Wireless whispering-gallery-mode sensor for thermal sensing and aerial mapping

The Internet of Things (IoT)^1,2 employs a large number of spatially distributed wireless sensors to monitor physical environments, e.g., temperature, humidity, and air pressure, and has many applications, including environmental monitoring³, health care monitoring⁴, smart cities⁵, and precision agriculture. A wireless sensor can collect, analyze, and transmit measurements of its environment^1,2. Currently, wireless sensors used in the IoT are predominately based on electronic devices that may suffer from electromagnetic interference in many circumstances. Being immune to the electromagnetic interference, optical sensors provide a significant advantage in harsh environments⁶. Furthermore, by introducing optical resonance to enhance light-matter interactions, optical sensors based on resonators exhibit small footprints, extreme sensitivity, and versatile functionalities^7,8, which can significantly enhance the capability and flexibility of wireless sensors. Here we provide the first demonstration of a wireless photonic sensor node based on a whispering-gallery-mode (WGM) optical resonator, in which light propagates along the circular rim of such a structure like a sphere, a disk, or a toroid by continuous total internal reflection. The sensor node is controlled via a customized iOS app. Its performance was studied in two practical scenarios: (1) real-time measurement of the air temperature over 12 h and (2) aerial mapping of the temperature distribution using a sensor node mounted on an unmanned drone. Our work demonstrates the capability of WGM optical sensors in practical applications and may pave the way for the large-scale deployment of WGM sensors in the IoT.