EXFO’s Olivier Côté digs into testing challenges — and outlines how methods are evolving to accommodate hollow core fiber’s physical peculiarities
One of hottest trends coming out of the optical fiber industry today is hollow core fiber (HCF).
HCF had a viral moment at MWC when it saw repeated mentions in connection with AI. Adding to the excitement, Chinese fiber giant YOFC unveiled its proprietary HCF technology at the event, aimed at AI data centers, financial trading firms, and submarine cable operators.
Speaking at a keynote address, Sir David Payne — a Fellow of the Royal Society and the Royal Academy of Engineering, and director of the Expert Advisory Committee at YOFC — touted HCF’s capability to surpass physical limitations of solid-core fiber, and positioned it as the fiber for future high-capacity, low-latency communication networks.
In a conversation with RCR Wireless News, Bernard Lee, senior director of strategic technology and innovation at SENKO Advanced Components added, “With the increase of AI, hollow core fiber plays a very crucial role…the optical signal travels 30% faster…the faster you can send the data, the faster the data center on the other side receive it and can process it.”
Compared to traditional single-mode fiber (SMF) which makes up the majority of the fiber deployment today, hollow core fiber makers can deliver up to three times the bandwidth, making it a great technology for AI data centers. But HCF also presents some very specific manufacturing, testing, and economic challenges that are holding companies back from wider adoption.
We recently had the opportunity to interview Olivier Côté, product line manager of Hollow-Core Fiber Solutions at EXFO to get a deep dive on it.
Hollow core fiber comes with some unique physical traits — high bending sensitivity and delicate microstructures, among other things. What kind of testing challenges emerge from these?
The challenges we hear about fall into several categories:
Impact of Low Internal Pressure and Gas Ingress: A hollowcore fiber operates with low internal pressure filled with inert gas, so if the fiber is left unsealed during installation—especially during splicing—it can suck in ambient gases such as CO₂ and hydrogen. This contamination alters the gas composition inside the core and leads to increased attenuation and absorption, particularly in the L bands. The Gas Filling Events (GFE) are measured with an OTDR (optical time domain reflectometer) and specialized analysis has been developed to manage the GFEs.
Mechanical sensitivity: HCF cabling is more delicate than conventional optical fiber. Its air-guided microstructure makes it more susceptible to macrobends than standard single-mode fiber, so handling and installation practices must be adapted accordingly.
Splice integrity: Joining HCF to standard SMF or to other HCF segments require specialized fusion splicing techniques—such as angle-cleaving and offset splicing—to manage the strong Fresnel back-reflection that occurs at the glass-to-air interface. Achieving low insertion loss (<1.5 dB) and acceptable back-reflection levels (<-40 dB) at these junctions is a primary field concern. Adapters like GRIN (graded Index) drastically improve losses and reflectance.
Spectral limitations: Some hollow-core fibers are limited to specific spectral windows, potentially complicating use in wavelength division multiplexing (WDM) deployments where broad spectral coverage is expected.
Connector cleanliness and alignment: HCF’s air-guided structure imposes stricter fiber alignment and cleanliness requirements at connection points, making contamination an even greater risk than with solid-core fiber.
Investment protection: HCF cables are significantly more expensive to manufacture and install than conventional fiber. Operators need rigorous certification from day one to protect their investment and ensure return on investment.
Taken together, this is not like deploying a new brand of fiber optic cable—it’s a dramatic change in the fiber technology itself, requiring new, specific test solutions and updated best practices.
What aspects of the current fiber testing methods must evolve and how in order to overcome these challenges and make testing at scale possible?
The good news is that HCF-ready test tools are available today and require relatively little additional training to use, particularly since HCF operates in the familiar 1550nm window, allowing field technicians to work with equipment and safety protocols they already know.
However, several things need to evolve for HCF testing at scale:
OTDR analysis must adapt: Traditional OTDR interpretation is unreliable for HCF due to its extremely low backscatter and irregular scattering patterns from the gas filling events and microstructural variations. Bi-directional analysis with HCF-specific algorithms is now essential for accurate splice loss and fault characterization.
Standardization is critical: Industry standards bodies are actively working on specifications for HCF performance, testing protocols, and compatibility requirements. Established standards will be essential for interoperability and to give operators confidence in multi-vendor deployments.
Test workflows must expand: Beyond traditional loss testing, teams will need to integrate dispersion testing and attenuation profile analysis optimized for HCF’s transmission bands into their standard procedures.
Best practices must propagate: Service providers, data center operators, and contractors will need to equip their deployment, testing, and monitoring teams with HCF- ready tools and updated best practices to address the needs of hybrid networks—particularly around splice validation, connector inspection, and hybrid link certification.
With hollow core fiber expected to coexist with single-mode fiber in hybrid networks, how does that interface between SMF and HCF affect testing?
Given that there are millions of miles of traditional optical fiber spanning the globe, it’s critical that emerging HCF technology can coexist and integrate efficiently with legacy infrastructure for seamless connectivity. In practice, this means hybrid deployments where HCF is used for latency-sensitive segments — such as data center interconnects — while the broader network continues to rely on existing single-mode fiber.
The SMF-HCF interfaces have evolved using GRIN as previously mentioned. Using a GRIN (gradedindex) adapter greatly simplifies hollowcore fiber deployment by providing a smooth optical transition between standard singlemode fiber and the larger, differently shaped mode field of hollowcore designs. By gradually reshaping the optical field inside the adapter, GRIN technology reduces insertion loss, lowers reflectance, and makes alignment far more tolerant, resulting in more reliable splices and easier field installation.
The key testing challenge lies at SMF-HCF interconnection points. When light transitions from the air-filled core of HCF into the solid glass core of standard fiber (or vice versa), strong Fresnel back-reflection occurs due to the refractive index mismatch if Graded index adapters (GRIN) are not used. This can degrade system performance and complicate OTDR-based diagnostics if not properly accounted for. Field teams must validate that these hybrid connections maintain acceptable insertion loss and back-reflection levels across the full link.
In your conversations with companies, what kind of interest or implementation of HCF and corresponding demand for specialized testing tools are you seeing?
Absolutely! We’re seeing accelerating demand for both HCF and HCF-ready test tools, driven primarily by hyperscalers and data center operators focused on AI workloads and ultra-low latency applications.
Major cloud providers have moved from laboratory trials to production deployments. Microsoft Azure is running live customer traffic over HCF links connecting data centers in major metro areas and has announced plans to significantly expand its HCF footprint. AWS has confirmed it is deploying HCF to connect data centers today and sees a much larger role for HCF in the future.
Financial trading firms are also early adopters, given the competitive advantage that even microsecond latency reductions can provide.
While supply remains constrained relative to demand—manufacturing at scale is still being ramped up by suppliers such as Corning, YOFC, Lightera, Prysmian, and others—every customer deploying HCF needs reliable test equipment from day one. HCF represents a larger per- kilometer investment than standard fiber, which makes rigorous testing and certification even more critical to protect ROI. EXFO is actively engaged with fiber manufacturers, specialist contractors, and leading hyperscalers to ensure our HCF test portfolio meets their certification, troubleshooting, and ongoing monitoring requirements.
Editor’s note: This interview has been mildly edited for clarity.
