Sheila, Camilla.
In 72604 Sheila and Vicky found the Quantum Efficiently of Homodyne PD-B was ~5% lower than expected and expected that the beam was not well aligned.
We tried to align the beam better, details below, but couldn't increase the current measured by PDB. The homodyne has been left in a configuration that isn't aligned to PDA.
We removed the steering mirror after the HD BS and adjusted the angle of the Homodyne to be closer to normal with the beam. We then tried adding a ROC 50mm and then 25mm lens directly before PDB to reduce the beam size. Each time after realigning we only got back to our starting value of ~0.56mA on H1:SQZ-HD_B_DC_OUT.
Re-taking the measurements roughly with the Thorlabs power meter didn't replicate the difference in QE Sheila and Vicky took, see below. We could try swapping to the spare HD.
| PDA | PDB | |
| Distance between BS and PD | 6.25" | 6.0" |
| Thorlabs Power Meter (not the calibrated Ophir PM) | 0.6mW | 0.576mW |
| H1:SQZ-HD_{A,B}_DC_OUT | 0.554mA | 0.565mA |
| Fudge Factor in H1:SQZ-HD_{A,B}_DC_GAIN | -1.09 | 1.11 |
| H1:SQZ-HD_{A,B}_DC_OUT with removed fudge factor | 0.508mA | 0.509mA |
| Responsivity | 0.847A/W | 0.884A/W |
| QE (??) Taken Responsivity/0.8582 from 72604 | 98.7% | 100.3% ?? |