Golay code modulation in low-power laser-ultrasound.

Area of Science:

• Physics
• Materials Science
• Signal Processing

Background:

• Standard techniques for impulse response recovery in modulated laser-ultrasonics often rely on coded sequences to improve signal-to-noise ratio (SNR).
• Existing methods provide improved SNR in the time domain but can be limited in frequency range specificity.

Purpose of the Study:

• To extend correlation-based detection for chirped waveforms, enhancing SNR within a well-defined frequency range.
• To apply this technique to modulated laser-ultrasonics for improved material characterization.

Main Methods:

• Modulating chirped waveforms with Golay codes for bandlimited carrier signal generation.
• Employing cross-correlation techniques to recover the system's response in the time domain with improved SNR.
• Analytical discussion of SNR improvement, showing proportionality to the square root of sequence length.

Main Results:

• Demonstrated experimental application in laser-ultrasound using modulated laser diodes on a 50μm plate with MHz range Lamb waves.
• Achieved experimental SNR improvements of approximately 1.83 (7 to 9 bits) and 2.17 (9 to 11 bits) Golay codes, closely matching theoretical predictions.
• Validated the technique's ability to recover waveforms for evaluating dispersion relations using multiple measurement points.

Conclusions:

• The developed correlation-based technique effectively enhances SNR for chirped waveforms in modulated laser-ultrasonics.
• The method offers a robust approach for investigating thin materials and layered structures via Lamb wave analysis.
• Recovered waveforms are suitable for characterizing materials by evaluating dispersion relations through inverse problems.