Ultra-thin fibre microphone monitors power grid systems

Researchers from Shanghai University have fabricated a hair-thin microphone made entirely of silica fibre that can detect a range of ultrasound frequencies beyond the reach of the human ear. Able to withstand temperatures up to 1000°C, the device could be used inside high-voltage transformers to detect early signs of failure before power outages occur.

Xiaobei Zhang, a researcher from Shanghai University, said conventional electronic sensors often fail under thermal stress or suffer from severe signal interference. “Our all-fibre microphone can survive in hazardous environments and is completely immune to electromagnetic interference while remaining sensitive enough to hear the subtle early warning signals of equipment failure,” Zhang said.

In the journal Optics Express, the researchers describe their new microphone, which is sensitive to frequencies from 40 kHz to 1.6 MHz. Unlike traditional microphones that rely on bulky housing, the new microphone is integrated within a fibre just 125 microns in diameter.

“Our all-fibre microphone can be placed directly inside voltage transformers to listen for tiny internal electrical sparks in real time, preventing blackouts or explosions and keeping our power supply safe,” Zhang said. “The microphone’s incredible durability and wide hearing range make it ideal for everything from industrial testing and medical imaging to monitoring aerospace engines and providing early natural disaster alerts.”

The researchers focused on detecting partial discharge inside high-voltage transformers, a type of small electrical fault that can signal equipment problems before wide power-grid failures occur. Detecting these signals directly inside the equipment is difficult with current sensors due to extreme heat and strong electromagnetic interference, making reliable monitoring a challenge for power systems.

To solve this challenge, the researchers developed a fibre-based microphone based on the photoelastic effect. This effect can be used to detect mechanical changes — like vibrations — that alter a light’s refractive index.

They developed a unique sound-sensing design that uses a vibration-sensitive membrane and an internal glass micro-beam suspended inside a single-mode optical fibre. Together, these components form a Fabry-Pérot interferometer that can detect extremely small vibrations, including the sparks produced by electrical discharges.

To sculpt the suspended structure deep within the hair-thin fibre, the researchers used picosecond laser-induced chemical etching, an advanced technique that can be used to create precise micro- and nanostructures.

“The entire interferometric structure is integrated directly within a hair-thin fibre,” Zhang said. “This self-packaged monolithic design enables seamless deployment in high-temperature and space-constrained environments without needing any additional protection.”

Looking ahead, the researchers plan to use a multi-laser additive and subtractive manufacturing platform, combining silica 3D printing with ultrafast laser micromachining, to create robust, all-silica packaging that will enhance both the sensing and mechanical performance of the microphone. This will make it possible to install the microphone inside real-world industrial equipment, like running power transformers, and to survive long-term in extreme conditions.

Image caption: The hair-thin microphone can detect a large range of ultrasounds and withstand temperatures up to 1000°C. It features a vibration-sensitive membrane and an internal glass micro-beam that is suspended inside a single-mode optical fibre. Image credit: Xiaobei Zhang, Shanghai University