The Deployment of Hollow Core Fiber (HCF) in Azure’s Network
Co-authors: Jamie Gaudette, Frank Rey, Tony Pearson, Russell Ellis, Chris Badgley and Arsalan Saljoghei In the evolving Cloud and AI landscape, Microsoft is deploying state-of-the-art Hollow Core Fiber (HCF) technology in Azure’s network to optimize infrastructure and enhance performance for customers. By deploying cabled HCF technology together with HCF-supportable datacenter (DC) equipment, this solution creates ultra-low latency traffic routes with faster data transmission to meet the demands of Cloud & AI workloads. The successful adoption of HCF technology in Azure’s network relies on developing a new ecosystem to take full advantage of the solution, including new cables, field splicing, installation and testing… and Microsoft has done exactly that. Azure has collaborated with industry leaders to deliver components and equipment, cable manufacturing and installation. These efforts, along with advancements in HCF technology, have paved the way for its deployment in-field. HCF is now operational and carrying live customer traffic in multiple Microsoft Azure regions, proving it is as reliable as conventional fiber with no field failures or outages. This article will explore the installation activities, testing, and link performance of a recent HCF deployment, showcasing the benefits that Azure customers can leverage from HCF technology. HCF connected Azure DCs are ready for service The latest HCF cable deployment connects two Azure DCs in a major city, with two metro routes each over 20km long. The hybrid cables both include 32 HCF and 48 single mode fiber (SMF) strands, with HCFs delivering high-capacity Dense Wavelength Division Multiplexing (DWDM) transmission comparable to SMF. The cables are installed over two diverse paths (the red and blue lines shown in image 1), each with different entry points into the DC. Route diversity at the physical layer enhances network resilience and reliability by allowing traffic to be rerouted through alternate paths, minimizing the risk of network outage should there be a disruption. It also allows for increased capacity by distributing network traffic more evenly, improving overall network performance and operational efficiency. Image 1: Satellite image of two Azure DC sites (A & Z) within a metro region interconnected with new ultra-low latency HCF technology, using two diverse paths (blue & red) Image 2 shows the optical routing that the deployed HCF cables take through both Inside Plant (ISP) and Outside Plant (OSP), for interconnecting terminal equipment within key sites in the region (comprised of DCs, Network Gateways and PoPs). Image 2: Optical connectivity at the physical layer between DCA and DCZ The HCF OSP cables have been developed for outdoor use in harsh environments without degrading the propagation properties of the fiber. The cable technology is smaller, faster, and easier to install (using a blown installation method). Alongside cables, various other technologies have been developed and integrated to provide a reliable end-to-end HCF network solution. This includes dedicated HCF-compatible equipment (shown in image 3), such as custom cable joint enclosures, fusion splicing technology, HCF patch tails for cable termination in the DC, and a HCF custom-designed Optical Time Domain Reflectometer (OTDR) to locate faults in the link. These solutions work with commercially available transponders and DWDM technologies to deliver multi-Tb/s capacities for Azure customers. Looking more closely at a HCF cable installation, in image 4 the cable is installed by passing it through a blowing-head (1) and inserting it into pre-installed conduit in segments underground along the route. As with traditional installations with conventional cable, the conduit, cable entry/exit, and cable joints are accessible through pre-installed access chambers, typically a few hundred meters apart. The blowing head uses high-pressure air from a compressor to push the cable into the conduit. A single drum-length of cable can be re-fleeted above ground (2) at multiple access points and re-jetted (3) over several kilometers. After the cables are installed inside the conduit, they are jointed at pre-designated access chamber locations. These house the purpose designed cable joint enclosures. Image 4: Cable preparation and installation during in-field deployment Image 5 shows a custom HCF cable joint enclosure in the field, tailored to protect HCFs for reliable data transmission. These enclosures organize the many HCF splices inside and are placed in underground chambers across the link. Image 5: 1) HCF joint enclosure in a chamber in-field 2) Open enclosure showing fiber loop storage protected by colored tubes at the rear-side of the joint 3) Open enclosure showing HCF spliced on multiple splice tray layers Inside the DC, connectorized ‘plug-and-play’ HCF-specific patch tails have been developed and installed for use with existing DWDM solutions. The patch tails interface between the HCF