Laser remote speech detection technology

Laser remote speech detection technology
Laser remote speech detection: Revealing the structure of the detection system

A thin laser beam dances gracefully through the air, silently searching for distant sounds, the principle behind this futuristic technological “magic” is strictly esoteric and full of charm. Today, let’s lift the veil on this amazing technology and explore its wonderful structure and principles. The principle of laser remote voice detection is shown in Figure 1(a). The laser remote voice detection system is composed of laser vibration measurement system and non-cooperative vibration measurement target. According to the detection mode of light return, the detection system can be divided into non-interference type and interference type, and the schematic diagram is respectively shown in Figure 1(b) and (c).

FIG. 1 (a) Block diagram of laser remote voice detection; (b) Schematic diagram of non-interferometric laser remote vibration measurement system; (c) Principle diagram of interferometric laser remote vibration measurement system

一. Non-interference detection system Non-interference detection is a very straightforward character of friends, through the laser irradiation of the target surface, with the oblique motion of the reflected light azimuth modulation resulting in changes in the receiving end of the light intensity or speckle image to directly measure the target surface micro-vibration, and then “straight to straight” to achieve remote acoustic signal detection. According to the structure of the receiving photodetector, the non-interference system can be divided into single point type and array type. The core of the single-point structure is the “reconstruction of the acoustic signal”, that is, the surface vibration of the object is measured by measuring the change of the detector’s detection light intensity caused by the change of the return light orientation. The single-point structure has the advantages of low cost, simple structure, high sampling rate and real-time reconstruction of acoustic signal according to the feedback of detector photocurrent, but the laser speckle effect will destroy the linear relationship between vibration and detector light intensity, so it restricts the application of single-point non-interference detection system. The array structure reconstructs the surface vibration of the target through the speckle image processing algorithm, so that the vibration measurement system has a strong adaptability to the rough surface, and has higher accuracy and sensitivity.

二. The interference detection system is different from the non-interference detection bluntness, interference detection has a more indirect charm, the principle is through the laser irradiation of the surface of the target, the target surface along the optical axis of the displacement to the back light introduces the phase/frequency change, the use of interference technology to measure the frequency shift/phase shift to achieve remote micro-vibration measurement. At present, the more advanced interferometric detection technology can be divided into two kinds according to the principle of laser Doppler vibration measurement technology and laser self-mixing interference method based on remote acoustic signal detection. Laser Doppler vibration measurement method is based on the Doppler effect of laser to detect sound signal by measuring the Doppler frequency shift caused by the vibration of the surface of the target object. The laser self-mixing interferometry technology measures the displacement, speed, vibration and distance of the target by allowing a part of the reflected light of the distant target to re-enter the laser resonator and cause the modulation of the laser field amplitude and frequency. Its advantages lie in the small size and high sensitivity of the vibration measurement system, and the low power laser can be used to detect the remote sound signal. A frequency-shift laser self-mixing measurement system for remote speech signal detection is shown in Figure 2.

FIG. 2 Schematic diagram of frequency-shift laser self-mixing measurement system

As a useful and efficient technical means, laser “magic” play remote speech can not only in the field of detection, in the field of counter-detection also has excellent performance and wide application – laser interception countermeasure technology. This technology can achieve 100-meter level interception countermeasures in indoor, office buildings and other glass curtain wall places, and a single device can effectively protect a conference room with a window area of 15 square meters, in addition to the rapid response speed of scanning and positioning within 10 seconds, high positioning accuracy of more than 90% recognition rate, and high reliability for long-term stable work. Laser interception countermeasure technology can provide a strong guarantee for users’ acoustic information security in key industry offices and other scenarios.