Fiber Loop Ring-Down Refractive Index Sensor Based on High- $Q$ Photonic Crystal Cavity

This paper proposed a novel ultracompact refractive index sensor formed by a high-quality (Q) photonic crystal (PhC) cavity-based fiber loop ring-down. In our proposed device configuration, we insert one pair of two air holes into the linear waveguide as two reflectors, and a point defect was defined between the two reflectors to form a nanocavity. Since the output resonant spectrum was extremely sensitive to a small refractive index variance attributed to the medium infiltrated in the holes, the ring-down time as well as the radiation intensity would be a function of the relative refractive index for input light at certain frequency. To improve the system sensitivity, the Q -factor of nanocavity was optimized by adjusting the radii of all air holes and the spacing between the two reflectors. The properties of the refractive index sensor were analyzed and simulated using the finite-difference time-domain method. Results showed that a quasi-linear measurement of refractive index was achieved with sensitivity of 20.34 ms per refractive index unit and Q -factor of 605. In addition, the sensing range could be broadened to 1.330-1.400 by using multiple operating frequencies.