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

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)