KEYWORDS: Connectors, Contamination, Particles, Temperature metrology, High power lasers, Air temperature, Reliability, Photonic integrated circuits, Optical surfaces
Co-Packaged Optics is a development of technology for high speed data switching, to be implemented widely in data center and high-performance computing architectures as a means to continue expansion of bandwidth and reduction of energy per bit. This development removes transceivers from the switch faceplate and replaces them with an optical link from the faceplate to transceiver PICs packaged on or near the ASIC switch substrate. Most approaches involve CW external lasers being carried over polarization-maintaining fiber to the PICs to be modulated for outgoing traffic. Lasers are active components with extremely high power densities and thus a relatively high failure rate, and they perform poorly at high temperatures such as prevail near the switch package. Therefore they will be remotely located or in removeable/front-panel pluggable packages that can be replaced with minimal disruption; this will require the use of optical fiber connectors. System reliability is significantly enhanced by using fewer, higher-power lasers, so very high powers are anticipated for these sources, up to and perhaps exceeding 250 mW, and any connector must be able to reliably tolerate these power levels over the lifetime of the laser or switch box. The use of expanded beam connectors reduces the optical intensity at exposed surfaces compared to PC connectors, and may mitigate some potential issues. We report on our initial studies to address this question of connector performance at these extreme conditions, with results on expanded beam single-mode connectors carrying high laser power in the O-band over many hundreds of hours.
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.
Based on a hypothetical 51.2 Tbps switch using co-packaged optics, we discuss key optical connectivity considerations and lay out different cabling options. Means are discussed to relax the length-accuracy requirements for fiber jumpers while maintaining a crossing-free cable layout for ease of assembly and serviceability. A key consideration is the mechanical stress in the fiber terminations which can lead to a degradation in the polarization integrity of external laser sources.
Fiber Optic Distributed Acoustic Sensing (DAS) and Distributed Strain Sensing (DSS) systems have widespread use for
asset and security monitoring. The acoustic signal from such sources as intruders, vehicles, or gunfire must be coupled
from the earth to an optical fiber which is then interrogated by DAS system technology. Because the optical fiber is the
sensing element, and because the cable is required to mediate the interaction of the fiber and its environment, the
selection of the optical fiber, cable design, and deployment conditions are critical to the performance of the system.
Cable designs specifically created for sensing are shown to achieve 20 dB higher signal-to-noise than standard telecom
designs, which correspond to an enhanced sensing range of more than 30 meters. In addition, directly burying the
sensing cable in the ground leads to 15 dB higher sensitivity than installing it in a duct. In many cases, standard cables
for telecommunications applications are designed to isolate and protect the fibers from the external environment;
therefore a cable designed for sensing applications and deployed specifically with this in mind leads to the highest
sensitivity with the largest sensing range.
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