Rousseau Guy, Blouin Alain
National Research Council Canada, 75 de Mortagne Boulevard, Boucherville, QC, J4B 6Y4, Canada.
Opt Express. 2012 Nov 5;20(23):25798-816. doi: 10.1364/OE.20.025798.
In state-of-the-art laser ultrasonics (LU), the signal-to-noise ratio (SNR) is limited by the shot noise of the detected laser radiation. Further improving the SNR then requires averaging multiple signals or increasing generation and/or detection laser intensities. The former strategy is time consuming and the latter leads to surface damages. For signal-independent limiting noises, Hadamard multiplexing increases the SNR by averaging multiple signals in parallel using a single detector. Here we consider the use of Hadamard multiplexing in LU for the non-contact ultrasonic inspection of materials. By using 31 element Hadamard masks to modulate the spatial intensity distribution of the generation laser beam, the measured SNR is improved by a factor 2.8, in good agreement with the expected multiplexing or Fellgett advantage. In contrast to many other applications of Hadamard multiplexing, the SNR is improved for shot-noise-limited measurements since the shot noise level is independent of the signal in LU. The Hadamard multiplexing of the detection laser beam is also considered but can only lead to a throughput or Jacquinot advantage. However, for pulse-echo LU, the Hadamard multiplexing of both generation and detection laser beams allows using the synthetic aperture focusing technique (SAFT).
在最先进的激光超声技术（LU）中，信噪比（SNR）受检测到的激光辐射散粒噪声的限制。进一步提高信噪比则需要对多个信号进行平均，或者提高激光产生和/或检测的强度。前一种策略耗时，而后一种会导致表面损伤。对于与信号无关的限制噪声，哈达玛复用通过使用单个探测器并行平均多个信号来提高信噪比。在此，我们考虑在激光超声技术中使用哈达玛复用进行材料的非接触超声检测。通过使用31元哈达玛掩模调制产生激光束的空间强度分布，测量得到的信噪比提高了2.8倍，与预期的复用或费尔盖特优势相符。与哈达玛复用的许多其他应用不同，由于散粒噪声水平与激光超声技术中的信号无关，所以对于散粒噪声限制的测量，信噪比得到了提高。还考虑了检测激光束的哈达玛复用，但这只会带来通量或雅克诺优势。然而，对于脉冲回波激光超声技术，产生和检测激光束的哈达玛复用允许使用合成孔径聚焦技术（SAFT）。
