The attached figure shows the dark noise and shot noise of CCFC error signal. The CCFC error signals were measured on 20210622 and the dark noise was measured on 20220218. Note that the DC port of CCFC PD should be terminated with 50Ohm for lower CCFC dark noise. This dark noise was measured by blocking the light before CCFC PD. This dark noise includes the noises of PD, RF amplifier, and mixer.

Estimation of shot noise

Measured CCSB power at CCFC PD when CCSB are on resonance: 0.8 uW

Typical efficiency of InGaAs photodiode at 1064 nm: 0.45 A/W

According to Yuhang's simulation, transimpedance of PD at 14MHz is 5900 V/A

Shot noise at 14MHz before amplification: sqrt(2*A*q)*R = sqrt(2*0.45*0.8e-6*1.6e-19)*5900 = 2e-9 V/rtHz

Shot noise at 14MHz after amplification: 2e-9*10^(34/20) = 1e-7 V/rtHz

Shot noise of CCFC error signal = amplitude of CCFC error signal*(shot nosie at 14MHz after amplification/amplitude of 14 MHz signal) = (154mVpp/2)*(1e-7 V/rtHz / 0.045 V) = 1.7e-7 V/rtHz 

where the amplitude of CCFC error signal and 14MHz signal (-14dBm = 0.045 V) are measured values.

I tried to measure the shot noise directly by injecting BAB to CCFC PD. The injected BAB power was 73uW. However, the shot noise could not be measured since the dark noise was still limiting.