October 13, 2015
NICT, Sumitomo Electric Industries, Ltd. (Sumitomo Electric), and RAM Photonics, LLC (RAM) set the new world record fiber-capacity of 2.15Pb/s, which advances over the previous record by more than a factor of two. Amid worldwide active studies on multi-core fibers as promising transmission media for ultra-high capacity, 31km-long transmission was experimentally demonstrated through a homogeneous 22-core single-mode multi-core fiber using a high performance optical frequency comb source that is able to simultaneously generate multiple signal wavelengths covering the entire C and L bands.
Demonstrated transmission experiments are boosting the realization of future ultra-high capacity digital coherent optical networks. The paper on this study was accepted and presented as a post-deadline paper at the 41st European Conference on Optical Communication (ECOC2015). The presented paper was regarded as the best post-deadline paper and was awarded the Nature Photonics Award.
A transmission capacity through an optical fiber must increase to keep pace with explosively growing telecommunication traffic. In order to expand fiber-capacity, new class of optical fibers including multi-core fibers and space division multiplexing (SDM) transmission using these fibers have been actively studied among global research institutes. Recently in NICT, various cutting-edge studies on multi-core fibers and their devices, as well as SDM transmission, have been energetically executed.
In order to develop practicable ultrahigh-capacity optical networks, however, synergies among the multi-core fibers, devices, and transmission equipment are essential. Considering actual deployments in long haul transmission systems, multi-core fibers must have low attenuation and low cross talk. A high performance laser source for a carrier signal is also one of the key devices in ultra-dense and broadband wavelength division multiplexing (WDM) systems, and therefore, realization of optical frequency comb sources that are able to generate high performance lightwaves with multiple wavelengths are strongly desirable.
Optical spectrum from high performance comb source
By employing a single-mode multi-core fiber and an optical frequency comb source, NICT demonstrated SDM/WDM transmission realizing the new world record fiber-capacity of 2.15Pb/s (1Pb/s=1015b/s), which advances over the previous record by more than a factor of two. In this experiment, the latest technologies including dual-polarization 64QAM super-channel transmitters, power optimized broadband optical amplifiers and digital coherent detections, were also applied in view of future ultrahigh capacity long haul transmission.
As a transmission line, a 31km-long homogeneous 22-core single-mode multi-core fiber, designed and fabricated for this work by Sumitomo Electric, was employed. Homogeneous single-mode multi-core fibers are suitable for long-haul SDM/WDM systems since signal lights propagating through different cores have the same latency. However, suppression of signal cross-talk requires advanced fiber design and fabrication processes, and 19 was the highest number of homogeneous single-mode cores set in a single fiber for long-haul transmission prior to this work.
A high performance optical frequency comb source was employed as the signal carrier source, which can simultaneously generate hundreds of signal carriers covering the C- and L-bands from 1510 to 1620 nm with equal 25GHz-spacing. The frequency comb source, custom designed and fabricated by RAM utilizing a custom highly nonlinear fiber fabricated by Sumitomo Electric, has superior performance (narrow linewidth, low noise, high frequency stability, high mutual coherency, and low power consumption) compared to the performance of existing laser arrays.
The paper on this study was accepted and presented on October 1, 2015 as a post-deadline paper at the 41st European Conference on Optical Communication (ECOC2015) held in Valencia, Spain from September 27 to October 1, 2015. The paper was regarded as the best post-deadline paper and was awarded the Nature Photonics Award in ECOC2015.
In this transmission experiment, NICT, Sumitomo Electric, and RAM demonstrated promising features of multi-core fibers and frequency comb sources to be applied in future ultrahigh capacity optical communication systems with digital coherent technologies. In an effort to deploy practical SDM/WDM transmission technologies, future research studies will continue to be executed.
 RAM Photonics, “TeraToneTM Low-Noise Frequency Comb”, http://www.ramphotonics.com/products/teratone/
 B. J. Puttnam, R. S. Luís, W. Klaus, J. Sakaguchi, J.-M. Delgado Mendinueta, Y. Awaji, N. Wada, Y. Tamura, T. Hayashi, M. Hirano and J. Marciante, “2.15 Pb/s Transmission Using a 22 Core Homogeneous Single-Mode Multi-Core Fiber and Wideband Optical Comb,” in Proc. ECOC2015, PDP.3.1.