Researchers at Washington University in St. Louis have used a wireless photonic sensor with a whispering-gallery-mode (WGM) architecture to record environmental data. The photonic sensors recorded air temperature in real time, achieving results comparable to those achieved by a commercial thermometer with a Bluetooth connection. The miniature WGM-based sensors could provide the IoT with highly sensitive, flexible sensing technology.

The node in the photonic sensor network is monitored by a customized iOS application that controls the remote system and collects and analyzes sensing signals. The iOS application can also monitor the spectral properties of the sensors in real time. The architecture of the sensor node consists of a sensing module, a microcontroller, a Wi-Fi unit, and a power supply.

“We developed a smartphone app to control the sensing system over Wi-Fi,” professor Lan Yang said. “By connecting the sensor system to the internet, we can realize real-time remote control of the system.”

The team was able to miniaturize bulky laboratory measurement systems. The mainboard of the WGM sensor, which integrates the system’s architecture, is just 127 x 67 mm (about 5 x 2.5 in.). The sensor, which is made of glass and is the size of one human hair, is connected to the mainboard by a single optical fiber.

“Thanks to their small size, the capability and flexibility of wireless photonic sensors can be improved by making them mobile,” Yang said.

In the sensor, light propagates along the circular rim of a structure like a sphere via continuous, total internal reflection, and light rotates inside the circular rim. Light waves detect environmental changes, such as changes in temperature and humidity.

A laser 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 including the laser current drive, monitoring circuit, thermoelectric cooler, and Wi-Fi unit.

The photonic sensors recorded data during the spring of 2017 under two scenarios: one was a real-time measurement of air temperature over a 12-hour period, and the other was an aerial mapping of temperature distribution with a sensor mounted on a drone in a St. Louis city park. When the drone flew from one measurement location to others, the resonance frequency of the WGM sensor shifted in response to temperature variations.

“The measurements matched well with results from the commercial thermometer,” Yang said. “The successful demonstrations show the potential applications of our wireless WGM sensor in the IoT. There are numerous promising sensing applications possible with WGM technology, including magnetic, acoustic, environmental, and medical sensing.”

Wireless sensors based on electronics can be hampered by electromagnetic interference, such as audio or visual signals caused by low-flying planes. Yang said that in contrast, optical sensors are immune to electromagnetic interference, and can therefore provide an advantage over electronic sensors in harsh environments.

“Optical sensors based on resonators show small footprints, extreme sensitivity and a number of functionalities, all of which lend capability and flexibility to wireless sensors,” Yang said. “Our work could pave the way to large-scale application of WGM sensors throughout the internet.”

The research was published in Light Science & Applications (doi:10.1038/s41377-018-0063-4).

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A team led by Lan Yang, professor of engineering and applied science at Washington University in St. Louis, has made two successful measurements using a photonics-based wireless sensor. The sensor, which belongs to a category called whispering-gallery-mode resonators, is not subject to electromagnetic interference. Photonic sensors are smaller and generally more flexible than the currently electronics-based technology. Courtesy of Washington University in St. Louis.

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