Visualization of light propagation in fs laser written waveguide arrays

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.