Reports until 17:39, Wednesday 15 March 2017
H1 TCS (TCS)
aidan.brooks@LIGO.ORG - posted 17:39, Wednesday 15 March 2017 - last comment - 10:54, Thursday 16 March 2017(34853)
H1-ITMX_HWS measurement indicates a strong point source thermal lens - need to check for any excess 1064nm

Following Betsy and Nutsinee's efforts to clean up the alignment of the ITMX_HWS probe beam yesterday, we reinitialized the HWSX code. The code has been running continuously since then. Last night there was a lock-loss in the IFO (at 1173593679) after being locked for about 2 hours. 

I pulled the HWS gradient field data from that time and have plotted it in the attached movie. I have rezeroed the HWS so that the reference wavefront was taken about 600s before the lock-loss, i.e when the IFO is hot. All subsequent gradient fields are plotted in reference to this initial state.

I then plot the difference in the gradient field from this reference time at intervals of 60s. Therefore, any thermal lenses you see in the gradient field appear as the negative of what would appear during a lock-acquisition (and when the cold state of the IFO is used for a reference wavefront). However, the heat flow is a linear process with regards to the input power, so analyzing the lens decay is fine for determining absorption.

From the point at which the lock-loss occurs:

Notes:

  1. there are few errant spots out near the edges of the HWS FOV. These are noise - just ignore them. I've not had an opportunity to remove them from the data
  2. At the time of the lock-loss, you can also see a bulk pitch towards the top left - this represents a tilt change in the optic.
  3. By and large, the gradient field shows no systematic noise on it, indicating that this measurement is quite clean.

The gradient field is shown above. The numerically integrated wavefront is shown below. There is some residual tilt in this wavefront.

The lens scale is about 60-70nm over a radius of about 20mm.

The next step is to compare this to COMSOL models of thermal lensing for heat sources of different sizes. This is to determine if this data matches a physical model.

Other possible explanations:

So far, I can only come up with one possible other interpretation. If there is excessive 1064nm light leaking onto the HWS, and the HWS doesn't filter it out enough with the bandpass filter (or this isn't in place), this might cause a systematic error in the HWS measurement. We can test to see if we're getting excess contamination of 1064nm light by leaving the HWS code running and turning off the SLED. The HWS code will record images on the HWS camera and when the IFO lock is lost or reacquired, we'll be able to see if there is excess 1064nm light on the CCD in the same position as the apparent thermal lens.

 

Images attached to this report
Non-image files attached to this report
Comments related to this report
kiwamu.izumi@LIGO.ORG - 05:35, Thursday 16 March 2017 (34865)

Looks familiar to me -- see the animations at the bottom of 29712.

aidan.brooks@LIGO.ORG - 10:01, Thursday 16 March 2017 (34868)

Based on fiducial reference points (baffle edges and earthquake stops) in the image of the ITMX HWS return beam, I estimate that the center of the ITM optic is [-6mm, +9mm] in the HWS coordinate system (as magified up to the ITM).This is illustrated in the attached image.

Orientation: the image is inverted vertically, the top of the optic is at the bottom of the image. We don't have the left-right orientation determined.

Best estimate: the displacement of the point source is about [20mm,25mm] from the optic center - toward the lower part of the optic.

A RH measurement would provide a more precise estimate.

 

Images attached to this comment
aidan.brooks@LIGO.ORG - 10:54, Thursday 16 March 2017 (34870)

Here is a zoom of the OPD. The tilt has been removed and the contours are labeled (they're every 10nm).

Images attached to this comment
Non-image files attached to this comment