Impact of high index buffer layer on the improvement of the MMI sensor sensitivity

The results of investigations are presented, of the impact of high index buffer layer on the improvement of the multimode interference (MMI) sensor sensitivity. The basis of the presented research is a gradient index MMI structure with a multimode section length relating to the length of an input field image formation. The multimode section is covered with a material which changes its optical parameters (refractive index and extinction coefficient) when exposed to appropriate gaseous environments. The coating parameters have an effect on light propagation conditions in a structure, which can be seen as an output signal variation. In this configuration a thin buffer layer of high refractive index is placed between the sensing layer and the gradient core. We present the results showing how these buffer layers lead to a sensitivity increase in the sensor based on the MMI structures. The performed analyses are based on numerical BPM calculations.