Ultrahigh-Resolution Ratio-Metric Wavelength Monitors Based on Tunable Microrings on Silicon

An ultrahigh-resolution ratio-metric wavelength monitor based on microring resonators (MRRs) is demonstrated on silicon. The theoretical wavelength resolution is related to the functional wavelength range and the quality (Q)-factor of the microring. We analyze the relationship and experimentally demonstrate that the functional range and the resolution can be adjusted by thermally tuning the resonance spacing of the MRRs. The resolution is also limited by the noise introduced in the measurements. An ultrahigh experimental resolution of 1.5 pm is obtained within a 0.72 nm functional range and an ultrahigh theoretical extreme resolution of ~0.4$ pm can be expected considering of the intrinsic systems noise only. The causes of the difference between the experimental and theoretical resolution and the measures to reduce the difference are also discussed.