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  11761   Fri Nov 13 15:48:16 2015 gautamUpdateLSCPhase tracker calibration using Rubidium standard

[yutaro, gautam]

Quote:

Summary:

I performed a preliminary calibration of the X and Y phase trackers, and found that the slopes of a linear fit of phase tracker output as a function of driven frequency (as measured with digital frequency counter) are 0.7886 +/- 0.0016 and 0.9630 +/- 0.0012 respectively (see Attachments #1 and #2). Based on this, the EPICS calibration constants have been updated. The data used for calibration has also been uploaded (Attachment #4).

Summary:

Having obtained a working FS725 Rubidium standard and syncing it to out GPS timing unit, I wanted to have one more pass at calibrating the phase tracker output, with the RF signal generator calibrated relative to an 'absolute' source. I also extended the range of frequencies swept over to 15MHz to 110MHz. We found that the phase tracker output appears linear over the entire range scanned, but taking a closer look at the residuals suggested some quadratic structure. Restricting the fitted range to [31MHz 89MHz] yields the following calibration constants for the X and Y arm respectively: 0.9904 +/- 0.0008 and 0.9984 +/- 0.0005. This suggests that out previous calibration was pretty accurate, and that it is valid over a wider range of frequencies, so we could plausibly fit in more FSRs in future scans if necessary. I have not updated these values on the EPICS screens (though judging by how close they are to 1, I wonder if this is even necessary)...

Details:

The principle change in the setup compared to that used to collect the data presented in elog 11738 was the addition of the FS725 rubidium standard. As detailed here, I synced the Rubidium standard to our GPS timing unit (this took a while - the manual suggests it should only take minutes, but it took about 10 hours - the two photos in Attachment #1 show the status of the front panel before and after it synced to the external 1PPS input). I then took 10 MHz outputs from the FS725, and ran one to the Fluke 6061A, and the other to the AG4395A. The Fluke 6061 A has a small switch at the back which has to be set to "EXT" in order for it to use the external reference (it has now been returned to the "INT" state). We then connected the output of the signal generator via a 3-way minicircuits splitter to the AG4395A, and the two beat channels. 

I cleared the phase history on the MEDM screen, and set the phase tracker UGF. We then swept through frequencies from 15MHz to 110MHz (using the AG4395 to verify the frequency at each step). I used the following command to record the average value (over 10 seconds) and the standard deviation: z avg 10 -s C1:ALS-BEATX_FINE_PHASE_OUT_HZ >> 20151113_PT_X.dat and so on.. The amplitude of the signal generated (i.e. before the splitter) was -18dBm (chosen such that the Q outputs of either phase tracker was between 1000 and 3000), while the gains were ~100 (X) and 50 (Y). I then downloaded the data and fitted it.

Fitting details:

The output of the phase tracker looks roughly linear over the entire range of frequencies scanned - but looking at the residuals, one could say there was some quadratic structure to it (see residual plots in Attachment #2). By looking at the shapes of the residuals, I judged that if we fit in the range [31MHz   89MHz] (for both X and Y), we should see negligible structure in the residuals. Attachment #3 contains the fits and residuals for these fits. One could argue that there is still some structure in the residuals, but is markedly less than over the entire range, and, I think, small enough to be neglected. The calibration constants quoted at the beginning of the elog are from the fits over this range. In principle, we could always break this down into smaller pieces and do a linear fit over that range. But this should allow us to scan through >5 FSRs.

Other remarks:

Since the beat signal also goes to the frequency counter via the couplers, I was also collecting the readouts of the frequency counter. Attachment #5 contains the data collected. It is interesting to note that the FCs fail at ~101 MHz (corresponding to ~6146 Hz after the dividers).

Also, we had taken another dataset last night, but found that there was an anomalous kink in the X phase tracker output at (coincidentally?) 89 MHz (I've attached the data in Attachment #6). I'm not sure why this happened, but this is what led me to take another dataset earlier today (Attachment #4).

Summary of Attachments:

  1. Attachment #1: Photos showing the front panel of the FS725 before and after syncing to the external 1PPS input.
  2. Attachment #2: Fits and residuals over the entire range scanned.
  3. Attachment #3: Fits and residuals over restricted range [31 89] MHz
  4. Attachment #4: Data used for phase tracker calibration.
  5. Attachment #5: Frequency counter data.
  11762   Fri Nov 13 17:33:39 2015 gautamUpdateLSCg-factor measurements
Quote:

ROC_ETMY = 59.3 +/- 0.1 m.

Summary:

I followed a slightly different fitting approach to Yutaro's in an attempt to determine the g-factor of the Y arm cavity (details of which are below), from which I determined the FSR to be 3.932 +/- 0.005 MHz (which would mean the cavity length is 38.12 +/- 0.05 m) and the RoC of ETMY to be 60.5 +/- 0.2 m. This is roughly consistent (within 2 error bars) of the ATF measurement of the RoC of ETMY quoted here.

