Coaxial cable has long been the conventional interconnection medium within data acquisition systems. However, technological advancement has now established fibre optic as a major competitive technology offering improved performances in several respects.

Hardwired copper cables cannot match the long distance signal transmission capabilities of fibre optic. Single mode fibre optic technology enables complete signal integrity to be retained over more than 150 kilometres (100 miles) before amplification is needed, while with coax a length of around 300 metres (1000 feet) can be achieved. That was not always the case as in the early days of fibre optic technology, only multimode fibres were available and they did not allow reliable data transmission over distances longer than 6 kilometres (4 miles).

As their large core allows multiple light rays to simultaneously travel through the fibre with various angles, the central rays arrive before the bouncing ones, making the signal spread and become unrecognisable over long distances. The emergence of single mode fibres, whose small core of approximately 9microns diameter only lets the central rays enter the fibre, combined with the use of high precision laser transmitters, enabled the elimination of the pulse spreading effect. This made long distance optical data transmission possible and largely contributed to the wide deployment of fibre optic.

Fibre optic cables provide higher data transmission performances than copper wires while being lighter, more compact and flexible. Optical fibre is significantly smaller than typical coaxial cable, but a copper wire of several centimetres diameter can carry less information than two strands of fibre. In single mode fibres, the use of laser transmitters increases the bandwidth enabling large volume of data to be sent simultaneously with high reliability, accuracy and repeatability. Additionally, fibre optics cables are immune to electromagnetic and radio frequency interferences, more protected against environmental phenomena like lightning, or ground loops, and less dangerous when operated in explosive environments. Finally, communication through fibre optic interconnections is more secured as light signals cannot be intercepted like electrical ones.

With the use of fibre optic technology, the ergonomics of measurement systems can be significantly improved while achieving superior data transmission. Designed to operate at long range without any signal degradation, lighter, more compact, less bulky, equipped with more flexible cables, a fibre-based system can be more easily embedded in fine or complex structures.

For example, strain monitoring systems are applications, for which the flexibility of fibre optic is especially well adapted. Used to measure and control in real time the strain levels experienced by structures in relation to external effects (water pressure, wind force, seismic waves, accelerations, vibrations, etc), they help operators to optimise the performances of their equipment. Optical sensors are embedded in the construction structure and linked to a central monitoring station that continuously analyses the data acquired and reports any malfunction or unusual information. Buildings, bridges, dams, sailing boats, icebreakers, oil tankers, floating rigs, pipelines, marine fuel load and unload facilities, aircrafts or high-speed trains are some examples of the possible applications.

Strain monitoring systems are often permanently installed on outdoor structures that are submitted to arduous conditions like uneven or unstable soils, water, salt spray, extreme temperatures, or vibrations. In those demanding applications, connectors have a special role to play in the protection of the fibres.

Two main options can be considered for fibre optic interconnections in specific environments: either use a rugged fibre optic connector specifically engineered to withstand difficult operating conditions, or – if having to use a standard fibre optic connector type (Multi Fibre Push-On, Mechanical Transfer Registered Jack, etc) – add a ruggedised protective housing around it (Fig.2).

One significant advantage of designing rugged fibre optic connectors from scratch is that they can be customised to fulfil the specific requirements of each application. Many parameters can be taken into account in the connector development, such as making it resistant to continual immersion (IP68), high pressures (more than 10 bars), corrosion, vibrations or other mechanical stresses. The functionalities of the connector can also be customised by accommodating mixed (hybrid) transmission needs, enabling a single connector to simultaneously transmit RF or low voltage signals as well as power (Fig.3).

As primary and pioneer user, the telecommunications industry played an important role in the deployment of fibre optic connector technology. The involvement of mainly a single industry led to the development of more or less standardised products, such as MPO/MTP, MTRJ, LC or SC. As they are now globally exploited, it can happen that for compatibility reasons, those standard connectors have to be integrated in applications having to face difficult operating conditions. However, standard fibre optic connectors are designed for standard environments – mostly indoor in a temperature controlled place – and with basic functionalities. For instance, if the connector frequently needs to be plugged and unplugged, manœuvrability becomes a critical parameter, which basic connector designs might not be able to respond to. So to protect and adapt the basic connection, a ruggedised housing can be developed.

Some ruggedised housing solutions are very easy to add on as they do not even require the original cable to be dismantled. On top of its main protective qualities like sealing, corrosion or shock resistance, the added housing can cumulate other functions and integrate, keying system facilitating the plug-in, or calibrated locking system securing the connection against accidental pull on the cable. To reduce as much as possible the space taken, lightness and compactness are critical parameters in the housing development.

In conclusion, even if often perceived to be complex and fragile, fibre optic can actually be used in demanding environments and this is opening new opportunities for the development of fibre-based monitoring equipment. By protecting the fibres, connectors strongly contribute to the final performance of the system developed. Either purpose built rugged or ruggedised afterwards, both connector solutions proved to be reliable.

Understanding the specific requirements each application demands early in the product development process should help identify which connector solution to choose.

David Magni is Engineering Project Leader, Fischer Connectors SA, Apples, Switzerland. www.fischerconnectors.com