Alexa, Evan, Kiwamu
We observed some new features which are related to the SRC mode hopping.
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We noticed that when SRM was slightly misaligned from the optimum angle in pithc by approximately 15 urad, it seems to reduce the number of mode hops in SRC.
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Of course, this reduced the power build up in the signal recycling cavity by some amount, but it was more stable due less mode hoppings.
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We starting putting an offset in the SRC operating point in order to study if the signal really has multiple zero crossings.
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When LSC-SRCL had an offset of -800 cnts it became much more stable than it was in the 10 W configuration. We could even stably go to -5000 cnts by expanding the linear range by the normalization with AS_RF90.
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On the other hand, when we went to the positive side in the offset (e.g. ~ 400 conts), we could easily lose the lock due to the instability or the hopping.
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This indicates that the error signal for SRCL is asymmetrical about 00-mode's zero crossing.
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We checked the same asymmetrical error signal both in 10 W and 1 W configurations and found that the instability seemed to be further away from the 00-mode in the case of 1 W.
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We also checked the level of RFAM in all the REFL detectors by directly detecting the prompt reflection of PRM while the interferometer was misaligned.
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Even though both in-vac and in-air RF45_I signals showed a discrete jump in the RFAM offset, the amount of the offsets were much smaller -- the biggest offset was about -22 cnts in REFL_A_RF45_I.
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Apart from REFL_RF45s, all the detectors seemed to have a linar relation in the RFAM offsets with respect to the PSL power.
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Note that we had to re-align the DRMI after three lock or so losses. Otherwise, we were not able to smoothly acquire the lock.
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 rangeclose to the edge of the linear range (because the linear range is20-ish nm40 nm in full width for SRCL). Something is not right.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].