Three-dimensional waveguide junctions inside glass

In photonic devices and networks, optical waveguides routinely need to be coupled, split, and switched; several methods are in use to fabricate those junctions. A promising technique for making three-dimensional junctions in transparent materials, such as silica glass, is to write them with femtosecond lasers. Under optimal laser-writing conditions, a focused spot photoexcites the glass through nonlinear absorption and locally changes the refractive index. Slowly moving the material stretches the spot into a line of altered index that acts as a waveguide. For creating junctions, however, it has proven difficult to precisely position the material for each branch. Now, researchers at Japan’s Kyoto University have sidestepped that issue by introducing parallel laser writing of multiple branches in three dimensions. The key is computer-generated holograms. With a CGH and a single laser, multiple beam spots can be focused at precise locations in the glass. As the material is moved, CGHs are sequentially swapped in and out to change the spots’ locations. Using 256 CGHs, the researchers fabricated a 20-mm-long continuous waveguide that split and spread into four branches about 85 µm apart. A schematic is shown on the left, and the output from a single incident 635-nm laser beam is shown on the right. Optimization of the method is under way. (M. Sakakura et al., Opt. Express 18 , 12136, 2010 http://dx.doi.org/10.1364/OE.18.012136 .)