Stefan, Thomas
In light of the new ITMX and ETMs being installed before O3, Stefan thought it would be a good idea to try to estimate the new contrast defect by calculating the fundamental mode of each arm cavity before and after the swap using data from the core optics galaxy page and propagating the beams to a common point and calculating the contrast defect via this equation:
Two methods:
- Stefan used handwritten MATLAB code,which is located in ligo/home/controls/sballmer/20171207, takes into account the arm cavity modes and the Schnupp asymmetry.
- I used Finesse with the aLIGO design and modifying the optics to the most in-situ numbers for ROCs (attached .kat parameter file and custom function to find eigenmodes). This model includes the arm cavities and the Schnupp asymmetry, as well as the thermal lens which is approximated at f=34.5km and the michelson length.
When looking at the contrast defect on the HR surface of the arm cavities, both models are consistent, which makes sense because its the simplest calculation. However, when propagating the beam to a different point such as the beamsplitter, things get more complicated because of the lenses caused by the AR surface of the ITMs and beamsplitter are estimates at best. Although there is a slight difference in the q-parameters between the two models because of these effects, the way we calculated the contrast defect is the same.
The parameters we used were as follows:
Both simulations used the same ROC numbers, arm cavity length of 3994.5 meters and Schnupp asymmetry = 0.08m
Before Swap
| S/N | RoC | |
| ITMX | 03 | -1939.90 |
| ITMY | 11 | -1939.20 |
| ETMX | 08 | 2241.54 |
| ETMY | 12 | 2238.90 |
After Swap
| S/N | RoC | |
| ITMX | 07 | -1940.30 |
| ITMY | 11 | -1939.20 |
| ETMX | 13 | 2242.20 |
| ETMY | 16 | 2246.90 |
Results
Finesse
| Location | Old Contrast Defect | New Contrast Defect |
| ITM HR Surface | 5.93e-6 | 3.87e-6 |
| BS HR Surface | 5.68e-6 | 4.59e-6 |
Stefan's Code
| Location | Old Contrast Defect | New Contrast Defect |
| ITM HR Surface | 5.93e-6 | 3.87e-6 |
| ITM + Schnupp | 5.63e-6 | 5.00e-6 |
In the end, the interesting part which both models agree on is that the contrast defect should be better with the new test masses.
It is important to note that we've only really considered the modal overlap, this has not incorporated the reflectivities of the ITMs which could make a difference in the end results if they are substantially imbalanced.