Reusable fiber to waveguide coupler

Most thin-film waveguide applications would greatly benefit from a simple and efficient method to couple light from an optical fiber to the planar waveguide. A new potential solution combines a gradient-index (GRIN) lens with a grating structure at its exit face to couple light from a single mode optical fiber directly to the waveguides. The new removable and reusable microlens device eliminates the costs associated with the lithographic grating couplers that are usually integrated into the waveguide devices. This is especially appealing in conjunction with disposable waveguides in single-use biosensing applications.

The efficiency of the external coupler depends on the characteristics of the grating and the distance to the waveguide surface. Finite-element method (FEM) calculations are used to model the coupling process. The results from the numerical simulations suggest that coupling efficiencies as high as 40% should be accessible.

The feasibility of the coupling scheme has been demonstrated using a 500 nm period grating on a collimating GRIN lens fabricated by direct laser ablation at 157 nm wavelength and yielding an experimental coupling efficiency of 13%.

An even simpler approach is to immediately use an optical fiber with an angle polished and grating structured end face as the external coupler by circumventing any further optical elements. In this case higher insertion losses must be tolerated.

Further information:

T. Fricke-Begemann, J. Ihlemann
Direct light-coupling to thin-film waveguides using a grating-structured GRIN lens
Optics Express 18, 19860 (2010)