Highly sensitive sensor based on an ultra-high-Q Mach-Zehnder interferometer-coupled microring

A high-Q Mach-Zehnder interferometer (MZI)-coupled microring is presented for optical sensing with high sensitivity and a large measurement range. By optimizing the length difference between the two arms of the MZI coupler, the MZI-coupled microring with a high-Q factor and high extinction ratio is obtained. In the present example, the Q factor of the designed silicon-nanowire-based microring is as high as 1.8×105 when the silicon nanowire has a propagation loss L=2 dB/cm. Due to this high-Q factor, the sensitivity for the change Δn of the effective refractive index is about 10−5-10−6 by measuring the shift of the resonant wavelength. Because of the wavelength dependence of the coupling ratio of the MZI coupler, it is possible to have only one resonant wavelength with a high extinction ratio in a very large wavelength span [i.e., the quasi-free spectral range of the MZI-coupler microring], which offers a very large measurement range covering the refractive index change of gas/liquid samples (e.g., 0≤Δn<0.48 in the present example).