Do polymer fibres make for good interconnect solutions?

William Heath, commercial director of Optoelectronic Manufacturing (OMC), picked up on three main reasons why they do. "If you have a high voltage that you want to isolate, then fibre optics is the only real way to do it. We have customers who use optical fibres in train braking systems, power stations, inside high voltage power supplies – all applications where you are using optical fibre not to achieve high speeds of transmission but because it is isolating. "You might have a high voltage at one end and something sensitive at the other end, like a computer system or a petrochemical tank, and what you don't want is to use copper cable and get high voltage tracking down your cable to computer control room and blowing up all your computers." The second main advantage, according to Heath, concerns removing interference. "If you take the example of trains, there is a lot of electrical interference and if you use a cable then you are going to pick up that noise. Basically, your cable is going to act as an antenna and there are some applications where any amount of shielding around your cable isn't enough. That is where optical fibre comes into its own because it doesn't pick up any interference at all. Another main reason you might use it is for security; it is very, very hard to tap into an optical fibre." So, given that isolation, interference and security issues are pointing towards a fibre optic solution, the first decision to be taken is whether to use single or multimode fibres. Superfast internet rifles through single mode fibres – a single beam of laser generated light that can contain the vast amounts of information required for telephone calls, cable TV and the like. For more general industrial applications, multimode fibres are usually just as efficient. And there are two principal reasons why they are far more cost effective – they can be made of cheaper material and can be powered by LEDs, rather than lasers. Glass fibres have a core diameter that is typically smaller than in polymer fibre, which means there is less dispersion and signals can be sent over longer distances and at higher speeds than would be the case with polymer fibre. On the other hand, polymer fibre is more robust, costs less and it is easier to handle. However, this does not mean that polymer fibre needs to be a compromise. Heath commented: "There has been room for some confusion because if polymer fibre is manufactured to the same standard that you would manufacture a glass fibre link to, then it can be just as reliable." There is a perception that polymer fibres are very easy to connect – a swift cut with a scalpel and it is ready to be 'shoved' into connector. Inevitably, if this approach is taken, then problems of reliability will occur. Any fibre optic link is comprised of three components – the transmitter, the cable and the receiver. Each of these has a tolerance: the launch power of the transmitter; losses in the cable; and the sensitivity of the receiver. The quality of the terminations can have an impact on all three. So while poorly prepared individual links may well work, or can be made to work by making secondary cuts in the fibre, over the course of hundreds or thousands of links there are bound to be failures. It is unusual for there to be a defect in the cable itself as these are detected during manufacture. It is far more likely that problems will occur when the cable is terminated, cutting the fibre and polishing both ends to an optical grade finish to provide a crystal clear window. The mechanical tolerances on the connectors can then result in potential losses. Equally, the tolerances within the transmitter and receiver housings, combined with the alignment of the emitting diode within the transmitter housing and the equivalent at the receiving end, all have an effect on the performance of the system. Optical budget These considerations form the start of the calculation of an optical budget. Heath explained: "To design your optical budget, you say this is how much light I have to a start with, this is how sensitive my receivers are, this is what I am going to lose in the cable, this is how much the LED and cable are going to degrade over 20 years or whatever the lifetime of the system is, this is the percentage that will back reflected – is it enough to be picked up by the receiver?; if so we need to lower our limit. "You need to consider the tolerances in all of these elements – best case and worst case. What you end up with is a window for each component in terms of its performance that you know will work and will do so reliably for the lifetime of your system. It is what we call the 'end of life margin'. Even in the worst case where everything degrades to its maximum tolerance, there will still be two or three dBs of margin before the system fails." The answer is not to flood the system at the beginning as this 'blinds' the receiver, saturating it with light and damaging its sensitivity. Too much light also can result in the problem of back reflection which Heath mentioned, where the small proportion of the signal that is back-reflected at the receiver end and returns to the transmitter and a small proportion of this is then reflected back down to the receiver. "If it is picked up by the receiver as an extra pulse it can play havoc with your data signal," observed Heath. Back reflection therefore becomes part of the optical budget design. It is only rarely that engineers will come to OMC armed with their optical budget already calculated. Heath concluded: "Our customers will be engineers who will be very competent at designing high voltage power supplies, high end military systems, braking systems and so on. They will not necessarily have expertise in the field of optical fibre, but will understand enough for us to ask them the right questions so that we can define the optical budget. Generally, what they say is that they have got an electrical signal at point A and they want to receive it again at point B and what they need is the stuff that converts it to light, transmits it and converts it back. Generally speaking, they don't want to get too involved in the optics of it."