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All-fiber laser Doppler vibrometer (LDV) systems have great potential in the application of remote acoustic detection. However, the system performance is impaired due to the serious intermediate frequency (IF) crosstalk signal, which is caused by the fiber circulator. In this paper, a system adopting a partial-fiber structure is proposed and analyzed. Simulative and experimental analysis indicate that the IF crosstalk signal is mainly caused by the fiber circulator, and it damages demodulation signal. In order to reduce the IF crosstalk signal, a polarization prism is employed to substitute the circulator. Experimental results reveal that the IF crosstalk signal can be eliminated by the partial-fiber LDV effectively. The comprehensible speech signals within the range of 50m can be obtained.

At present, remote human signature detection plays an increasingly important role in the field of anti-terrorism and security defense all around the world. In order to acquire remote, non-cooperative human signature signals, a double mode (audio/video) surveillance system is developed. The system mainly consists of a Laser Doppler Vibrometer (LDV), a pan-tilt-zoom (PTZ) camera and a theodolite. The LDV is used to acquire remote audio by detecting the vibration of the object (caused by the acoustic pressure). The PTZ camera is used to capture the video of the human body, and track the body when he/she moves, then analyze the image to select a proper vibrating target for LDV measurements. The theodolite is applied to control the orientation of the LDV. For the reason that many noise sources disturb the LDV-measured signals, such as laser speckle noises, environmental noises and the noises caused by sensor moving, a kind of speech enhancement technology (OM-LSA algorithm) is used to improve the intelligibility of the noisy voice signals detected by the LDV system. Experiments results indicated that the SNR and MOS of the LDV speech signal (the range is 150 m) can be increased by 135% and 26% respectively by using the speech enhancement technology, and the remote speech and video signals (the range is 30 m) can be obtained by the double-mode surveillance system.

At present, laser hearing has played more and more important role in the field of anti-terrorism and security defense all around the world. In order to acquire remote voice, a Laser Doppler Vibrometer (LDV) is established, the voice signal demodulation method is based on the arctangent phase algorithm. On the basis of the system, a kind of speech enhancement technology is used to improve the intelligibility of the noisy voice signals detected by the LDV system. First, based on the heterodyne detection theory, the detection principle and method which acquire voice by detecting throat vibration is introduced. Then a kind of speech enhancement technology is used to improve the intelligibility of the noisy voice signals detected by the LDV system. Finally, to validate this system and speech enhancement technology, some experiments are performed and the results indicated that the comprehensible speech signals within the range of 75 m can be obtained by self-made LDV. On the other hand, the speech enhancement technology can improve the intelligibility of the noisy voice signals detected by the LDV system effectively.