Details:

I set up the problem as follows: we have a bunch of peaks that have been identified as TEM00, TEM10... etc, and from the fitting, we have a bunch of central frequencies for the Lorentzian shapes. The equation governing the spacing of the HOM's from the TEM00 peaks is:

\Delta f_{HOM_{mn}} = \frac{FSR}{\pi} (m+n)cos^{-1}(\sqrt{g_1 \times g_2})

The main differences in my approach are the following:

  1. I attempt to simultaneously find the optimal value of FSR, g1 and g2, by leaving all these as free parameters and defining an objective function that is the norm of the difference between the observed and expected values of \Delta f_{HOM_{mn}} (code in Attachment #1). I then use fminsearch in MATLAB to obtain the optimal set of parameters.
  2. I do not assume that the "unknown" peak alluded to in my previous elog is a TEM40 resonance - so I just use the TEM10, TEM20 and TEM30 peaks. I did so because in my calculations, the separation of these peaks from the TEM00 modes are not consistent with (m+n) = 4 in the above equation. As an aside, if I do impose that the "unknown" peak is a TEM40 peak, I get an RoC of 59.6 +/- 0.3 m.

Notes:

  1. The error in the optimal set of parameters is just the error in the central positions of the peaks, which is in turn due to (i) error in the calibration of the frequency axis and (ii) error in the fit to each peak. The second of these are negligible, the error in my fits are on the order of Hz, while the peaks themselves are of order MHz, meaning the fractional uncertainty is a few ppm - so (i) dominates.
  2. I am not sure if leaving the FSR as a free parameter like this is the best idea (?) - the FSR and arm length I obtain is substantially different from those reported in elog 9804 - by almost 30cm! However: the RoC estimate does not change appreciably if I do the fitting in a 2 step process: first find the FSR by fitting a line to to the 3 TEM00 peaks (I get FSR = 3.970 +/- 0.017 MHz) and then using this value in the above equation. The fminsearch approach then gives me an RoC of 60.7 +/- 0.3 m

 

 

  11800   Mon Nov 23 20:32:43 2015 KojiUpdateLSCFrequency source fixed, IMC LO level adjusted

The frequency source was fixed. The IMC LO level was adjusted.

IMC is locked => OLTF measured UGF 144kHz PM 30deg.

  11801   Mon Nov 23 21:48:49 2015 KojiUpdateLSCFrequency source fixed, IMC LO level adjusted

The trouble we had: the 29.5 MHz source had an output of 6 dBm instead of 13 dBm.

The cause of the issue: A short cable inside had its shield cut and had no connection of the return.


- The frequency source box was dismantled.
- The power supply voltages of +28 and +18 were provided from bench supplies.
- The 29.5 MHz output of 5~6 dBm was confirmed on the work bench.
- The 11 MHz OCXO out (unused) had an output of 13 dBm.

- Once the lid was opened, it was immediately found that the output cable for the 29.5 MHz source had a sharp cut of the shield (Attachment1).
- OK. This cable was replaced. The output of 13 dBm was recovered.

- But wait. Why is the decoupling capacitor on the 29.5 MHz OCXO bulging? The polarity of the electrolytic capacitor was wrong!
- OK. This capacitor was replaced. It was 100 uF 35 V but now it is 100 uF 50 V.

- I further found some cables which had flaky shields. Some of them were twisted. When the panel cable s connected, the feedthroughs were rotated. This twists internally connected cables. Solder balls were added to the connector to reinforce the cable end.

- When the box was dismantled, it was already noticed that some of the plastic screws to mount the internal copper heat sinks for ZHL-2's were broken.
They seemed to be degraded because of the silicone grease. I didn't try to replace all as it was expected to take too much time, so only the broken screws
were replaced with steel screws with shoulder washers
at the both side of the box.

- After confirming the circuit diagram, the box was returned to the rack. The 29.5 MHz output of 13 dBm there was confirmed.

  11803   Mon Nov 23 23:42:56 2015 ericqUpdateLSCALSY recovered

[ericq, gautam]

Gautam couldn't observe a Y green beatnote earlier, so we checked things out, fixed things up, and performance is back to nominal based on past references. 

Things done:

  • Marconi carrier output switched back on after Koji's excellent RF maintence
  • BBPD power supplies switched on
  • Removed a steering mirror from the green beatY path to do near/far field alignment. 
  • Aligned PSL / Y green beams 
  • Replaced mirror, centered beam on BBPD, moved GTRY camera to get the new spot.
  • POY locked, dither aligned, beatnote found, checked ALS out-of-loop noise, found to be in good shape. 
  11804   Tue Nov 24 01:14:23 2015 KojiUpdateLSCALSY recovered

Sorry, I completely forgot to turn the Marconi on...

  11806   Tue Nov 24 14:58:40 2015 yutaroUpdateLSCITMX misaligned

I misaligned ITMX. The oplev servo for ITMX is now turned off. You can restore ITMX alignment by running "restore".

  11810   Wed Nov 25 16:40:32 2015 yutaroUpdateLSCround trip loss of Y arm

I measured round trip loss of Y arm. The alignment of relevant mirrors was set ideal with dithering (no offset).

Summary:

 round trip loss of Y arm: 166.2 +/- 9.3 ppm

(In the error, only statistic error is included.)

How I measured it: 

I compared the power of light reflected by Y arm (measured at AS) when the arm was locked (P_L) and when ETMY was misaligned (P_M). P_L and P_M can be described as 

P_M=P_0(1-T_\mathrm{ITM})

P_L=P_0\left[1-(1-\alpha)\frac{4T_\mathrm{ITM}}{T_\mathrm{tot}^2}T_\mathrm{loss}\right].

