On-Train Fiber Optic Connectivity
The railway industry still hesitates to make systematic use of fiber optic technology on board rolling stock. HUBER+SUHNER explains how recent advances in cable and connectors have made on-train fiber optic connectivity a reality.
Over the past few years, the emergence of Ethernet as an industry standard for communication on board trains has moved from concept to reality. Pushed by new applications such as on-board passenger entertainment, passenger information, video monitoring, on-board Internet, or modern train control systems, the demand for greater bandwidth of train backbone networks continues to increase. In this context, fiber optic technology has earned its place, at least from a system performance point of view.
Nevertheless, the railway industry is, in general, still hesitant to make systematic use of this technology on board rolling stock. The industry is concerned about fiber optics’ suitability for use on trains due to harsh environmental conditions; a train is a moving object subject to strong mechanical stresses and high safety requirements.
Of course, most of the concerns are focused on the area where fiber optic connectivity components would be subject to the most severe environmental conditions – inter-vehicle jumper systems. Today’s connectivity technology and experience, however, tend to demonstrate that fiber optics are an appropriate technology for the rail industry now and in the future.
Rolling Stock Fiber Optic Cables
Railway cables must comply with two types of requirements:
- Fire safety
Fire safety is a hot topic in the railway industry. Public transport operators must ensure that, in the case of a fire on board, passengers and staff can be safely evacuated without injury due to smoke inhalation.
As a leading market for railway technology, Europe is intently working on the final development of a European fire safety standard for rolling stock equipment designated EN 45545. Ultimately, such a standard should supersede current national standards applicable in individual countries (e.g. BS6853, NF F 16-101, DIN 5510, UNI CEI 11170).
Nonetheless, other standards might well remain in place on other continents. This means that in the near future, all cables installed on a train – including fiber optic cables – will have to comply with the requirements of this standard.
Although environmental requirements are very well defined by European standards for electrical cables (EN 50306, EN 50264, EN 50382), there is no standardized requirement defined yet for special cables such as fiber optics. Environmental requirements should, in principle, cover characteristics such as fire resistance, abrasion resistance, fluid resistance, traction resistance, water absorption, etc.
HUBER+SUHNER solved this problem with its RADOX material technology. Based on an electron-beam cross-linking process, this technology has been widely used for more than 25 years on electric cables.
However, in contrast to electrical cables made of copper, the operating principle of electron-beam cross-linking is theoretically not compatible with the physical properties of optical fibers.
Electron-beam cross-linking is the interconnection of adjacent long molecules with networks of bonds (cross-linking) induced by electron-beam treatment. Electron-beam processing of thermoplastic material results in several enhancements, such as an increase in tensile strength and resistance to abrasions, stress cracking, and solvents.
High-performance optical fibers normally used for high data-rate transmission are made of silica (glass), and the effects of high-energy (MeV) electron beaming on this material considerably degrades its light transmission properties. More precisely, if a fiber optic cable is considered, the thickness of the cable jacket may not be sufficient to stop the electron beam before it reaches the glass fiber. Upon impact with the glass material, the kinetic energy of the electrons transforms into heat. Therefore, condensing an electron beam onto particles of glass may generate enough heat to even create defects in the structure of the material itself. As a consequence, this might bring the optical glass fibers to a state where it is no longer possible to use them for high bit-rate transmission.
However, solutions are now available, such as RADOX fiber optic cables. In order to meet the requirements of European standards, these cables use the standardized RADOX EM 104 jacket material, fulfilling all the environmental requirements defined in table 4 of EN 50254-1.
Rolling Stock Fiber Optic Connectors
The connector technology applied to rolling stock applications has today reached a high level of maturity and reliability for inter-vehicle applications. Although qualification processes and related norms and standards (NF F 61030, EN 50467, IEC 61373) are very demanding in the railway industry, there are several suppliers throughout the world who are capable of delivering state-of-the-art connector solutions either with circular or rectangular footprints. However, this situation applies to electrical connectors for power and signals; there is still a way to go before reaching an equivalent situation for fiber optic connectors.
Nevertheless, thanks to the specific adaptation of connector inserts, solutions are already available that offer both the reliability of proven and familiar railway connector body designs and the performance of fiber optic technology.
Indeed, one of the difficult aspects in the design of such connectors is to combine mechanical resistance needed for stringent environmental conditions (shock, vibration) with low optical attenuation performance into a unique design. The goal is to achieve a sustainable, minimum-signal attenuation in order to guarantee a high transmission bandwidth.
Degradation of the quality of optical contacts (scratches, dust, etc.) or misalignment are responsible for major signal losses. The connector design should obviously take these critical aspects into consideration.
Additionally, the effects of mechanical stress on the optical fibers within the connector should be minimized, as they are a cause of unintentional increase in signal attenuation. A proper strain relief management at the termination point between the optical cable and the backshell of the connector ensures that a pulling force on the cable does not directly induce tension on the fibers or on the contacts.
Today, reliable fiber optic railway connectors can be found with circular or rectangular footprints; with variable fiber counts; and with butt joint or expanded-beam technologies; thus responding to a wide range of application constraints and system performance requirements.
As far as fiber connection technology is concerned, the selection is given by various criteria. As an example, in applications where train vehicles need to be regularly reconfigured, expanded beam solutions may be preferred, which are less sensitive to dust contamination and consequently require less systematic cleaning of contact surfaces. In this case, optical performances are limited (typical insertion loss <1dB for single mode at 1310nm and <0.7dB for multimode at 850nm), possibly requiring additional active equipment.
On the other hand, in applications where optical link budgets are tight, butt joint technology may be preferred, because it offers improved optical performance (typical insertion loss <0.5dB for single mode at 1310nm and <0.2dB for multimode at 850nm), but it requires a more systematic cleaning of the contact surfaces.
As far as connector body technology is concerned, the choice here is also driven by different criteria. As an example, installation may require rectangular bodies, so that solutions with fiber optic inserts and contacts are preferred. Where installation requires circular bodies, solutions based on MIL-DTL-5015 or MIL-DTL-38999 design standards with fiber optic inserts and contacts are also available. Such solutions offer the advantage of using existing connector designs, which are already familiar and qualified in the rail industry, thus limiting intensive qualification process requirements.
The ability of the industry to deliver state-of-the-art fiber optic connectivity components for rolling stock applications does not necessarily imply that reliable cable system solutions are easy to achieve.
In this respect, inter-vehicle jumper systems are a perfect example. Because such assemblies are permanently exposed to dynamic mechanical and environmental constraints, their design is the key to meeting the service life expectations of the rolling stock industry.
Indeed, as the connector is often a fixed part in these assemblies, the cable and its termination into the back shell of the connector can be challenging. Under these circumstances, fiber optic cable construction, system design, and termination process remain areas where know-how, competence, and experience are crucial to meet demanding dynamic test conditions induced by inter-vehicle movements over millions of cycles on multiple axes.
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