All-fiber low-frequency shifter based on acousto-optic interaction and its heterodyne vibration response

We demonstrate two all-fiber low-frequency shift schemes based on the acousto-optic interaction in a few-mode fiber (FMF). Two acoustically induced fiber gratings (AlFGs) are cascaded in reverse to achieve an efficient cycle conversion between LP11 and LP01 core modes in the FMF while obtaining a frequency shift of 1.8 MHz. In addition, a long-period fiber grating (LPFG) is employed to replace the AIM, which achieves a lower frequency shift of 0.9 MHz, and its tunable wavelength range exceeds 100 nm. Both schemes show the characteristics of an upward frequency shift. Moreover, we also present a heterodyne detection system based on the above frequency shift schemes, which is verified in response to micro-vibration signals ranging from tens to hundreds of kilohertz, as well as speech signals in a lower frequency range. The experimental results show that these all-fiber frequency shift schemes have potential applications, such as in fiber optic hydrophones, laser speech detection, and fiber optic sensors. (C) 2022 Optica Publishing Group

Automated summary

This article demonstrates two all-fiber low-frequency shift schemes based on acousto-optic interaction in a few-mode fiber (FMF). The schemes achieve efficient conversion between LP11 and LP01 core modes in the FMF, resulting in frequency shifts. Additionally, a heterodyne detection system based on these schemes is proposed and validated for the detection of micro-vibration and speech signals.