The reason why P_L takes this form is: (1-alpha)*4T_ITM/(T_tot)^2 is intracavity power and then product of intracavity power and loss describes the power of light that is not reflected back. Here, alpha is power ratio of light that does not resonate in the arm (power of mismatched mode and modulated sideband), and T_tot is T_ITM+T_loss. Transmissivity of ETM is included in T_loss. I assumed alpha = 7%(mode mismatch) + 2 % (modulation) (elog 11745)

After some calculation we get

1-P_L/P_M\simeq \frac{4(1-\alpha) T_\mathrm{loss}}{T_\mathrm{ITM}}-T_\mathrm{ITM}.

Here, higher order terms of T_ITM and (T_loss/T_ITM) are ignored. Then we get

(1-\alpha) T_\mathrm{loss}=\frac{T_\mathrm{ITM}}{4}(1-P_L/P_M+T_\mathrm{ITM}).

Using this formula, I calculated T_loss. P_L and P_M were measured 100 times (each measurement consisted of 1.5 sec ave.) each and I took average of them. T_ETM =13.7 ppm is used.

Discussion: 

-- This value is not so different from the value ericq reported in July (elog 10248).

-- This method of measuring arm loss is NOT sensitive to T_ITM.  In contrast, the method in which loss is obtained from finesse (for example, elog 11740) is sensitive to T_ITM.

In the method I'm now reporting, 

\Delta T_\mathrm{loss}/T_\mathrm{loss}\simeq\Delta T_\mathrm{ITM}/T_\mathrm{ITM},

but in the method with finesse,

\Delta T_\mathrm{loss}\simeq\Delta T_\mathrm{ITM}.

In the latter case, if relative error of T_ITM is 10%, error of T_loss would be 1000 ppm.

So it would be better to use power of reflected light when you want to measure arm loss.  

  11813   Wed Nov 25 22:37:12 2015 yutaroUpdateLSCremoval of Gautam's cable in 1Y2 and restoration of POYDC

[yutaro, Koji]

We disconnected the cable that was connected to CH5 of the whitening filter in 1Y2, then connected POYDC cable to there (CH5). This channel is where POYDC used to connect.

Then we turned on the whitening filter for POYDC (C1:LSC-POYDC FM1) and changed the gain of analog whitening filter for POYDC from 0 dB to 39 dB (C1:LSC-POYDC_WhiteGain).

  11814   Wed Nov 25 22:59:42 2015 yutaroUpdateLSCAS table optics realignment

I slightly changed the orientation of a few mirrors on AS table that are used to make the AS light get into PDs, in order to confirm that the strange behavior of ASDC (I will report later) is not caused by clipping related to these mirrors or miscentering on PDs.

Then output level of ASDC, AS55, and AS165 could have changed.   

So take care of this possible change when you do something related to them. But the relative change of them would be at most several %, I think.

 

  11815   Wed Nov 25 23:17:34 2015 yutaroUpdateLSCstrange behavior of ASDC

[yutaro, Koji]

I noticed that ASDC level changes depending on the angle of ITMY when trying to take some data for loss map of YARM. We finally found that ASDC level behaves strangely when the angle of ITMY in yaw direction is varied, as you can see in Attachment 1. Now, AS port recieved only the reflection of ITMY. 

NOTE: This behavior indicates that angular motion could couple to length signal in AS port.      

Koji suggested that this behavior might be caused by interference at SR2 or SR3 between main path light and the light reflected by the AR surface. By rough estimation, we confirmed that this scenario would be possible. So it would be better to measure AR reflection of the same mirror to ones used for SR2 and SR3 in term of incident angle.     

Ed by KA: This senario could be true if the AR reflection of teh G&H mirrors have several % due to large angle of incidence. But then we still need think about the overlap between the ghost beam and the main beam. It's not so trivial.

  11816   Wed Nov 25 23:34:52 2015 yutaroUpdateLSCround trip loss of Y arm

[yutaro, Koji]

Due to the strange behavior (elog 11815) of ASDC level, we checked if it is possible to use POYDC instead of ASDC to measure the power of reflected light of YARM. Attached below is the spectrum of them when the arm is locked. This spectrum shows that it is not bad to use POYDC, in terms of noise. The spectrum of them when ETMY is misaligned looked similar.

So I am going to use POYDC instead of ASDC to measure arm loss of YARM. 


Ed by KA:
The spectra of POYDC and ASDC were measured. We foudn that they have coherence at around 1Hz (good).
It told us that POYDC is about 1/50 smaller than ASDC. Therefore in the attached plot, POYDC x50 is shown.
That's the meaning of the vertical axis unit "ASDC".

  11817   Thu Nov 26 19:39:27 2015 KojiUpdateLSCCurrent state of the frequency source, and possible improvement

Uploaded on T1000461 too.

  11818   Fri Nov 27 03:38:23 2015 yutaroSummaryLSCround trip loss of Y arm

Tonight I measured "loss map" of ETMY. The method to calculate round trip loss is same as written in elog 11810, except that I used POYDC instead of ASDC this time.

How I changed beam spot on ETMY is: elog 11779.

I measured round trip loss for 5 x 5 points. The result is below.

