A scientific research team from Shanghai University recently developed an ultra-thin microphone made entirely of silica optical fiber. Its diameter is as thin as a hair, but it can detect a wide range of ultrasonic signals far beyond the upper limit of human hearing, and can continue to work in environments up to 1,000 degrees Celsius. Because the entire device uses a glass optical structure instead of traditional electronic components, it can still maintain stable performance in extreme high temperatures and strong electromagnetic interference environments. It is regarded as an important attempt to break through the limitations of traditional sensors under extreme working conditions.
The research team said that a long-term target application of this all-fiber microphone is to be placed directly inside a high-voltage transformer to "listen" for weak acoustic signals of early equipment failures and issue early warnings before the problem evolves into a large-scale power outage or explosion. One of the authors of the paper, Zhang Xiaobei from Shanghai University, pointed out that traditional electronic sensors are prone to failure in high-temperature environments and are highly susceptible to interference in strong electromagnetic fields. However, the new fiber-optic microphone can survive in dangerous environments and is sensitive enough to capture subtle signals in the budding stage of equipment failure.
The research results have been published in Optics Express, a journal of Optica Publishing Group. This microphone can respond to a frequency range from 40 kilohertz to 1.6 megahertz, far beyond the audible range of the human ear. The entire sensing structure is "encapsulated" inside a single-mode optical fiber with a diameter of only 125 microns, eliminating the need to rely on a bulky external housing like traditional microphones. Researchers said that the all-fiber design allows it to be deployed directly inside industrial equipment with limited space and harsh environments to achieve real-time monitoring.
In terms of specific application ideas, the team focused on research on partial discharge signal monitoring in high-voltage transformers. Partial discharge is a type of tiny electrical fault that can act as a "harbinger" before a large-scale failure occurs. However, due to the high temperature and strong electromagnetic noise inside the transformer, it is difficult for traditional sensors to accurately capture these signals while the equipment is running. To solve this problem, researchers used the photoelastic effect in optical fibers—the phenomenon in which mechanical vibrations trigger small changes in the refractive index of light in the optical fiber—to construct an acoustic detection solution entirely based on light.
This fiber optic microphone integrates a sophisticated sensing structure, including a vibration-sensitive microdiaphragm and glass microbeams suspended inside a single-mode fiber. The two together form a high-precision Fabry-Perot interferometer structure for measuring extremely small vibrations. To create a suspended structure inside such a narrow optical fiber, the research team used a picosecond laser-induced chemical etching process. This advanced manufacturing technology can precisely process micro- and nanoscale structures inside solid materials.
In order to verify its performance in extreme environments, the researchers tested the microphone in a furnace at 1,000 degrees Celsius for 100 minutes. The results showed that its structure and signal transmission remained stable. Experiments also show that the sensor has reliable acoustic response within an ultra-wide frequency band of 40 kilohertz to 1.6 megahertz, and can work normally in different media such as air and underwater, showing its adaptability in a variety of application scenarios.
Zhang Xiaobei pointed out that the entire interference structure is integrated into a "hair-thin" optical fiber, allowing this self-encapsulated monolithic design to be directly deployed in high-temperature, space-limited environments without the need for additional protective shells. The team predicts that this microphone is expected to be used in fields such as online monitoring of high-voltage power equipment status, industrial non-destructive testing, medical imaging, aerospace engine monitoring, and natural disaster early warning, providing earlier and more accurate acoustic diagnosis methods for critical infrastructure.
Looking to the future, the research team plans to further integrate acoustic functional modules into the device to continue to improve the sensitivity limit. They also plan to use a multi-laser composite additive and subtractive manufacturing platform to combine silicon dioxide 3D printing with ultra-fast laser micromachining to create a stronger integrated all-silicon packaging structure, thereby further improving the mechanical strength and sensing performance of the microphone. Researchers say this will pave the way for its long-term installation and application in real industrial environments, especially inside operating power transformers, making this type of fiber optic microphone truly a reliable "stethoscope" in extreme environments.