Maxwell's fisheye lens as efficient power coupler between dissimilar photonic crystal waveguides

The imaging properties of the Maxwell's fisheye (MFE) lens makes it a viable candidate to implement power coupling between different types of waveguides. A coupler based on the MFE lens is designed to couple a square lattice photonic crystal to a triangular lattice one. The MFE lens is implemented as ring-based multilayer and graded photonic crystal (GPC) structures. The performance of the ring-based MFE lens is better than the GPC-based one in the C-band of optical communication. The proposed ring-based MFE coupler has a footprint of $3.62\times 3.62\mu m$ and covers the entire C and U bands. The S and L bands are partially covered. The average insertion loss of 0.1dB and the maximum return loss of -11dB in the C-band is achieved.