SNR enhancement for composite application using multiple Doppler vibrometers based laser ultrasonic propagation imager

•We propose a hardware-based signal-to-noise (SNR) enhancement technique using multiple Laser Doppler Vibrometers (MLDV) for the Ultrasonic Propagation Imager (UPI) system.•Experiment shows that the SNR enhancement is closed to theoretical prediction.•We use the UPI system for damage visualization of a 2 mm-thick carbon-fiber-reinforced-plastic (CFRP) wing box with impact damage and an 8 mm-thick plastic 3D-printed object with internal damage.•The UPI system with MLDV improves the detectability of damages due to the higher SNR of the measurement signals.

In recent years, the technology of using laser ultrasonic propagation imaging for damage visualization of composite structures were applied to real-world applications. Among many choices of sensor for the Ultrasonic Propagation Imager, the laser interferometry has several advantages: it is non-invasive, and portable, and with extraordinarily long-range measurement. However, the critical issue with interferometry sensing is its low signal-to-noise ratio (SNR), where the background noise can mask the damage-induced waves and making it impossible to identify the damages, especially in composite structures. In this paper, we propose a hardware-based SNR enhancement technique using multiple Laser Doppler Vibrometers (LDVs). The out-of-plane mode of ultrasonic signals are measured by multiple LDVs at a common sensing point and then averaged in real time. We showed that the SNR enhancement in experiments was consistent with the theoretical prediction, and also the test results showed a clear improvement for damage visualization of structures using Ultrasonic Wave Propagation Imaging and Ultrasonic Wavenumber Imaging algorithms.