Press Release | Successful Development of World’s First 19-Core Simultaneous Pumped Optical Amplifier | NICT-National Institute of Information and Communications Technology

Points

Developed a prototype of 19-core EDFA (erbium-doped fiber amplifier) for demonstrating the principle of simultaneous pumping which enables long-distance optical transmission with a large capacity equivalent to 19 individual optical fibers.
By adopting a free-space optical system using lenses, a single optical amplifier can be used for all cores, increasing efficiency compared to conventional single mode fiber technology requiring 19 optical amplifiers.
Expected to allow realization of long-distance multicore-fiber transmission systems required to support continuously increasing communication traffic.

The National Institute of Information and Communications Technology (NICT, President: Dr. Masao Sakauchi), in collaboration with Furukawa Electric Co., Ltd. (Furukawa Electric, President: Mitsuyoshi Shibata), its subsidiary OFS Fitel, LLC (OFS, CEO and Chairman: Timothy Murray), and Optoquest Co., Ltd. (Optoquest, President: Noboru Higashi), has succeeded in developing the world’s first “19-core simultaneous pumped optical amplifier” for multicore fiber transmission.

The development of multicore fibers as a next-generation technology that will increase the transmission capacity per single optical fiber has been highly anticipated. Since multiple cores (light paths) exist in a multicore fiber, however, it is difficult to integrate optical amplifiers compactly, which are needed for long-distance transmission. This has been an obstacle to the practical introduction of long-distance transmission systems using multicore fibers. The 19-core simultaneous pumped optical amplifier is capable of simultaneously amplifying optical signals transmitted through 19 cores. The technology is a leap forward in the practical realization of large-capacity, long-distance optical communication using multicore fibers.

Background

With the recent growing popularity of smartphones and high bandwidth applications, Internet traffic is increasing at an annual rate of up to 40%, and there is a limit to the optical energy that can be launched into optical fibers that will be used to support this increasing traffic. Then it has been pointed out that the physical limit of transmission capacity may soon be reached if the current situation continues. Thus, in order to go beyond this limit, research on multicore fibers - in which multiple cores (light paths) are provided inside a single optical fiber - has been conducted. However, optical amplifiers, which are needed for long-distance communication using multicore fibers, have only been reported as proof-of-concept demonstrations or have been demonstrated by transmission experiments based on separate pumping systems, which require the same number of optical amplifiers as the number of cores. Accordingly, there is still a need for developing practical, compact, efficient, and economical optical amplifiers.

Achievements

19-core optical fiber amplifier

We have succeeded in developing a “19-core simultaneous pumped optical amplifier,” which is compatible with a 19-core fiber (press released on March 8, 2012). The optical amplifier consists of a multiplexer that multiplexes signal light and pumping laser light, an optical fiber for amplification, and an isolator. NICT worked out the basic design and tested the transmission, Furukawa Electric and OFS fabricated the 19-core fibers for transmission and amplification, and Optoquest fabricated the optical multiplexer and the isolator.

A feature of the optical amplifier is that, by employing a free-space optical system using lenses, an optical multiplexer that efficiently multiplexes optical signals and pumping laser beams for multiple cores and a 19-core simultaneous isolator that prevents amplified communication signal light from being reflected and reentering the amplifier are implemented without depending on complex optical fiber machining technology. Thus, we succeeded in developing a prototype for demonstrating the principles of a compact, energy-saving, and economical optical amplifier that is capable of efficiently amplifying optical signals for 19 cores without the need for the same number of amplifiers as the number of cores. Since the free-space optical system readily allows the number of cores to be reduced, it is possible to develop compact, efficient, and economical optical amplifiers for multicore fibers with various numbers of cores up to 19.

Furthermore, in this research, we revised the arrangement of the cores in order to suppress core-to-core interference noise, which has been an issue in 19-core fibers (see Fig. 2 in the appendix). It is expected that the practical introduction of long-distance transmission systems using multicore fibers will accelerate in order to handle communication traffic that will drastically increase in the future.

Future Prospects

Aiming at early practical introduction of multicore-fiber long-distance transmission, NICT will cooperate more closely with telecom carriers and device manufacturers to promote the practical application of this new technology to multicore fibers having up to 19 cores. NICT will also work to improve the performance of optical amplifiers in areas such as reducing noise and enhancing output power.

