A high-performance micro-GRIN-chip spot-size converter formed with focused ion beam

A high-performance spot-size converter composed of a micrograded-index slab chip with a hemi-cylindrical endface for conversion between single-mode fibers (SMFs) and fine-core waveguides has been fabricated using the focused ion beam etching technique. The microchip has the index difference as much as 1.8 in the index-gradient direction and the input hemi-cylindrical endface of curvature radius as small as 6.3 μm. The measured full width at the 1/e
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maximum of the resulting focused spot intensity distribution is 1.8×2.5 μm
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when the input end is excited by an SMF of 10.8-μm-spot diameter at the wavelength of 1.55 μm.