DOUGLAS CLAGUE HOLLOW-CORE TESTING
RETHINKING TESTING AND CERTIFICATION FOR HOLLOW-CORE FIBRE Fibre‑optic cables deliver extraordinary data transmission capacity, with modern systems sending and receiving hundreds of terabits per second. But the bandwidth and latency demands of today’s AI‑driven data centres are stretching traditional networks to their limit. Enter the transformative potential of Hollow- Core Fibre (HCF), which is moving rapidly from promise to reality as lab research becomes field trials become early operational deployments, writes VIAVI Solutions Fibre Optic Field Solutions Marketing Manager Douglas Clague .
HCF’s ability to guide light through a predominantly air filled core rather than solid glass promises transformative performance gains through lower attenuation, reduced latency, and less signal distortion. However, the same physical properties that enable these benefits introduce new complexities when it comes to testing and certification. WHAT ARE THE BENEFITS OF HOLLOW-CORE FIBRE? Compared with traditional single mode fibre (SMF), HCF offers clear and measurable benefits. Because light travels through air rather than solid glass, HCF achieves significantly lower latency, has lower chromatic dispersion, and its reduced light and glass interaction means nonlinear effects are negligible. In certain wavelength bands, advanced HCF designs have even demonstrated attenuation below 0.1 dB/km. Individually, each of these benefits is attractive, but together they enable and drive specific applications for DCI and AI workloads and next-generation optical transport, where every microsecond and every fraction of a decibel of optical loss can influence overall efficiency and scale. HOLLOW-CORE FIBRE DEMANDS A DIFFERENT TESTING APPROACH As operators consider deploying HCF at scale, the industry must rethink how this new class of fibre is validated. Conventional fibre test methods and assumptions, developed for solid core single mode fibre (SMF), are inadequate when applied directly to HCF. Applying them without adaptation risks inaccurate measurements and misleading conclusions, which undermine the
confidence needed to deploy such a higher-cost infrastructure. HCF’s air guided design reduces interaction between light and glass, which, in turn, alters the optical signatures relied upon by traditional Optical Tome-Domain Reflectometer (OTDR) test techniques. An OTDR maps the condition of an optical fibre by sending short pulses of light into the fibre and analysing the tiny amount of light that returns, using Rayleigh backscatter and Fresnel reflections to identify loss, events, and distance. In HCF, Rayleigh backscatter is much weaker, typically around 14 to 20dB lower than in SMF. Therefore, to be able to perform OTDR measurement, it requires high dynamic at short pulse width. Unlike SMF, where backscattering is relatively uniform, HCF may show non-uniform backscattering along the fibre length due to microstructural variations. Infiltration of ambient air
components around the splice point could generate high reflection that produces different OTDR signatures compared to SMF. Standard OTDR settings may fail to detect key features, requiring different pulse widths and averaging. This complicates OTDR trace interpretation and requires different test settings than for SMF (index of refraction (IOR) and backscatter coefficient factor (K)). Variable backscatter coefficient also means that bidirectional OTDR test and analysis is the only method suitable for characterising fibre and splice losses. These effects make these familiar single‑ended OTDR measurements unreliable if they are interpreted in the same way as regular SMF results. Without selecting test equipment with the correct performance and adapting test methodologies, operators risk either overlooking real impairments or misclassifying benign features as defects.
Regular uni-directional OTDR trace showing the transitions from SMF to HCF, variation of RBS coefficient in the trace and reflective splices
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| ISSUE 44 | Q3 2026
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