• A world record for high-capacity, long-haul transmission in standard diameter optical fibers was achieved in coupled-3-core multi-core fiber with characteristics similar to multi-mode fibers.
  • The signal processing complexity is significantly reduced compared to multi-mode fibers.
  • The fiber type is promising for early adoption in backbone high-capacity transmission systems as it can be cabled with the same technology.
In a collaboration, led by RADEMACHER Georg between researchers from the Network Systems Research Institute at the National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.) and researchers from NOKIA Bell Labs (Bell Labs, President: WELDON Marcus), led by RYF Roland, transmission of 172 terabit/s over 2,040 km was successfully demonstrated, using a standard outer diameter (0.125 mm) coupled-3-core optical fiber.
Using the product of data-rate and distance as a general index of transmission capacity, we achieved 351 petabit/s x km, more than doubling the current world record in standard outer diameter optical fibers employing space-division multiplexing. The used coupled-core multi-core fiber requires signal processing on the receiving side after transmission, but the signal processing load is less compared to more commonly investigated few-mode fibers. In addition, the used fiber has the same outer diameter as standard optical fibers which allows to convert such a fiber into a cable with existing technologies and equipment, simplifying a timely adoption of coupled-core multi-core fibers in the industry.
     The results of this experiment were presented at the 43rd International Conference on Optical Fiber Communications (OFC 2020) where it was accepted as a Post Deadline Paper.


Figure 1
Figure 1: Data-rates and distances reported to date with standard cladding diameter optical fibers
In order to cope with ever-increasing communication traffic, research on new-types of optical fibers that can exceed the limits of conventional optical fibers and large-scale optical transmission experiments using them are actively conducted around the world. In research pursuing ultimate high capacity, multi-core and multi-mode fibers that increase the number of optical fiber cores and transmit optical signals of different modes to each core are being studied. On the other hand, in research aimed at early commercialization, research is being carried out on multi-core or multi-mode optical fibers with a standard outer diameter (0.125 mm) in consideration of manufacturing methods and ease of handling.


NICT constructed a large-capacity, long-distance transmission system based on the results of Bell Labs' long-distance transmission demonstration experiment using the suppressed modal dispersion characteristics of a coupled-core multi-core fiber. 359 wavelength channels were modulated by 16QAM signals, and a total data-rate of 172 terabits per second was successfully transmitted over 2,040 km. Converted to the product of transmission capacity and distance, which is a general indicator of transmission capacity, 351 petabit per second x km was achieved, which is more than twice the current world record.
When using coupled-core multi-core fibers for transmission, it is necessary to eliminate the interference between optical signals between cores by signal processing (MIMO processing) on the receiving side. To date, transmission over coupled-core multi-core fibers has been performed only in a limited signal band (less than 5 nanometers in wavelength range), and it was unclear whether it is possible to achieve both long-distance transmission characteristics and large-capacity transmission in coupled-core multi-core fibers.
In this experiment, using a standard outside diameter optical fiber, we succeeded in transmitting 17 times the backbone communication capacity of Japan over a distance of 2,040 km. The standard outside diameter optical fiber is compatible with conventional fiber cables, increasing prospects for early commercialization of large-capacity backbone communication systems.
Figure 2
Figure 2: Experimental demonstrations of advanced optical fibers by NICT

Future Prospects

We will work on research and development of future optical communication infrastructure technology that can smoothly accommodate traffic such as 5G-based services and international communications via submarine cables.
The paper on the results of this experiment was published at the 43rd International Conference on Optical Fiber Communication (OFC 2020, March 8 (Sun) - March 12 (Thu)), one of the largest international conferences on optical fiber communication held in San Diego, USA. It was highly evaluated and was presented in the Post Deadline session, known for release of latest important research achievements, and published on Thursday, March 12 2020.


International Conference: 43rd International Conference on Optical Fiber Communications (OFC 2020) Post Deadline Paper
Title: 172 Tb/s C+L Band Transmission over 2,040 km Strongly Coupled 3-Core Fiber
Authors: Georg Rademacher, Ruben S. Luís, Benjamin J. Puttnam, Roland Ryf, Sjoerd v. d. Heide, Tobias A. Eriksson, Nicolas K. Fontaine, Haoshuo Chen, René-Jean Essiambre, Yoshinari Awaji, Hideaki Furukawa, and Naoya Wada

Previous NICT Press Releases


1. Description of the novel transmission system

Figure 5
Figure 5: Schematic diagram of transmission system
① Simultaneous generation of 359 laser lines at different wavelength
② Modulation of all laser lines with polarization multiplexer 16-QAM signals with quasi independent data 
③ Each signal is launched into a different core of the coupled-3-core multi-core fiber
④ The signals are launched multiple times through the 60 km coupled-3-core multi-core fiber to emulate transmission of a total of 2,040 km distance
⑤ The signals in all cores are received and 6 x 6 MIMO processing is used to separate the signals from each other

2. Experimental Results

Figure 6
Figure 6: Experimental results
Each dot in figure 6 represents the data-rate that could be achieved for a wavelength channel. The data-rates per channel were between 400 and 550 Gb/s for each channel and the total data-rate was greater than 172 Tb/s, being the sum of the data rates of each wavelength channel.


Figure 3
Figure 3: Cross-section of a common standard single-mode optical fiber
One terabit is one trillion bits and one petabit is one thousand trillion bits. Total download traffic in Japan in May 2019 is about 12 terabits per second.
optical fibers with standard outer diameter
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 150 terabits per second.
coupled-core multi-core fiber
In order to reduce crosstalk between cores in (non-coupling type) multi-core optical fibers, the separation between cores is appropriately increased to confine the signals in each core. In long distance transmission, signal distortion occurs due to linear and nonlinear optical effects, and after transmission, signal processing needs to be performed individually for each core.
A multi-mode optical fiber has a larger core diameter and multiple fiber modes can propagate independently. During transmission, modes generally mix with each other. Hence, the receiver side generally requires MIMO processing to invert the mixing that occurred during transmission. Signals in different modes generally arrive at different times at the receiver, increasing the computations load for MIMO processing.
In the coupled-core multi-core fiber, the cores are arranged close together with large crosstalk on the premise that signal interference between the cores is compensated (removed) by MIMO processing on the receiver side. Since the core separation is small, a high core density can be obtained. Although the signals propagate through each core, the cores are randomly coupled, creating mixing similar to multi-mode fibers. This means that the receiver must perform MIMO processing to separate the signals. However, the fast random coupling means smaller temporal spread is observed and MIMO signal processing requires less computational load compared to multi-mode fiber transmission.  
Figure 4
Figure 4: Comparison of coupled-core and uncoupled-core multi-core fibers
product of data-rate and distance
One of the great advantages of optical fiber transmission is a large capacity, based on many wavelength channels using the wide wavelength range of light, 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.
previous experiments by Bell Labs using coupled-core multi-core fibers
While different research institutes have demonstrated transmission experiments on various multi-mode fibers, transmission using coupled-core multi-core fibers were mainly carried out at Bell Labs. A coupled-core multi-core fiber is a kind of multi-core fiber proposed by Prof. KOKUBUN Yasuo (Yokohama National University) and Prof. KOSHIBA Masanori (Hokkaido University) in 2009. In the Bell Labs transmission experiments, the propagation delay difference (mode dispersion) between cores was suppressed to a minimum compared to a single-core multi-mode fiber, and the signal processing load for MIMO equalization was reduced.

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