Photonic sensor attached to a drone mapping temperature distribution. (Credit: Washington University)
Photonic sensor attached to a drone mapping temperature distribution. (Credit: Washington University)

A team of researchers at Washington University in St. Louis was the first to successfully record environmental data using a wireless photonic sensor resonator with a whispering-gallery-mode (WGM) architecture. The sensors recorded data during the spring of 2017 under two scenarios: one was a real-time measurement of air temperature over 12 hours, and the other was an aerial mapping of temperature distribution with a sensor mounted on a drone. Both measurements were accompanied by a commercial thermometer with a Bluetooth connection for comparison purposes. The data from the two compared very favorably.

In the world of the “Internet of Things” (IoT), there are vast numbers of spatially distributed wireless sensors predominately based on electronics. These devices are often hampered by electromagnetic interference, such as disturbed audio or visual signals caused by a low-flying airplane. But optical sensors are immune to electromagnetic interference. Wireless photonic sensor resonators with a whispering-gallery-mode (WGM) architecture have small footprints, extreme sensitivity, and a variety of possible functions. These sensors get their name because they work like the famous whispering gallery in St. Paul’s Cathedral in London, where someone on the one side of the dome can hear a message spoken to the wall by someone on the other side. Unlike the dome, which has resonances in the audible range, the sensor resonates at light frequencies and also at vibrational or mechanical frequencies. In contrast to existing table-sized lab equipment, the mainboard of the WGM sensor is a mere 127 millimeters by 67 millimeters — roughly 5 inches by 2.5 inches — and integrates the entire architecture of the sensor system. The sensor itself is made of glass, is the size of a human hair, and is connected to the mainboard by a single optical fiber. A laser light is used to probe the sensor. Light coupled out of the sensor is sent to a photodetector with a transmission amplifier. A processor controls peripherals such as the laser current drive, monitoring circuit, thermo-electric cooler, and Wi-Fi unit.

In the WGM, light propagates along the circular rim of a structure by constant internal reflection. Inside the circular rim, light rotates 1 million times. Over that space, light waves detect environmental changes, such as temperature and humidity, for example. The sensor node is monitored by a customized operating systems app that controls the remote system and collects and analyzes sensing signals.

Wireless sensors, whether electronic or photonic, can monitor such environmental factors as humidity, temperature, and air pressure. Applications for wireless sensors encompass environmental and health-care monitoring, precision agricultural practices, and smart cities data-gathering, among other possibilities.

The researchers had to address stability issues, which were handled by the customized operation systems app they developed and also miniaturization of the bulky laboratory measurement systems. They developed a smartphone app to control the sensing system over Wi-Fi. By connecting the sensor system to the Internet, they were able to realize real-time remote control of the system.

In June 2017, they mounted the system on the outside wall of a building, accumulated a plot of the frequency shift of the resonance, and compared their data with a commercial thermometer. The researchers also mounted their system on an unmanned drone alongside a commercial thermometer. When the drone flew from one measurement location to others, the resonance frequency of the WGM shifted in response to temperature variations. The measurements matched well with results from the commercial thermometer. There are numerous promising sensing applications possible with WGM technology, including magnetic, acoustic, environmental and medical sensing.