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Reports until 21:21, Monday 20 October 2014
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kiwamu.izumi@LIGO.ORG - posted 21:21, Monday 20 October 2014 - last comment - 12:56, Tuesday 21 October 2014(14531)
indeed multiple zero crossing in SRCL

Alexa, Evan, Kiwamu

We observed some new features which are related to the SRC mode hopping.

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kiwamu.izumi@LIGO.ORG - 10:41, Tuesday 21 October 2014 (14543)

The SRCL error signal was calibrated in [nm] from a measurement of the open-loop transfer function last night. However the number does not seem right.

Last night, the UGF of the loop was estimated to be 27 Hz, which corresponded to an optical gain of about 5.0 x 1010 [cnts /meters] at the input of the LSC-SRCL filter. Therefore an offset of -800 cnts that we introduced at the SRCL input corresponds to a displacement of 16 [nm] ... which is actually already out of the linear range  close to the edge of the linear range (because the linear range is 20-ish nm 40 nm in full width for SRCL). Something is not right.

kiwamu.izumi@LIGO.ORG - 12:56, Tuesday 21 October 2014 (14550)

I made an independent and more accurate calibration for SRCL. The result suggested that my previous calibration was off by roughly a factor of 2. The optical gain of SRCL should be 1.65 x 1011 [cnts/meters].

Therefore the 800 counts offset that we put yesterday should correspond to a displacement of 4.8 nm. We could sweep SRCL up to 6000 counts or 36 nm in one side of the fringe yesterday.

 

(Calibration method)

In the previous entry, I used the SRCL UGF in order to estimate the optical gain in counts/meters. This time, I used a sideband build-up signal which should give us a direct measure of the SRCL linewdith or liner range.

The plot below shows time series of some signals when we were changing the SRCL offset last night:

As shown in the plot, as we swept the offset of SRCL, the sideband power of SRC observed by AS_RF90 decreased/increased. When the sideband power becomes the half of the maximum,  SRCL must be at the point where the linear range ends. Since we already know how big the linear range should be in terms of the SRCL displacement, we can calibrate the optical gain.

 

The plot below shows a x-y projection of AS_RF90 and SRCL_OFFSET from the same data as shown above:

By performing fitting, I was able to estimate the half-wdith at half-maximum (HWHM). I found the HWHM to be 3300 counts in terms of SRCL_OFFSET. According to galaxy (https://galaxy.ligo.caltech.edu/optics/), the transmissivity of SRM is T_{srm} = 37% for SRM-w14 and this gives a finesse of about 13. Therefore the HWHM should be (1064 nm ) / 4 / finesse = 20 nm. 

Finally the calibration is calculated as (3300 counts) / (20 nm) = 1.65 x 1011 [counts/meters].

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