Being a theorist, most of time you are facing a difficult problem, so basically you know that’s gonna take a long time to find the solution. Being a experimentist, the problem is that you have no idea how long a simple task will take.
At the end of last week, i found out the frequency of repumper laser (driving the P1/2 transition) is off by 0.04nm. For the fibre laser we are using, it’s not a hard problem. I just need to reduce the temperature of the fibre by 6 degree. Sounds easy, but when i actually did that, my whole LBO cavity screwed. Before i can easily get more than 240mW, now i tried two days and still can only get 70mW.
The reason i can think of is thermal lensing. At low power, i can couple about 90% of infra-red light into the cavity, but at high power, i can barely reach 50%. That becauses inside the cavity, the higher power produces heat which will change the index of refraction of the crystal. The weird thing is this never happened before in my cavity. Why suddenly it becomes an issue?
After about a week of struggling and trying different things, i finally figured out how to bring the power back: just translate the output coupler away from the crystal by about 1/2 turn, which is only about 0.1mm. My power jumped to 200mW. Man, this is just experimental physics. I just changed the wavelength by 0.04nm, which is only 0.04/560=0.007%. Being a experimentist, you have to be aware those tiny changes.
Update: just notice there is a quote from Quotes of the Day:
“In theory, there is no difference between theory and practice; In practice, there is.”
- Chuck Reid