(unit: ppm)

494.9 +/- 7.6       356.8 +/- 6.0       253.9 +/- 7.9       250.3 +/- 8.2       290.6 +/- 5.1      
215.7 +/- 4.8       225.6 +/- 5.7       235.1 +/- 7.0       284.4 +/- 5.4       294.7 +/- 4.5      
205.2 +/- 6.0       227.9 +/- 5.8       229.4 +/- 7.2       280.5 +/- 6.3       320.9 +/- 4.3      
227.9 +/- 5.7       230.5 +/- 5.5       262.1 +/- 5.9       315.3 +/- 4.7       346.8 +/- 4.2      
239.7 +/- 4.5       260.7 +/- 5.3       281.2 +/- 5.8       333.7 +/- 5.0       373.8 +/- 4.9 

The correspondence between the loss shown above and the beam spot on ETMY is shown in the following figure. In the figure, "downward" and "left" indicate direction of shift of the beam spot when you watch it via the camera (ex. 494.9 ppm corresponds to the lowest and rightest point). 

Edited below on 28th Nov. 

To shift the beam spot on ETMY, I added offset in YARM dither loop. The offset was [-30,-15,0,15,30]x[-10,-5,0,5,10] for pitch and yaw, respectively.  How I calibrated the beam spot is basically based on elog 11779, but I multiplied 5.3922 for vertical direction and 4.6205 for horizontal direction which I had obtained by caliblation of oplev (elog 11785).

Edited above on 28 th Nov.    

 

   

 

I will report the detail later. 

  11819   Fri Nov 27 22:20:24 2015 yutaroUpdateLSCround trip loss of Y arm

Here, I upload data I took last night, including the power of reflected power (locked/misaligned) and transmitted power for each point (attachement 1).

 

And I would like to write about possible reason why the loss I measured with POYDC and the loss I measured with ASDC are different by about 60 - 70 ppm (elog 11810 and 11818). The conclusion I have reached is: 

It could be due to the strange bahavior of ASDC level. 

This difference corresponds to the error of ~2% in the value of P_L/P_M. As reported in elog 11815, ASDC level changes when angle of the light reflected by ITMY changes, and 2% change of ASDC level corresponds to 10 urad change of the angle of the light according to my rough estimation with the figure shown in elog 11815 and attachment 2. This means that 2% error in P_L/P_M could occur if the angle of the light incident to YARM and that of resonant light in YARM differ by 10 urad. Since the waist width w_0 of the beam is ~3 mm, with the 10 urad difference, the ratio of the power of TEM10 mode is (10\,\mu \mathrm{rad}/ \theta_0)^2\sim0.01, where \theta_0=\lambda/\pi w_0. This value is reasonable; in elog 11743 Gautam reported that the ratio of the power of TEM10 was ~ 0.03, from the result of cavity scan. Therefore it is possible that the angle of the light incident to YARM and that of resonant light in YARM differ by 10 urad and this difference causes the error of ~2% in P_L/P_M, which could exlain the 60 - 70 ppm difference. 

  11820   Sat Nov 28 11:46:40 2015 yutaroUpdateLSCpossible error source of loss map measurement

I found that TRY level degraded and the beam shape seen with CCD camera at AS port was splitted when the beam spot on ETMY was not close to the center. This was because dither started not working well. I suspect so because in such a case TRY level went up when I did iteration with TT1 and TT2 after freezing dither. Splitted beam shape indicates that incident light did not match well with the cavity mode.

TRY level for each point was this:

TRYDC
[[ 0.6573      0.8301      0.8983      0.8684      0.6773    ]
 [ 0.7555      0.8904      0.9394      0.8521      0.6779    ]
 [ 0.6844      0.8438      0.9318      0.8834      0.6593    ]
 [ 0.7429      0.8688      0.9254      0.8427      0.6474    ]
 [ 0.7034      0.8447      0.8834      0.8147      0.6966    ]]

 In the worst case, TRY level was 70 % of the maximum level. Assuming that this degrade was totally due to the mode mismatch, this corresponds to ~50 urad difference between the angle of incident light and resonant lighe in the arm (see elog 11819).

  11821   Sun Nov 29 05:23:57 2015 ranaUpdateLSCCurrent state of the frequency source, and possible improvement

I need some more hints to understand the improvement, although its generally good to re-build it considering the sad state of the assembly/installation that you found.

I see that the current design brings the 11 MHz signal to -2 dBm before intering the first ZHL-2+, but since that has a NF of 9 dB, that seems to only degrade the phase noise to -2 - (-174 +9) = -163 dBc. That seems OK since we only need -160 dBc from this system. Probably the AM noise is worse than this already (we should remember to hook up a simple AM stabilizer in 2016, as well as the ISS).

What else are the main features of this improvement? I can reward a good summary with some Wagonga.

  11822   Sun Nov 29 12:32:26 2015 KojiUpdateLSCCurrent state of the frequency source, and possible improvement

I'm not claiming we need to modify the frequency source immediately as we are not limited by the oscillator amplitude or phase noise.
I just wanted to note something in mind before it goes away quickly.

Alberto's T1000461 tells us that the oscillator and phase noise are degraded by factor of ~3 and ~5 due to the RF chanin.
My diagram is possible removal of up-down situation of the chain.

Maybe more direct improvement would be:

- Removal of two amplifiers out of four. The heat condition of the box is touch thought it is not critical.

- The modification will allow us to have a spare 11MHz channel at 1X2 rack that would be useful for 3f modulation.

  11823   Mon Nov 30 10:41:45 2015 yutaroUpdateLSCDoes a baffle in front of ETMY have effect on loss map measurement?

It might have, so I think I need to estimate shift of beam spot more preciely.