transmission and SMF active and passive equipment, each containing two SMF compatible connectors, coupled to the ISP HCF cable. In image 6, this has been terminated to a patch panel and mated with existing DWDM equipment inside the DC. Image 6: HCF patch tail solution connected to DWDM equipment Testing To validate the end-to-end quality of the installed HCF links (post deployment and during its operation), field deployable solutions have been developed and integrated to ensure all required transmission metrics are met and to identify and restore any faults before the link is ready for customer traffic. One such solution is Microsoft’s custom-designed HCF-specific OTDR, which helps measure individual splice losses and verify attenuation in all cable sections. This is checked against rigorous Azure HCF specification requirements. The OTDR tool is invaluable for locating high splice losses or faults that need to be reworked before the link can be brought into service. The diagram below shows an OTDR trace detecting splice locations and splice loss levels (dB) across a single strand of installed HCF. The OTDR can also continuously monitor HCF links and quickly locate faults, such as cable cuts, for quick recovery and remediation. For this deployment, a mean splice loss of 0.16dB was achieved, with some splices as low as 0.04dB, comparable to conventional fiber. Low attenuation and splice loss helps to maintain higher signal integrity, supporting longer transmission reach and higher traffic capacity. There are ongoing Azure HCF roadmap programs to continually improve this. Performance Before running customer traffic on the link, the fiber is tested to ensure reliable, error-free data transmission across the operating spectrum by counting lost or error bits. Once confirmed, the link is moved into production, allowing customer traffic to flow on the route. These optical tests, tailored to HCF, are carried out by the installer to meet Azure’s acceptance requirements. Image 8 illustrates the flow of traffic across a HCF link, dictated by changing demand on capacity and routing protocols in the region, which fluctuate throughout the day. The HCF span supports varying levels of customer traffic from the point the link was made live, without incurring any outages or link flaps. A critical metric for measuring transmission performance over each HCF path is the instantaneous Pre-Forward Error Correction (FEC) Bit Error Rate (BER) level. Pre-FEC BERs measure errors in a digital data stream at the receiver before any error correction is applied. This is crucial for transmission quality when the link carries data traffic; lower levels mean fewer errors and higher signal quality, essential for reliable data transmission. The following graph (image 9) shows the evolution of the Pre-FEC BER level on a HCF span once the link is live. A single strand of HCF is represented by a color, with all showing minimal fluctuation. This demonstrates very stable Pre-FEC BER levels, well below < -3.4 (log 10 ), across all 400G optical transponders, operating over all channels during a 24-day period. This indicates the network can handle high-data transmission efficiently with no Post-FEC errors, leading to high customer traffic performance and reliability. Image 9: Very stable Pre-FEC BER levels across the HCF span over 20 days The graph below demonstrates the optical loss stability over one entire span which is comprised of two HCF strands. It was monitored continuously over 20 days using the inbuilt line system and measured in both directions to assess the optical health of the HCF link. The new HCF cable paths are live and carrying customer traffic across multiple Azure regions. Having demonstrated the end-to-end deployment capabilities and network compatibility of the HCF solution, it is possible to take full advantage of the ultra-stable, high performance and reliable connectivity HCF delivers to Azure customers. What’s next? Unlocking the full potential of HCF requires compatible, end-to-end solutions. This blog outlines the holistic and deployable HCF systems we have developed to better serve our customers. While we further integrate HCF into more Azure Regions, our development roadmap continues. Smaller cables with more fibers and enhanced systems components to further increase the capacity of our solutions, standardized and simplified deployment and operations, as well as extending the deployable distance of HCF long haul transmission solutions. Creating a more stable, higher capacity, faster network will allow Azure to better serve all its customers. Learn more about how hollow core fiber is accelerating AI. If you are attending the Optical Fiber Communication (OFC) Conference in San Francisco, US (March 30 – April 3 2025), find out more during a panel discussion given by Russ Ellis (Azure Principal Cloud Network Engineer), titled ‘Deployment of Hollow Core Fiber (HCF) in the Microsoft Azure Cloud’. The session is at 16:00-18:30 on Sunday March 30 meeting room 201-202 (Level 2): OFC Program
