jennifer.wright@LIGO.ORG - posted 11:56, Thursday 17 July 2025 - last comment - 15:32, Monday 21 July 2025(85795)
ISS array work - horizontal scan
Jennie, Rahul
On Tuesday Rahul and I took the measurements for the horizontal coupling in the ISS array currently on the optical table.
The QPD read 9500 e-7 W.
The X position was 5.26 V, the Y position was -4.98 V.
| PD | DC Voltage [mV] pk-pk | AC Voltage [mV] pk-pk |
| 1 | 600 | 420 |
| 2 | 600 | 380 |
| 3 | 600 | 380 |
| 4 | 600 | 420 |
| 5 | 800 | 540 |
| 6 | 800 | 500 |
| 7 | 600 | 540 |
| 8 | 800 | 540 |
After thinking about this data I realise we need to retake it as we should record the mean value for the DC coupled measurements. This was with a 78V signal applied from the PZT driver and an input dither signal of 2 Vpp at 100Hz on the oscilloscope and I think 150 mA pump current on the laser.
Rahul, Jennie W
Yesterday we went back into the lab and retook the DC and AC measurements while horizontal dither was on while measuring using the 'mean' setting and without changing the overall input pointing from what it was in the above measurement.
QPD sum is roughly 5V.
Next time we need to plug in the second axis of the PZT driver so as to take the dither coupling measurement in the vertical direction.
horizontal dither calibration = 10.57 V/mm
dither Vpk-pk on QPD x-direction = 2.2V
dither Vpk-pk on QPD y-direction = 0.8V
dither motion in horizontal direction in V on QPD = sqrt(2.2^2 + 0.8^2)
motion in mm on QPD that corresponds to dither of input mirror = sqrt(2.2^2 + 0.8^2) / 4.644 = 0.222 mm
Code is here for calibration of horizontal beam motion to QPD motion plus calibration of dither measurements.
To work out the relative intensity noise:
RIN = change in power/ power
= ( change in current/ current) / responsivity of PD
= (change in voltage/voltage) / (responsitvity * load resistance)
Therefore to minimise RIN we want to minimise change in voltage / voltage for each PD.
To get the least coupling to array input alignment we work out
relative RIN coupling = (delta V/ V) / beam motion at QPD
This works because the QPD is designed to be in the same plane as the PD array.
These are all a factor of 50 higher than those measured by Mayank and Shiva but after discussion with Keita either we need higher resolution measurements or we need to further optimise input alignment to the array to minimise the coupling.