Laser-induced sound (LGS) technology is an effective method for achieving node-free communication from air to underwater environments. This paper introduces a frequency-shift keying (FSK) modulation method to enhance the stability of LGS communication. By controlling the time intervals between laser pulses to generate laser pulses with different repetition frequencies, sound signals are produced underwater based on the thermal expansion mechanism. At the receiving end, decoding is achieved by leveraging the energy differences in laser-induced sound signals of varying repetition frequencies. Experimental results demonstrate that the FSK modulation method provides superior communication performance. Compared to the traditional on-off keying (OOK) modulation method, FSK modulation effectively prevents decoding errors caused by interference between pulses, thereby enhancing the stability and reliability of communication.
The work presented in this paper focuses on a novel method that is laser-induced ultrasonic (LIU) imaging detection based on interferometric synthetic aperture focusing technology (InSAFT). The approach adopted is combing interferometry technology with synthetic aperture focusing technology (SAFT). A set of numerical simulations to test the validity of LIUInSAFT imaging detection were performed by the finite element method (FEM). The results of the simulation indicate that the height of target determined by LIU-InSAFT agrees well with that of setting value in the mode of simulation. All the preliminary results throw light on the nature of LIU-InSAFT applied to height detection or inner roughness of inner surface in materials.
Laser-EMAT (electromagnetic acoustic transducer) technology has the advantages of laser induced ultrasound and
EMAT simultaneously. This paper introduced a novel simulation about laser-EMAT testing the surface crack with
various depth in the aluminum block. A finite element model of laser-EMAT detection was set up for investigating the
influence of groove-type crack on ultrasonic propagation in the way of numerical simulation. Then, the response curve of
voltage in time domain was obtained by the testing coil in EMAT, which is proposed to determine the depth of crack
above. Good coupling could be found between voltage of signal received by EMAT coil and amplitude of ultrasound
generated by the laser. In addition, the snapshot of ultrasound field at different time demonstrates mode conversion
occurs when the surface wave propagated through the crack. The simulation results show the relative error of
determining crack depth by the proposed method is less than 6.5%.
Laser-induced acoustic (LIA) technology is of significance for the communication from the air platform to the
underwater platform. This paper describes a method of nonlinear LIA in the liquid based on the mechanism of optical
breakdown. Acoustic signals with high energy were generated by using a Nd: YAG pulsed laser with 10 ns width. A PZT
hydrophone was used to receive the LIA signals for cases of the linear and nonlinear regime in the water. Meanwhile,
simulation of LIA in a distance of 400 m between laser spot on the water surface and the hydrophone was performed by
the COMSOL Multiphysic software. The simulated results show the communicated ability of LIA technology in the long
distance.
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