Fibre provider

The 50 µm, 62·5 µm and 9 µm refer to the diameter of the inner glass core in which the light travels. The 125 µm figure refers to the diameter of the glass outer cladding. Because each fibre shares the same outer diameter the mechanical properties of the fibres are identical. However the optical properties vary significantly.

Multimode fibres have a large core which allows for less critical alignment and can be used with low cost led technology, however because of the core diameter the bandwidth is limited.

Single mode fibre on the other hand has almost unlimited bandwidth due to the small core only supporting one light mode. The disadvantage of this is the need for very high precision alignment required in both joints and connectors and the need to use expensive laser technology to drive the fibre. These factors combine to make a single mode installation approximately four times the cost of a multimode installation.

How is the distance over which a fibre can be used related to attenuation and bandwidth?

When specifying a fibre, the main two attributes are the attenuation and bandwidth. Put simply, the attenuation of a fibre (or link) is the light loss through the fibre (or system) measured in decibels. The task of measuring the loss of a link is a relatively simple process using a light source and power meter at the required wavelength. This attenuation component of the fibre or link, along with the bandwidth of the fibre, characterises the fibre link and data rate/distance capability of that link.

Bandwidth over the fibre was never much of an issue in the past – there was always far more bandwidth than the electronics of the day could interface to. Now, however, bandwidth has become a significant factor, due to the emergence of new applications such as the Gigabit Ethernet standards – presently 1 Gigabit is available, 10 Gigabit standards are in the pipeline. In the past, at low data rates the useable distance over fibre links was dictated by the attenuation, ie by the link loss budget.

Now, due to the much greater bandwidth requirements of Gigabit Ethernet we are finding that some multimode fibre types are limited in usable distance by bandwidth rather than loss.

The data rate versus length of a fibre link is related to the bandwidth of the fibre but is also a factor of the transceiver technology. The simplest way to demonstrate the length limitations of the various protocols of fibre types is in the form of a table.

Fibre seems relatively fragile. What is its bending capability?

Contrary to first impressions, optical fibre is very flexible and capable of relatively tight bending. The light continues to remain in the core of the fibre up to a fairly tight bend radius.

In typical fibre applications no noticeable loss effects are seen until the 20 mm radius is exceeded. At around 10 mm radius the loss can vary between 0·5 dB and 2 dB depending on the fibre size and wavelength (62·5µm/50µm, 1300 nm – 850 nm). However, small bend radii over long periods of time can cause eventual failure of the fibre, from a mechanical standpoint, and even very tight bending for short periods of time can cause stress cracking which could cause long term eventual failure.

For these reasons TIA 568-A allows for no more than a 30 mm bend radius for two and four fibre cables. Some manufacturers claim a long-term bend radius as low as 25 mm. So, with some care, fibre cable can prove to be a surprisingly robust medium.

Should we use 62·5 µm or 50 µm fibre? What are the deciding factors?

In an industry which, in the UK, is predominantly based around 62·5 µm multimode fibre, 50 µm fibre generates a lot of questions regarding compatibility and the suitability of present 62·5 µm hardware with 50 µm fibre.

Some of the more common questions are answered below:

Is 50 µm fibre accepted by the standards?

Yes, 50 µm is soon to be accepted by all major standards.

Is 50 µm fibre compatible with all active devices used for 62·5 µm?

Yes, all active equipment is compatible with both fibre types.

Is 62·5 µm fibre and 50 µm fibre interchangeable?

In many cases they can be interchanged, but there is a caveat. In figure 2, you will see that there is no attenuation loss when changing from 50 µm to 62·5 µm fibre. However, there is a one-time attenuation loss (theoretically 1·9 dB) when launching from 62·5 µm fibre into 50 µm fibre. How significant this is depends on the loss budget for the system and the fibre run length. In a lot of cases it is acceptable and does not affect the system.

Is 50 µm fibre compatible with standard multi mode connectors?

Yes, the outside diameter of the glass cladding on 62·5 µm and 50 µm fibre is identical. 50 µm fibre was actually developed in 1976 and is widely used in the European market today.

Why has the profile of 50 µm fibre suddenly been raised in the industry?

With the future requirements for ever increasing bandwidths it is now recognised that the bandwidth of 50 µm is superior to that of 62·5 µm, and so it has the advantage of increased distance capability.