Novel discrete frequency-phase modulated excitation waveform for enhanced depth resolvability of thermal wave radar

Abstract

Abstract Thermal wave radar (TWR) is a state-of-the-art non-destructive testing method, inspired by radio wave radar systems, in order to increase depth resolution and signal to noise ratio of optical infrared thermography through pulse compression. Analogue frequency modulation (i.e. frequency sweep) and Barker binary phase modulation are the two popular and widely researched pulse compression techniques in TWR among which Barker coding has shown the highest performance. This paper introduces a novel modulated waveform with variable discrete frequency-phase modulation (FPM) which distinctively enhances the depth resolvability of TWR compared to the existing techniques. The pulse compression quality and depth resolvability of the novel FPM waveform is initially evaluated through a 1D analytical solution. The analogue frequency modulated and discrete phase modulated waveforms as well as mono-frequency excitation (i.e. lock-in thermography) are also evaluated at the same central frequency as the reference. Objective functions are defined and a large search space is explored for optimal modulation codes. Two FPM waveforms are selected based on their maximized depth resolvability through resultant lag and phase in the output channel of TWR. Furthermore, the excellent performance of the selected FPM waveforms is validated by 3D finite element simulation. A delaminated glass fiber reinforced polymer (GFRP) laminate is simulated in order to evaluate the impact of a dominant lateral heat diffusion on the performance of the novel FPM waveforms. The superior depth resolvability of the introduced FPM waveforms is confirmed and their robustness at various noise levels is demonstrated.