Visualization of light propagation in fs written waveguide arrays
2007
For various applications it is interesting to directly visualize the propagation of light in waveguides. For this
purpose, we used special fused silica glasses with a high content of OH. This leads to the formation of color
centers when waveguides are written with fs laser pulses. When light is launched into the waveguides the color
centers are excited and the fluorescence can be directly observed. This is especially interesting in waveguide
arrays for the visualization of the evanescent coupling, since the discrete light evolution exhibits many features
which are in strong contrast to propagation in common isotropic media. As an example for the visualization
we will discuss here the possibility to excite a completely incoherent propagation within the waveguide array
although the sources are fully coherent. When multiple waveguides are excited, the light evolution in the array
can be described as a superposition of the single propagating amplitudes. The formula for the resulting intensity
contains an interference term. One can explicitly show that this interference term vanishes for certain excitation
patterns. When for instance two adjacent waveguides are excited the light propagates as there was no interference
term, which is equivalent to the simple sum of the two intensities of the single amplitudes. This suggests the term
"quasi-incoherent" for this new kind of propagation effect. In contrast a coherent superposition including the
interference term is obtained for an excitation of two waveguides when there is one waveguide located between
the two excited ones.
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