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Multicore Fiber Optimization for Application to Chip-to-Chip Optical Interconnects - By : Véronique François, Francois Laramée,

Multicore Fiber Optimization for Application to Chip-to-Chip Optical Interconnects


Véronique François
Véronique François Author profile
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.

Francois Laramée
Francois Laramée Author profile
François Laramée has completed a master’s degree at ÉTS He is a lecturer and a lab instructor at ÉTS, and an Associate for the Regional Network Services Global at Ciena, Montreal, Canada.

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Picture of a QLogic fibre channel Storage Area Network switch. Source [Img2].

Picture of a QLogic fibre channel Storage Area Network switch. Source [Img2].

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 [1], [2]. 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.

Rack-to-rack communications in a A 19-inch rack used for switches at the DE-CIX in Frankfurt, Germany. Source [Img3].

Rack-to-rack communications in a A 19-inch rack used for switches at the DE-CIX in Frankfurt, Germany. Source [Img3].

Modern supercomputers have fiber counts in the 105 range for rack-to-rack and board-to-board communications [3]. 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 [4]. 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 [8]. 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 [9]. In particular, transmission capacity in excess of 1 Pb/s/fiber has been achieved using a 12-core fiber and advanced modulation [10]. A hexagonal arrangement of the cores maximizes channel density. Such all-solid fibers have been demonstrated with heterogeneous [11] or trench assisted [12], [13] cores, as well as in a ring configuration [14] in order to minimize crosstalk. Rectangular arrangements designed to match two-dimensional arrays of VCSELs have also been demonstrated [15]. 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 [19] and hole-wall-assisted [20] MCFs have been proposed to reduce both crosstalk and bend loss in long-haul transmission at 1550 nm.

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Fig. 1. Cross-section of the MCF showing 37 space-division-multiplexed cores in a 100×100 µm2 area. Source [Img4].

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.

Conclusion

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.

Fig. 2. (a) Computed mode profile at 850 nm of the 7-rod MF with = 2 µm and F = 0.5. (b) Dual-core microstructure with 5 rings of holes used for the calculation of crosstalk. Source [Img4].

Fig. 2. (a) Computed mode profile at 850 nm of the 7-rod MF with = 2 µm and F = 0.5. (b) Dual-core microstructure with 5 rings of holes used for the calculation of crosstalk. Source [Img4].

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

Author's profile

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 

Author profile

Francois Laramée

Author's profile

François Laramée has completed a master’s degree at ÉTS He is a lecturer and a lab instructor at ÉTS, and an Associate for the Regional Network Services Global at Ciena, Montreal, Canada.

Program : Electrical Engineering 

Author profile


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