Hollow-Core Fibers in Data Transmission
Technical advantages and specific requirements for splicing and measurement
Hollow-core fibers are widely regarded as a key technology for the future of data transmission. Unlike conventional optical fibers, they guide light through an air-filled core, resulting in lower latency, reduced dispersion, and exceptionally high power margins.
At the same time, hollow-core fibers introduce new challenges for the industry. Established splicing and measurement techniques were developed for solid-core fibers and offer only limited capabilities when it comes to processing and accurately characterizing this fundamentally different fiber type.
Physical Advantages
Practical Implications
The structure of hollow-core fibers differs fundamentally from that of conventional single-mode fibers, which consist of a solid glass core surrounded by cladding. In hollow-core fibers, the cladding is not a homogeneous glass region but a finely structured silica microstructure made up of thin-walled capillaries. This structure surrounds the air-filled core and confines the light within the cavity through anti-resonant or photonic effects.
This design offers several important advantages. While light propagating in solid glass experiences a refractive index of approximately n ≈ 1.44–1.47, propagation in air occurs with minimal optical interaction at a refractive index of about n ≈ 1.0003. As a result, signals can be transmitted at speeds up to roughly 30% faster. Because more than 99% of the optical power is guided through the air core, hollow-core fibers also provide significantly higher power reserves than conventional solid-glass fibers. These characteristics make them highly attractive for transmission links where speed, signal integrity, and power margins are critical. At the same time, however, their unique structure introduces new challenges for technicians and network operators, particularly in the installation, splicing, and qualification of fiber connections.
Low-Loss Connections Despite a Delicate Microstructure
The light-guiding microstructure of hollow-core fibers is extremely sensitive to heat input and mechanical misalignment. If the fine cavities in the splice region collapse, this can result in significant signal loss. A gentle splicing process is therefore essential—one that minimizes attenuation while preserving the mechanical integrity of the connection as much as possible.
Light guidance in hollow-core fibers relies on precisely engineered hollow structures. Even minor misalignments can cause unwanted modes to be excited, leading to measurable transmission losses. Simple core centering is therefore insufficient. Instead, precise alignment in the X, Y, and Z axes is required—and for certain fiber designs, additional rotational and pitch/yaw alignment is also necessary. As a result, splicing equipment must provide high-resolution imaging, dedicated alignment algorithms, and tightly controlled process management.
Heat input is particularly critical. Conventional arc fusion can deform or collapse the delicate microstructure in the air-core region, causing substantial attenuation. To address this, splicing system manufacturers are increasingly adopting advanced approaches that enable more precise control of the temperature profile. In three-electrode systems, individual arcs can be regulated independently, allowing heat to be distributed more evenly. This helps preserve the internal structure while achieving low splice loss combined with sufficient mechanical strength. Under optimized conditions, splice attenuation can be limited to approximately 0.1 dB.
In practice, this means that hollow-core fibers require not only specialized equipment, but also experienced operators with a deep understanding of the process. Splicing performance depends strongly on parameters such as fiber geometry, alignment accuracy, and process reproducibility. Only once a stable, low-loss connection has been achieved can the fiber link be reliably evaluated and qualified using appropriate measurement techniques.
Reliable Qualification Requires Dedicated Measurement Technology
Hollow-core fibers exhibit fundamentally different backscattering characteristics compared to conventional single-mode fibers. Rayleigh backscattering in the air-filled core is approximately 14 to 20 dB lower, which significantly complicates standard OTDR analysis. In addition, reflections at splice points or at transitions between single-mode fibers and hollow-core fibers can create dead zones that obscure adjacent events. As a result, conventional measurement approaches are often insufficient for the reliable evaluation of HCF links.
Consequently, OTDR test parameters must be selected with considerably greater care when measuring hollow-core fibers. A sufficiently high dynamic range, pulse widths matched to the link length, and appropriate test wavelengths are critical. In field applications, measurements are typically performed at 1550 nm or 1625 nm. By contrast, the 1310 nm wavelength commonly used for shorter distances often fails to deliver meaningful results in many HCF applications. Bidirectional measurements are also essential, as the backscatter coefficient and event representation can vary along the link. Only with carefully adapted settings can splices, transitions, and attenuation values be evaluated reliably.
Comprehensive qualification of hollow-core fiber links requires additional measurement parameters. The attenuation profile (AP) provides insight into spectral loss behavior across a defined wavelength range and reveals potential limitations caused by absorption bands. Measurements of chromatic dispersion (CD) and polarization mode dispersion (PMD) deliver further critical information on transmission performance and suitability for high data rates. Especially in new installations, these measurements help validate manufacturer specifications and enable a reliable assessment of overall link quality.
Field qualification is further complicated by the absence of established standard procedures for hollow-core fibers. This makes thorough documentation of all measurement directions, parameters, and detected events all the more important to ensure traceability and long-term comparability of results.
Conclusion
In summary, hollow-core fibers offer clear technical advantages for data transmission. Lower latency, minimal dispersion, reduced nonlinear effects, and high power margins make them an attractive solution for demanding applications. Practical experience shows, however, that these benefits can only be fully realized when splicing and measurement techniques are consistently adapted to the fiber’s unique structure. Low-loss splices require highly precise alignment and carefully controlled heat input. In addition to properly configured OTDR measurements, supplementary testing of attenuation profile (AP), chromatic dispersion (CD), and polarization mode dispersion (PMD) is essential for a robust qualification of the fiber link. Successfully deploying hollow-core fibers therefore requires not only the right fiber, but also appropriate processes, suitable tools, and specialized expertise.
