A micro light-beam spot-size converter using a hemicylindrical GRIN-slab tip with high-index contrast

A simple light-beam spot-size converter (SSC) using a microscale graded-index (GRIN)-slab tip with a hemicylindrical endface for efficient coupling between a single-mode fiber and a fine-core waveguide is reported. Numerical examples based on the finite-difference time-domain (FDTD) method have shown that the tip made of a-SiOx : H is capable of coupling a narrow field, 0.5 mu m in radius at a wavelength of 1.55 mu m, to a 5.3-mu m radius standard single-mode fiber with a coupling loss,as low as 0.07 dB. A micro GRIN slab with an index difference of as much as 34% was successfully fabricated. It has been confirmed experimentally that a GRIN slab witha hemicylindrical endface of 27-mu m radii, a thickness of 33 mu m, and a length of 35 mu m, is capable of converting a spot radius of a single-mode fiber (5.3 mu m) into a spot radius of 1.1 mu m at a wavelength of 1.55 mu m.