The achievements of this research have been accepted for the European Conference on Optical Communication (ECOC) 2013 (22 - 26 Sept., London, UK), and have been presented on Sept. 26.

Appendix

Fig.1 19-core simultaneous optical amplifier configuration diagram

The following describes the features of the individual component devices of the 19-core simultaneous optical amplifier that we have newly developed.

(1) Optical multiplexer

By adopting a free-space optical system, it is possible to perfectly multiplex 19 signal beams and 19 pumping laser beams of a multicore fiber, enabling wasteless energy supply.

(2) 19-core optical fiber amplifier

A 19-core optical fiber amplifier with a revised core arrangement to reduce signal noise caused by core-to-core interference. Erbium ions are added in each core so that they may amplify a signal beam, when pumped by a pumping laser beam.

(3) 19-core simultaneous isolator

A 19-core simultaneous isolator that realized simultaneous isolation of multiple signals has been achieved for the first time by adopting a free-space optical system. It suppresses reflection of amplified optical signals, to prevent generated signal interference.

Fig.2 Comparison of new 19-core fiber and conventional 19-core fiber

In conventional 19-core fibers, cores have been arranged in the form of a hexagonal lattice. In contrast, in the new 19-core fiber, cores are arranged concentrically, so that core-to-core interference noise is suppressed, allowing long-distance transmission.

Glossary

Systems for multiplexing signal light and pumping laser light

In order to amplify a transmitted optical signal directly, the signal light is first combined with light of a different wavelength (pumping light), which is generated by a laser, and the combined light is made to enter an optical fiber for amplification. In the amplifying optical fiber, the energy of the pumping light is supplied to the signal light via rare-earth ions (usually erbium ions), which causes amplification of the signal. In a multicore fiber having multiple cores, it is necessary to combine signal light and laser light for each core. Although several methods have been investigated for this purpose, an optical amplifier that can be readily connected to an optical fiber, that incurs only small loss, and that has high amplification efficiency has not yet been developed.

・Separate pumping system: The individual cores of a multicore fiber are connected to single-core fibers by means of fan-in/fan-out, and then the single core fibers are individually connected to amplifiers designed for single-core fibers, which perform amplification processing, including multiplexing. It is considered that practical implementation is difficult because of the issue of how to connect a multicore fiber with single-core fibers and because of the need for the same number of amplifiers as the number of cores.

・Cladding pumping system: Laser light is emitted toward the periphery (cladding) of each core of a multicore fiber so that a part of the laser light will be multiplexed with the signal light in that core. Since light leaks to cores other than the target core, there is a considerable loss of energy, leaving the need to improve efficiency.

Separate pumping system

19-core fiber

NICT published a press release on March 8, 2012 (https://www.nict.go.jp/press/2012/03/08-1.html).
A multicore fiber having multiple light paths (cores) is very effective in increasing the transmission capacity, and the 19-core fiber is the world’s highest-density multicore fiber suitable for high capacity transmission.

Isolator

An isolator is an element that permits transmission of a signal only in one direction while prohibiting transmission in the opposite direction. It is an important element in an optical amplifier. Specifically, it suppresses reflection of optical signals to prevent laser oscillations which may interfere with the amplified signals. An element that can simultaneously isolate signals in multiple cores has not previously been developed.

See Fig. 1-(3) in the appendix.

Free-space optical system

As opposed to the approach of implementing a coupling system by using special optical-fiber machining technology, the inputs and outputs of the individual cores of a multicore fiber are simultaneously manipulated by using lenses. This technology is highly compatible with space division multiplexing using a multicore fiber, and it is possible to flexibly deal with an increase in the degree of multiplexing or the addition of optical elements, etc.

See Fig. 1-(1)in the appendix.

Core-to-core interference noise

In a multicore fiber, since cores exist in proximity to each other, there are cases where a signal that has leaked from one core enters another core, causing interference and generating noise. In the 19-core optical fiber, the distance between cores is very small, so that sophisticated technology is required in order to suppress noise caused by core-to-core interference.

Fan-in/fan-out

Fan-in/fan-out refers to a device (optical element) for connecting each core of a multicore fiber with a single-core fiber in a multicore fiber transmission system. There are various issues to be addressed with this device, such as suppressing core-to-core signal interference in the connector, reducing insertion loss, minimizing variations among the individual cores, and ensuring the ease of mass production, and various implementations are being investigated.

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