Figure 2 XCT data and density segmentation revealing cracks (red) and air bubbles (blue) inside the coated sample B.
2.4 | Optical coherence tomography systems
Two in-house-built spectral-domain OCT systems were used for imaging. The first is based on a commercial, near-infrared SC source (SuperK Extreme EXR-9, NKT Photonics) filtered around the 1.3 μm wavelength range (1.07-1.48 μm). The system is able to achieve an axial (depth) resolution of around 3 µm divided by the refractive index (n ) of the sample, and a lateral resolution of 6 µm using a galvanometric scanning system. The system operates at a line-scan (A-scan) rate of 76 kHz with a signal sensitivity of 89 dB (amount of attenuation required to achieve a signal-to-noise ratio of one) for 4 mW average power on the sample. For more details on the system, see Ref. [12].
The second OCT system is illustrated in Figure 3(a). It is based on an in-house built SC source that covers from 1-4.5 μm, a Michelson interferometer, a galvanometric scanning system, and an upconversion spectrometer (see ref. [21] for details). The SC output is filtered to obtain a spectrum covering 3.5-4.5 μm with around 20 mW average power on the sample. The scanning beam is focused onto the samples using a 30 mm BaF2 lens, resulting in a lateral resolution of ~30 µm. Due to the wide bandwidth of the laser, the system achieves a high axial resolution of ~8 µm (divided by the refractive index n ). The line rate of the system depends on the chosen integration time of the spectrometer, which influences the maximum imaging depth as shown in Figure 3(b). The images show a near-surface cavity in sample B with some debris inside the cavity. It is evident that the imaging depth is drastically improved when increasing the integration time from 300-1800 μs, but only a minor improvement is obtained by doubling the integration time to 3600 μs. This indicates that it is close to the penetration limit of the sample, given the properties of the material and the sensitivity roll-off of the OCT system [22]. For the best imaging in these highly scattering coatings, the integration time was therefore set to 3600 μs, corresponding to a line rate of 280 Hz. A total of 400 lines were captured during a 3 mm scan, providing a cross sectional image (B-scan) roughly every 1.5 seconds with a spatial oversampling of 4. It should be noted that the depth information in OCT is obtained from the difference in optical path distance (OPD) of the sample signal relative to the stationary reference mirror path in air. Due to the refractive index of the sample, the measured OPD (and therefore the scale bar) is longer than the physical distance by a factor of n .