Eleonora and Yuhang

To have an idea about how much amplification is required for demodulated signals from QPD, we compared the QPD demodulated signal with DGS and network analyzer.

The test is done by taking signal from QPD2 segment1 with green beam reflected form the locked filter cavity. The beam was centered with galvo on the QPD and power was ~1.5mW

In the first attached figure, the in-phase demodulated signal (measured in DGS/network analyzer) is shown together with DGS ADC noise. We  see that the measurement in DGS and network analyzer agree with each other. However, in the measurement of DGS, the peaks are barely higher than the ADC noise.

In the second plot, there is a comparison of demodulated in-phase/in-quadrature signals.

The large peak at 88 Hz is due to galvo loop, we adjusted the gain of galvo  servo and it is reduced.