Analysis of narrow gap induced additional micro-ring loss

We have analyzed the coupling gap dependent micro-ring loss in a single ring all-pass filter configuration using the two dimension (2D) finite difference time domain (FDTD) and EIM (effective index method). We utilized a new analysis scheme by calculating the transmission signal as a function of input wavelength and fitting the transmission spectrum with a phenomenological ring loss parameter. This novel scheme circumvents the complex waveguide mode analysis process, when the coupling gap is small and the all-pass coupling region becomes multi-mode. We find that the radiation loss increases rapidly with decreasing coupling gap width when the gap reaches 200 nm or below. Our initial results show that the intrinsic bending loss of a silicon micro-ring (on oxide) with a radius of 2.5μm is about 3dB/cm. The total loss of the micro-ring with 150 nm coupling gap reaches 21.82dB/cm.