We now have green light at the Y end.
The set-up (with careful instructions from Kiwamu) - setting up with 100mW of IR into the oven.
Input IR power = 100mW measured.
Output green power = 0.11mW
(after using 2 IR mirrors to dump IR light before the power meter so losing a bit of green there light too)
And it's pretty circular-looking too. Think there might be a bit more efficiency to be gained near the edges of the crystal with internal reflections and suchlike things but that gives us an UGLY looking beam. Note - the polarisation is wrong for the crystal orientation so used a lambda/2 plate to get best green power out.
Efficiency is therefore 0.11/100 = 0.0011 (0.11%) at 100mW input power.
Temperature of the oven seems to be around 35.5degC for optimal conversion.
Took a picture. Ta-dah! Green light, and lots more where that came from! Well... about 3x more IR available anyway.