Hidden hazards

But optical fibre is decidedly nasty stuff. Or at least it can be if you're not fully tuned into the pitfalls. So let's cut straight to the chase and examine those areas for concern.

Fibre hazards come in two basic varieties: hazards to humans and those to fibres. The first kind could cost your health, the latter your job.

Protect yourself
Although fibres don't bite, they can harm you in several ways. The first of these is laser light hazard to your eyesight. The risk is minimal if you take care, but oh-so underhand if you're not aware.

Fibres carry a very pure and concentrated form of light that is perfectly capable of damaging your sight – permanently. This intense light can burn a spot in the retina without you knowing. There's no pain, no smell of burning, just a delayed realisation that your eyesight's not as sharp as it used to be in one spot and you have to shift your vision.

If it's infra-red, the light's not even visible. Light emitting diode (led) based launch amplifiers cannot cause this problem, but those using lasers definitely can. With the move to Gigabit Ethernet and OM3 fibre, the majority of new systems use lasers. The only safe approach is to treat every fibre as active and hazardous and never look at them end-on.

When might this be relevant? Well, some incident might slice open a cable or a connector might part from its fibre. Alternatively, you might be inspecting terminations using a connector microscope. In this situation the fibre is close to your eye and pointed directly at it for an extended period of time, plus it's magnified. Don't assume that the microscope has an infra-red filter built in; instead make sure the system is not live.

There are hazards from the ultraviolet light sometimes used to cure adhesives in splices and connectors. Safety procedures are provided with the materials required and these should be followed to the letter.

The arc is the hazard when it comes to fusion splicing. When used in confined spaces atmospheric testing must be carried out for combustible gases and vapours. In well-ventilated areas there is little risk, but in manholes it's an entirely different matter. Heavier-than-air mixtures can accumulate and you must follow the prescribed safety practices to avoid the inherent risks.

Rather less obvious is the danger of fibre ends entering the skin and causing infection. Even if there's no infection, glass in the skin can be extremely irritating or positively painful, as anyone who has laid glass fibre loft insulation will know.

Technically, optical fibre is made of silica rather than glass, but the effect is the same. What's more, because silica is totally inert, it will never break down in the body. Once there, it's there for good. Because silica is so brittle, it just shears off at the surface of the skin too – with wood splinters you stand a chance of retrieving them but not with optical fibre.

Treat fibre with respect and sweep up offcuts religiously. Use either a purpose-made disposal container or a short length of really sticky duct tape (gaffer tape) as a magnet and disposal wrapper for the fibre scraps.

There are other ways fibres can enter the body. Just think, a tiny fibre scrap clinging to your clothing could end up on your sandwich or in your mug of tea, and you'd never know. At least not until a fortnight later, when you try to describe those odd internal pains to your doctor. Keep it clean and always wash your hands before rubbing your eyes or using the toilet or you could have a painful experience.

The final personal injury caused by fibre cables is cuts from sharp, badly sheared, plastics tie-wraps. Some end-users now have an outright ban on the use of these because they cannot afford to pay compensation claims for industrial injury and due to the damage they can do to cables if over-tightened. These days it's worth checking out the rules before using cable ties.

Protecting fibres
Are fibres at risk and do they really need cosseting? You bet they do if you want them to obey and reward you. Optical fibres may not need the regular care and feeding that household pets do, but they'll certainly serve you better if you make sure they are bedded down properly in the first place.

The most obvious risk during fibre cable installation is damage to the fibre through rough handling. This includes bending too sharply; tie-wrapping too tightly; the microscopic bending of fibres by crushing and pulling; and the macro-bending of fibres through poor management and cable congestion. Distributors sell plenty of first-rate cable runway products and flexible conduit tubes that can help to avoid or entirely prevent these problems.

Other kinds of harm include the kinking of fibre element tubes at cable breakout and routing points, while the improper cutting of module leads can result in sufficient mechanical shock to cause fibre shift and misalignment of power coupling.

Training and good installation practice will minimise the risk of these problems but you can never be too careful, especially as fibre damage is often invisible. By the time the fractures come to light and complaints are received it's too late and the cost of removing and renewing an entire system will cost someone dearly. But it still happens with monotonous regularity. Make sure it doesn't happen to you.

We've already touched on tidiness but we must not forget the need for cleanliness either. Contamination or other kinds of damage to the mating surface of the optical connector may result in degraded performance due to power loss or reflection.

Poor connector assembly is another cause of optical losses; the light-carrying, core diameter of single-mode fibre is only eight to ten microns (approximately 0·01 mm), making tolerances extremely critical.

Following the rules
Good working practice will eliminate these problems, along with the risk of electrostatic discharge damage to the electronics associated with optical systems.

All told, the bottom line is very simple: you can save your job and health by taking proper care when installing fibre optics. But don't be fooled, what you can't see definitely can hurt you.