jeffrey.kissel@LIGO.ORG - posted 11:37, Wednesday 21 August 2019 - last comment - 14:45, Friday 23 August 2019(51440)
IFO's DARM Opto-mechanical Response (Sensing Function) Vs. Spot Position and SRCL Offset
S. Dwyer, J. Kissel We've repeated Monday's measurements (see LHO aLOG 51393) of the IFO's DARM Opto-mechanical Response (Sensing Function) Vs. SRCL Offset now at the "July" spot positions (where Monday's were at the "August" positions -- see attachments to LHO aLOG 51436 for what this means in terms of A2L gains). There is similar drastic impact on the sensing function. See attached: 2019-08-21_H1_100ctSRCLOffset_sensingFunction_referenceModel_vs_allMeasurements.pdf I'm again using the "No Spring" loop model, which has H_c Optical Gain 3.13e6 [ct/m] f_cc Cavity Pole Frequency 411 [Hz] and *no* optical spring from SRC detuning, in order to emphasize / better quantify the effects of either actual detuning or remaining L2A2L issues. In the July Spot positions, with a 100 ct SRCL offset, both of these effects are minimized, and the sensing function appears quite flat, where the other three configurations have interesting features. In fact, the cavity pole frequency also best matches the model at 411 Hz in the July positions and with a 100 ct offset, where the other configurations have lower frequency (the worst being 395 Hz from Monday's Aug positions and no SRCL offset [determined by "by-hand" fitting from LHO aLOG 51393]). Looking at the phase, my preliminary interpretation of these data are that (a) Changing the spot position moves the detuned spring frequency to lower frequency but keeps the detuning Q / amplitude level about the same, and (b) Adjusting the SRCL offset reduces "the amount" (the Q and amplitude) of detuning. I'm not yet ready to say this is definitively true, since -- in the beginning of O3 up to July 31, the low-frequency response was bouncing around from week to week (see LHO aLOG 50498), which may be a result of the L2A2L issues. Monday's measurement of Aug spot positions with 100ct SRCL offset show evidence of the "unphysical" phase turn-up and funky magnitude response, yet today's July spot positions, with no SRCL offset don't appear reproduce the unphysical issues -- when we would expect them to be quite similar to all previous O3 sensing function measurements before the spot move. More to come, I'm sure...
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J. Kissel
In the above aLOG, I've compared all of the measurements to the same, no spring, low-frequency optical response model to emphasize how much it changes with spot position and srcl offset. We see that there is not only a spring-like effect, but also some (what we believe to be) parasitic L2A2L coupling of some unknown shape or form (especially evident in the requested 100ct SRCL Offset, August Spot position data).
We also know that our MCMC algorithm for fitting this data, for which we only give it the poles and zeros to model a spring (2 zeros at 0.0 Hz, and an arbitrary set of complex poles at frequency f_s with phase separation defined by Q), cannot accurately give us approximations for what at least the spring part is doing because it's getting confused by the parasitic L2A2L response.
However, in order to make progress on how much detuning we have, and how the requested 100 ct SRCL offset changes that detuning in physical units, we need *something*.
So -- I've noodled around with the optical plant parameters that we *do* have in order to make a "best" by-hand fit of the data. Here, "best" is based on minimizing the residual (ratio) between measurement and model, acknowledging that I will never be able to get an actually good fit, because I'm not using enough poles and zeros to cover the L2A2L effect. Also, acknowledge that my by-eye happiness is determining the uncertainty in the parameters, so we have no nice neat, multi-dimensional posterior distribution that exposes the (very real) covariance between the parameters.
Given those caveats, I attach the results.
Here's a table of the resulting fit parameters.
No Offset 100 ct SRCL Offset
Aug spots Jul Spots Aug Spots Jul Spots
optical gain, H_c [1e6 ct/m] 3.13 (0.01) 3.16 (0.01) 3.13 (0.01) 3.16 (0.01)
caviy pole frequency, f_cc [Hz] 395 (3) 405 (3) 409 (3) 415 (3)
optical spring frequency, f_s [Hz] 7.2 (0.5) 6.0 (0.5) 5.0 (1.0) 0.0 (1)
optical spring quality, Q [n.a.] 10 (5) 35 (5) 10 (5) 1.0 (1)
spring type [n.a.] pro pro anti? no?
measurement date [YYYY-MM-DD] 2019-08-19 2019-08-21 2019-08-19 2019-08-21
attachment page page 1 page 2 page 3 page 4
Again, in the 100ct SRCL offset data, there's not enough poles and zeros in the fit, and/or not a visible enough spring to really trust the spring-related assessment last two columns, and I thoroughly recommend we map out the parameter space better and develop a model (or get rid of) the supposed L2A2L coupling so we can really do a proper fit on this.
We can at least confidently say that Jul spot positions, with a requested 100 ct SRCL offset is best for the optical gain and cavity pole.
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