the accuracy necessary for highly efficient in-coupling into waveguides is generally associated with high effort and thereby high costs. as screen printing of waveguides is a cheap production technique, we suggest an according coupling scheme that does not rely on precise alignment of the light source with respect to the waveguide position. of course, this larger tolerances lead to less efficient coupling but in applications like sensing moderate coupling efficiencies are tolerable.
our coupling scheme relies on dye molecules emitting fluorescent light into the waveguide. these molecules are excited by an led. a spot of dye containing ps anisol solution is screen printed on top of a waveguide close to one of its ends. the adjustment requirements regarding the led are moderate due to the rather large lateral dimensions of the fluorescent spot of 250 ไมโคร
to provide both, high absorption of excitation light and a large stokes shift to minimize re-absorption of fluorescent light,we employed a light harvesting system consisting of two types of molecules, coumarin 545T ,
modeling the molecules as randomly oriented dipoles within a ps layer on a pmma substrate direction as 3.6% taking further into account the losses due to excitation light transmitted by the fluorescent spot 25% and the energy loss per photon due to the frequency shift of the fluorescent light 22% the upper limit for the coupling efficiency is 2.1%