stimulated Raman scattering (SRS), when signal photons induce the inelastic scattering of a lower-wavelength 'pump' photon in a non-linear optical medium. As a result additional signal photons are produced, with the surplus energy resonantly passed to the vibrational states of molecules in the fiber core. This process, as with other stimulated emission processes, allows all-optical amplification in optical fibers with the gain depending on the material of the fiber core.

Fig. 5
Fig. 5 Comparison of transmission loss when Raman amplification is not performed and after Raman amplification

New optical fiber

Currently, standard single-core single-mode fiber, which is widely used for medium- and long-distance communication, is considered to have a capacity limit of about 100 terabits per second in the conventional C- and L-bands and 200-300 terabits per second if adopting additional bands. In order to further increase transmission capacity, research on multi-core fibers with more cores (light paths) and multi-mode fibers has been performed extensively in recent years.

Fig. 6
Fig. 6 Main standard outer diameter optical fibers that NICT has conducted transmission experiments

16QAM

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 in each. The spectral density of 16 QAM is therefore 4 times higher than for simple modulation formats such as on-off keying. Modulation methods that can transmit 5 times (32QAM) and 6 times (64QAM) information of OOK can also be used, but 32QAM and 64QAM but make the system more vulnerable to signal distortions such as optical amplifier noise and are not suitable for long-distance transmission. 16QAM is considered to be a practical multi-level modulation methods because it can reach medium and long distances sufficiently while increasing the information density per symbol.

Technical Contact

Benjamin J. Puttnam
Photonic Network Laboratory
Photonic ICT Research Center
Network Research Institute

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