Chien-Ming, Yu-Hang,

We want to reduce the coherent noise coupled into the homodyne by reducing the intensity of CC beam entering the OPO. However, this action will cause the demodulation 14 MHz error signal (for coherent control of the pump beam phase) to become smaller. So, we tried to increase the gain of the photodetector at 14MHz by changing the capacitor CX and the amplifier resistors R1, R2, see Fig.1.

We let the capacitor CX become a plug-in base and test some combinations of capacitors. Currently we find the biggest signal at 14 MHz when using CX as 150pF+33pF. What is puzzling is that when we replace this combination with a single 180 pF capacitor, the signal is reduced by 8 dB on the spectrum analyzer. So, we are now using 150pF+33pF, and R1=500 ohms R2=5k ohms. The S/N now is improved 5 times more as shown in Fig.2.