A Practical Introduction to Fibre Optics November 2014 Colin G4GBP

The history of fibres There are references going right back to the Roman times of glass being pulled into strands and being used as ‘light pipes’. Our real interest starts in about 1970 when Corning started doping silica glass and Bell Labs demonstrating that a semi-conductor laser could be used to send signals down the fibres with very low losses. The first practical use of fibre optic cables was by Dorset Police in 1975

How are fibre optic cables made? From very pure glass. 1 Making a preform glass cylinder (seed) 2 Drawing the fibres from the cylinder 3 Testing the fibres

Drawing fibres from the seed Nn The ‘seed’ gets heated to 1900⁰C The first glob drops by gravity and forms a thread Then it is drawn down by tractor Heat and micrometer control dia. Feed rate between 10-20m/s About 1.5 miles fed on to drum

Sizes of fibres The normal fibre optic cables that are in common use today are shown below: 62.5µm known as OM1 50µm known as OM2 9µm known as OS1

OM1, OM2 & OS1 – why? OM1 with its larger internal core diameter allows the light to scatter more. Narrowing the diameter of the core reduces this effect. This allows greater distances and higher data rates

I don’t see the light… Visible Red light has a nominal wavelength of 650nm Fibres use 850nm for short distances using OM1 & 2 whereas OS1 uses the lower wavelength for greater distances/higher data rates

The Fusion Splicer Here is a picture of my fusion splicer. Behind the black Hood is an LCD screen In front is the fuser mechanism itself.

Splicing step 1 To join fibre optic cables we need to strip the outer ‘buffer layer’ off. Then we need to cut or cleave the core & cladding. This must be a ‘perfect’ Cleave so that the two joints can be spliced together with the fusion splicer.

Splicing step 2 The ends are ‘cleaved’ This is done by putting a very small ‘nick’ in the glass and then ‘breaking’ the glass fibre.

Slicing step 3 Each ‘cleaved’ end is laid into the fusion splicer and held down in precise ‘V’ blocks.

Splicing step 4 The fusion slicer has two sets of cameras, Servos adjust the jaws so that the fibres are in exact alignment before applying a carefully metered arc to weld the fibres. It measures the losses across the joint and tests for tensile strength.

Splicing step 4 The last stage is to protect the joint with a heat shrinked sleeve. The sleeve has a metal bar through it to stop it bending. Job done!