Cheryl Daniel
The IOT2L table drawing can be found here: D1300357. We installed a 2" turning mirror after IO_MCR_M3 a the location of IO_MCR_BD2. The beam is sent to the right on the drawing where it encounters a 2" lens with focal lens 100mm. A Thorlabs PDA100A2 was installed somewhat prior to the focus of the beam and acts the trigger PD. Its gain was set to 0dB. A 14mm reflective shutter with was put in front of the IMC LSC PD. When closed the reflected beam is steered onto a razor blade beam dump.
Next we swapped IO_MCR_BS1 with a 90:10 beamsplitter and increased the power using the halfwave plate IO_MCR_HWP1.
The power on the photodetectors before and after:
| in lock |
Before |
Before 35W |
After 1.85W |
After 35W |
|---|---|---|---|---|
| IMC REFL DC | 0.08mW | 1.4mW | 0.54mW | 9.15mW |
| WFS_A DC | 0.037mW | 0.65mW | 0.028mW | 0.46mW |
| WFS_B DC | 0.029mW | 0.51mW | 0.022mW | 0.36mW |
The wavefront sensors seem to have lost about 30% of power, whereas the DC PD power has increased by about 7.
We reduced the IMC IN1GAIN by 20dB. This gives us a ugf around 42kHz. Later in the locking process this gain will be increased by another 3dB which would give us a ugf around ~65kHz. This is a little bit puzzling, since we would expect a 16dB gain to give us the old ugf, which was around 70kHz.
The -20dB was put in the IMC_LOCK DOWN state (for IMC acquisition) as well as lscparams.IMC_IN1_gain. This should be all of the places that the gain is hard-coded. Everywhere else is relative (eg. CARM_TO_ANALOG has a +=3 to add 3dB to whatever is the current value).
I forgot to save a screenshot of the IMC OLG measurement when we had IMC-only (no IFO lock), but that had a UGF of about 41 kHz. (I have the data, but will have to remind myself how to call the plotting scripts...)
Attached is a screenshot of the IMC OLG while the full IFO was locked at Engage_ASC_for_full_IFO, so 2W lock, CARM on its final sensor, DARM still on RF. Looks like ~58kHz after the +3dB from CARM_to_analog.
Attached are the IMC and CM transfer functions when fully locked at 35W. The IMC ugf is around 72kHz (excellent phase margin!), whereas the CM ugf is around 20kHz.
This seems to confirm that the optical gain has changed by a factor of 10, whereas the power increase when locked is only around 7. One way to resolve this is with a small amount of light in the wrong polarization incident to the IMC. Due to the halfwave plate most of it would be directed onto the photodetectors, whereas the correct polarization would be attenuated. This would mean the shot noise limited sensitivity has gotten better by approximately 3.8, rather than 2.6 which we would have expected form a factor of 7 increase in power.
Here is a snap shot of the updated medm screens.
Remeasured the error and control spectra of the IMC REFL servo and the LSC REFL servo signals. The IMC signals are mostly the same. The CM servo shows significant improvements at frequencies above ~30Hz. The improvement at 1kHz is a factor of ~5.7, when compare with a lock just 13 hours ago. Since we were only detecting 1.4mW of light previously, the dark noise was probably at a similar level as the shot noise.