[Aritomi, Yuhang, Matteo]
We found that we still had saturation problem of CCFC RF and LO so we reduced them.
Then we measured CCFC error signal with different CCFC demodulation phase (Pic. 1). AOM FM freq is 300 mHz and deviation is 2kHz, so AOM scan speed for IR is 4kHz/(5/3 s)/2 = 1.2 kHz/s. CCFC amplitude for normalization is 28 mV. The calibration factor of CCFC error signal is determined by fitting the blue curve around 0, which is 1191 Hz/V.
We locked CCFC with 70 deg and 250 deg CCFC demodulation phase (both are I phase, but sign is opposite) and compared the locking accuracy with CCFC lock (Pic. 2). We found that CCFC locking accuracy with 250 deg is smaller than 70 deg above 1kHz. Changing CCFC demod by 180 deg means that CCSB on resonance and off resonance are swapped. CCSB noise on resonance is filtered out by cavity pole while the noise of other CCSB is not. If noise of upper/lower CCSB are different, this noise difference can happen.
IR filter is 500 gain and 30 Hz low pass filter.
Anyway now CCFC locking accuracy is below 1Hz if the calibration factor is correct. Strange thing is that locking accuracy above 10kHz is much better than BAB locking accuracy with green lock.
12dB attenuator was added for RF signal (before the 32dB amplifier)
12dB attenuation was applied to LO signal (DAC current control was reduced from 1/2(-12dBm) to 1/8(-24dBm))
Current RF amp: -15dBm
Current LO amp: -24dBm
I fitted the measured CCFC error signal by fitting the CC detuning (with respect to carrier), demodulation phase, and starting time (first plot). In this plot, misalignment effect is not considered.
In the second plot, I added the misalignment effect in theoretical curve by fixing the mode matching to 94%.