camilla.compton@LIGO.ORG - posted 16:51, Wednesday 23 November 2022 - last comment - 17:21, Monday 10 February 2025(66011)
ETM HWSs reflecting off ITM surface
In alog 65804 Ross, Mitchell and I adjusted and dithered ITMX to see how much reflection off the ITM the ETM Hartmann sees. See attached for the ETMX HWS beam refected off the ITMX. This is a known issue in both ETM HWSs. It may explain why ETM ring heater tests look okay but not powerups.
How much 520nm ETM HWS beam do we expect to refect off the ITM vs ETM:
Assuming beam in 520nm but it is really 520+/-10nm M1900163. Using spec sheets C1103238 for ETM and C1103261 for ITM.
Reflection from ETM HR for 520nm: (T ETM_AR )2 . R ETM_HR = 0.932 . 0.25 = 20%
Reflection from ITM HR for 520nm: (T ETM_AR . T ETM_HR)2 . R ITM_HR = (0.93 x 0.75)2 . 0.99 = 48%
Images attached to this report
Comments related to this report
Re-calculating for current 530nm M530F2 HWS beams, you can clearly see why the retrofections off the ITM are less of an issue with the 530nm source.
Using spec sheets C1103238 for ETM and C1103261 for ITM.
Reflection from ETM HR for 520nm: (T ETM_AR )2 . R ETM_HR = 0.932 . 0.75 = 65%
Reflection from ITM HR for 520nm: (T ETM_AR . T ETM_HR)2 . R ITM_HR = (0.93 x 0.25)2 . 0.99 = 5%