Some more words on yesterday's REFL path work.
The 90/10 BS that splits the light between REFL11 and REFL55 was placed back in August 2013, to compensate for the fact that REFL11 has a much larger RF transimpedance than REFL33. See elog 9043 for details.
We had been operating for a long time with an embarrasingly small amount of light on the REFL PDs. REFL11 used to have 80 uW, REFL33 used to have 400 uW and REFL55 used to have 700 uW. REFL 165 was the only sane one, with about 15 mW of light.
After yesterday's work, the situation is now:
| |
Power incident [mW] |
PD responsivity [A/W] |
photocurrent [mA] |
shot noise intercept
current [mA]
|
Ratio (photocurrent) /
(shot noise intercept current)
|
| REFL 11 |
1.3 mW |
0.7 |
0.91 mA |
0.12 mA |
7.6 |
| REFL 33 |
13 mW |
0.7 |
9.1 mA |
0.52 mA |
17.5 |
| REFL 55 |
12 mW |
0.7 |
8.4 mA |
1.6 mA |
5.3 |
| REFL 165 |
50 mW |
0.15 |
7.5 mA |
1.06 mA |
7.1 |
As an aside, I was foiled for a while by S vs. P polarizations of light. The light transmitted through the PBS was P-pol, so the optics directing the beams to REFL11, 33 and 55 were all P-pol. At first I completely removed the PBS and the waveplate, but didn't think through the fact that now my light would all be S-pol. P-pol beam splitters don't work for S-pol (the reflection ratios are different, and it's just a terrible idea), so in the end I used the PBS to set the half waveplate so that all of my light was P-pol, and then removed the PBS but left the waveplate. This means that all of the old optics are fine for the beams going to the 3 gold-box REFL PDs. We don't have many S-pol beamsplitter options, so it was easier to use the waveplate to rotate the polarization. |