PIETRO BERNASCONI FIBRE CONNECTION LOSSES
FIBRE CONNECTION LOSSES MATTER MORE THAN EVER
From AI data centres and advanced sensing systems to biomedical instruments and quantum photonic platforms - where even small losses can degrade performance, reduce sensitivity, or compromise information integrity - the demand for ultra- low-loss fibre connections has become a critical requirement rather than a desirable feature. Yet fibre connectors remain indispensable for building flexible, modular, and maintainable optical systems writes Diamond SA Technical Key Account Manager Pietro Bernasconi .
A look at the mechanisms responsible for insertion loss in fibre connectors, in particular on fibre core misalignment reveals how high-accuracy connector manufacturing processes can impact system performances across the entire spectral range, especially at shorter wavelengths. WHEN LOW LOSS IS NOT LOW ENOUGH In optical networks, power losses can trigger a broad variety of undesired
situations may also be encountered in medical and scientific instrumentation like optical coherence tomography (OCT), distributed fibre sensing, or astronomical interferometry that operate at the limits of detection sensitivity. In all these cases, optical losses are not merely costly, they are true showstoppers. The takeaway message is clear: minimize optical losses from the very beginning, before they propagate into system-level complexity and cost.
the ferrule that encapsulates it. On the fibre side, the key parameters are the eccentricity of the core relative to the cladding, the uniformity of the outer diameter, and, possibly, its oval cross- section. On the ferrule side, the bore that houses the fibre must be precisely centred and sized closely enough to the fibre’s outer diameter to prevent lateral displacement during the glueing process. Each of these tolerances contributes independently, and their combined effect determines the residual axial offset of the fibre cores at the connector interface. For good-quality single-mode fibres operating at 1550 nm with typical MFD of approx. 10 mm, inside state-of-the-art ceramic ferrules the total core offsets typically fall in the range of 0.5–1.0 µm. When two ferrules are mated face-to- face, these individual offsets may push the effective misalignment further and yield losses of about 0.15–0.25 dB. Though such values appear modest, they may not be acceptable in many loss-sensitive systems. The situation deteriorates significantly at shorter wavelengths with much smaller fibres MFDs. There, the same 0.5–1.0 µm offset that could be acceptable at 1550 nm produces losses of 0.8–1.0 dB at 800 nm. This sharp penalty would make standard connector technology inadequate for many emerging applications that operate in the visible and near-infrared range. FIBRE CENTRING IN METAL- CERAMICS FERRULES To reduce core misalignment between mating connectors the manufacturing tolerances of both fibres and ferrules should be tightened. However this approach is often impractical or too costly. A more effective solution relies upon a two-part ferrule: a zirconia ceramic outer shell housing an internal titanium capillary that eventually holds the fibre. By plastically deforming the titanium, the fibre’s core can be displaced and moved to the very centre of the ferrule as shortly described in the following.
THE ORIGIN OF FIBRE CONNECTION LOSSES
consequences. Along any signal transmission line, losses cause
performance degradations that may need to be compensated for through infrastructure upgrades - such as more powerful transmitters, more sensitive receivers, or additional amplifiers - each carrying clear engineering and economic challenges. However, these remedies, though necessary, often give rise to a whole new set of problems related to an increased system energy consumption. Specifically, an increase in the power dissipation triggers thermal management challenges within and around opto- electronic systems that, in a vicious circle, can jeopardise or even undermine the effectiveness of the solutions. These issues have been accompanying the telecom and datacom industries for decades, and despite relevant technical progress, many concerns persist or have even worsened in modern data centres and AI computing clusters that often operate under very tight power budgets. In some other domains, countermeasures are not even an option. Quantum communication and quantum computing systems for instance often rely on the transmission of individual photons or delicate entangled states. In such environments, any loss is irreversible and directly corrupts the information integrity. Other examples based upon single-photon detection range from quantum key distribution (QKD) to fluorescence lifetime imaging (FLIM) and photon-counting LiDAR systems. Similar
Frequently underestimated contributors to optical loss are fibre connectors. These opto-mechanical components are key to systems’ flexibility since they allow optical networks to be modular, reconfigurable, and maintainable. Despite their deceptively simple function, the engineering behind them can be quite complex. For a connector to perform reliably, the cores of the two mated fibres must be mutually aligned with sub-micron accuracy, in a consistent and repeatable manner. Any deviation at this junction introduces insertion losses and, potentially, back- reflection, both of which may cripple the system performance. Achieving low insertion loss at a fibre connector interface requires precise control of two main factors: mechanical alignment of the fibre cores and optical surface quality. Assuming the fibre end- faces are properly polished and clean, the dominant mechanical contributions to loss arise from misalignment between the cores of the two mated fibres, which can manifest as a combination of lateral offset and angular tilt. In practice, lateral offset is the primary driver and typically dominates the loss since its effect scales exponentially with the ratio of displacement to fibre’s mode field diameter (MFD). In a fibre connector arrangement, these misalignments are the cumulative result of manufacturing tolerances across the two most critical components inside a connector body: the fibre itself and
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| ISSUE 44 | Q3 2026
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