Lab-on-a-Chip: Direct coupling of light into a planar thin film waveguide
As disposable products, waveguide-based biochips offer a wide of possibilities, but relatively high manufacturing costs often make their use uneconomical. The most important cost factor is the structuring of the chip, which enables effective coupling of the laser light. A novel coupler makes it possible to couple light into chips without structuring.
Planar thin film waveguides have become indispensable tools in various fields such as telecommunication technology, biosensors and material characterization. In biosensor technology, for example, they can be used to efficiently excite dye molecules and to follow changes in the surface coating extremely precisely. A decisive point in the use of waveguide-based biochips is the coupling of the laser beam into the waveguide.
Due to the low layer thickness and the high refractive index, coupling via the end surface of the waveguide or prism coupling is not feasible.
Figure 1: Function principle of evanescence field microscopy: The light of a laser is coupled into a microscope slide via a grating structure. The evanescent field stimulates optical transitions in molecules located on the surface of this carrier. (Source: Fricke-Begemann)
With the coupler presented here, the grating for coupling the light is not first structured on the chip, but already on the light feed line. For this purpose, the light of a light-guiding fiber is expanded and collimated with the aid of a GRIN lens. The second end face of this lens is structured in the form of a grating, which induces the modulation of the refractive index necessary for coupling in this waveguide when it is close to the actual waveguide surface. To adjust the coupling angle, the structured end face can be ground at a defined angle. A GRIN lens, a rod lens or - with previous collimation - a simple glass or quartz cylinder can be used for collimation
Figure 2: Functionality of the simplified structure: The light is guided via a glass fibre to a lens. This is ground on the side facing the waveguide and provided with a grating. This means that the grating no longer has to be applied to the light-conducting layer of the waveguide. If there is sufficient proximity between the grating and the waveguide, the laser light is coupled in. (Source: Fricke-Begemann)
With the use of waveguides as sample carriers, an evanescence field excitation can also be performed on any microscope (including simple, appropriately retrofitted microscopes). Highly specialized TIRF microscopes and special objectives are no longer required.
In order to be able to estimate the achievable coupling efficiency, simulations were carried out with which it could be shown that, in principle, efficiencies of up to 40% can be achieved with a given coupling length.
Granted German patent: DE102010029612 (B4)
Dr. Ireneusz Iwanowski