Sensitive detection of remote vibration owns promising potential applications such as military reconnaissance, geological exploration, structural health monitoring and other fields. Nevertheless, how to detect remote vibration information with high sensitivity and anti-disturbance has become a major challenge. At present, various vibration measurement methods such as electrical, imaging, and traditional heterodyne interferometry are difficult to simultaneously possess characteristics of high light intensity sensitivity, long working distance and anti-disturbance of ambient light. Laser feedback interferometry technology can amplify the weak light signal returned from a distance through secondary stimulated radiation in the laser cavity, and it belongs to coherent measurement, so it is expected to achieve non-contact detection of remote vibration with high sensitivity.

However, the traditional laser feedback system has the problem of parasitic noise interference in remote measurement. In the laser feedback system, the output laser needs to be modulated by the frequency shifter and shaped by other optical elements such as the lens group before it irradiates the target. The surfaces of these components will also generate weak reflected light signals, which are returned to the laser cavity and amplified by the feedback effect. These signals generated in addition to the target reflection are called parasitic noise. When the intensity of the target reflection signal is low, the truly useful signal will be submerged in the parasitic noise, which greatly limits the actual detection ability of the laser feedback system for weak signals.

In order to solve the parasitic noise problem and realize remote vibration measurement, researchers from the State Key Laboratory of Precision Measurement Technology and Instruments of Tsinghua University present a new polarization-modulated laser frequency-shifted feedback interferometry measurement method, which can effectively suppress the intensity of parasitic noise without affecting the useful signal, and thus obtain remote vibration information. This method can directly measure the vibration of non-cooperative targets at 300 m, and can further realize voice reconstruction at 300 m based on the vibration information. The relevant research results were recently published in Photonics Research, Volume 12, Issue 9, 2024, titled "Ultrasensitive detection of remote acoustic vibrations at 300 m distance by optical feedback enhancement".[ Mingwang Tian, Xin Xu, Sihong Chen, Zhipeng Feng, Yidong Tan, "Ultrasensitive detection of remote acoustic vibrations at 300 m distance by optical feedback enhancement," Photonics Res. 12, 1962 (2024)]

Figure 1 Schematic diagram of the structure of the laser feedback remote vibration measurement system. L1: convex lens; BS: beam splitter; FR: Faraday rotator; HWP: half-wave plate; AOM1, 2: acoustic-optic modulator; AP: aperture; L2: concave lens; L3: convex lens; QWP: quarter-wave plate; T: target; PD: photodiode.

In the proposed polarization-modulated laser feedback system, the parasitic light reflected by the surface of the acousto-optic frequency shifter crystal is modulated by the Faraday rotator and the polarization state when it returns to the laser cavity is orthogonal to the light field in the resonator, so it cannot be amplified by the feedback effect; while the signal light reflected from the target is modulated by the Faraday rotator and the quarter-wave plate. When it returns to the laser cavity, the polarization direction is consistent with the light field in the cavity, so it can be amplified by the feedback effect and output again. After the output laser is received by the detector and photoelectrically converted, the vibration information carried in it can be demodulated. The experimental results show that the working distance of the proposed vibration measurement system can reach 300 m, the amplitude response sensitivity at 300 m can reach 0.72 nm/Hz^1/2 at 1 kHz, and the vibration frequency measurement error does not exceed 0.1%. In addition, the system achieves long-distance voice reconstruction based on vibration measurement. It clearly restores the test voice "Is this your handbag?" at 80 m, 240 m, and 300 m, respectively. The voice files restored at different distances can be correctly recognized by the mature audio-text conversion website (NetEase Jianwai), proving that the voice restored by the system is understandable and distinguishable. The system has the ability to reconstruct voice at long distance.

Figure 2 The corresponding spectrograms of the restored voice signals obtained from an ordinary carton at distances of 80 m, 240 m, and 300 m, respectively. (a) 80 m. (b) 240 m. (c) 300 m.

Professor Yidong Tan, the corresponding author, commented: This work achieves a non-contact, highly sensitive vibration measurement with anti-interference characteristic, providing new insights for research in this field. The proposed polarization-modulated method successfully suppresses the parasitic noise in the laser feedback system and realizes the coherent detection of weak signals of non-cooperative targets at 300 m. Furthermore, the proposed system realizes long-distance voice reconstruction, which is expected to be applied in military reconnaissance, public security and other fields.

The team's subsequent work will focus on the instrumentation innovation of the proposed principle system to enhance its flexibility and operability in practical applications. In addition, spectrum analysis technology research will be carried out on the basis of vibration measurement to achieve accurate identification and analysis of vibration characteristics of long-distance targets, broadening the breadth and depth of the technology applications.