Daniel, Nutsinee
TTFSS
Instead of a phase-frequency discriminator we now have a IQ demod inside the new TTFSS. It's working, but not quite as it should. The input plus/minus switch needs fixing, UFG looks miserable (33kHz zero crossing, used to be 300kHz and can be pushed to 500kHz), need more gain. We are now lock our pump laser to the OPO error signal with this TTFSS (S1812013).
Here's a transfer function taken at the common path.
Pump refl PD saturation
Looking at the RF mon signal (signal before going to demod board), not only we saw 80MHz and 160MHz (2 omega) but also
35.3MHz, 44.4MHz, 70.6MHz, 115.4MHz, and 199.9MHz
35.3 MHz is likely our 35.5MHz modulation frequency for the SHG (I used peak search button on the Agilent not sure how accurate the frequency is). Though we didn't expect it to be as high as the 80MHz. From my note we were driving the EOM with 1.5Vpp, the EOM modulation depth spec is 15mrad/V. So total is 22.5 mrad/2 ~ 0.01 modulation depth. This is Newport 4004. Could it be that our carrier is so large that .01 of that is equal to 80MHz sb magnitude? SHG linewidth for green is 30MHz. 35.5MHz will mostly reflects back and goes to OPO. SHG is locked on transmitted red.
Here attached a photo from Agilent screen, left is when the cavity is unlocked (but hanging on resonance), and right is when it's locked. This was taken with ND1 filter in front of the PD. RF mon has 23 dB attenuation, ND1 counts for 20 dB. So before when we weren't locked at the peak the PD used to have -30+23+20 = 13dBm of 80MHz.
Don't know where the rest of the lines came from. They're not that small either. All of these probably contributed to our refl PD saturation problem.
These RF levels cannot explain the observed "saturation" and the required locking offset at higher intensity. Looking back at the sweeps without the ND filter, the signal actually get squashed at higher power. The next thing to check is the size of the beam on the PD. A beam that is way too small could locally saturate the charge carriers.
The observed modulation strength of the 35.5 MHz sidebands on the green light is about a factor 10 higher than expected. It should be much smaller than the 80 MHz modulation index.
With the ND1 filter in front of the OPO REFL PD, the TTFSS doesn't have enough common gain. This explains some of the funny transfer function. The ugf needs to be well above 100 kHz for the EOM/PZT crossover to work correctly.