Forming Light Projected From Fibres

Optical fibres were a brilliant piece of engineering combined with a bit of physics knowledge. When light is travelling in a block and reaches the surface, it will normally refract out into the air. But if the angle of incidence is large enough total internal reflection occurs where all of the light is reflected back into the material and so none is lost. Optical fibres work by constantly bouncing the light using total internal reflection as shown in the diagram below:

the light ray enters the fibre at one end and is reflected out at the other end
Thank you to the BBC Bitesize for this image

Of course things never can be that easy. Real optical fibres are not so crude as to simply reflect the light back and forth. Instead the optical fibre is formed of circular layers of differing optical density. This means the light actually bends back, and rather than bouncing, snakes its way along the fibre. There  are also such things as multi-mode fibres that are thicker (which can sometimes even be an advantage) but have the ability to transfer multiple light beams as there is a greater variety of paths the light can travel through when the fibres are thicker and so multiple communication beams can be sent parallel to each other. Something that has not seen much research is whether it is possible to alter the beam of light as it enters and exits the fibre using various modification practices applied to the fibre itself. The study today has aimed to alter the output of the multi-mode fibre by using the adaptive wavefront shaping technique. The technique involves fitting a deformable mirror (only possible because of the multi-mode’s increased size) to the end of the optical fibre bundle. The deformable mirror does as it sounds and is uneven deliberately to distort a wave falling on it normally to correct any aberrations that have already occurred. The experiments showed that the adaptive mirror combined with an amplification technique could produce light shapes such as a single focus, multiple spots or a ring formation. These were all shown to be creatable even when a multi-mode fibre with 127 separate light beams was employed, clearly this is a robust and effective light manipulation technique.

Paper links: Shaping the light amplified in a multimode fiber


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