Acousto-Optic Comb Interrogation System for Random Fiber Grating Sensors with Sub-nm Resolution.

Area of Science:

• Photonics and Optical Sensing
• Materials Science and Engineering
• Acoustics and Ultrasonics

Background:

• Ultrasound detection of nanometer-range displacements is crucial for characterizing small cracks and structural health monitoring.
• Conventional interferometric techniques struggle to detect sub-nano-strain to nano-strain perturbations (sub-nm displacements).
• These perturbations represent a ~0.1% wavelength change at 1550 nm, posing detection challenges.

Purpose of the Study:

• To propose and demonstrate a novel demodulation system for detecting sub-nanometer surface acoustic waves (SAWs).
• To enable precise ultrasound detection in the MHz frequency range for non-destructive evaluation (NDE) applications.
• To overcome the limitations of conventional interferometric techniques for high-frequency, low-amplitude displacement sensing.

Main Methods:

• Development of a demodulation system utilizing a self-heterodyne acousto-optic frequency comb.
• Employing a self-heterodyne approach to extract phase and amplitude modulated signals from random fiber grating spectra.
• Calibration of the system using phase detection with a heterodyne interferometer (limited to 200 kHz).

Main Results:

• Successful detection of surface acoustic waves with sub-nanometer amplitudes in the frequency domain.
• Demonstrated capability to detect acoustic frequencies up to 1 MHz and associated displacements.
• Established a pathway for achieving sub-nanometer strain detection at MHz frequencies with random fiber gratings.

Conclusions:

• The proposed self-heterodyne acousto-optic frequency comb system offers a promising solution for high-sensitivity ultrasound detection.
• This technology can significantly advance structural health monitoring and non-destructive evaluation capabilities.
• Further development aims to achieve sub-nanometer strain detection at MHz frequencies, enhancing NDE precision.