This is in reponse to LLO alog 27053 : " ..... Would be helpful if DetChar can tell us if the OL lasers have been glitchy in the last several months. ...."
Summary:
| Oplev |
Data segment (1Hr long) |
Sum (counts) | Glitchy? | Remarks |
|---|---|---|---|---|
| ETMX | 01-08-2016 13:00 UTC | 41000 | No | Seems good. One more stage of whitening may also be ok. |
| ETMY | 01-08-2016 10:00 UTC | 28000 | No | All Good. Could copy these settings to ETMX. |
| ITMX | 01-08-2016 10:00 UTC | 33000 | No | The periodic pulses seen in the SUM is probably due to the temperature controller in the diode laser having railed or become unstable. |
| ITMY | 01-08-2016 10:00 UTC | 30000 | No | Does not seem to have whitening filters since we can see in the SUM spectra that it is being limited by ADC noise beyond 5Hz. |
I dont think the ITMX laser is mode hopping. I did not notice any step like changes in the SUM signal, though it is hard to tell due to the periodic pulses.
The latter can be fixed by tweaking the PID setttings of temperature controller and its set point, I think.
Both ITM oplevs have large resonances at ~60 and ~90 Hz. These are much attenuated in ETMs. The mounting of ITM oplevs could perhaps be examined to see if there is something out of place.
QPD Sum calibration:
At the laser wavelength 632 nm, Si QPD efficiency is ~0.35 A/W; Transimpedance=10kOhm according to D1100290 ; Differential Ouput gives x2 ; 16bit-ADC gives (2^16)/20 counts/V;
Using these factors : 1mW of incident light on the QPD would give us ~23k counts. Since ETMX, ITMX and ITMY oplevs have 18dB, 21 dB and 15 dB whitening gain,
the incident power on these QPDs is
| ETMX | ETMY | ITMX | ITMY |
| 0.32 mW | 1.22 mW | 0.13 mW | 0.23 mW |
And 1/4 of that per quadrant !!
Note: I'm not sure over what time duration Suresh made his analysis, but Jason has recently tweaked the gain and whitening settings on the ITMX, ITMY, and ETMX optical levers last week Tuesday (July 28). See LHO aLOG 28645. Likely, Suresh's suggestions to improve the whitening were based on data prior to this change, and the setting are now acceptable. See Keita's post-change assessment in LHO aLOG 28661.
The data segment analysed is from 1st Aug 2016 at about 10:00 UTC.
And yes ... I had initially analysed data from before the change and had to redo the job after I saw the post by Jason on the 26th Aug (and also the request from Keita). This analysis is after all those changes.
And just in case I goofed up something, I rechecked the time stamp on the data plotted in the attached pdf file. All data is from 1st Aug.