 

According to Steve's drawing, radius of the hole of the baffle is 19.8 mm.

Intensity distribution of fundamental mode in x axis direction is this (y is integrated out):

I(x)=\frac{1}{\sqrt{2\pi(w/2)^2}}e^{-\frac{1}{2}\left(\frac{x}{w/2}\right)^2}

With the radius of curvature of ETMY of 60 m and the arm length of 37.78 m, the beam width w on ETMY is estimated to be 5.14 mm. From this expression of the intensity, \int^\infty_{x=9.56\mathrm{mm}}\mathrm{d}xI(x)=100\,\mathrm{ppm},\,\int^\infty_{x=10.00\mathrm{mm}}\mathrm{d}xI(x)=50\,\mathrm{ppm}, for example. If round trip loss is considered, these values are doubled.

Although maximum shift of beam spot from the ideal spot on ETMY is estimated to be sqrt(6.0^2+(1.7+1.7)^2)=6.9 mm, this value could have error of several tens of % because I am not sure to what exten the calibration is precise, which means that the maximum shift could be ~10 mm and seperation between the baffle and the beam could be ~10 mm.

Therefore, I need to check how much the beam spot shifts with another way, maybe with captured image of the CCD camera. 

 

  11825   Mon Nov 30 14:12:14 2015 ericqUpdateLSCLO level check for the LSC RF distribution box

I checked the RF levels at the LSC LO distribution box, with the agilent scope and a handful of couplers. This was all done with the Marconi at +13dBm. 

I only checked the channels that are currently in use, since the analyzer only measures 3 channels at a time, and rewiring involves walking back and forth to the IOO rack to make sure unpowered amps aren't driven, and I was getting hungry. 

For the most part, the LO levels coming into the LSC demod boards are all around +1.5dBm (i.e. I measured around -18.0dBm out of the ZFDC-20-5 coupler, which has a nominal 19.5dB coupling factor)

The inputs piped over from the IOO rack, labeled as "+6dBm" were found to be 4.7dBm and 2.9dBm for 11Mhz and 55MHz, respectively. 

The 2F signals were generally about 40dB lower, with two exceptions:

  • REFL165's ~332MHz signal was around -18dBc
  • POP22 had many more visible harmonics than any other LO signal
    • 11MHz: -32 dBc
    • 33MHz: -32 dBc
    • 44Mhz: -15dBc 

Here are the raw numbers I measured out of the couplers, all in dBm:

  • 11MHz in: -14.8
  • 55MHz in: -16.6
  • POX11:    -18.7
  • POY11:    -18.0
  • REFL11:   -18.0
  • REFL33:   -18.3
  • POP110:   -17.9
  • AS110:    -18.1
  • POP22:    -19.9
  • REFL165:  -18.5
  • AS55:     -18.6
  • POP55:    -18.8 (this port is used as the REFL55 LO)
  11826   Mon Nov 30 15:17:57 2015 KojiUpdateLSCLO level check for the LSC RF distribution box

T1000461 tells us that the nominal LO input is 2dBm although we don't know what's the LO level is at the mixers in the demod boards.

  11827   Mon Nov 30 16:33:06 2015 ericqUpdateLSCstrange behavior of ASDC

One possible explanation of this behavior is simply poor centering of the AS beam on AS55 (whose DC level provides ASDC, if memory serves me correctly).

I misaligned ETMY, and moved ITMY through its current nominal alignment while looking at the POYDC and ASDC levels. 

In both pitch and yaw, the nominal alignment is fairly close to the "plateau" in which the AS beam is fully within the PD active surface. I.e. it doesn't take much angular motion to start to lose part of the beam, and thus introduce a first order coupling of angle to power. (Look at the plateaus at around -2min and -0.5min, and where the rapidly changing oplev trace crosses zero)

Furthermore, POYDC seems to be in some weird condition where it is actually possible to increase the reported powerwhen misaligning in pitch, but somehow there is more angular coupling in this state. 

In any case, I would advise that the POY11 and AS55 RFPDs have their spots recentered with optics in their nominal aligned states. In fact, given how we found REFL11 alingment to be less-than-ideal not so long ago, all of the RFPDs could probably use a checkup. 

  11828   Mon Nov 30 17:17:30 2015 yutaroUpdateLSCDoes a baffle in front of ETMY have effect on loss map measurement?

With captured images of ETMYF, I measured the shift of the beam spot on ETMY.

The conclusions are:

the baffle would have almost no effect on loss map measurement and

the calibration of beam spot shift is confirmed to be not so bad.

 

What I did:

I captured ETMYF images in the cases that (i)beam spot is centered on ETMY, beam spot is at the rightest and lowest point of my loss map measurement (corresponding to [0,0] component of the matrix shown in elog 11818), and beam spot is at the leftest and highest point of my loss map measurement ([4,4] component). Each captured image is attached.

Then using ImageJ, I measured the shift of the beam spot. I calibrated lengh in horizontal direction and vertical direction with the diameter of the mirror. 

Results:

The amount of the beam shift was 7.2 mm and 8.0 mm for each case.

These values indicate that clapping loss due to the baffle is less than 10 ppm in a round trip.

Today's results support the previous calibration with oplev, which says the amount of the beam shift is 7.0 mm. Two values derived by different calibrations coincide within ~10 % though they are totally different methods. This also support the calibration of the oplev for ETMY (elog 11785) indirectly. 

 

 

  11829   Mon Nov 30 18:27:30 2015 KojiUpdateLSCstrange behavior of ASDC

It wasn't fully mentioned in ELOG 11814.
We checked the PD first and this behavior didn't change after the realignment of the AS55PD.
Yutaro confirmed that this effect is happening in the vacuum chamber.

  11836   Wed Dec 2 03:34:30 2015 ericqUpdateLSCSRCL OLG weirdness

[gautam, ericq]

Since ETMX seems to have been on good behavior lately, we tried to fire the IFO back up. 

We had a fair amount of trouble locking the DRMI with the arms held off resonance. For reasons yet to be understood, we discovered that the SRCL OLG looks totally bananas. It isn't possible to hold the DRMI for very long with this shape, obviously. 

With the arms misaligned and the DRMI locked on 1F, the loop shape is totally normal. I haven't yet tried 3F locking with the arms misaligned, but this is a logical next step; I just need to look up the old demod angles used for this, since it wasn't quickly possible with the 3F demod angles that are currently set for the DRFPMI. 

  11838   Wed Dec 2 15:52:28 2015 yutaroUpdateLSCrestoration of POXDC

I disconnected the cable that was connected to CH6 of the whitening filter in 1Y2, then connected POXDC cable to there (CH6). This channel is where POXDC used to connect.

Then I turned on the whitening filter for POXDC and POYDC (C1:LSC-POXDC FM1, C1:LSC-POYDC FM1) and changed the gain of analog whitening filter for POXDC and POYDC from 0 dB to 45 dB and from 0 dB to 39 dB, respectively (C1:LSC-POXDC_WhiteGain, C1:LSC-POYDC_WhiteGain).

  11839   Wed Dec 2 17:14:33 2015 yutaroUpdateLSCBeam on POX11 is possibly not centered well

I checked how POXDC level changes when the angle of ITMX is varied. ETMX was misaligned.

Then I found that in YAW direction the POXDC level is maximized but it doesnt have plateau, and in PIT direction it is not maximized so that it is at the slope and it doesnt have plateau, as shown in attached figures. These results indicate that the beam size on POX11 is not small enough compared to the size of the diode and it is not centered well.

  11841   Thu Dec 3 03:01:07 2015 gautamUpdateLSCCalibration of C1CAL

[ericq, gautam]

While trying to resolve the strange SRCL loop shape seen yesterday (which has been resolved, eric will elog about it later), we got a chance to put in the correct filters to the "CINV" branch in the C1CAL model for MICH, PRCL, and SRCL - so we have some calibrated spectra now (Attachment #1). The procedure followed was as follows:

  1. Turn on the LO gain for the relevant channel (we used 50 for MICH and SRCL, 5 for PRCL)
  2. Look at the power spectra of the outputs of the "A" and "CINV" filter banks - the former has some calibrated filters in place already (though I believe they have not accounted for everything).
  3. Find the peak height at the LO excitation frequency for the two spectra, and calculate their ratio. Use this to install a gain filter in the CINV filter module for that channel. 
  4. Look at the spectrum of the output of the "W" filter bank for that channel - the plot attached shows this information.

The final set of gains used were:

MICH: -247 dB

PRCL: -256 dB

SRCL: -212 dB

and the gain-only filters in the CINV filter banks are all called "DRMI1f".

Once we are able to lock the DRFPMI again, we can do the same for CARM and DARM as well...

  11845   Thu Dec 3 19:10:28 2015 yutaroUpdateLSCXARM lock with ITMX actuated and related change on ASS

To avoid the strange kicking of ETMX, I locked XARM with ITMX actuated instead of ETMX so that I changed elements of C1LSC_OUTPUT_MTRX; before: XARM=ETMX, after: XARM=ITMX.

And I change C1:LSC-XARM_GAIN from 0.007 to 0.022.

 

With this setup, I ran dither but then error signals of dithering oscillated as shown in the figure below.

Then I found that if C1:ASS-XARM_ETM_PIT_L_DEMOD_SIG_GAIN / C1:ASS-XARM_ETM_YAW_L_DEMOD_SIG_GAIN in C1ASS_LOCKINS_XARM.adl are changed as 0.200 -> 0.100 and 0.200 -> 0.100, respectively, the dithering works well.

But the burt file that is loaded when you let dithering "ON" is not changed, because now I don't know how to update a burt file. So, if you let dithering "ON", the dithering will run with the condition that C1:ASS-XARM_ETM_PIT_L_DEMOD_SIG_GAIN / C1:ASS-XARM_ETM_YAW_L_DEMOD_SIG_GAIN are not 0.100 but 0.200.

 

   

  11847   Fri Dec 4 12:33:52 2015 yutaroUpdateLSCBeam on POX11 is possibly not centered well

To focus POX beam on POX11 PD, I added an iris and a lens before POX11 PD as you can see in Attachment 1.

It seemed that the beam is well focused, but the behavior of POXDC has not changed, as shown in Attachments 2 & 3.    

  11850   Fri Dec 4 23:02:13 2015 yutaroUpdateLSCBeam on POX11 is possibly not centered well

[yutaro, Koji]

Now, the beam on POX11 PD is well centered and well focused.

