I have tried several different cavity designs but haven’t really been able to get the green output power past about 2.5 watts. I would expect better conversion if the rest of the math is working out: I should see about four watts for my bench setup.
The most likely issue here is that the beam and KTP are not perfectly phase-matched. KTP loses efficiency rapidly if the input beam phase is not matched to the ideal crystal rotation and temperature. How does this work? KTP has a different refractive index depending on beam polarization. What you want to do is rotate the crystal so a given part of the polarized beam hits each refractive index. This means the net refractive index in the crystal is variable depending on the angle the crystal is rotated. In addition this refractive index changes with temperature, so you need to hold the crystal at a constant temperature for a given rotation angle.
I do have a temperature controller on my crystal, but the temperature set point is done via a small potentiometer. Therefore while I can hold the temperature constant I can’t make minute changes to the temperature. This makes it impossible to really dial in the phase: normally you change the crystal angle to get the best conversion and then tweak the temperature to dial it in.
The TEC controller I’m using is a controller I designed as a prototype when building my laser. It has a digital interface, but I don’t have anything to plug into it. But, I have plenty of spare parts lying around and it only took me a few days of hardware and programming to get a simple display and some buttons working to control it. I can now control the temperature in .1ºC increments.
The results were revealing…and disappointing. The video below shows just how critical the temperature is. in it, I change the temperature of the KTP by just one degree C, and that change causes a 3x change in the beam power output.
Temperature sensitivity of KTP second harmonic generation.
The disappointing part of this is that even after I dialed in the temperature for maximum conversion I still can’t get over about 1.5 watts out of it.
I am also seeing an aggressive drop off in power. This could be caused by my pump cooling. I am cooling the pump but not really regulating it or cooling it aggressively. But this seems pretty extreme given vanadate’s broad absorption spectrum:
