Design and angular robustness test of chirped periodically poled lithium niobate crystal for 1064 nm second-harmonic generation experiment

Abstract

Ultrasonic phase changes carry critical information regarding tissue structures, and phase weighting can enhance the sharpness of ultrasonic images. Addressed here are challenges, such as the faint scattering echoes from folds, substantial noise interference, and the lengthy processing time involved with time-domain corrected imaging. This paper proposes a frequency-domain coherent imaging method based on the coherence factor of the phase imaginary part. Firstly, the phase information in the wavefield signal is extracted, and then the phase imaginary part matrix is extracted using Circular Statistics. Subsequent construction of the Phase Imaginary Coherence Factor (PICF) involves multiplying this matrix with the original frequency-domain matrix used in Phase Shift Migration (PSM) imaging. By incorporating the PICF into phase migration imaging and adjusting the PICF of the migrating wavefield at each layer, fibre texture information can be efficiently recovered by frequency domain signal multiplication. This paper applied the technique to carbon-glass fiber composite boards with an 18mm thickness. The experimental outcomes reveal that without PICF weighting, phase migration imaging loses the fiber layout information beyond a 10mm depth and fails to detect deeper region defects. PICF-weighted PSM imaging identifies three fibre folds with depths of 11 mm, 15 mm and 16 mm, respectively. This approach improves the imaging clarity and textural detail of the fold defects and maintains the detection error of the fold angles to about 10%. The imaging time is only 1.5 seconds, which is at least 8.67 times more computationally efficient than time-domain TFM imaging.