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World's first demonstration of optical transmission and switching of 15-mode multiplexed signals on a field-deployed multi-mode fiber network

- Technology development focusing on ICT infrastructure after Beyond 5G -

November 22, 2022
(Japanese version released on September 22, 2022)

National Institute of Information and Communications Technology

Highlights

  • Demonstration of spatial-division-multiplexed network based on the first field-deployed 15-mode fibers
  • The world’s first successful optical switching of 15-mode multiplexed signals by spatial optical switch prototype
  • A major step toward the realization of a backbone communication system that supports various information communication services after Beyond 5G
A group of researchers from the Network Research Institute of the National Institute of Information and Communications Technology (NICT, Japan) led by Ruben Soares Luís, in collaboration with the University of L'Aquila (Italy), Heinrich-Hertz-Institute, Fraunhofer Institute for Telecommunications (HHI, Germany), Finisar Australia (Australia), Prysmian Group (Italy, Holland, France), and Nokia Bell Labs (Bell Labs, USA) achieved the first demonstration of a field deployed 15-mode optical fiber network, equivalent to 15 parallel fiber networks using a single fiber installed in the city of L’Aquila, Italy.
So far, mode-division-multiplexed fiber communication networks have been demonstrated only in a laboratory environment and the number of switching modes was limited to 10. In this demonstration, the world’s first field-deployed 15-mode fiber with standard cladding diameter, installed in the city of L’Aquila, Italy, was used to support a network node capable of switching up to six 15-mode spatial super channels, each carrying up to 5 terabit per second (Tbit/s) signals using mode-multiplexing. This was equivalent to implementing 15 parallel optical fiber networks using a single fiber and a single network node, demonstrating the potential for spatial division multiplexing (SDM) systems to greatly extend the capacity of current fiber networks in a realistic environment. The metropolitan 15-mode fiber network was a 6.1 km long and installed in an underground tunnel in the city of L’Aquila. The 15-mode network node was the first 15-mode reconfigurable optical add and drop multiplexer (ROADM), which used reprogrammed conventional wavelength selective switches (WSS). This allowed using the same switch fabric to simultaneously switch multiple modes to greatly reduce the overall cost of the system.
This demonstration shows that mode-multiplexing over multi-mode fibers with a standard cladding diameter, which are compatible with current cabling technologies and have a high spatial channel count, can be adopted in deployed networks and is a significant step toward the realization of a backbone communication systems to support various information communication services after Beyond 5G.
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.

Background

High-capacity SDM technologies have been under research and development, in order to cope with the ever-increasing traffic volume. Previously, NICT has successfully demonstrated high-capacity transmission and switching supported by multi-core optical fibers. However, the use of multi-mode fiber network systems, which have improved spatial density (number of channels per fiber) have been in development. Optical switching of 10 modes or less per multi-mode fiber in a laboratory environment has been demonstrated. Nevertheless, further expansion of the number of modes and verification in a real-world environment are to be expected.

Figure 1
Figure 1: 15-mode multiplexed network constructed on the testbed in the city of L’Aquila, Italy

Achievements

NICT and its partners have implemented a 15-mode fiber network utilizing multi-mode fibers of a field deployed testbed in the city of L’Aquila, Italy. The deployed 15-mode fibers were used to demonstrate mode-multiplexed transmissions up to 48.8 km (8 rounds). In addition, optical switching of 15-mode multiplexed signals using field-deployed fibers was achieved for the first time.
The SDM optical switch prototype was constructed using conventional wavelength selective switches (WSS) programmed to handle signals from multiple modes simultaneously. Effectively, each WSS was equivalent to 5 wavelength cross-connects (WXCs), each with a 2×2 switching capability. A total of 3 WSS were used to switch a total of 15 modes per fiber. A transmitter and the WXC node supporting 15-mode multiplexed signals were located in the facilities of the University of L’Aquila and connected to a field-deployed 6.1 km 15-mode fiber ring.
In the demonstration of optical switching, 15-mode signals with 6 wavelengths were generated. Each wavelength corresponded to a 15-mode spatial super channels with 5 Tbit/s capacity and the total data rate per fiber was 30 Tbit/s. At the network node, the path of each wavelength of the mode-multiplexed signals was directed according to the programmed configuration. Three general functions of a ROADM system were evaluated, including add/drop of all wavelengths, express pass of all wavelengths, and partial express or add/drop of individual wavelengths. For all cases, mode-multiplexed signals were appropriately received after optical switching.
Although mode multiplexing requires digital signal processing at the receiver to compensate for a mode mixing, a mode-multiplexed network utilizing multi-mode fibers with standard diameters can provide a high-density, high-capacity network at low cost. The multi-mode fibers are easy to manufacture, compatible with existing cable technology, and can provide a high-density, high-capacity network at low cost. This demonstration of optical transmission and switching of mode-multiplexed signals on the testbed is an important step toward accelerating research on mode-multiplexed communications and realizing a backbone communication system that supports the evolution of various information services through Beyond 5G.

