Aim: We made some measurements with the quadrants to check if both RF and DC are working fine

For testing the quadrant, we took a beam from the main laser with a beam splitter (BS1). Scheme in figure 4.

At the first tried, we just put another beam splitter (BS2) after the first one, then EOM in one of the path with a modulation frequency of 23MHz. One lens was put between BS1 and BS2 in order to have the beam waist inside the EOM. After combining two pathes with BS3, we put a filter with OD=1 to avoid injecting too much power on the quadrant. Then we place the quadrant with the beam more or less centered.

But we could not see any beats from the RF output of the quadrant. We thought one possible reason could be the phase modulator makes 2 sidebands (one above and one below the light frequency), both of them interfere with the non-modulated beam. Probably two beat signals cancel each other almost perfectly because the beam path is the same (there is no such thing as a cavity that can treat the two sidebands differently).

So we decided to add the AOM on the non-modulated path with modulation frequency of 80MHz. With two beams of similar size and well aligned on the quadrant, we could see the 80MHz beat in the spectrum. 

Then we demodulated the RF signal with the box. By checking the in-phase(I) and quadrature(Q) of one of quadrant output , we could see that not only the power between I and Q is changing, but also the total power (both signals are moving up and down together).

Since our oscilloscope was not able to calculate the total power of two signals, we decided to put the second quadrant at the other output of BS3 and accquired one group of data while we knocked on the bench. The raw data we got show in figure 1. From this figure, we confirmed the quadrants are working fine. The I and Q are perfectly out of phase for two quadrants. The fact that the I and Q signals of the same QPD do not have the same peak-to-peak amplitude could be explained by a path length change of less than one wavelength.

Then Martin did some analysis of the data.  In figure 2, he calculated the I^2+Q^2 for both quadrants, and also the sum of them. The amplitude of two quadrants have more or less each others inverted signal. The fluctuation in the sum is smaller than that of the individual amplitudes. Hence the total power is almost conserved. The remaining fluctuation in the sum could be due to unequal beat signal amplitude (for instance due to a difference in ND filter, or a difference in overlap of the beams on the different QPDs).

In figure 3, the phase of both quadrants are moving in the same way, it could be caused by the path length difference before BS3, so it has same effect for both the quadrants. But there is this factor of 2 difference in the phase changing, which we didn't really understand. Part of it could be caused by the small DC offset of the raw signal.

After testing the RF signal of the quadrants, we also tested the DC with the galvo. We simply put the galvo in front of one of the quadrant (see the bench picture in figure 5), and did some rough alignment to make sure that the beam is inside the diode. Then by sending four quadrant DC outputs to the galvo controlling board, two outputs of this board will be used to control the x and y direction of the galvo. Then we checked the x and y error signal through the monitor port of this board. As long as the galvo loop is closed, the galvo is able to bring the error signal back to 0, which means the beam is centered on the quadrant.

Conclusion: With two quadrants, we got figure 1 that confirm both of them are working as we expected. For the DC, the galvo is able to center the beam on the quadrant. Now everything has been removed from the bench and packed. We will send them to NAOJ today or latest next Monday.