Investigating attenuation in optical fibres

Essay by ruby86College, UndergraduateB, April 2004

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My aim is to investigate the factors which affect the attenuation of infrared radiation in an optical fibre modelled by transparent jelly. I will determine how the length and width of the jelly affect how the radiation is attenuated. The loss of the intensity of the infrared radiation is represented by the amount of voltage across the infrared detector. If the intensity of the radiation is high, then the voltage across the infrared detector will also be high.

Apparatus (before pilot experiments)

Infrared transmitter and detector

Ruler / vernier callipers


Multimeter (

Wires x 2

Bunsen burner

Plastic container

Tripod and gauze

Glass rod

Clear gelatine powder

2 x 250cm3 beakers


Power pack

Pilot experiments

I carried out a series of pilot experiments in order to determine the range of lengths and widths that I will be investigating. I also needed to find the correct proportions of the gelatine powder and water to use for the jelly to be firm and consistent.

Modifications made during pilot experiments

I found that using 400cm3 of water and 1 sachet of gelatine left the jelly to runny to be used in the experiment. I therefore used 500ml of water and 2 ½ sachets of gelatine (44.25g).

I found that there was a potential difference across the infrared detector even when the infrared emitter was switched off. The IR radiation being picked up is from the immediate environment of the detector. Whilst carrying out my experiment it would pose a huge problem to allow any background radiation to reach the IR detector.

When I vary the dimensions of the jelly, I could obtain anomalous results. For example, if the length of the jelly were shortened, the voltage across the infrared detector should increase to a certain value. However, if...