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AdV-COM (AdV commissioning (1st part) )
vardaro - 19:46 Wednesday 27 November 2019 (47808) Print this report
In february we measured anti-squeezing from 35Hz, in May from 55Hz. Can this effect be related to the photodiodes flicker noise?

I'm performing new analysis on the data taken in February and in May during the shifts in which we changed the injected squeezing level into the ITF by changing the green pump power. The descripiton of the activity performed during these shifts can be found in the entries #44846 and #45918.

If we plot the spectrum of hrec with different pump power level (i.e. different level of injected squeezing) we can see that in February (fig 1) we measured anti-squeezing from 35 Hz whereas in May (fig 2) only from 55 Hz.

I remember that in February we had the old photodiodes whereas in May the HQE photodiodes. 

In February we could see anti-squeezing (so also squeezing) from lower frequencies, this could be one of the limiting factors of why in the last months the sensitivity of the interferometer increases less injecting squeezing.


A question: less anti-squeezing can be due to higher losses in the squeezing injection, thus this effect can be explained with higher noise at low frequencies caused by the photodioed electronic flicker noise?

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mwas - 16:43 Friday 29 November 2019 (47818) Print this report

Figure 1 shows the projection of the photodiodes flicker noise. I do not think it has a significant impact on measured anti-squeezing.

When analyzing squeezing/anti-squeezing, optical losses and technical noises need to be treater separately, as they have a different impact on the measured squeezing/anti-squeezing (technical noises increase the noise level in squeezing and anti-squeezing measurements, whereas losses increase the noise in squeezing measurement and decrease the noise in anti-squeezing measurements).

In the February data it is hard to see anti-squeezing between 35Hz and 55Hz, in particular in the 9mW green pump data there appears to be a noise bump in that band, that could be related to the changes in scattered light visible below 30Hz.

A large change between February and May is that in between a large amount of scattered light polluting DARM, MICH and PRCL started after the injection system realignment (I am not able to find the logbook entries related to this at the moment).

Is there expectations to see anti-squeezing in the 35Hz-55Hz band? The optical losses have the same impact at all frequencies, so using the observed anti-squeezing at a few kHz the amount of anti-squeezing below 100Hz can be predicted and compared to the other noises already present (and the radiation pressure should actually decrease when injecting anti-squeezing).

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