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 .