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Optical glass fibres are ideal transportation media for light. Not only telecommunications benefit from this fact; a lot of other applications also apply this idea. At the beginning and at the end of transportation via the fibre, the light needs to be conditioned. This is done through elements we know from classical optics. Besides others, the following techniques are commonly applied.

Experimental Set-up

Beam splitting
For this task, either beam splitter plates or cubes are used. Beam splitter cubes (BSC) have the advantage that no beam displacement occurs, as is the case when using plates. The incident light is splitted into two fractions. The splitting ratio depends on the coating design of the beam splitting cube. Commonly, this ratio is 1:1. This means that both beams have half the incident intensity. Furthermore, the direction of one beam is changed by 90° with respect to the other.

Quarter wave plate
Such a plate is commonly manufactured from natural quartz and has a certain thickness. If the thickness is a multiple of a quarter for a given wavelength, then the plate effects the polarisation state of light which passes the plate. Hereby, the orientation of the optical axis of the quartz plate, with respect to the polarisation direction of the light, plays an important role. Using a quarter wave plate between two crossed polarisers, for example, the outgoing light intensity can be modified by turning the plate.

Optical diode
Finally, the combination of a quarter wave plate and a polarising beam splitter cube can also be used as a so-called optical diode. This means that light can pass only in one direction via this combination. To verify this within the experiment, a mirror is used to change the direction of the light. If the quarter wave plate is turned into the position where it changes the polarisation direction of the back coming light by 90°, then it will be reflected at the polarising beam splitter cube. Such arrangements are very important to protect the laser source against back scattered light which would cause unstable operation of the laser.