Optical fibres dominate the telecommunication industry for data transfer and are also used as an active gain medium (doped with rare earth elements) in fibre lasers. Both these application of optical fibres use single mode beams. Dr Peter Horak pointed out that if single mode optical fibres are to be used in telecommunications there is a predicted crunch in the amount of data that will be able to be transferred. A possible solution to this would be to introduce multimode optical fibres, which would introduce a new degree of freedom.
Optical fibre LASERS produce an extreme amount of power at the output (in the order of kW). Although at large powers there is damage to the fibre, optical Nonlinearities occur due to large intensity and thermal issues are also observed. A solution to this is to increase the radius of the fibre but this in turn produces higher order modes. Yet it has been found that at high powers the output beam gets distorted which is undesirable.
The disadvantage of optical fibres is that they have a loss, very small but it is there. In order to counteract this optical amplifiers are used and if the optical fibre has multiple modes, multimode optical amplifiers need to be used. In optical telecommunications there is a need for all the modes to be amplified by the same amount. Opposed to this optical fibre LASERS need the first mode to be amplified and/or the lower modes to have a higher loss for a cleaner output.
In order to address the above; the pulses within the optical fibres need to be modelled. Dr Horak mentioned that one could use the Beam propagation method, but this becomes computationally tedious. An alternative is to use the Generalized nonlinear Schrödinger equation, which is sufficient for the first few modes. On top of that amplifier models need to be used such as: Gain medium (rate equations), Parametric amplification and Raman gain. Finally if one wants to model LASERS there is a need to add optical resonators to the model.
Posted by Ioannis Begleris