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-SiO/sub x/:H is capable of coupling a narrow field, 0.5 /spl mu/m in radius at a wavelength of 1.55 /spl mu/m, to a 5.3-/spl 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 with a hemicylindrical endface of 27-/spl mu/m radii, a thickness of 33 /spl mu/m, and a length of 35 /spl mu/m, is capable of converting a spot radius of a single-mode fiber (5.3 /spl mu/m) into a spot radius of 1.1 /spl mu/m at a wavelength of 1.55 /spl mu/m.