Summary:
QE and transimpedance combined for Baffle PD1 and PD4 on all ITMs and ETMs agree with each other within 10%. In the table below, QE/transimpedance combined for each PD is normalized such that the average of all diodes becomes 1.
| X | Y | |||
| PD1 | PD4 | PD1 | PD4 | |
| ETM | 1.01 | 1.00 | 1.09 | 1.03 |
| ITM | 0.94 | 0.98 | 1.00 | 0.95 |
I haven't done any error analysis, but the biggest error should be the systematic in ITM-ETM comparison that was introduced by an assumption that IR mode matching is perfect.
PD1-PD4 comparison on the same baffle (e.g. ETMX PD1 and PD4) should be quite good.
Details:
This is yet another baffle health check.
The other day I used some short time available to misalign TMS and steer MMT3 to point IR beam on ETM baffle PD4, first on ETMX and then on ETMY. This could be used to make ETMX(PD4)/ETMY(PD4) comparison, assuming that the IR straight shot beam size is the same on ETMX and ETMY.
Then I proceeded to align MMT3, align ETMs to point the beam to PD4 on ITM baffles. Since ETM reflectivity is basically 1, and since the mode matching cannot be as bad as green, we can use these measurements combined with nominal IR beam size to make ETM(PD4)/ITM(PD4) comparison.
Also, when people do initial alignment they are basically doing PD1/PD4 comparison on a single baffle using green beam. (You cannot make comparison across different baffles using green because of the beam size difference described in my previous alog.)
So, stitching all these together, if we believe that the QE variation of diodes for green is the same as that for IR, we can map out the QE/transimpedance combined for all baffle diodes.
In the table below of the raw data, photo current in mA for straight shot measurements of green on PD1 and PD4 as well as IR on PD4 on all arm cavity baffles are shown.
| PD1 green | PD4 green | PD4 IR | |
| EX | 0.0453 | 0.0448 | 0.0127 |
| EY | 0.0111 | 0.0104 | 0.0130 |
| IX | 0.123 | 0.129 | 0.0176 |
| IY | 0.0243 | 0.0230 | 0.0170 |
Green data can be used to make PD1/PD4 ratio on each of the optics.
IR ETM data is used to make PD4 ETMX/ETMY ratio.
IR ITM data, combined with the nominal IR beam radius on ITM and ETM (53.4 and 63.5 mm, respectively), is used to make PD4 ETMX/ITMX and ETMY/ITMY ratio.
After some math we get the table at the top of this entry.