Abstract
GRIN medium with a lateral sech refractive index variation, can make normal-incident light beam gradually curve to the medium with a larger refractive index, and periodically converge the light beam to a point smoothly and continuously. This property of GRIN medium can be used as a coupl er to realize a mode spot size conversion. This paper mainly discussed the transmission property of Gaussian light beam in a sech GRIN medium by numerical simulation. SOI waveguide is widely used in the photonic integrated circuit. To achieve a higher coupling efficiency between single mode optical fiber and single mode SOI slab waveguide, which suffer a great light beam coupling loss for the mismatch of the spot size. The GRIN medium coupling structures are designed as a coupler, with symmetric refractive index distribution and asymmetric refractive index distribution. The insertion loss calculated in theory are 0.71dB and 1.35dB respectively, which has a significant improvement in coupling loss, compared with the 30dB coupling loss caused by direct butt -joint transmission.
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