Future Prospects

In the future, we will establish a foundation for future large-capacity optical transmission technology while working to extend the distance of large-capacity multimode optical fiber transmission, expand the scale of optical switching, and pursue the possibility of merger with multi-core technology.
The results of this experiment were published at the 48th European Conference on Optical Communications (ECOC 2022, September 2022), one of the largest international conferences related to optical fiber communications held in Basel, Switzerland. It was selected as the best hot topic paper (Post Deadline Paper) and published on September 22 (Thursday).

Responsibilities of each organization

NICT: Demonstration experiment of optical switching, prototype of 15-mode optical switch, analysis of experimental data
L'Aquila University: Construction of a 15-mode optical fiber in the testbed
HHI: Building a 15-mode transceiver
Finisar: Development of control program for wavelength selective switch for 15-mode optical switch
Prysmian: Provide 15-mode fiber cable
Bell Labs: Construction of 15-mode fiber connection

References

European Conference on Optical Communication (ECOC2022)

Title: Characterization of the First Field-Deployed 15-Mode Fiber Cable for High Density Spatial Division Multiplexing
Authors: Georg Rademacher, Ruben S. Luis, Benjamin J. Puttnam, Giammarco Di Sciullo, Robert Emmerich, Nicolas Braig-Christophersen, Andrea Marotta, Lauren Dallachiesa, Roland Ryf，Antonio Mecozzi, Colja Schubert, Pierre Sillard, Frank Achten, Giuseppe Ferri, Jun Sakaguchi, Cristian Antonelli, Hideaki Furukawa
Title: Demonstration of a Spatial Super Channel Switching SDM Network Node on a Field Deployed 15-Mode Fiber Network
Authors: Ruben S. Luis, Georg Rademacher, Benjamin J. Puttnam, Giammarco Di Sciullo, Andrea Marotta, Robert Emmerich, Nicolas Braig-Christophersen, Ralf Stolte, Fabio Graziosi, Antonio Mecozzi, Colja Schubert, Frank Achten, Pierre Sillard, Roland Ryf，Lauren Dallachiesa, Satoshi Shinada, Cristian Antonelli, Hideaki Furukawa

Previous NICT press releases

Appendix

1. Newly developed transmission system

Figure 6
Figure 6: Configuration of 15-mode fiber network in the city of L’Aquila 　

Figure 6 shows the configuration of the 15-mode fiber network. A transmitter, an optical switch, and a receiver are located in the university of L’Aquila and connected to a 15-mode fiber deployed in the city of L'Aquila.

15-mode Transmitter (TX)
Wavelength/polarization-multiplexed 16QAM signals (50 Gbaud) are generated by using 6 external cavity lasers and a polarization-multiplexed IQ modulator. Signals divided into 15 are spatially multiplexed by using a 15-mode multiplexer (MPLC: Multi Plane Light Converter). As a result, 30 Tbit/s (5 Tbit/s x 6 wavelengths) 15-mode multiplexed signal is generated from the transmitter.

15-mode optical switch
15-mode multiplexed signals with 6 wavelengths are spatially-demultiplexed by the 15-mode demultiplexer (DeMUX) at the input of a switch. Each signal (wavelength multiplexed signal) is switched to an adjacent node (express) or its own node (drop) by a 2x2 wavelength cross connect (WXC). The express signals are spatially-multiplexed again by the 15-mode multiplexer (MUX) at the output of the switch, and 15-mode multiplexed signals except the dropped wavelengths are transmitted to the adjacent node through the 15-mode optical fiber. The dropped wavelengths are processed at the own node without being converted into a mode-multiplexed signal. In this demonstration, three commercially available 4x16 wavelength selective switches were used to form 15 2x2 WXCs.

15-mode Receiver (RX)
The receiver has a 15-mode demultiplexer at the input, and each of the separated 3 modes (groups) is processed. A wavelength to be processed is selected by a wavelength selective filter and received by a coherent receiver. The signal is demodulated by digital signal processing (MIMO).

