Design, fabrication and characterization of SU-8 and PMMA grating couplers

Grating couplers utilize light diffraction to achieve vertical coupling between waveguides and optical fibers. We proposed using SU-8 or polymethyl methacrylate (PMMA) polymer materials as the waveguide core layer and polydimethylsiloxane (PDMS) as the cladding layer for the grating coupler and completed the fabrication of both uniform and convergent grating couplers. We studied the effect of waveguide layer thickness, etching depth, and grating period on the uniformity of grating coupling performance. The influence of the convergence angle on the coupling performance of the convergence grating is investigated, and the optimal parameters are finally determined. The experimental results show that the coupling efficiency of the SU-8 uniform grating coupler reaches 30.1% and the 3 dB bandwidth is 70 nm when the grating period length is 4μm. For the SU-8 convergent grating coupler, the coupling efficiency reaches 40.0%, and the bandwidth is 65 nm when the convergence angle is 20°. The coupling efficiency of the PMMA grating coupler is lower than that of the SU-8 grating coupler; the coupling efficiency reaches 29.0% when the grating period of the uniform grating coupler is 4μm and 35.1% when the convergence angle of the convergent grating coupler is 20°.