Results of investigations of input/output couplers basing on a 2D refractive index gradient are presented. A new simple method of its forming has been suggested and realized. This method allows to achieve a considerable narrowing of the directional beam diagram of such elements to the values no more than 25' - 30'. The whole (100%) light output was experimentally observed for all studied samples. The waveguide launching efficiency of about 20% was obtained using a He-Ne laser incident beam without any additional optimizing transformations on its width and divergence.
Dmitry V. Svistunov,
"Highly effective input/output coupler in graded index waveguide", Proc. SPIE 2096, Lasers in Optical Systems and Devices, (10 August 1994); doi: 10.1117/12.183104; https://doi.org/10.1117/12.183104