2. Results of this experiment

Figure 7
Figure 7: Switching scenarios to demonstrate 15-mode ROADM
Figure 7 shows three switching scenarios in this demonstration of 15-mode ROADM.
     
Scenario A: Add full wavelengths of 15-mode (from Node A to Network)
Drop full wavelengths of 15-mode (from Network to Node B)
Scenario B: Full Express wavelengths of 15-mode (from Node A to Node C)
Scenario C: Partial Express wavelengths of 15-mode (from Node A to Node C)
Partial Drop wavelengths of 15-mode (from Node A to Node B)
Partial Add wavelengths of 15-mode (from Node B to Node C)
　
Figure 8
Figure 8: Received signal quality after each optical switching

Figure 8 shows the received signal quality (quality factor Q) after optical switching. In all scenarios, the received signal quality after optical switching exceeds the minimum receivable level (FEC threshold), indicating that the 15-mode multiplexed signal can be demodulated even after switching.

Glossary

The city of L’Aquila, Italy (Testbed environment)

Testbed for research and development of new optical fiber communication is constructed in the city. Novel fiber cables, such as multi-core fiber, multi-mode fiber, are deployed in the underground tunnel of 6.1 km per lap. In this demonstration, 15-mode transceiver and 15-mode optical switch were in the university of L’Aquila and connected with the deployed 15-mode fibers.


Mode division multiplexing

Each propagation mode of a multi-mode fiber can be used as an independent transmission channel, and a transmission capacity can increase with the number of multiplexed modes. The multiplexed modes can be demultiplexed and demodulated by a mode-demultiplexer and a digital signal processing in the receiver.

Figure 2
Figure 2: Cross section of a 15-mode optical fiber and conceptual image of multi-mode propagation
[Click picture to enlarge]
15-mode fiber (multi-mode fiber)
Multi-mode fiber has a larger core diameter than a conventional single mode fiber. The larger core can generate more vibration states (propagation modes). In general, multi-mode fiber can be used for a short-distance communication (less than some hundreds of meters), which has less effect by the modal dispersion.
15-mode fiber is a multi-mode fiber whose core diameter is designed to support up to 15 propagation modes. The core diameter is 0.028 mm and its modal dispersion is designed to be small.

Figure 3
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 250 terabits per second. Research and development of new standard cladding diameter optical fibers is actively conducted.


Spatial super channel

A spatial super channel is a block of information that is transmitted by multiple spatial channels in parallel. The information is divided into as many parts as spatial channels for independent transmission. As an example, a 5 Terabit per second spatial super channel transmitted by 5 spatial channels means that each spatial channel will transmit 1 Terabit per second. Spatial channels can take the form of cores of a multi-core fiber or modes in a multi-mode fiber. They can also take the form of independent fibers. For the transmission of spatial super channels, it is important that the corresponding spatial channels are synchronized. Otherwise, some synchronization procedure for temporal realignment may be required.

Figure 4
Figure 4: Examples of spatial super channels using groups of single-mode fibers (fiber bundles), multi-mode fiber, or multi-core fiber
[Click picture to enlarge]

Terabit per second (Tbit/s)

One terabit is one trillion bits, and one terabit per second is equivalent to 10 thousand channels of 8K broadcasting per second.


Spatial division multiplexing (SDM)

SDM is a multiplexing technique that spatially increases transmission paths and expands transmission capacity. The capacity of the standard single-core single-mode fiber currently used for short and long-distance optical communication systems is considered to be limited to about 250-300 terabits per second. In order to solve this problem, SDM research has been advanced on multi-core fibers with increased number of cores (transmission path) and multi-mode fibers (mode division multiplexing).


Reconfigurable optical add and drop multiplexer (ROADM)

A reconfigurable optical add and drop multiplexer is a typical form of network node capable of directing traffic in a wavelength division multiplexing (WDM) network. A conventional ROADM will have 2 or more line sides, each handling an optical signal with multiple wavelengths. A wavelength selective switch is used to separate the different wavelengths of the signals and direct them to one of the other line sides (express) or to drop and add new signals (add and drop). In the case of advanced ROADM technologies, such as spatial division multiplexing ROADMs, the switching of wavelengths may be swapped by switching of spatial channels. In this case, the wavelength selective switch may be replaced with an appropriate spatial switching device, such as a “core selective switch” when using multi-core fibers.

Figure 5
Figure 5: Structure of a conventional ROADM
[Click picture to enlarge]


Wavelength division multiplexing (WDM)

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).


MIMO (Multiple-input-multiple-output)

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 propagation speed difference of all fiber modes increases.

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