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May 30, 2017

Successful field trial of all-Raman optical amplification technology for multiband large capacity transmission of 400 Gbps signals in installed fiber ~ Double the usable bandwidth of installed fiber

Nippon Telegraph and Telephone Corporation (Headquarters: Chiyoda-ku, Tokyo, President and CEO: Hiroo Unoura, hereinafter NTT) announced that, in conjunction with Nippon Telegraph and Telephone West Corporation (Headquarters: Osaka Prefecture Osaka City, President: Kazutoshi Murao), successfully conducted a field trial on widely-installed optical fiber cable, in which, through the appropriate use of "all-Raman optical amplification technology" ^*1 as transmission line repeaters, 400 Gbps signals were transmitted in multiple bands at the same time. Furthermore, we confirmed, through this field trial, that we can add signals in a new band without affecting the signals in the existing band.

The results of this field trial suggest the possibility of achieving ultra-high capacity transmission speeds of up 30 Tbps. These speeds, together with the ability to upgrade systems already in use will be needed to satisfy the demands imposed by communication between data centers. We are continuing to verify the ultimate performance limits in detail as well as operational aspects, with the goal being to contribute to various fields that require ultra large capacity transmission.

1.Background

Our vision of future communication traffic sees the wide-spread distribution of high-resolution images and video such as 4K / 8K and full-scale popularization of M2M services. Since the obvious assumption is that capacity demands will skyrocket, NTT has been actively pursuing a wide spectrum of studies into realizing ultra-large capacity transmission of signals exceeding 100 Gbps. Furthermore, with the recent increase in traffic between data centers, greater capacity must be matched by greater cost-savings.
Greater capacity is most directly achieved by using multiple bands (C band ^*2 + L band ^*3 etc.) which expands the usable bandwidth of the optical fiber.
Several problems arise if we attempt to transmit multiband signals by wavelength division multiplexing (WDM) over dispersion shifted optical fiber (DSF: Dispersion Shifted Fiber) cable. The zero dispersion wavelength ^*4 of DSF lies in the extended band (C band) and around this wavelength, the signal is degraded by nonlinear effects (especially four wave mixing ^*5). The common solution, unequally spacing the signal wavelengths in the C band, degrades the wavelength utilization efficiency. This is a barrier to further capacity increases.

2.Research Results

Our approach, applying "all-Raman optical amplification technology" in both the existing band (L band) and extended band (C band), allowed us to transmit multiple 400 Gbps signals (each consisting of two 16-QAM modulated signals) over 200 km of installed DSF cable with sufficient transmission margin. The usable bandwidth is doubled by combining the existing band and the extended band. Since signal degradation due to four wave mixing increases with the signal's optical power, low optical powers are essential to suppressing nonlinear effects. All-Raman optical amplifiers are superior to general EDFAs (Erbium-Doped Optical Fiber Amplifiers) as they allow low signal transmission powers to be used. Our approach also improves frequency utilization efficiency in the C band by equally spacing the wavelength intervals.
The use of Raman amplifiers raises safety issues because high power pump lights are used. Our field trials were designed to fully comply with IEC ^*6 standards. Furthermore, we confirmed that by appropriate control of the Raman amplifiers made it possible to add C band optical signals without altering the transmission quality of the L band signals. This confirms that capacity can be upgraded without disrupting L-band services already in use.

3.Future Improvements

We will continue to promote R&D on large-capacity optical transmission technologies including 400 Gbps multiband optical signal transmission and all-Raman optical amplification.Thus we will utilize these results, to realize large capacity networks that are very economical.

The outcome of this field trial is expected to enhance research in various fields that require super high capacity transmission, such as using existing fiber to accommodate datacenter traffic.

Figure 1 Field trial configuration (left) Signal power image plot with all-Raman optical amplification (right)

Glossary

^*1All Raman optical amplification technology
Raman amplifiers are a type of optical amplifier. They are based on a nonlinear effect of fiber called Raman scattering. Pump light that has shorter wavelength than the wavelength band being used is injected into the fiber, which makes amplification in an arbitrary band possible. All Raman optical amplification is the method of setting Raman amplifiers in all repeaters in the optical transmission line.

^*2C band
One of the wavelength ranges used for optical communications specified by ITU-T (International Telecommunication Union Telecommunication Standardization Sector), it is the light wavelength band of 1530 nm to 1565 nm.

^*3L band
One of the wavelength ranges used for optical communication specified by ITU-T, it is the light wavelength band of 1565 nm to 1625 nm.

^*4Zero dispersion wavelength
The propagation speed of optical signals in optical fiber varies with wavelength. The wavelength at which the rate of change in speed is zero is called the zero dispersion wavelength.

^*5Four wave mixing
Four wave mixing is a kind of nonlinear effect that occurs in optical fiber. When two or more different wavelengths of light transit a fiber, a new light with a wavelength different from any of the incident lights is generated. WDM optical transmission with equi-spaced wavelength intervals triggers four wave mixing where the new light overlaps with the signal lights, which degrades signal quality.

^*6IEC
IEC (International Electrotechnical Commission) is an international standardization body working in the technical fields of electricity and electronics.

For inquiries concerning this release, please contact:

Nippon Telegraph and Telephone Corporation
Information Network Research Institute
Public Relations
inlg-pr@lab.ntt.co.jp

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