TECHNICAL
fibre is realised at the service level, not just inferred from physical-layer characteristics. These measurements are essential for applications such as data centre interconnects (DCI), financial trading networks, and other time- sensitive infrastructures—where error-free transmission is critical and microseconds matter.
are still essential, especially in high- bandwidth applications. While CD in hollow-core fibre is typically below 4ps/ nm·km, PMD is generally higher than in conventional SMF fibre and can vary by manufacturer. In real-world deployments, networks often include hybrid links combining SMF and hollow-core fibre where cumulative dispersion effects must be fully understood. Verifying CD and PMD ensures predictable system performance, supports interoperability with existing infrastructure, and helps identify any deviations from expected behaviour before service activation. Can single-ended CD/PMD testing really work on hollow-core fibre? Yes—and it has been optimised, tested and validated in both lab and field trials. The FTBx-570 single-ended dispersion analyser uses an end-of-fibre reflection, not RBS, to perform CD and PMD measurements. As a result, the low backscatter coefficient of hollow-core fibre does not limit measurement range. Only the reflectivity at the fibre end matters. In validation trials, EXFO successfully measured HCF links exceeding 100km using a standard UPC connector— demonstrating that long-distance characterisation can be achieved efficiently with a single setup. Dual-ended PMD measurements are not the norm for most HCF applications. Single-ended testing, by contrast, provides a highly practical, scalable, and field-friendly solution—especially when rapid validation or troubleshooting is required. Can BER or latency testing be performed directly over hollow-core fibre? Absolutely. With the FTBx-88810 Series tester and the BERT application, EXFO enables end-to-end bit error rate (BER) and latency measurements from 1G to 800G directly over hollow-core fibre. BER validation is particularly important, as it allows network operators to detect transmission impairments or link degradation—including effects related to gas absorption in the hollow core— under real network conditions. Latency measurements complement BER testing by confirming that the expected delay reduction delivered by hollow-core
A full characterisation of these GFEs can provide valuable information about the quality of the installations, including not only splice losses but also the levels of hydrogen and carbon dioxide present in the fibre after splicing. What critical parameter is often overlooked when validating hollow-core fibre (HCF)? Latency. When hollow-core fibre is introduced into a network, validating latency is just as important as characterising dispersion—especially in hybrid architectures that combine SMF and HCF. In these scenarios, latency must be assessed as a function of wavelength to confirm that replacing portions of SMF with HCF delivers the expected end- to-end delay reduction across the full operating band. Transmission delay variation with wavelength is particularly important in latency-sensitive applications and cannot be inferred from CD measurements alone.
What about attenuation profile measurements?
Hollow-core fibre’s ultra-low attenuation is one of its most compelling advantages, but total insertion loss alone does not provide a complete picture of fibre performance. For HCF, attenuation can vary with wavelength due to fibre design, microstructured cladding geometry, and coupling conditions—particularly across the C and L bands used in high-capacity and DCI applications.
Attenuation profile measurements make it possible to:
n Verify that low-loss performance is maintained across the entire operating band n Identify wavelength-dependent loss features that may impact system margin
EXFO’s FTBx-570 CD/PMD analyser is uniquely suited for this task, as it:
n Supports refractive indices down to 1.00
n Compare measured performance against manufacturer specifications
n Measures CD, PMD, and wavelength- dependent latency in a single-ended configuration n Achieves reaches exceeding 200km on HCF, compared to SMF, thanks to HCF’s lower attenuation In practice, single-ended testing delivers significant operational advantages: no second unit, no second technician, and faster troubleshooting, particularly during early deployments and field validation scenarios. polarization mode dispersion (PMD) be measured on hollow- core fibre (HCF)? A common misconception is that because HCF inherently exhibits low chromatic dispersion and minimal nonlinear effects, CD and PMD do not need to be verified. In practice, measurement and validation Why should chromatic dispersion (CD) and
n Validate consistency between fibre spans, lots, or installation conditions
n This level of insight is especially important during technology trials, acceptance testing, and early deployments, where HCF behaviour is still being closely evaluated. To support these use cases, EXFO offers complementary tools for attenuation profiling:
n CTP10: provides high-resolution
attenuation analysis for detailed fibre characterisation in a lab environment
n FTBx-5255 Optical Spectrum Analyzer: paired with a broadband source, enables visualisation of attenuation profiles across the C+L bands with 0.017nm resolution Together, these solutions allow engineers to move beyond simple loss measurements and gain a wavelength- resolved view of attenuation behaviour,
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MAY 2026 Volume 48 No.2
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