As usual, members of this forum have taken the time to contribute valuable suggestions. Here was the original query:
"This is a legacy single oscillator cell Nd:YAG laser. All electronics seem to be in quite good operational shape. All optical components present do not show any obvious damage. There is an optical schematic on the internet:
We don't have the spatial filter components only. We aligned using HeNe lasers; the HeNe goes in and out on the center axis of the YAG rod. After alignment, we can get quite robust free lasing, ~150 mJ per shot. The problems come with Q switching. The Pockels cell was aligned with the HeNe to give a cross image through crossed polarizers; the Q switch timing was initially set to what should be set to a near optimum time based on observing the flash output with a photodiode. Q switched output does take place. However, instead of an expected circular burn, the burns are narrow vertical ellipses. It is the kind of burn pattern you might perhaps expect if the YAG rod had a horizontal rotation--yet the HeNe alignment, and good free lasing, do not seem to be consistent with that.
So, what's going on? Without the spatial filter there may be less mode specificity, but I don't see how that could cause something this extreme. What (silly) things are we doing or overlooking?"
Some possible suggested causes were:
1. Nd:YAG rod reversed 180 degrees
2. Q switch components (mainly Pockels cell) put in cavity only after free lasing alignment
3. Q switch delay time well off optimum
In this instance, #1 and #2 were not factors. (In fact, for this laser, if the Pockels cell is put in place only after alignment for free lasing, all lasing will be lost.) Factors such as #3 turned out to be a partial, though not majority, contributor. No care was made to optimize the Q switch timing and, further, the flashlamp voltage was intentionally turned down--both of these for "conservative operation" reasons. In fact, with optimum Q switch timing and higher lamp voltage, the burn pattern looked much better, although there was still slight vertical ellipticity.
The majority of the solution came from a cavity alignment that was NOT constrained by the factory tapped holes for fastening components in the laser head. A variety of alignments had been tried within the range of component placements possible using the tapped holes. This principally involved slight movements of the output coupler perpendicular to the alignment ray pathway as well as the bending mirror on the other side of the Nd:YAG cell. Though the alignment HeNe was always on axis in and out of the YAG rod, some burn ellipticity persisted for every one of these numerous alignments. Finally, I milled wider slots for the output coupler mount and the bending mirror mount, so that there was greater freedom in placing them. Once this was done all traces of the burn ellipticity problem disappeared.
Thanks again for all the helpful suggestions!
New Orleans, LA USA
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