Advanced frequency or/and phase modulated excitation waveforms for enhanced depth resolvability of thermal wave radar

In this paper, a detailed investigation of the depth resolvability performance of thermal wave radar (TWR) in the inspection of composites through advanced frequency or/and phase modulated excitation waveforms is presented. In addition to conventional linear frequency modulated waveform (LFM) and Baker phase modulated waveform, two advanced excitation waveforms, i.e., orthogonal phase-coded linear frequency modulated waveform (OPCLFM) and nonlinear frequency modulated waveform (NLFM), are introduced to enhance depth resolvability of TWR. To evaluate fairly their performance, all aforementioned excitation waveforms are formulated based on the same central frequency. Moreover, both phase contrast and delay time contrast are obtained to quantitatively characterize the depth resolvability of these excitation waveforms by a one-dimensional photothermal diffusion wave model of carbon fiber reinforced polymer (CFRP) laminate. Finally, we demonstrate that a proper window function can further significantly enhance the depth resolvability of TWR.