We found out why POXDC had behaved as reported in elog 11839. There were a few reasons: the beam was not focused enough, hight of a mirror was not matched to the beam well, path of the light reflected by misaligned SRM was occasionally close to the path of POX beam.

Then, What we did is following:

- changed orientation of SRM slightly

- changed the hight of the mirror whose hight had not matched well, by changing the pedestal (hight of which mirror was changed is shown in Attachment 1.)

- put a lens with f=250 mm (where the lens is located is shown in Attachment 1.)

- refined alignment for the POX beam to hit on the center of POX11 PD.  

As a result, POX DC level behaved as shown in Attachment 2&3 when the orientation of ITMX was varied (Attachment 2: POX DC vs ITMX PIT, Attachment 3: POX DC vs ITMX YAW). 

You can see broad plateau when varied in both PIT and YAW directions, and the beam is at the center of the plateau if ITMX is aligned ideally.

 

 

  11857   Mon Dec 7 11:11:25 2015 yutaroSummaryLSCround trip loss of X arm

On the day before yesterday and in this morning, I measured loss map of ETMX. I reported the method I used to change the beam spot on ETMX below.

Round trip loss was measured for 5 x 5 points. The result is below.

(unit: ppm)

455.4 +/- 21.1       437.1 +/- 21.8       482.3 +/- 21.8       461.6 +/- 22.5       507.9 +/- 20.1      
448.4 +/- 20.7       457.3 +/- 21.2       495.6 +/- 20.2       483.1 +/- 20.8       472.2 +/- 19.8      
436.9 +/- 19.3       444.6 +/- 19.7       483.0 +/- 19.5       474.9 +/- 20.9       498.3 +/- 18.7      
454.4 +/- 18.7       474.4 +/- 20.6       487.7 +/- 21.4       482.6 +/- 20.7       487.0 +/- 19.9      
443.7 +/- 18.6       469.9 +/- 20.2       482.8 +/- 18.7       480.9 +/- 19.5       486.1 +/- 19.2 

The correspondence between the loss shown above and the beam spot on ETMX is shown in the attached figure. In the figure, "up" and "right" indicate direction of shift of the beam spot when you watch it via the camera (ex. 455.4 ppm corresponds to the highest and rightest point in the view via the camera). 

This result is consistent withe previous result of 561.19 +/- 14.57 ppm ericq got with ASDC and reported in elog 10248 if the discussion I reported in 11819 is taken into account. Elog 11819 says in short that the strange behavior of ASDC could give us 60-70 ppm error.

The reason why the error is larger than that of the measurement for ETMY is that the noise of POX is larger than that of POY. But I am not sure to what extent the statistical error needs to be reduced.

How I shifted the beam spot on ETMX:   

Basically, the method was same as one used for Y arm. Different point is: for Y arm we have two steering mirrors TT1&2, but for X arm we have only one steering mirror BS. Then in order to shift incident beam so that the beam spot on ITMX does not change, I ran the dithering of X arm as well as that of Y arm and added offsets to both dither loops that caused same amount of shift on ETMX and ETMX. Thanks to the symmetry between X arm and Y arm, the dithering of Y arm ensured that the beam spot on ITMX was unchanged as well as that of ITMY. The idea of this method is schematically shown in Attachment 2. 

The calibration of how much the beam spot shifted is based on the results of elog 11846 . The offset was [-15,-7.5,0,7.5,15]x[-5,-2.5,0,2.5,5] for pitch and yaw, respectively.  

 

  11860   Mon Dec 7 15:56:35 2015 yutaroUpdateLSCXARM lock with ITMX actuated and related change on ASS

I changed the snapshot file for ASS, /opt/rtcds/caltech/c1/scripts/ASS_DITHER_ON.snap as follows:

L124 >  C1:ASS-XARM_ETM_PIT_GAIN 1 -5.000000000000000e-02

        => C1:ASS-XARM_ETM_PIT_GAIN 1 -1.500000000000000e-02

L128>   C1:ASS-XARM_ETM_YAW_GAIN 1 5.000000000000000e-02

        => C1:ASS-XARM_ETM_YAW_GAIN 1 1.500000000000000e-02

The purpose of this change is to avoid the oscillation when the dithering of X arm is running.

  11864   Tue Dec 8 15:57:16 2015 yutaroSummaryLSCPower recycling gain estimation from arm loss measurement

I estimated power recycling gain with the results of arm loss measurement.

From elog 11818 and 11857, round trip losses including transmittivity of ETM of Y arm and X arm (let us call them T_\mathrm{loss,Y} and T_\mathrm{loss,X}) are 229+13.7=243 ppm and 483+13.7=495 ppm, respectively.

 

How I calculated:

I used the following formula.

Amplitude reflectivity of an arm cavity r_\mathrm{FP}

r_\mathrm{FP}=\sqrt{1-\frac{4T_\mathrm{ITM}T_\mathrm{loss}}{T^2_\mathrm{tot}}}   (see elog 11816)

Amplitude reflectivity of FPMI r_\mathrm{FPMI}

r_\mathrm{FPMI}=\frac{1}{2}(r_\mathrm{FP,X}+r_\mathrm{FP,Y})

With power transmittivity of PRM T_\mathrm{PRM} and amplitude reflectivity of PRM r_\mathrm{PRM}, power recycling gain is

\mathrm{PRG}=\frac{T_\mathrm{PRM}}{(1-r_\mathrm{PRM}r_\mathrm{FPMI})^2}.

