DOUGLAS CLAGUE HOLLOW-CORE TESTING
RETHINKING CERTIFICATION FOR HOLLOW-CORE FIBRE
For established fibre types, certification is often reduced to a small set of acceptance
metrics. With HCF, that approach is no longer sufficient. A credible approach must focus on characterisation rather than simple pass or fail validation. A meaningful certification outcome should demonstrate true linear attenuation, accurate identification and quantification of splices, dispersion characteristics appropriate for the intended transmission application, and stable performance across the relevant wavelength range. Hollow‑core designs are still evolving, and deployments represent significant capital investment. Certification must therefore provide assurance not just for day‑one activation, but for long‑term operational viability. THE ROLE OF BIDIRECTIONAL OTDR TEST AND ANALYSIS One of the most important differences between HCF and conventional fibre lies in how attenuation is measured. Because backscatter levels in HCF are both weaker and less uniform, single‑ended OTDR measurements can distort apparent loss. Changes in backscatter levels may appear as loss events even when no additional attenuation is present. Bidirectional OTDR analysis overcomes this limitation by combining measurements taken from opposite ends of the fibre. By aligning the traces and applying a bidirectional calculation, backscatter‑related artefacts can be suppressed, revealing the true “loss profile” of the link. However, standard OTDR post- processing algorithms are often insufficient for the task. Custom software or expert manual interpretation is often needed to assess splice loss and distributed attenuation. For HCF certification, this is not an enhancement. It’s a necessity. It is also important to note that not every OTDR is suitable for HCF. This is because, the combination of low backscatter, variable backscatter coefficient and highly reflective splices places stringent demands on OTDR performance and configuration flexibility. The lower backscatter levels requires an OTDR with higher dynamic range performance. Also, as splices could be 2 to 4 km apart, the OTDR must offer high dynamic range at short pulse widths so each splice can be measured cleanly without events merging. At short pulse widths, usable dynamic range matters far more than the headline figures quoted at long pulses. Without enough performance margin, dead zones and trace tailing can mask important events and compromise measurement accuracy.
Loss profile trace obtained by performing bidirectional OTDR analysis
WAVELENGTH CONSIDERATIONS AND SPECTRAL BEHAVIOUR OF HOLLOW-CORE FIBRE Many HCFs are designed to guide light more effectively at longer wavelengths, typically starting in the S-band (approximately 1450 nm). Because of this optimisation, measurements at 1310nm may have limited diagnostic value, with testing at 1550 nm and above being more representative of operational performance. Also, long wavelength tests are more sensitive to bending loss and spectral anomalies, making them better for detecting subtle issues. Certification strategies should therefore align test wavelengths with the fibre’s intended transmission bands rather than relying on legacy defaults. Beyond individual test wavelengths, the overall spectral attenuation behaviour matters as well. Differences in fibre design, manufacturing, or gas infiltration during splicing can introduce absorption features outside the main transmission band. Therefore, measuring attenuation across a wide wavelength range is becoming a core part of HCF certification. These measurements help operators verify transmission bands, identify absorption features and detect non- uniform spectral behaviour. This spectral insight is especially relevant in data centre and transport environments, where flexibility and upgrade headroom are strategic requirements. DISPERSION STILL NEEDS TO BE TESTED IN HCF While HCF has the benefits of inherently low chromatic dispersion and modest polarisation mode dispersion, testing is still important. HCF manufacturing processes continue to evolve, and installed links often contain multiple splices between fibre segments. Dispersion measurements provide a baseline characterisation that supports future transmission upgrades and helps rule out unexpected behaviour introduced during installation. OTDR‑based dispersion tests do not work well on hollow‑core fibre, because HCF’s extremely low backscatter prevents accurate measurement for medium to long distances (metro to long-haul). Therefore, dedicated optical source and
receiver-based technique that don’t depend on backscatter are far better suited. In the context of HCF, dispersion testing is less about troubleshooting and more about deployment confidence, ensuring that controlled laboratory performance translates into predictable field behaviour. ENSURING OPERATIONAL CONFIDENCE FOR OPERATORS Hollow-core fibre is poised to reshape the performance limits of next-generation optical networks, particularly as AI- driven data centres push bandwidth and latency requirements to new extremes. Its successful adoption, however, hinges on testing and certification practices that account for its distinct physical behaviour. It relies on bidirectional analysis to obtain an accurate loss profile, careful interpretation of splices and bends to ensure events are correctly understood, dispersion characterisation to stablish a reliable performance baseline, and wide‑band spectral attenuation profile measurements to validate transmission windows. Combined, these practices form the foundation of a modern HCF testing framework. For operators, effective testing and certification safeguards high‑value infrastructure investments and provides the confidence needed to deploy HCF at scale. In this sense, testing is not just a deployment step, it is the enabling discipline that turns HCF from a promising innovation into a production‑ready optical technology.
Douglas Clague Fibre Optic Field Solutions Marketing Manager
www.opticalconnectionsnews.com
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ISSUE 44 | Q3 2026
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