Design, prototyping, and polarity management of all-port flipped 19-core fiber splitters for broadcast-and-select-type spatial cross-connects

A hierarchical optical cross-connect that combines a spatial cross-connect (SXC) based on spatial division multiplexing technology with a conventional wavelength cross-connect can economically accommodate the continuing exponential growth in Internet traffic. The SXC has a core selective switch (CSS) in a route-and-select configuration. The CSS is a multi-core fiber (MCF)-based optical switch that can switch optical signals on a per-core basis without going through fan-in fan-out (FIFO) devices. To reduce the cost of the SXC, a broadcast-and-select (B&S) configuration is promising, where the ingress CSSs in the SXC are replaced by MCF splitters (SPLs). In this paper, we propose a construction for ${1} \times N$ MCF SPLs and a method to manage the polarity and core arrangement flipping when using them for the SXC. In addition, we report experimental results using ${1} \times {2}$ all-port flipped 19-core fiber (19-CF) SPLs constructed to verify the effectiveness of the proposed method. The unique feature of the proposed method is that the core arrangement of all MCF output ports is inverted with respect to that of the input port. This cancels the core arrangement flipping that occurs in the egress CSS in the B&S SXC. Furthermore, based on a comparison with MCF-based optical devices, such as the CSS and FIFO, which have polarity, we clarify that MCF SPLs have no polarity, similar to MCF patch cords. Therefore, a single type of MCF SPL can be used without worrying about orientation. Three constructed all-port-reversed ${1} \times {2}$ 19-CF SPL prototypes exhibit low loss (${\lt}{7.5}\;{\rm dB}$), low polarization-dependent loss (${\lt}{0.5}\;{\rm dB}$), and low total inter-core cross talk (${\lt} {-}35.7\;{\rm dB}$) at C-band wavelengths. In addition, no optical signal-to-noise ratio penalty is observed when 100-Gb/s-based wavelength-division-multiplexed signals are input to all cores. Finally, we show that an all-port non-flipped ${1} \times {4}$ 19-CF SPL can be constructed by connecting three all-port flipped ${1} \times {2}$ 19-CF SPLs in a tree structure.