I went back through more of the recent mode matching measurements in HAM7 to help us track how the changes we're making are impacting the astigmatism of the squeezed field going into the interferometer.
Below I tabulate the 1D overlap between the vertical and horizontal q parameters. Since the PSAM settings seem to cause higher order aberrations in addition to astigmatism, I also tabulate the M2 values for each set of measurements, though its less clear what we can conclude from the M2 parameter alone since it doesn't tell us what higher order mode content is being generated by the PSAMs.
Jun 26: aLog 90783
For this set of measurements, we were using the nominal setting for the ZM2 PSAM and taking measurements on SQZT7 after the beam diverter using the Thorlabs beam profiler.
XY 1D overlap:
| ZM4\ZM5 SG (V) | -4.5 | -2.0 | 0 | 2.0 |
|---|---|---|---|---|
| 2.0 | 0.995 | 0.996 | 0.990 | 0.991 |
| 4.0 | 0.996 | 0.994 | 0.992 | 0.991 |
| 6.0 | 0.995 | 0.995 | 0.993 | 0.993 |
| 8.0 |
0.995 |
0.994 | 0.994 | 0.994 |
M2:
| ZM4\ZM5 SG (V) | -4.5 | -2.0 | 0 | 2.0 |
|---|---|---|---|---|
| 2.0 | 1.35, 1.32 | 1.27, 1.23 | 1.17, 1.18 | 1.17, 1.19 |
| 4.0 | 1.33, 1.29 | 1.24, 1.21 | 1.16, 1.16 | 1.16, 1.17 |
| 6.0 | 1.32, 1.29 | 1.22, 1.20 | 1.16, 1.16 | 1.16, 1.17 |
| 8.0 | 1.31, 1.27 | 1.20, 1.19 | 1.16, 1.16 | 1.17, 1.18 |
It appears that the astigmatism gets worse for higher values of the ZM5 strain but the M2 gets better. The dependence on ZM4 is less obvious, but that is expected since the beam spot size on ZM4 is only 1 mm.
Jun 30: alog 90827 For this set of measurements, we were measuring the beam using a pickoff mirror between ZM4 and ZM5. Given that we don't expect ZM4 to have that large of an impact due to the small spot size, one assumes that the astigmatism and M2 values will be similar to the data right before ZM4 taken on Jun 29: see the data in alog 90815 and the analysis in alog 91185
Oddly, it doesn't work out this way. The astigmatism still seems worse when ZM2 is changed from its nominal value, but the astigmatism coming from ZM2 (assuming that is the source) seems lower in this dataset than in the Jun 29 dataset. However, the M2 values are significantly higher. Perhaps there's some hysterisis or other non-stationarity in the astigmatism and higher order abarations coming from the PSAMs? This seems plausible for a strain induced effect like this.
Another possibility is that the beam was clipping somewhere. That could explain a high M2 value. Unfortunately, that sort of thing can happen with an improvised pick-off path like this one.
Data with ZM2 = 3.15 V
| ZM4 SG (V) | 1D Overlap X/Y | M2 |
|---|---|---|
| 2.0 | .995 | 1.78, 1.73 |
| 4.0 | .999 | 1.81, 1.73 |
| 6.0 | .999 | 1.84, 1.75 |
| 8.0 | .998 | 1.90, 1.81 |
Data with ZM2 = 4.5 V
| ZM4 SG (V) | 1D Overlap X/Y | M2 |
|---|---|---|
| 6.2 | .986 | 1.79, 1.64 |
Jun 30_2: alog 90841
This dataset looks at the mode after the beam diverter. The ZM2 setting has been changed to 2.4 V for most of this dataset, a value which may give improved mode matching into the Filter cavity. We expect that changing the ZM2 settings will impact the beam quality at this location. This dataset is difficult to compare with the others since there isn't much overlap in the settings. The M2 values after ZM5 look much better than in the data taken before ZM5 earlier in the day. Odd, since higher order aberrations aren't something that can be easily undone.
Data with ZM2 = 2.4 V
| ZM4, ZM5 SG (V) | 1D Overlap X/Y | M2 |
|---|---|---|
| 2.0, 0.8 | .993 | 1.24, 1.26 |
| 4.0, 0.8 | .991 | 1.20, 1.23 |
| 4.0, 2.0 | .997 | 1.33, 1.30 |
| 6.0, 2.0 | .992 | 1.30, 1.30 |
Data with ZM2 = 3.15 V
| ZM4, ZM5 SG (V) | 1D Overlap X/Y | M2 |
|---|---|---|
| 2.0, 4.4 | .998 | 1.39, 1.32 |
July 10: alog 90986
These measurements were taken after the beam diverter on SQZT7 shortly after the ZM4 preloading was adjusted. In the first two measurements, the beam spot postion on ZM2 is varied to see the impact on the outgoing mode:
| ZM2 Spot Position | 1D Overlap X/Y | M2 |
|---|---|---|
| 5.85" (nominal) | .995 | 1.35, 1.35 |
| 5.75" | .997 | 1.28, 1.25 |
Centering the spot better does seem to improve things a bit.
Next, they did a sweep of ZM4 and ZM5 with the new ZM4 preloading. It is unclear from the aLog which ZM2 spot position was used for this dataset. Note that the ZM5 strain gauge was broken for this measurement, so I record the bias voltage for ZM5 instead
| ZM4 SG, ZM5 Bias (V) | 1D Overlap X/Y | M2 |
|---|---|---|
| -4.1, 20 | .997 | 1.41, 1.34 |
| -4.1, 100 | .995 | 1.31, 1.28 |
| -2.9 , 100 | .996 | 1.29, 1.26 |
| -1.1, 100 | .996 | 1.28, 1.25 |
| -1.1, 20 | .996 | 1.37, 1.33 |
| 1.0, 100 | .996 | 1.26, 1.23 |
| 2.7 , 100 | .995 | 1.25, 1.23 |
| 2.7, 20 | .997 | 1.37, 1.31 |