May 30, 2017
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.
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.
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.
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.
Nippon Telegraph and Telephone Corporation
Information Network Research Institute
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