Participants: Eleonora, Matteo L, Yuefan

We did a preliminary try to implement the dithering technique to keep the beam direction aligned with the cavity axis by acting on BS pitch and yaw.

We started with yaw:

1) We inject a sine perturbation in BS yaw with frequency 10 Hz and amplitude 3mV.

2) We acquired the transmitter green power in labview using one of the spare channel of the "telescope" ADC board.

3) We demodulated it by multipling it for a sine with the same frenquency and filtered it with a lowpass (butterworth 4th order, cutoff frequency 1 Hz)

4) We filtered the error signal with another lowpass (butterworth 1th order with cut off frequency at 0.01 Hz and adjustable gain).

5) We summed the correction signal to the "manual offset" in the yaw local control loop which is usually set by hand during the manual alignement procedure.

In Pic. 1 there is an "explained" scheme of the labview frontpanel, in Pic. 2 there is the block diagram of the vi.

The attached video shows the effect of the loop when we change the manual offset of BS yaw. The starting position is 0.02. We change it to 0.01 and to 0.

(See Pic.1 for a reference of the different controls and graph shown) 

It seems that the loop is somehow able to bring back the offset to a position which makes the transmitted power less sensitive to the modulation. We need to check the long term performances and implement the same loop on the other degree of freedom.

DEMODULATION PHASE ISSUE

We tried to adjust the demodulation frequency by adding a tunable phase difference between the signal sent to the BS and the one used for the demodulation. With the loop open, we tried to change the demodulation phase in order to maximize the error signal but we couldn't see any change. We suspect that there is a problem with the reset of the subvi used to generate the sine wave. We might have found a solution that we will try soon. Anyway for the moment the demodulation phase is not optimized.

Link to the video in mp4 format.

https://drive.google.com/file/d/178Y6unT0S023VQCVl7pdYEdYgbTta22U/view?usp=sharing