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

Abstract

The broad-frequency response and nanometer-range displacements of ultrasound detection are essential for the characterization of small cracks, structural health monitoring and non-destructive evaluation. Those perturbations are generated at sub-nano-strain to nano-strain levels. This corresponds to the sub-nm level and, therefore, to about 0.1% of wavelength change at 1550 nm, making it difficult to detect them by conventional interferometric techniques. In this paper, we propose a demodulation system to read the random fiber grating spectrum using a self-heterodyne acousto-optic frequency comb. The system uses a self-heterodyne approach to extract phase and amplitude modulated signals to detect surface acoustic waves with sub-nanometer amplitudes in the frequency domain. The method can detect acoustic frequencies of 1 MHz and the associated displacement. The system is calibrated via phase detection with a heterodyne interferometer, which has a limited frequency response of up to 200 kHz. The goal is to achieve sub-nanometer strain detection at MHz frequency with random fiber gratings.

Review of Random Fiber Lasers for Optical Fiber Sensors

“…RFGA was introduced into the loop by a circulator. Similarly, RFGA-based ultrasonic testing had a high resolution, which was calibrated by combining self-heterodyne acousto-optic frequency comb interrogation technology and a heterodyne interferometer [87], as shown in Figure 18. An AOM generated an acousto-optic comb, which was amplified by an EDFA, and the comb was injected into RFGA through a circulator.…”

Microsphere Coupled Off-Core Fiber Sensor for Ultrasound Sensing

“…For ultrasound detection, Fan et al detail that the importance of high reflectivity, relatively large contrast, and linewidth would benefit the high frequency ultrasound sensor response [ 28 ]. Because the displacement of ultrasound sensing at MHz can be as small as sub-um [ 29 ], which is impossible to be detected with interferometers due to the diffraction limit of the laser wavelength, to increase the detection sensitivity for intensity detection, the sensor must operate at a quadrature point where relative intensity change versus wavelength shifts due to the displacement of the ultrasound signal, defined as intensity slope. To increase the sensitivity of the ultrasound sensing at the highest frequency, the intensity slope should be at its maximum.…”