On Thursday and Friday we worked at the optimization of the coherent control loop. The goal of the shift was to minimize the phase noise and improve the long term stability of the coerent control loop.
Structure of the loop:
The coherent control loop is closed with two actuators. A fast phase shifter PZT (PI S-303.CD ) placed on the mirror LO_M1 and a slower phase shifter (PI S-325.30L) placed on the mirror LO_M4 (HD_CC_COARSE loop). The error signal of the Fast CC loop is the SQZ_EQB1_HD_DIFF_RF_4MHz_I channel (i.e. the RF Difference channel of the homodyne detector demodulated at the Coherent control laser frequency offset), whereas for the HD_CC_COARSE loop the error signal can be selected between the correction signal of the Fast Coherent control (SQZ_EQB1_HD_CC_Corr_50kHz) or the Error signal of the fast Coherent control loop ( SQZ_EQB1_HD_DIFF_RF_4MHz_I). We can select between the two error signal by using the relay SQZ_EQB1_HD_CC_COARSE_ERR_SELECT (0 = 4_MHz_I, 1=CC_Corr). We added also a protection for the COARSE CC loop: if the correction of the Fast CC loop is lower than-2V or higher than 2 V the Coarse CC Loop has to be open. This protection is implemented with the relay HD_CC_COARSE_TRIG.
During these two shifts we optimized both the loop and in particular we use the COARSE CC LOOP to keep the correction of the FAST CC LOOP around zero.
Transfer functions measurement:
We performed a noise injection in both the loops. Both the loops were simultaneously closed. The loop filter used for the fast CC Loop was a pure integrator and a double pole at 20kHz with Q=1. The gain of the loop was 15000 at 1Hz. The loop filter used for the COARSE CC loop was a pure integrator and a double pole at 500 Hz Q=0.7. The gain of the loop was 0.05 at 1 Hz
- 8th April at 11:59:00 UTC we took five minutes of clean data with both the loop closed
- 8th April at 12:14:00 UTC we performed a noise injection on the Fast CC loop with a white noise (amplitude 50uV). Figure 1: OLG of Fast CC Loop
- 8th April at 14:24:00 UTC we performed a noise injection on the Coarse CC loop (amplitude 25mV). Figure 2: OLG of COARSE CC Loop
Proposal of new filters:
We tried to improve both the loops. In particular we added a boost at low frequency to the COARSE CC LOOP, we removed the integrator in the FAST CC LOOP, we increased its bandwith and we added a boost berween 0.011Hz and 50 Hz to the Fast CC Loop.
We saved the new controller for the Fast CC Loop in the filter configuration file with the name "CC_flt_boost". The gain is 150=50x3 @ 1 Hz: where 50 is a comoensation factor and 3 is due to the fact that we have increased the gain by a factor 3 respect to the previous filter.
ACL_FILTER_SET "CC_flt_boost" 1 150.0 1.0 20
ACL_FILTER_POLES "CC_flt_boost" 0.1 1.0
ACL_FILTER_ZEROS "CC_flt_boost" 50.0 0.0
ACL_FILTER_ZEROS "CC_flt_boost" 5000.0 0.7
ACL_FILTER_POLES "CC_flt_boost" 20000 1
The new filter for the HD COARSE LOOP is saved with the name "HD_COARSE_CC_BOOST_FLT"
ACL_FILTER_SET "HD_COARSE_CC_BOOST_FLT" 1 1 1 20
ACL_FILTER_POLES "HD_COARSE_CC_BOOST_FLT" 0 0
ACL_FILTER_POLES "HD_COARSE_CC_BOOST_FLT" 0.001 0
ACL_FILTER_ZEROS "HD_COARSE_CC_BOOST_FLT" 6.0 1.0
ACL_FILTER_POLES "HD_COARSE_CC_BOOST_FLT" 1000 0.5
Figure 3 shows the comparison between the new and the old loop filter for the HD CC Coarse. The new proposal has an UGF around 2 Hz
Figure 4 shows the comparison between the new and the old loop filter for the HD CC Fast. The new proposal has an UGF around 600 Hz
Figure 5 shows the FFT of the Slow CC loop error signal with the old filter and the projection with the new filter
Figure 6 shows the FFT of the FastCC loop error signal with the old filter and the projection with the new filter. We expect a 12% reduction of the pahse noise with the new filter
Test of new filters
All the measurements are taken with the Cpherent control demodulation phase equal to 1.75, i.e. all the CC signal was in the Q Quadrature. Moreover all the measurement are taken after a switch off of the HD AA dither lines.
- 9th April at 11:55:00 UTC 2 mins of data with the old filter (Fast CC Loop G=15000 Coarse CC Loop G=0.05)
- 9th April at 11:57:30 UTC 2 mins of data with the new Fas CC Loop Filter G=15000 and with the old Coarse CC Loop filter G=0.05
- 9th April at 12:07:00 UTC 2 mins of data with the new Fast CC Loop Filter G=15000 and with the new Coarse CC Loop Filter G=3
- 9th April at 12:11:00 UTC 2 mins of data with the new Fast CC Loop Filter G=45000 and with the new Coarse CC Loop Filter G=3
- 9th April at 12:19:00 UTC 2 mins with both the loops open
- 9th April at 12:25:30 UTC 2 mins of data with both the loops open and a Phase scan with freq = 3 Hz and amplitude equal to 1 V with the Fast CC PZT. These data are used for the error signal calibration Figure 7
- 9th April at 14:54:00 UTC 2 mins of data with both the loop open and the IR Shutter closed (we measured the sensing noise)
Figure 8 shows the residual phase noise of the coherent control loop in 4 different configurations. Blue: with the Old filters (16.4mrad of residual phase noise), Red with the new filter only in the Fast CC Loop (13.44 mrad of residual phase noise), Cyan with both the new filters (17.42 mrad of residual phase noise), Black with both the new filters but G-45000 for the fas CC Loop (6.58 mrad of residual phase noise).
Observing the cyan spectrum we noticed that increasing the slow CC loop caused a loose of gain in the fast cc loop, thus we compensated this by increasing the gain of the fast CC Loop of a factor 3 (black curve). We keep this as final configuration for the CC Loop
Figure 9: is the comparison between the phase noise with the Coherent control loop closed with the new filters (black), with the CC Loop Open Blue (phase noise 139 mrad) and the homodyne sensing noise red (phase noise contribution 0.87 mrad)
From Figure 9 is visible that the high CC Loop gain below 10 Hz is useless because we are limited by the sensing noise. By comparing the Blue and the black curves we can observe that between 500Hz and 5000 Hz we are slightly reintroducing phase noise.
Strategy used to engage the new CC Loop filters
- Use the SQZ_EQB1_HD_CC_CORR signal as error signal for the COARSE CC LOOP. This means set HD_CC_COARSE_ERR_SELECT = 1
- Close the Coarse CC Loop with gain = 3. This means set HD_CC_COARSE_GAIN = 3
- Select the old filter for the fast CC Loop (Filter 0) and reset it
- Close the Fast Coherent control loop with Gain = 15000
- Select the new fitler for the fast CC Loop (Filter 1) before reset it
- Increase the gain by a factor 3. I.e. Fast CC Loop Gain = 45000
When you want to open the loop open before the phast CC Loop and after the Coarse CC Loop.
We will write soon a python function to open/close the coherent control loop and we will implement soon the automation for the HD detection