Cost-effective single-etched TM-mode SOI grating couplers for broadband perfectly vertical coupling

In this paper we present a uniform fully-etched TM-mode grating coupler for vertical coupling of light into SOI photonic integrated circuits and a chirped alternative to increase its bandwidth by using the same fabrication steps and maintaining its coupling efficiency. The first design refers to a uniform grating consisting of 22 periods with 670 nm period length, exhibiting 5.6 dB coupling losses at 1564 nm and a 3dB bandwidth of 32 nm. The 3dB bandwidth is extended from 32 to 76 nm by adding a chirped section at the front end of the uniform section in the second design. The ultra-wideband coupler can be used across all C-band as well as in S and L bands, it is realized at no expense of fabrication complexity while coupling efficiency is maintained. The coupling efficiency can be improved if the grating gap is decreased below 80 nm yet increasing fabrication resolution requirements. Theoretical and experimental analysis is presented for the coupling efficiency versus structure period and gap width while angle alignment tolerance is also investigated.