Laser and Locking troubleshooting
The laser system is functioning in a non-optimal mode due to issues with the fiber amplifier. The normal laser setup and locking procedure would be:
- Turn on seed laser
- Turn on fiber amplifier at 5 W
- Optimize PPLN temperature
- Lock cavity
In the current operating mode, it is clear that the light coming out of the fiber amplifier is much more sensitive to the seed laser conditions. One symptom of this is that the green power depends a great deal on the seed temperature. In addition, the coupling into the cavity does as well. Here is a rough procedure that seems to work "OK".
- Turn on seed laser (set power to ADJ+5 - you will change this later).
- Turn on fiber amp at 5 W.
- Wait for the power to ramp up and stabilize (could take more than an hour). Should get to 900 mW or so.
- Turn on the seed laser ramp - observe the power fluctuations. If they are very large, play with the seed laser power (maybe lower it?) and the PPLN temperature. There may be a different optimum PPLN temperature at each seed power.
- Be careful with the seed power - if too large, you will horrendous noise on PDR photodiode.
- Once you have minimized the power variation with seed temperature, try locking and cycling. If it works fairly reliably, you are done. If not, play with seed power and PPLN temperature some more, and hope you find a stable running condition.
- In some cases, I have seen the power droop gradually over time. I have fixed this sometimes by nudging the seed power a little higher. Other times, I have tweaked the PPLN temperature.
- "Resting" the fiber amplifier seems to help for some reason - if you just can't find a stable operating mode, consider turning the whole system off, and waiting a day or so.