EDIT: The calibration has been corrected to include the right NPRO temp control coefficient as measured by Rana at the 40m. This makes it of roughly the same level as the incoherent noise in the MZ measurement, but still different in shape. I think the coherent noise dominates in this measurement because of the shorter path length, so it may be that the MZ noise limits us more in the real gyro (this will likely change after we put in the windows).
Here is a spectrum of the slow control signal to the NPRO calibrated to meters. The calibration is:
6.103 x 10-4 V/ct * 1.1 GHz/V * (c / 1064 nm) * 0.75 m = 1.78 x 10-9 m/ct
Unlike yesterday's measurement, this does not have the right shape to be our current limiting noise source. It does, however, pose an even bigger threat than the noise measured yesterday given our current understanding of how noise couples in. That is, if in fact the differential-mode noise is limiting us now because of our week feedback loops, then once we take care of that we will have to deal with this 0.1-um level noise and its FSR coupling, which raises the "displacement noise altering FSR" curve on the NB by about an order of magnitude.
The fact that this noise is directionally preferential---it shows up along one leg but not in the difference between two like paths, as yesterday---leads me to believe that it is not air noise. The calibration may be off in one of the two (or both) measurements but the data are qualitatively different, and I don't think we would see this if air noise were dominant.
Let me clarify what I think is happening:
There are two types of noise we are considering here
- Incoherent noise from the shaking of the mounts and from air currents. This shows up in the MZ-type measurement and in the linear cavity measurement.
- Coherent noise from the stretching of the table. This shows up only in the linear cavity measurement
- If our locking loops have infinite gain and bandwidth, then we only see either source as it affects the area of the cavity. That is, we don't see direct length-to-frequency coupling from cavity noise, but rather only the modulation of the 1-FSR offset due to a CM change in cavity size. In this case, we will be concerned with the loudest of the above sources of noise, which I believe (given these past two measurements) is (2).
- Since our locking loops are far from perfect, we are seeing the noise from (1) couple in much more directly than it will in the final gyro. I do not propose a mechanism, but, for example, it could be that we are seeing noise from the (sometimes shakier) non-cavity optics that should be suppressed by the loops.
If I am right, then we may seriously start to consider the idea of locking the laser to a stable reference, then locking the CCW length of the gyro cavity to the laser via PZT.
Maybe the word "mode" isn't the right choice. I'm not talking about a vibrational mode, but rather the low-frequency thermal expansion of the table. I find it hard to believe that the perimeter of the table won't change by more than 10-10 m or so over 1-10 mHz timescales.
Regarding the AC on/off spectrum, do you mean a spectrum of the gyro signal, or some other configuration?
I have a feeling that Alastair will know better what to do about the windows, but I can try and think of something in his absence.
About the word wrapping, I don't know; I've never had an issue in Safari (OS X) or Firefox (Linux).
I don't think there's any breathing mode being excited. At low frequencies, we are most likely measuring the air noise. Since the table has its first eigenfrequency above 100 Hz, the flexing at 100 mHz is suppressed by a factor of lots.
Most likely, we just need to plug the giant holes on that box. Where are those windows??? Also, take a spectrum with the AC turned off and make a comparison with it on/off.
P.S. Why do the entries in this elog not word-wrap in some browsers, while the ones in the TCS lab tab do word wrap?