Planar hyperbolical lenses with an average efficiency of 85% for coupling a 40 μm wide beam into a 4 μm wide waveguide are realised. The total length required for the beam compression is 600 μm, with a lateral index contrast of 0.01. Good agreement is found between the BPM predicted and the measured lens efficiencies.
Hamnes, P. C. A., Krijnen, G. J. M., & Driessen, A. (1990). High effiency small-size planar hyperbolical lenses. Electronics letters, 26(14), 1064-1066. [10.1049/el:19900689]. https://doi.org/10.1049/el:19900689