In current data-center switches, external fiber-optic connections are terminated in pluggable transceivers at the faceplate of the housing. The signals within the switch are transported electrically via copper traces on printed circuit boards. With increasing data rates, these electrical connections are becoming progressively more lossy, and increasing the electrical power to compensate for this loss negatively impacts the operational cost, electric power infrastructure, and waste heat management. To address these problems, the industry is envisioning placing the transceivers inside the housing very close to where the signals are generated: co-packaged with the switch ASIC. This approach effectively replaces the high-loss copper lanes with low-loss optical-fiber. However, to achieve this low-loss optical connectivity, the optical fibers must be single-mode for short ~0.5 m application lengths to avoid introducing signal impairments due to multi-path interference. It is also desirable that these fibers have good bend performance and mode-field diameters compatible with the installed base of single-mode fiber. In this paper, we will summarize the design of a new fiber optimized for these co-packaged applications and present data on developmental prototypes that demonstrates their suitability for use in short-length optical interconnects. We will also present a novel concept for management of the hundreds to thousands of fibers within the switch housing in which variable lengths of cable are neatly secured inside stackable accumulators. This tailoring of the length for each cable path results in no cable crossovers and will thus facilitate lower-cost and less error-prone assembly and easier maintenance of the switch.
Optical interconnects in data centers have traditionally used 850 nm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) in combination with multimode fiber, having a reach up to 100 m in length. Longer links typically use standard single-mode fiber in conjunction with either InP-based edge-emitting lasers or silicon photonic transmitters operating in the 1310 nm or 1550 nm window. Single-mode GaAs-based VCSELs operating at 1064 nm offer another path for achieving longer system reach. Potential advantages of these VCSELs include better power efficiency, modulation speeds reaching 50 Gbps and large-scale fabrication volumes. The longer wavelength is also beneficial due to the lower attenuation and chromatic dispersion of optical fibers at that wavelength. However, one practical issue for single-mode transmission is that the G.657 standard for single-mode fiber requires that the 22-meter cable cutoff wavelength be less than 1260 nm, and these fibers are typically few-moded at 1064 nm. The large differences between the group velocities of the LP01 and LP11 modes can lead to degradation of the system performance due to multi-path interference if the higher order modes are present. To resolve this quandary, we have designed and validated the performance of a new optical fiber which is single-moded at wavelengths less than 1064 nm, but also has G.657- compliant mode field diameter and dispersion characteristics that enable it to be used in the 1310 nm window.
Recently it has been shown that standard single-mode fibers, which support two LP modes around 850 nm, can yield high modal bandwidth with graded-index profile design. A transmission system using such fibers along with 850 nm single mode VCSEL transceivers offers a potentially cost-effective high-bandwidth solution for data center applications and future high-speed short distance communications. The system reach highly depends on the modal bandwidth of the fiber. In this context, it is of interest to explore the characterization method of the modal bandwidth of two-mode and few-mode fibers, especially if the method can be simpler than traditional methods used for 50-μm core multimode fiber. To address this issue, we propose a simple and robust method for two-mode and few-mode fiber modal delay and bandwidth measurements using frequency domain method. An analytical transfer function model was formulated and achieved excellent agreement with experimental results. The model allows one to extract the modal delay based on one single measurement, regardless of the launch condition. The transfer function and hence modal bandwidth with arbitrary launch condition can be calculated, from which we define a worst-case modal bandwidth that can gauge the fiber modal bandwidth under general conditions. The analytical model is also generalized to consider higher-order modes and additional bandwidth degradation effects. Through the detailed study, we show that the simple frequency domain measurement method as facilitated by the analytical model can deliver a full set of modal delay and modal bandwidth information that otherwise requires more complex method like differential mode delay measurements.
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