Grating couplers in polymer with a thin Si3N4 layer embedded

Polymer has been considered to be an ideal material option for integrated photonics devices. To measure these devices, normally the route of horizontal coupling is chosen to couple the light into or out of the polymer waveguide. Due to the relatively low refractive index, implementing the surface grating coupler in this material system remains to be a challenge. In this paper, we present a polymer based surface grating coupler. Rather than expensive CMOS fabrication, the device is fabricated through a simple and fast UV based soft imprint technique utilizing self-developed low loss polymer material. The coupling efficiency is enhanced by embedding a thin Si3N4 layer between the waveguide core and under cladding layer. Around -19.8dB insertion loss from single-mode fiber (SMF) to single-mode fiber is obtained for a straight waveguide with grating coupler at each end. If collected with multi-mode fiber (MMF), it can be reduced to around -17.3dB. The 3dB bandwidth is 32nm centered at 1550nm. The proposed surface grating coupler and its easy fabrication method would be attractive for practical applications.