%Check for accidental resonances of HOM in the arms (maybe due to
%sidebands). At the moment there is only data for the y arm.
%Stuff one might change often
modeorder = 0:5; % Look for TEMmn modes where m,n run over modeorder
Kevin and I meaured the transfer function of the photodiode circuit using the Jenne laser and agilent in the 40m lab. The attached figures depict our measured transfer function over the modulation frequency ranges of 30kHz-30MHz and 1kHz-30MHz when the power of the laser was set to 69 and 95 μW. These plots indicate a clear roll off frequency around 300 kHz. In addition, the plots beginning at 1kHz display unstable behavior at frequencies below 30kHz. I am not sure why there is such a sharp change in the transfer function around 30kHz, but I suspect this to be due to an issue with the agilent or photodiode.
I've restarted the NDS2 process on Megatron so that we can use it for getting past data and eventually from outside the 40m.
1) from /home/controls/nds2 (which is not a good place for programs to run) I ran nds2-megatron/start-nds2
2) this is just a script that runs the binary from /usr/bin/ and then leaves a log file in ~/nds2/log/
3) I tested with DTT that I could access megatron:31200 and get data that way.
There is a script in usr/bin called nds2_nightly which seems to be the thing we should run by cron to get the channel list to get updated, but I' m not sure. Let's see if we can get an ELOG entry about how this works.
Then we want Jamie to allow some kind of tunneling so that the 40m data can be accessed from outside, etc.
I have done the following:
* installed the nds2-client in /ligo/apps/nds2-client
* moved the nds2 configuration directories to /ligo/apps/nds2/nds2-megatron
* set up a cron job to update the channel list every morning at 5 am. The cron line is:
15 5 * * * /usr/bin/nds2_nightly /ligo/apps/nds2/channel-tracker /ligo/apps/nds2/nds2-megatron
cron will send an email each time the channel list changes, at which point you will have to restart the server with:
* restarted nds2 with updated channel lists.
I have set the cron job up to restart the nds2 server automatically if the channel list changes. The only change is that the cron command was changes to /ligo/apps/nds2/nds2-megatron/test-restart.
The upgrade of megatron broke the nds2 service. I have fixed things by
1) installing the latest version of framecpp (1.19.32) from the lsc debian repository (this was necessary because I couldn't link to the existing version)
2) built nds2-server-0.5.11 and installed it in the system directories (/usr/bin)
3) there were a few scripts/links/etc that didn't seem to be set up correctly and I fixed them to correspond with my preious message.
nds2 is now running and the channel list should be updated regularly and the service restarted as appropriate.
Here's a Finesse modeling of what we're expecting to observe with this test. It uses Gautam's base model of the 40m IFO with appropriate modifications for the needed configuration.
The idea is to lock the IFO in the SRMI configuration, with the phase-locked AUX beam injected from the AS port. The AUX beam is imprinted with AM sidebands and slightly misaligned relative to the SRC so as to transfer power into HOM1. The RF network analyzer provides the drive signal for the AOM, and its frequency is swept to coherently measure the transfer function [reflected AUX beam / drive]. The reflected AUX beam is sensed by the AS110 PDA10CF.
It is also possible to drive PM sidebands as Koji suggests, but the squeezer group has encouraged using AM for practical advantages. The SNR with AM is a bit higher (less power lost into harmonics at large modulation index), there is a broadband AOM already available aligned to the SQZ beam at LLO, and there is also concern that driving strong PM could interfere with the SQZ control loops.
Attachment #1 shows the expected response to swept-AM in SRMI. Resolving just the FSR and the first-order mode splitting is sufficient to extract the SRC Gouy phase.
Since the 40m has not been opearted in SRMI since ~2016 (last done by Eric Q.), Gautam believes it may take some time to relock this configuration. However, the modeling indicates that we can likely obtain sufficient sensitivity in DRMI, which would allow us to proceed faster. Attachment #2 shows the expected response to swept-AM in DRMI. The PRC leakage signal turns out to be significantly smaller than the SRC reflection (a factor of ~30 in amplitude), so that the signal still retains its characteristic shape to a very good approximation. The tradeoff is a 10x reduction in SNR due to increased PSL shot noise reaching AS110.
Based on this, we should proceed with DRMI scans instead of PRMI next week.
The PRC FSR is, of course, very close to twice of our f1 moudlation frequency (11MHz x 2 = 22MHz) .
I still don't understand what response the measurement is looking for. I understood the idea of using the subcarrier as a stablized carrier to the PRC with a certain freq offset from the main carrier. I suppose what was swept was the AOM modulation frequency (i.e. modulation frequency of the AM applied to the subcarrier). If that is the case, the subcarrier seemed fixed at an arbitorary frequency (i.e. 50MHz) away from the carrier. If one of the AM sidebands hits the PRC resonance (i.e. 22, 44, 66MHz away from the main carrier), you still have the other sideband reflected back to the AS. Then the RF signal at the AS is still dominated by this reflected sideband. I feel that the phase modulation is rather suitable for this purpose.
If you are talking about ~MHz AM modulation by the AOM and scanning the PLL frequency from 1MHz to 60MHz, the story is different. And this should involve demodulation of the AS signal at the AM modulation frequency. But I still don't understand why we don't use phase modulation, which gives us the PDH type signal at the reflection (i.e. AS) port...
Not much progress today with the AUX cavity scans. I've determined there still are some alignment issues.
At the start of today a large AUX/PSL beat note was visible on the AS110 sensor, at a similar power as where we left off last night (-60 dBm). Proceeding from there, I attempted to reproduce Johannes' measurement of the cavity transmission resonances. I misaligned the X-arm, locked the Y-arm cavity, and scanned the AUX RF offset approximately 8 MHz in 2 kHz steps. This should have swept through two FSRs, but nothing was visible.
Further inspection revealed that none of the PSL light was making it back to through the AUX fiber to the PSL table. I take this to mean that the beam seen earlier on AS110 was the ITMY reflection, and that the AUX injection axis was no longer reaching ETMY. I also found that the AUX beam size just after the 90/10 beasmsplitter looks anomolously large. Maybe a lens was recently changed? In any case, the mode-matching looks like it is going to need to be readjusted.