Grating Coupler With Reduced Back Reflection Using λ/4 Offset at Its Grating Sub-Teeth

We demonstrate a grating coupler with reduced back reflection on silicon-on-insulator (SOI) platform. Each grating tooth of the grating coupler is uniformly divided into multiple sub-teeth with
λ
/4 offset between two adjacent sub-teeth, reducing the Fresnel reflection from interfaces between core and cladding materials in transmission direction. The back reflection can be theoretically suppressed from −12 dB to below −30 dB for a focusing grating coupler working at transverse magnetic (TM) polarization. Experimental results show that the effect of Fresnel reflection on transmission spectrum of the proposed grating coupler is significantly reduced compared to conventional grating coupler. Furthermore, a microring resonator using the proposed focusing grating coupler to couple light into and out of waveguide is fabricated. The measured spectra show that fluctuations on transmission spectra of the ring resonator are effectively suppressed, especially for the drop-port, a well-shaped resonance peak is clearly observed, making the resonance wavelength easily distinguishable, which is highly required in sensing applications using wavelength interrogation.