Hadamard multiplexing in laser ultrasonics

Area of Science:

• Materials Science
• Optics and Photonics
• Non-Destructive Testing

Background:

• State-of-the-art laser ultrasonics (LU) faces signal-to-noise ratio (SNR) limitations due to detected laser radiation's shot noise.
• Current SNR improvement methods involve time-consuming signal averaging or potentially damaging high laser intensities.

Purpose of the Study:

• To investigate the application of Hadamard multiplexing to enhance SNR in laser ultrasonics for non-contact material inspection.
• To evaluate the effectiveness of Hadamard multiplexing in overcoming shot-noise limitations in LU.

Main Methods:

• Utilized 31-element Hadamard masks to modulate the spatial intensity distribution of the generation laser beam.
• Applied Hadamard multiplexing to both generation and detection laser beams in pulse-echo LU configurations.
• Explored the use of synthetic aperture focusing technique (SAFT) with multiplexed beams.

Main Results:

• Achieved a measured SNR improvement by a factor of 2.8, consistent with the theoretical Fellgett advantage.
• Demonstrated SNR enhancement in shot-noise-limited measurements, a unique benefit for LU.
• Showcased the potential for Hadamard multiplexing of both beams to enable SAFT in pulse-echo LU.

Conclusions:

• Hadamard multiplexing offers a viable strategy to significantly improve SNR in laser ultrasonics without detrimental side effects.
• This technique provides a novel approach for more sensitive and efficient non-contact ultrasonic material inspection.
• The combined multiplexing of generation and detection beams opens avenues for advanced imaging techniques like SAFT.