Interconnection
Fibre-optic connectors for performance- critical applications in harsh environments
By Phil McDavitt, strategic business manager, Lane Electronics I
n traditional data-centre applications fibre is extensively used in equipment installed in a carefully regulated and protected operating environment. The popular fibre connector types such as LC, SC, ST and multi-fibre MPOs are most commonly used for interconnection in such installations. This type of connector uses a carefully polished physical contact with a very small-diameter fibre core of less than 50 microns (0.05 millimetres) - about the size of tiny dust particles. These connectors use a finely polished contact end that accurately and physically butts against a polished end of the mating fibre to provide a near invisible joint with minimal transmission loss and will offer excellent performance and reliability in such protected environments. The light-transmitting fibre core needs to align perfectly with the connected fibre core to avoid transmission problems and without sealing or protection the very small size means it will be very sensitive to any dirt or moisture that might contaminate the end of the light- transmitting fibre.
Reliable and secure, performance-critical data-based communication systems in more demanding harsh environments and applications require a more ruggedised connector system to support them. It is essential that the optical contacts remain protected and in general a modified fibre-optic connector technology is used to deliver a level of in-use reliability that fibre connector types such as LC, SC, ST and multi-fibre MPOs cannot match.
Typical applications include military and aerospace in tactical communications, avionics, and battlefield networks, oil and gas for downhole drilling, subsea communication and pipeline monitoring, industrial and transportation, railway signalling, automation and heavy machinery, medical and scientific imaging systems, sensors and diagnostics in demanding settings.
In these domains there will be different approaches to help provide this extra resilience, essentially these can be grouped into two main criteria. i) adding mechanical and environmental protection to the connector system through seals and materials and
50 September 2025
within each mating half. Unlike physical contact technology, the expanded beam connection is contactless, with the expanded light beam passing over a small gap between the two lenses within the connector system once mated. This significantly improves resistance to vibration and mechanical wear and can increase the reliability and lifetime of the connector. The air gap also means that any dirt or contamination on the lenses will not damage the mating half in use. A good test for expanded beam connectors that are designed for harsh environments is to drop disconnected connectors in dirt, then rinse them off with water or wipe with a cloth and then mate them – a good connector system should not be affected and work straight away, every time.
II) expanded beam technology. Different construction of the connector type allows these additional features to be introduced and has resulted in a number of different connector series which are now the go-to solution.
The first approach is perhaps the most obvious and easily understood. Adding protection to light-transmitting fibre ends will prevent possible degradation and failures of the system through dust or moisture. For use in harsh environments, these features most typically appear as metal connectors bodies and elastomeric seals which become compressed through joining two connector halves together. The metal construction adds considerable mechanical strength and selection of materials like aluminium, bronze and stainless steel can then address specific performance needs like chemical resistance and salt-spray corrosion. Environmental seals add reliable protection when mated or sealed protective covers will provide the same when the connectors are not in use. When added shock and vibration become additional considerations, some of the higher- reliability solutions will provide additional alignment features which take any mechanical stresses away from the light-transmitting fibre contacts, extending operational life and reliability.
A good example of this approach would be the EATON Souriau range of connectors
Components in Electronics
fitted with high-reliability ELIO fibre-optic contacts. The 8D range in particular is based on a popular derivative of the military- proven MIL-DTL-38999 circular connector style. The connector construction provides comprehensive sealing when mated and the circular metal shell can be specified in aluminium, composite, marine bronze and stainless steel with a range of different plating finishes. The singlemode and multimode ELIO fibre-optic contacts use a robust 2.5mm ferrule with precision alignment to provide performance and resilience under high levels of shock and vibration.
The alternative approach is expanded beam technology which uses two matching lenses, one at the end of the transmitting fibre from which the light beam is emerging, and the other at the end of the fibre it is being connected to. The first lens expands the emerging beam and the second lens than refocuses the light, reconstructing a beam in the second fibre of the same size of the original beam. As the beam of light passing between the two lenses is expanded to have a much larger diameter than the beams inside the fibres, this means that any particle of dust or dirt will be a much smaller proportion of the transmitted light beam, and it therefore has less effect on blocking the light and affecting the signal. Specifically designed expanded beam connectors incorporate this lens technology
Low insertion loss to ensure minimal signal loss (typically <0.5 dB for physical contact connectors and <1.0dB for expanded beam)
High return loss in order to reduce signal reflections and maintain data integrity
Shock and vibration resistance: connectors must meet military and industrial standards (e.g., MIL-DTL-83526) to withstand mechanical stress
Impact protection: Use of reinforced housings to prevent damage from physical impact
Wide operating temperature range, typically -55°C to +85°C
IP67 or IP68-rating for protection from water and environmental conditions
Stainless steel or composite plastic materials to resist exposure to fuels, oils, and other harsh substances A good example of expanded beam technology would be the CINCH Fibreco Junior range of connectors. Derived from the military-proven MIL-DTL-83526 connector style, the range features sealed singlemode and multimode expanded beam inserts within a fully sealed hermaphroditic circular metal shell. The circular metal shell can be specified in aluminium, marine bronze and stainless steel with a range of different plating finishes. All Cinch
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