Air-Suspended SU-8 Polymer Waveguide Grating Couplers

We present SU-8 polymer waveguide grating couplers for TE mode with enhanced refractive index contrast by employing air as upper and lower cladding layer. Numerical simulations predict a coupling loss of 4.9 dB, indicating a coupling efficiency of 32% at a maximum spectral response of 1550 nm. Based on a polymer lamination method, air-suspended waveguide grating couplers were successfully fabricated and characterized. Due to current limitations in the fabrication process, the perturbation of the experimental grating couplers is weaker than the original simulation. However, transmission measurements have shown that a single grating coupler exhibits approximately 8 dB coupling loss at a center wavelength of 1557 nm, indicating a coupling efficiency of 16% with respect to a single-mode optical fiber. The demonstrated parallel fabrication method employing the widely used SU-8 photoresist makes the polymer waveguide grating couplers very attractive for a wide range of low-cost polymer photonic applications.