Loss In Fiber Optics
Any impurities that remain in the fiber after manufacture will block some of the light energy. The worst culprits are hydroxyl ions and traces of metals. The hydroxyl ions are actually the form of water which caused the large losses at 1380 nm. In a similar way, metallic traces can cause absorption of energy at their own particular wavelengths. These small absorption peaks are also visible. In both cases, the answer is to ensure that the glass is not contaminated at the time of manufacture and the impurities are reduced as far as possible. We are aiming at maximum levels of 1 part in 109 for water and 1 part in 1010 for the metallic traces. Now for the second reason, the diversion of the light.
This is the scattering of light due to small localized changes in the refractive index of the core and the cladding material. The changes are indeed very localized.
We are looking at dimensions which are less than the wavelength of the light. There are two causes, both problems within the manufacturing processes. The first is the inevitable slight fluctuations in the ‘mix’ of the ingredients. These random changes are impossible to completely eliminate. It is a bit like making a currant bun and hoping to stir it long enough to get all the currants equally spaced. The other cause is slight changes in the density as the silica cools and solidifies. One such discontinuity is illustrated in Figure below and results in light being scattered in all directions. All the light that now finds itself with an angle of incidence less than the critical angle can escape from the core and is lost. However, much of the light misses the discontinuity because it is so small. The scale size is shown at the bottom.
The amount of scatter depends on the size of the discontinuity compared with the wavelength of the light so the shortest wavelength, or highest frequency, suffers most scattering. This accounts for the blue sky and the red of the sunset. The high frequency end of the visible spectrum is the blue light and this is scattered more than the red light when sunlight hits the atmosphere. The sky is only actually illuminated by the scattered light. So when we look up, we see the blue scattered light, and the sky appears blue. The moon has no atmosphere, no scattering, and hence a black sky. At sunset, we look towards the sun and see the less scattered light which is closer to the sun. This light is the lower frequency red light.
On a point of pronunciation, the ‘s’ in Fresnel is silent.
When a ray of light strikes a change of refractive index and is approaching at an angle close to the normal, most of the light passes straight through as we saw in a previous chapter.
Most of the light but not all. A very small proportion is reflected back off the boundary. We have seen this effect with normal window glass. Looking at a clean window we can see two images. We can see the scene in front of us and we can also see a faint reflection of what is behind us. Light therefore is passing through the window and is also being reflected off the surface.
We are most concerned about this loss when considering the light leaving the end of the fiber as shown in Figure above. At this point, we have a sudden transition between the refractive index of the core and that of the surrounding air. The effect happens in the other direction as well. The same small proportion of light attempting to enter the fiber is reflected out again as in Figure below.
The actual proportion of the light is determined by the amount by which the refractive index changes at the boundary and is given by the formula:
To see how bad it can get, let’s take a worst-case situation – a core of refractive index 1.5 and the air at 1.0
Divide out the terms in the bracket:
So, 96% of the incident light power penetrates the boundary and the other 4% is reflected. This reflected power represents a loss of 0.177 dB. It may be worth mentioning in passing that if we try to squirt light from one fiber into another, we suffer this 0.177 dB loss once as the light leaves the first fiber and then again as the light attempts to enter the other fiber. Remember that these figures are worst-case. We get up to all sorts of tricks to improve matters as we shall see when we look at ways of connecting lengths of optic fibers together.
Making use of Fresnel reflection
The return of the Fresnel reflection from the end of a fiber gives us a convenient and accurate method of measuring its length. Imagine a situation in which we have a drum of optic fiber cable marked 5 km.
Does the drum actually contain 5 km? or 4.5 km? or is it in five separate lengths of 1 km? It is inconvenient, to say the least, to uncoil and measure all fiber as it is delivered.
The solution is to make use of Fresnel reflection that will occur from the far end.
We send a short pulse of light along the fiber and wait for the reflection to bounce back. Since we can calculate how fast the light is traveling and can measure the time interval, the length is easily established. This magic is performed for us by an instrument called an optical time domain reflectometer (OTDR).
A sharp bend in a fiber can cause significant losses as well as the possibility of mechanical failure. It is easy to bend a short length of optic fiber to produce higher losses than a whole kilometer of fiber in normal use. The ray shown in Figure below is safely outside of the critical angle and is therefore propagated correctly. Remember that the normal is always at right angles to the surface of the core.
Now, if the core bends, as in Figure 6.5, the normal will follow it and the ray will now find itself on the wrong side of the critical angle and will escape. Tight bends are therefore to be avoided but how tight is tight? The real answer to this is to consult the specification of the optic fiber cable in use, as the manufacturer will consider the mechanical limitations as well as the bending losses. However, a few general indications may not be out of place.
A bare fiber – and by this is meant just the core/cladding and the primary buffer – is safe if the radius of the bend is at least 50 mm. For a cable, which is the bare fiber plus the outer protective layers, make it about ten times the outside diameter or 50 mm, whichever is the greater.
The tighter the bend, the worse the losses. Shown full size, the results obtained with a single sample of bare fiber were shown in Figure below. Attached instruments indicated a loss of over 6 dB before it broke. The problem of macrobend loss is largely in the hands of the installer.
These are identical in effect to the macrobend already described but differ in size and cause. Their radius is equal to, or less than, the diameter of the bare fiber – very small indeed.
These are generally a manufacturing problem. A typical cause is differential expansion of the optic fiber and the outer layers. If the fiber gets too cold, the outer layers will shrink and get shorter. If the core/cladding shrinks at a slower rate, it is likely to kink and cause a microbend.
With careful choice of the fiber to be installed, these are less likely to be a problem than the bending losses caused during installation since optic fiber cables are readily available with a wide range of operating temperatures from 55°C to 85°C.