I've been working on a large DPSS green for a couple of years and am trying to implement a design that uses passive Q-switching to increase the intracavity intensity for green conversion. The design I'm trying is a z-fold configuration where two concave mirrors form a focal point to provide the right intensity to the saturable absorber, a fairly large pump mode volume, and about a 2x reduction in area going into the KTP. I've been using the free software Rezonator to work out the cavity and the software predicts its stable down to about 30cm of thermal lensing in the vanadate crystal. Here is a diagram of the resonator design:
Attachment 60238
I have this lasing on my workbench in CW mode -- all the mirrors are in place but there is no Cr:YAG or KTP crystal in the cavity. The problem I'm having is there is a very narrow range of pump powers where it lases. If I turn up the pump power past this range lasing stops. I think this is thermal: If I start with the pump up I'll get lasing and then the beam will fade, flicker and finally go out within a few seconds. I don't have a TEC on the vanadate but I do have it in a copper block and the block doesn't get warm so I don't think adding a TEC will help. A thermal camera on the vanadate shows it's only about 80ºF. I think something about this cavity is magnifying the thermal lensing of the vanadate.
I've done some simple thermal lensing calculations and they show the vanadate should be nowhere close to the 30cm of lensing needed to lose stability. This is a pretty long cavity design (about 650mm) and there is some astigmatism caused by the beam hitting the concave mirrors at an angle (max about 10º). I don't know how much of a difference that makes (Rezonator shows the astigmatism actually decreases with more lensing).
I had a theory that maybe the lensing is causing the beam angle to change in the cavity and this is causing it to walk off the pump mode overlap. As a test I turned the pump up until the laser stopped, and then adjusted the X-axis of the pump and viola! I got the beam back. There is increased deflection as I increase the pump, and if I back the pump power off the deflection decreases so there is really only a narrow sweet spot of pump power where it works, but I can tune that spot for any power.
As lensing increases the focal point between the concave mirrors shifts to the left. I assume my alignment is not perfect enough in the vanadate and the beam is not completely parallel. But when I aligned this I used several 1mm pinholes and the beam seems to be going straight through the vanadate. I don't know how I could make this more precise.
Can anyone think of any other issues here that may be causing the beam angle to walk in the vanadate as the power is increased? Any idea how to eliminate this?
Thx,
Brian