Laser Tuning and Measurement

Regen alignment

Regen tuned up by Endo-san (the Coherent engineer) and LC which gave a major improvement in regen output spatial quality (figure 1 and figure 2) and improved the energy output to ~19mJ.

Figure 1 : Regen spatial profile Oct 2011

Figure 2 : Regen spatial profile after tuning Dec 2011

The regen cavity was quite badly misaligned in October so in future if we adjust the regen for energy we need to monitor the spatial profile as well so the cavity doesn't walk away from good alignment.

Laser Energy

The beam was realigned through the rest of the laser (spatial filter, linear amps, SHG crystal). The energy after the linear amps and the crystal was higher than for running in Dec 2010. Endo-san didn't know why the energy had dropped then because there is nothing obviously wrong with the linear amps, so we think that the higher output energy is linked to the better spatial profile of the regen input to the linear amps extracting the gain more efficiently (figure 3).

Figure 3 : Green and ir energy Dec 2011

The conversion efficiency to the green is a bit less than before (24% at high energy, cf 36% measured previously) but this may be due to the different size and divergence of the ir beam.

Laser M2

The laser M2 was measured with a 1.5m focal length lens and gave M2 = 1.01 and 1.07 in the two axes (figure 4), which is an improvement on previous measurements. The beam is still strongly astigmatic though (because of the tilted etalon in the regen to stretch the pulse duration and avoid damaging the linear amps).

Figure 4 : Green M2 Dec 2011

There is visible damage on the etalon (new one ordered so this will be changed when the replacement arrives), linear amps, image rotator between the amps and the SHG crystal. However these have been noted since at least 2005 and possibly earlier so we have been taking data with these damaged components for years. The quality of the beam at the focus was good (figure 5).

Figure 5 : Green beam at 310mm in the M2 measurement