Comparison of robust coupling techniques for planar waveguide immunosensors

We have analyzed and fabricated two different coupling schemes to meet the requirements for a convenient means of coupling into a planar waveguide immunosensor that is relatively insensitive to beam alignment. These are the "launch" coupler and the grating coupler. Each possesses advantages and disadvantages, depending mainly on die thickness (mode number) of the waveguide to be illuminated. For example, the launch coupler is best suited to a thick (highly multimode) waveguide and is less efficient for a thin (few mode) guide. Our experimental results verify predictions of a ray theory developed to give coupling efficiency for a variety of coupling parameters.