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Figure 6. Interaction of laser light with polished surface( left) and ground surface( right), with I 0 = Input intensity, I T = transmitted intensity and I A = absorbed intensity.
surface roughness values between 1 nm and 2.18 lm Rq( Tab. 1).
A constant laser power of 500 W and an effective duration of 30 s was applied to the prepared samples and temperature variance and transmitted radiant power were then recorded.
4 Laser induced heating of fused silica
First, the measurement results from the contact thermometer are observed and visualized in dependency of the samples surface roughness( Fig. 5). The temperature increase due to laser radiation is considered. As the theoretically low absorption of the laser wavelength 1070 nm suggests, the polished sample with very low surface roughness experienced only a modest temperature increase of approximately ΔT = 30 K. In contrast, samples with higher surface roughness exhibited a significantly greater temperature increase, exceeding ΔT = 100 K.
A significant correlation between roughness and laser induced heating of the fused silica samples is observed. A temperature increase is apparent until a certain roughness is achieved, afterwards a saturation or slight decrease happens.
The experimentally determined temperature values and their functional behavior indicate that the hypothesis proposed in Chapter 2 could be fundamentally confirmed. The enhanced heating of rough glass surfaces by an infrared( IR) laser can be explained by the increased occurrence of multiple reflections within the microstructure of the surface. When the glass surface exhibits pronounced peaks and valleys due to higher roughness, the incident laser light is scattered and reflected multiple times within these structures.
This process effectively increases the optical path length of the laser light within the roughened region, allowing for more opportunities for energy absorption at each interaction with the surface. Since the absorption of laser energy depends on the number of photon-surface interactions, the multiple reflections lead to a substantial amplification of the overall absorbed energy, even if the intrinsic absorption coefficient of the glass material remains low.
Moreover, the altered surface topography enhances the probability of Fresnel absorption, which occurs at each reflection due to the change in refractive index between air and glass. This effect is particularly pronounced for light at oblique angles of incidence, which are more likely to occur within the irregular structure of a rough surface.
Combined, these factors lead to the observed increase in localized heating of the glass surface under IR laser irradiation.
This enhanced heating of rough glass surfaces by IR laser can be attributed to multiple reflections occurring within the peaks and valleys of the rougher surface, which substantially amplify the effective absorption of the laser energy( Fig. 6). In addition to the assumed processes of multiple reflections that lead to absorption, it should not go unmentioned that laser losses due to reflections and in particular scattering processes on the component surface are also to be expected. These can also represent a significant error influence for the absolute measured temperature values. This topic is an important aspect for future considerations.
The power measurement of the laser radiation transmitted through the fused silica samples is evaluated in the analogy to the thermometer measurement. The lower the transmitted laser power, the higher the laser absorption within the glass sample. Therefore, the resulting graph in Figure 7 is inversed compared to the contact temperature measurement.
The results of transmitted laser power generally support the observation of a roughness dependency. Again, a correlation between roughness and laser transmission of the fused silica samples is visible. The transmission decreases until a certain roughness is achieved, afterwards a saturation or slight increase can be seen.
5 Challenges in thermography measurements
Comparative measurements with the thermography camera showed significantly different temperature values with respect to the contact thermometer. Figure 8 illustrates the maximum temperature values from both devices for the same laser processed samples. In most cases the values gained by thermography are much too low compared to the contact thermometer. The biggest deviation of measured temperatures between contact thermometer and thermography in the experiments is found with a value of 70 K( sample roughness 2.2 lm).
Thermography clearly can be useful for qualitative evaluations of an areal temperature distribution, compared to the solely point application of a contact thermometer. However, the emission coefficient e is relevant for radiation-based temperature acquisition which can cause issues regarding quantitative evaluations. The value of e must