V. COMPARISON OF ZnGeP 2 OPTICAL PARAMETRIC OSCILLATOR AND
5.4: Results and Discussion
5.4.3: Brightfield Imaging
energy falls below the 3 times threshold mark. Hopefully, this will show a trend that will provide more insight into this result to help determine whether the observed effect is a result of the macropulse duration or the micropulse structure of the two lasers.
While further experimental investigation is limited, it is possible to suggest a few possible mechanisms. Since the FEL macropulse is long compared to ablation times, it is possible that plume screening could play a role in the reduced efficiency seen by the FEL, as ejected material from ablation early on in the FEL macropulse could screen energy from the latter part of the macropulse from reaching the tissue and thus reducing the efficiency. The energy density of the OPO is also higher which could lead to a red- shifting of the spectrum of the tissue causing an increase in the relative absorption and thus an increase in the efficiency. In addition, by looking at the ablation event from a modeling perspective, if the blow off ablation model is applied to the OPO, in which all of the energy is essentially dumped into the system before the onset of ablation, and the steady state ablation model, in which the target tissue reaches the onset of ablation during laser irradiation and then continues to absorb energy at a constant rate during the remainder of the pulse, then we would expect to see ~50% deeper craters associated with the ZGP-OPO.
the OPO laser. Figure 5.6 shows the OPO comparison for three times the ablation threshold at 6.1 mm. Figure 5.7 and 5.8 shows the same comparison for 6.45 and 6.73 µm respectively.
The images show the classic ablation mechanism from the initial onset of the ablation plume through the expansion of the ablation plume and the subsequent collapse and recoil phases. The same mechanism is seen for the OPO and FEL at all three wavelengths. No differences are seen between the two laser sources with respect to the dynamics of plume formation and collapse except for the timescale. A small difference in the size of the plume is seen between the FEL and OPO due to the small difference in spot size. The size of the plume increases with increasing wavelength due to the increase in the penetration depth, as well as an increase in the energy delivered. A larger volume of water is being ablated leading to a larger ablation plume.
The one difference that can be seen as a result of the imaging is in the timing of the ablation event due to the difference in pulse duration. The onset of the ablation event due to the OPO pulse is seen at 1 µs after the onset of the laser pulse, which corresponds to 0.9 µs after the end of the laser pulse. In contrast, the onset for the FEL does not begin until 10 µs after the start of the laser pulse, which corresponds to 5 µs after the end of the laser pulse. Similarly, the largest ablation plume is seen at 5 µs (4.9 µs after the end of the pulse) for the OPO laser, while the largest plume is seen at 25 µs (20 µs after the end of the pulse) for the FEL. These results are consistent for all three wavelengths, indicating that it is a pulse duration dependent event and not a wavelength dependent one.
This finding reinforces the results of the crater depth measurements. The energy is being delivered much more quickly by the OPO when compared with the FEL which shows
Figure 5.6 The results of the bright-field (pump-probe) imaging are shown for 6.1 µm.
The scale bar represents 1 mm. The images presented were taken at the time intervals shown in microseconds after the start of the subsequent laser pulse. The top eight frames are of the FEL, while the bottom eight frames are of the OPO. A similar ablation mechanism can be seen for both lasers while the time course of the OPO begins much earlier when compare with the FEL. The spot size of the FEL was ~90 µm while the OPO was ~60 µm. The radiant exposure was 3 times threshold for the given combination of laser wavelength and pulse duration.
Figure 5.7 The results of the bright-field (pump-probe) imaging are shown for 6.45 µm.
The scale bar represents 1 mm. The images presented were taken at the time intervals shown in microseconds after the start of the subsequent laser pulse. The top eight frames are of the FEL, while the bottom eight frames are of the OPO. A similar ablation mechanism can be seen for both lasers while the time course of the OPO begins much earlier when compare with the FEL. The spot size of the FEL was ~90 µm while the OPO was ~60 µm. The size of the ablation plume and thus the amount of material ejected is increased relative to the 6.1 µm images (Figure 5.6) as expected due to the increased penetration depth and pulse energy at this wavelength.
Figure 5.8 The results of the bright-field (pump-probe) imaging are shown for 6.73 µm.
The scale bar represents 1 mm. The images presented were taken at the time intervals shown in microseconds after the start of the subsequent laser pulse. The top eight frames are of the FEL, while the bottom eight frames are of the OPO. A similar ablation mechanism can be seen for both lasers while the time course of the OPO begins much earlier when compare with the FEL. The spot size of the FEL was ~90 µm while the OPO was ~60 µm. The size of the ablation plume and thus the amount of material ejected is increased relative to the 6.1 and 6.45 µm images (Figure 5.6 and 5.7) as expected due to the increased penetration depth and pulse energy at this wavelength.
that the ablation process for the OPO is more energetic (5-10 times faster) which could lead to a much more efficient ablation process.