Full-Range Detection in Cascaded Microring Sensors Using Thermooptical Tuning

We present detailed analysis and experimental demonstration of full-range detection in cascaded microring sensors by using thermooptical tuning. A cascaded microring sensor translates spectrum shifts into intensity changes eliminating the need for expensive spectral analysis equipment for sensing applications. Without tuning capability, this configuration provides high sensitivity, but a small detection range since the intensity output decays toward zero for low overlap between spectra of individual microrings. We overcome this limitation by using thermooptic tuning in one of the microrings for extending the detection range to span an entire free spectral range of the probing microring. Additionally, we analyze the impact of resonance linewidths and differences in free spectral ranges (FSRs) of microrings on the widths of output intensity peaks. With full FSR thermooptical tuning, we demonstrate a total detection range of 0.0241 refractive index units (RIU) and a limit of detection of 4.6 x 10(-5) RIU for microrings with 100-mu m radii. Photodetector-based signal readout combined with CMOS compatible manufacturability makes this device design attractive as a silicon-based optical sensing platform.