05 Nov 2013 |
Research article |
Information and Communications Technologies
Multicore Fiber Optimization for Application to Chip-to-Chip Optical Interconnects
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The very high aggregate bandwidth demands for storage area networks, large scale server warehouses, traffic routers, and high-performance computers have opened up opportunities for optics to compete with electrical interconnects at shorter and shorter distances , . We present the design of a holey microstructured multicore optical fiber optimized to meet the stringent requirements of chip-to-chip optical interconnects, namely, be compatible with high-speed vertical-cavity surface-emission lasers (VCSELs), feature ultra-high channel density, low crosstalk, and millimeter-bend resistance to sustain the tight bends required on an electronic circuit board.Modern supercomputers have fiber counts in the 105 range for rack-to-rack and board-to-board communications . To respond to the severe requirements of future systems, optical interconnects (OIs) will need to continue to improve in density. Current state-of-the-art commercial OIs achieve 12×10 Gb/s transmission rates using parallel arrays of vertical cavity surface-emission lasers (VCSELs) and multimode fiber ribbon cables . With a channel-to-channel pitch of 250 µm, these interconnects were developed for rack-to-rack applications and do not meet the very high density requirement for chip-to-chip optical interconnects (C2OIs), of the order of 1 Tb/s.cm2 . Multicore fibers (MCFs) have the potential to dramatically increase the channel density of optical interconnects and active optical cables through space division multiplexing [5–7].
Indeed, they are naturally compatible with 2D arrays of VCSEL transmitters. MCFs were proposed in the early days of optical fiber developments . However, it is only recently that they became the subject of intense investigation, as potential candidates for transport of the ever-increasing traffic of our information society . In particular, transmission capacity in excess of 1 Pb/s/fiber has been achieved using a 12-core fiber and advanced modulation . A hexagonal arrangement of the cores maximizes channel density. Such all-solid fibers have been demonstrated with heterogeneous  or trench assisted ,  cores, as well as in a ring configuration  in order to minimize crosstalk. Rectangular arrangements designed to match two-dimensional arrays of VCSELs have also been demonstrated . However, these technologies rely on conventional solid optical fiber and feature the centimeter-limited bend radius typical of these fibers. On the other hand, it has been shown that single-core holey microstructured fibers (MFs) can be tailored to much lower bend radius capabilities, typically < 0.01 dB/loop at radius R = 5 mm, compared to a 20 dB/loop for standard single-mode fiber, and are robust as well [16–18]. To take advantage of this, hole-assisted  and hole-wall-assisted  MCFs have been proposed to reduce both crosstalk and bend loss in long-haul transmission at 1550 nm.Therefore, the objective of the present research is to investigate the feasibility of a holey microstructured MCF featuring the highest possible density and the lowest possible bend loss for 1-m long transmission links operated at 10 Gb/s. We present the systematic modeling of an all-silica hexagonal microstructure with 1-rod cores and discuss the design tradeoffs. We then present the optimization of the microstructure using 7-rod cores and show that this design meets all the set objectives properly.
We modeled a microstructured multicore fiber that satisfies the stringent requirements of the microelectronics industry for C2OI links. We demonstrated that the conventional microstructure with 1-rod cores is not suitable, whereas the one with 7-rod cores meets all the performance objectives nicely. For transmission at 10 Gb/s over 1 m, a minimum core-to-core pitch of only 14 µm is sufficient to allow low enough crosstalk and millimeter bend radii; there is sufficient room available to increase this pitch in order to address longer links and a 25 Gb/s rate.Using forward-error correction-algorithms, the 14 µm core-to-core pitch can also provide for higher bit rate and distance. Hence, the proposed 7-rod microstructure design indeed provides the highest aggregate capacity envisioned to date for single-wavelength space-division multiplexed C2OIs. The holey microstructure is instrumental in achieving the required bend performance in C2OIs, which is much tighter than in telecommunication-distance applications. Yet, the very dense and tiny silicate microstructure may prove a challenge to manufacture, and so the simplification of the microstructure is our next objective. In particular, the replacement of the arrayed microstructure with a nanostructure of random holes similar to what is used in some bend-resistant fibers for the FTTx industry is under investigation.
To understand more about Multicore Fiber Optimization for Application to Chip-to-Chip Optical Interconnects, we invite you to read the Research Paper available at the following link:
Francois V. and F. Laramee (2013). Multicore Fiber Optimization for Application to Chip-to-Chip Optical Interconnects. Accepted paper for a future publication of Journal of Lightwave Technology. PDF
Véronique François is a professor in the Department of Electrical Engineering at ÉTS. Her research interests are photonics, optical instrumentation, agile optical amplifiers, optical fibers and doped materials.
Program : Electrical Engineering