craig.cahillane@LIGO.ORG - posted 06:21, Saturday 19 January 2019 - last comment - 18:40, Sunday 20 January 2019(46541)
RIN Investigations
Koji, Craig Koji has the idea that maybe our IMC FSR is not exactly equal to our RF 9 modulation frequency, and this is responsible for additional 9 MHz RF AM seen by Sheila and Jenne. So today Koji and I moved the modulation frequency around. We used the double modulation technique for finding the FSR by looking at the 24 + 9 = 33.178 MHz peak and trying to minimize it. We found that the frequency was already pretty well tuned. We moved it around from 9.100230 MHz and found the best rejection of 33.178 MHz peak at 9.100225 MHz. Larger movement of the modulation frequency (on the order of 10s of Hz) resulted in much higher 33.178 MHz peaks. Then we tried repeating Sheila and Jenne's OMC 9 MHz locking test. The results are plotted below. The first attachment shows the OMC DCPD spectra and RF AM monitor spectra, each one appropriately calibrated into RIN, for different RF9 modulation slider values. We see a moderate (factor of 2) win from going from +27 dB to 23.4 dB, otherwise 9 MHz RIN remains about constant. The second attachment shows the effect of changing the 9 MHz modulation frequency by 5 Hz. The 9 MHz RIN seems to be independent of our frequency move. We note that the move from 23.4 dB to 27 dB on the RF9 slider actually does increase the modulation depth, since the OMC DCPD SUM increases when this is done. Koji suspects that his measurements from before measuring the response of the EOM suffered from broadband saturations spoiling the linear response. Going above 23.4 dB on the 9 slider is still not recommended. We moved the modulation frequency back to 9.100230 MHz and left it there. ------------------------------------------------------------------------- We also injected a line into the EOM driver for the 9 to calibrate the REFL A 9I pd into units of RIN. It seems that REFL A 9I is a pretty poor RIN sensor relative to the OMC and RF AM monitor. (attachment 3)
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Optics Express 23 (2015) 19417; http://dx.doi.org/10.1364/OE.23.019417 and also alog 13378.
Back then, we measured 9.100235.6 Hz (paper) and 9.100229 Hz (alog). So the FSR changed by less than 11 Hz, or ~1 ppm. Or in other words, the cavity length of ~16.5 m changed by less than 20 µm, since the last measurements 5 years ago.
Some notes about the measurement that were not mentioned before: - We measured out of IMC REFL RF OUT with the IMC locked with 25 watts input requested. - Attachment one is the data on the 33 MHz Peak vs 9 MHz changes we acquired. - Attachment two is the calibration of REFL A/B 9I RIN we got by injecting AM into the 9 MHz stabilizer.
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