Consignas de identificación estudiantil

iments on Kerr-lens mode-locking of the more-broadband gain medium Yb:Lu2O3. The

designed resonator was of convex-concave type similar to the resonator described in sec.4.2 with an identical focusing arrangement consisting of R1=R2=300 mm concave mirrors and having similar misalignment sensitivity. The pump head was aligned for 24 passes, providing a pump spot 2.8 mm in diameter on the Yb:Lu2O3 disk. The initial (unpumped)

disk curvature was _∼2 m. We used VBG-stabilized pump diodes lasing at 976 nm with a FWHM emission bandwidth of _∼2 nm for efficient pumping (see Fig.2.2).

4.5.1

Technical obstacles

Four disks were available for these experiments (from the group of Prof. Huber). All of them were soldered to metal heat sinks. Two disks with a large diameter of 10 mm were from the same boule and two smaller-diameter disks were from a different boule, grown at a somewhat earlier time.

Yb:Lu2O3 disks with 10-mm diameter

Preliminary diagnosticvi _{of the 10-mm disks in a microscope with cross-polarizers showed}

crystal inhomogeneities for both disks. In Fig.4.19 both Yb:Lu2O3 disks are shown for

comparison. One of the disks was edge-bevelled after soldering . This resulted in small

homogeneous region

(a) (b)

Figure 4.19: Cross-polarized microscope images of two 10-mm-diameter Yb:Lu2O3

disks.

4.5 Kerr-lens mode-locking of Yb:Lu2O3 oscillator 69

damage of the disk near the edge. This disk was destroyed under moderate pumping of below 50 W.

The second large disk was tested in a multi-mode CW cavity and showed no lasing. Just replacing the output coupler by an HR mirror could allow lasing in the multi-mode regime, which at least indicates high losses introduced by the disk. Moreover, partial darkening of the pump spot was observed. In particular, at increased pump powers (>100 W) part of the pump spot became dark, which completely distorted the laser performance. The disk could withstand up to 200-W pump power, exhibiting this darkening effect. Better performance could not be reached by changing the pump spot position on the disk.

Yb:Lu2O3 disks with 4-mm diameter

The disks 4 mm in diameter showed substantially better performance in comparison with the larger disks. Thus, in the multi-mode regime the optical-to-optical efficiency of 50% could easily be reached while pumping with 200 W. Nevertheless, another technical problem appeared: the pump spot diameter on the disk could not be reduced below 2.8 mm. This is due to the relatively large 1-mm-diameter coupling fibre of the pump diodes and constraints on the size of the collimating optics. Moreover, the disks had parts of the solder at the edges, which absorbed fluorescence light and heated up the disk. The temperature rise over 50 ◦C between the pump spot and the disk edges is plainly visible in Fig.4.20. Operation

(a) (b)

Figure 4.20: The temperature gradient of 4-mm-diameter Yb:Lu2O3 disks under

the 100 W pump with no lasing. (a): 1 % at. disk; (b): Zoomed pump spot region of 3 % at. disk.

of both disks under these conditions is risky, because it may lead to disk damage and induces strong thermal disk distortions, which degrade the oscillator performance in the fundamental mode regime.

4.5.2

Results

A 1-mm-thick glass plate was chosen as Kerr medium and introduced a round-trip disper- sion of -10000 fs2_{. By gradually increasing the distance d between mirrorsR1 andR2, the}

resonator was brought to the stability edge. A hard-aperture was installed between mirror

R2 and the focus. No SESAM was used for this experiment in order to completely elim- inate uncertainties from its curvature and thermal lens. By pushing mirror R1 mounted to the translation stage the oscillator could be routinely mode-locked. The oscillator was running with rather poor mode quality and strong beam pointing fluctuations were induced by the strong thermal aberrations in the disk. Neither the CW nor the ML regime showed much difference in terms of these fluctuations. In this mode of operation an output power of 7.5 W could be achieved with 3.5% OC and _∼ 130 W pump power . The pulses are rather long with _∼430 fs duration.

1 2 3 4 0 1 2 3 4 5 6 7 8 9 Intensity (a.u.) Time (ps) 7,5 W 7,5 W sech2-fit 1030 1035 1040 0,0 0,2 0,4 0,6 0,8 1,0 Inte ns ity (a .u. ) Wavelength (nm) 7,5 W 7,5 W sech2-fit

(a)       (b)

Δλ≈2.5nm

Figure 4.21: Yb:Lu2O3 oscillator characteristics operating with 1-mm-thick fused-

silica plate, 3.5% OC and hard-aperture. (a): Optical spectrum at 7.5 W with CW component; (b): Autocorrelation trace at 7.5 W.

4.5.3

Conclusion

The preliminary results on the mode-locking of the Yb:Lu2O3 oscillator are rather moder-

ate. Nevertheless, KLM could be realized even with the strong thermal problems from the disk and consequently poor mode quality and large beam pointing fluctuations. Histori- cally, this is the second thin-disk oscillator, mode-locked via a Kerr lens. Decreasing the pump spot size on the disk or working with larger disk diameters will dramatically improve