Long-term stability

The laser was designed for long-term stability on a daily lab basis instead of short living records. Stable mode-locked laser performance of several weeks and even months was required and achieved.

The results of a 16 hours over night test with constant pump power of 140 W are shown in Fig.7.5. The power fluctuates only about 0.3 W, corresponding to an outstanding small relative value of 0.07 % of the average output power. Nevertheless, in the zoom two effects are obvious: A long-periodic change and short frequent oscillations.

The output power shows a long-periodic modulation with a period time in the range of 1000 minutes. The power increase within the first 100 minutes is explained by the fact that the laser was aligned when warmed up whereas the power log was started when the system was still cool so that the resonator had to heat up again to reach its optimum configuration. After 200 minutes the power decreases slightly again to the start value and rises again after 800 minutes. This modulation might come from mini-mal temperature and humidity changes in the working atmosphere during the night

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Figure 7.4: Experimental results of SESAM mode-locked Yb:YAG thin-disk laser with 22 AMC passes. Left: Experimental data and sech²-fit of intensity autocorrelation (AC) of the shortest pulses corresponding to an output power of 44 W. Assuming a sech² pulse shape as it is for soliton mode-locking nearby transform limited pulses with a duration of 0.87 ps were achieved. Right: Corresponding laser spectrum (logarithmic scale) centered at 1030.0 nm with a spectral width of 1.3 nm.

when no people work in the lab. This long-term change is also represented in the beam walk. In the bottom panel of Fig. 7.5 the position of the laser spot 4 meters behind the output coupler is recorded. In horizontal as well as in vertical direction a general shift of the beam position is visible. The change in spot position is stronger during the heating up phase and weakens afterwards. However, the beam walk in vertical direction is approximately 0.15 mrad whereas in horizontal direction it is only 0.10 mrad. This indicates that this long-term change in power leads to a change of the beam position mainly in vertical position.

After 100 minutes a higher frequent oscillation with a period duration of nearly 30 minutes and a power amplitude of 0.1 W is visible. Since this power fluctuation begins just after the heat-up phase after 100 minutes and since the power oscillates abso-lutely regularly, it seems to be a feedback effect due to thermal misalignment of optical components. Pictures of a thermal camera have shown that especially the dispersive mirrors heat up due to their layer structure. Based on a resonator length of nearly 45 m minimal misalignment of the mirrors affects the beam pointing. A minimal change of the beam position leads to a minimal misalignment that results in a slight decrease of the intracavity power. Due to this power decrease the thermal load in the mirrors is reduced, too and the mirrors slightly change their position going back to the previ-ous configuration again. Since this configuration was optimized for highest power, the power increases again and the cycle starts again. This beam walk is also visible in the monitored spot position in the bottom panel of Fig. 7.5. The beam position shows a period duration of nearly 30 minutes corresponding to the high frequent oscillations in the output power. That confirms that the high frequent power oscillations may be due to a thermal misalignment and lead to a feedback loop.

7.3 Experimental results: Laser specifications with SESAM mode-locking

Figure 7.5: Long-term stability test over a duration of 1000 minutes with a constant pump power of 140 W. Top: Output power, the inset shows a zoom for better clarity of the high frequent oscillations. Bottom: Position of the laser spot 4 meters behind the output coupler. The inset shows a zoom for better clarity of the high frequent oscillations. In the inset the scaling of the two y-axes is different. The high frequent oscillations in the beam position and the output power exhibit a period duration of nearly 30 minutes. The oscillations in the output power may come from a feedback loop due to thermal misalignment as it is confirmed in the beam pointing log showing exactly the same oscillations. See text for more details.

Figure 7.6: Long-term stability test: Mode-locked pulse on the oscilloscope. More than 1.6 Mio sweeps measured over a time duration of 1000 minutes are plotted, ex-hibiting a fluctuation of the pulse shape of just 0.5 % standard deviation.

In the inset of Fig. 7.5 it is clearly visible that the high frequent fluctuations are stronger in the horizontal direction, where the position change is 1.5 mm, respectively 0.37 mrad, than in the vertical direction, where the walk-off is just 0.3 mm, respectively 0.07 mrad. This might indicate that the mirror mounts allow better for horizontal tilt-ing than for vertical tilttilt-ing.

Furthermore, the laser worked in mode-locked operation. A screen shot of the oscil-loscope transient in Fig. 7.6 confirms an excellent stability. The pictures shows more than 1.6 Mio sweeps with no single pulse missing measured over a time duration of 1000 minutes. The pulse shape exhibits a standard deviation of less than 0.5 %. This confirms the stability of the SESAM mode-locked Yb:YAG thin-disk laser.

A long-term power log of 45 days is plotted in Fig. 7.7. The laser was not realigned or re-optimized during this time. After ten days the laser had to be turned off for a few hours for construction work in the lab. It was powered up afterwards without any changes in the configuration. The sharp dips in the power curve at day 4 and after 30 days result from the fact that we split the laser beam into two beams, one for experiments, one for the power log. Hence, the power going into the power meter was slightly reduced. However, the stability of the output power is astonishing. Without any pump power stabilization, output power fluctuations in the range of just 1.5 W are observed, corresponding to a fluctuation of only 3.5 % within a time duration of 45 days. Stable mode-locked operation was controlled with an oscilloscope. No pulse instabilities occurred during this time.

The two long-term tests presented in this section show that the choice of the laser resonator design, the SESAM parameters, and the balance between GDD and SPM is appropriate for stable long-term use. It was even possible to run the laser nonstop for several months without any realignment.

7.3 Experimental results: Laser specifications with SESAM mode-locking

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Time (days)

Output Power (W)

Figure 7.7: Long-term stability test: Output power log over a duration of 45 days for a constant pump power of 140 W. During this time no alignment or any other changes were done. The sharp dips in the output power are explained in the text. After 10 days the laser war turned off for a few hours.

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The pulse train of the SESAM mode-locked Yb:YAG thin disk laser presented in Section 7.3 was used with a pulse duration of approximately 1 ps.

7.4 Experimental results: Laser specifications with