- Have been investigating 316 Hz noise in the control signal for the DOPO lock. Here is a list of some things that have been ruled out, mostly electrical:
- EOM power supply --> noise still present in DOPO transmission
- RFPD DC out --> no funky ground loops with scope (also looking at demod signal in different channel), noise still visible in transmission
- RFPD power supply --> noise still visible in transmission...
- Pump laser intensity (upstream pickoff) --> not a great test because pickoff optics are also on the optical table..
- 2 x SR560s --> No effect after bypassing
- Marconi --> same result as with anything in the loop after RFPD demod
- Things left to rule out:
- Fume hood exhaust fan ** highly suspected, my phone's own cheap-o microphone power spectrum shows peaks at 316.5 Hz (!) when near the exhaust fan
- NPRO temp controller fan --> phone audio spectrum shows line noise (60 Hz) mostly, and also 188 Hz... need to test further independently of the fume hood...
In ruling out the 6-axis translation mount on the DOPO cavity, I removed the PPKTP crystal + oven temporarily but still saw the noise. Since the resonator was no longer stable without the crystal, I needed to bring the mirrors closer and realign the output coupler from scratch.
Restored DOPO cavity with crystal, alignment. MM efficiency ~ 35%... still optimizable.