Summary:
For the upcoming HAM1 vent (E2200228), we might replace an HR mirror for P-pol at one corner in REFL path where AOI~50deg with 50:50 BS for P-pol that is nominally for 45deg AOI (E1000671).
I quickly measured the BS reflectivity (~43%) and AR (~0.18%) of the potential replacement optic at AOI=50deg in the lab. This seems to be usable.
Details:
See cartoon. I used Prometheus laser coupled to fiber collimator as the light source. The 2" BS was mounted on a rotation stage so I can easily change AOI. I used two power meters, one to monitor the slow drift of the laser power and the other to measure the power impinging the 2" BS ("A" position), reflected by the 2" BS ("B") and by AR ("C").
After setting up the collimator, PBS/BD and 1" BS, the collimator was adjusted to level the beam with the table surface using a ruler and a viewer card. Angle of the BS, after establishing the zero AOI position, was read off of the rotation stage that has 2 deg graduation dial. The reading error is about +-0.2deg I'd say. (Zero AOI position was established by retroreflection where the beam goes back to the fiber collimator within +-1mm over 16" distance, i.e. AOI is within +-1/(25.4*16)/2=+-1.2mrad=+-0.07deg. This is much smaller than the reading error of the rotation stage and is ignored.)
In addition to 50deg, I also measured BS reflection for 45 and 55 deg (but not AR).
For each of the data points, I measured with/without blocking the light to subtract the dark offset. Only one of these offsets are noted in the table below becase they were in general quite inconsequential (dark<1e-3*bright).
| BS angle relative to retro | Witness [mW] | A [mW] | B [mW] | C [uW] |
| NA | 1.09 | 1.125 | - | - |
| 45deg | 1.10 | - |
0.562 |
- |
| 50deg | 1.20 | - | 0.531 | - |
| 50deg | 1.28 | - | - |
0.773-0.023=0.750 |
| 55deg (BS) | 1.17 | - | 0.429 | - |
From the table above, BS and AR power reflectivity are calculated by
R_BS=(B/Witness(B))/(A/Witness(A))
and
R_AR=C/(1-R_BS)^2,
respectively.
| AOI (deg) | R_BS | R_AR |
| 45+-0.2 | 0.495 | Not measured |
| 50+-0.2 | 0.430 | 1.75e-3 |
| 55+-0.2 | 0.356 | Not measured |
Reference to this alog will be added to E2200228.
FYI, ATF measurement in E2200228 shows that BS transmission was 50.315% at 45deg for their witness sample.
Also FYI, the fiber collimator, PBS and 1" 50:50 P-pol were left on the optics table in the optics lab in case Craig wants to measure the transmission of double-glassed viewport for REFL parking beam dump, but note that he will have to get creative as he needs S-pol.