 I assumed T_\mathrm{ITM}\simeq T_\mathrm{tot}=\frac{2\pi}{401}=0.01566T_\mathrm{PRM}=0.05637, and r_\mathrm{PRM}=\sqrt{1-T_\mathrm{PRM}}, and then I got

PRG = 9.8.

Since both round trip losses have relative error of ~ 4 % and PRG is proportional to inverse square of T_\mathrm{loss} up to the leading order of it, relative error of PRG can be estimated as ~ 8 %, so PRG = 9.8 +/- 0.8

 

Discussion

According to elog 11691, which says TRX and TRY level was ~125 when DRFPMI was locked, power recycling gain was \mathrm{PRG}=125\times T_\mathrm{PRM}=7.0 at the last DRFPMI lock.

Measured PRG is lower than PRG estimated here, but it is natural because various causes such as mode mismatch between PRC mode and arm cavity mode, imperfect contrast of FPMI, and so on could decrease PRG, which Eric suggested to me. 

 

Added on Dec 9

If T_\mathrm{loss,X} were as small as T_\mathrm{loss,Y}, PRG would be 16.0. PRC would be still under coupled.  

  11870   Thu Dec 10 12:33:04 2015 yutaroUpdateLSCstrange behavior of ASDC

I did additional tests for the strange behavior of ASCD. ETMY, ETMX and ITMY were misaligned so that only light reflected by ITMX went into AS port. I had done similar measurement before with ITMY YAW varied.

Attachment 1 shows how ASDC level changed when ITMX PIT varied.

Attachment 2 shows how ASDC level changed when ITMX YAW varied.

Attachment 3 shows how the power of light measured by a power meter just after the AS view port varied when ITMX YAW varied.

 

Comparing 1 & 2, we can say that this behavior is not unique to YAW direction.

From 2 & 3, we can say something strange is happening inside the chamber.   

 

  11871   Thu Dec 10 19:53:22 2015 yutaroUpdateLSCstrange behavior of ASDC

To check if the strange behavior of ASDC is caused by SR2/SR3 or not, I did the following measurement:

ASDC measures the power of the light reflected by ITMX. POXDC measures the power of the light reflected by ITMX and SRM successively. Then I varied the angle of ITMX in YAW direction and compared the behaviors of ASDC and POXDC.

The results are shown in Attachments 1-3.

As you can see in these figures, the strange up-and-down behavior appeared ONLY in ASDC. Therefore, the cause of this behavior exists between AS table and SRM (I had confirmed that the angle of SRM did not affect ASDC).

And this behavior is fringe-like, as can be seen in the figures (there seems to be 3 "peaks" and 2 "valleys"), so the cause could be interference between main path and not good AR reflection at a mirror after SRM before AS table (I suspect a mirror is flipped mistakenly).   

  11872   Fri Dec 11 09:35:44 2015 yutaroUpdateLSCPower recycling gain estimation from arm loss measurement

I took PR3 AR reflectivity and calculated PRG (PR3 is flipped and so AR surface is inside PRC).

As shown in attached figure, which shows AR specification of the LaserOptik mirror (PR3 is this mirror), AR reflectivity of PR3 is ~0.5 %. Since resonant light in PRC goes through AR surface of PR3 4 times per round trip, round trip loss due to this is ~2 %. Then I got

PRG = 7.8.    

 

  11873   Fri Dec 11 13:28:36 2015 KojiUpdateLSCPower recycling gain estimation from arm loss measurement

Can I ask you to make a plot of the power recycling gain as a function of the average arm loss, indicating the current loss value?

  11874   Fri Dec 11 15:37:50 2015 yutaroUpdateLSCPower recycling gain estimation from arm loss measurement

Attached is the plot of relation between the average arm round trip loss and power recycling gain. 2 % loss due to PR3 AR reflection is taken into account.

  11889   Thu Dec 17 01:55:16 2015 ericqUpdateLSCUncooperative AUX X

[ericq, Gautam]

We were not able to fix the excess frequency noise of the AUX X laser by the usual laser diode current song and dance. Unfortunately, this level of noise is much too high to have any realistic chance of locking.  angry

We're leaving things back in the IR beat -> phase tracker state with free running AUX lasers, on the off chance that there may be anything interesting to see in the overnight data. This may be limited by our lack of automatic beatnote frequency control. (Gautam will soon implement this via digital frequency counter). I've upped the FINE_PHASE_OUT_HZ_DQ frame rate to 16k from 2k, so we can see more of the spectrum.

For the Y beat, there is the additional weird phenomenon that the beat amplitude slowly oscillates to zero over ~10 minutes, and then back up to its maximum. This makes it hard for the phase tracker servo to stay stable... I don't have a good explanation for this. 

  11892   Fri Dec 18 17:37:04 2015 ranaUpdateLSCUncooperative AUX X

Here's how we should diagnose the EX laser:

  1. Compare IR RIN of laser out to 100 kHz with that of another similar NPRO.
  2. Look at time series of IR beat signal with a fast scope. Are there any high frequency glitches?
  3. Disconnect all of the cables to the EX laser PZT and temperature control. Does the frequency noise change?
  4. Change the temperature by +/- 1 deg to move away from mode hop regions. Remeasure RIN and frequency noise and plot.
  11894   Mon Dec 21 02:29:49 2015 ericqUpdateLSCAUX X RIN measurements

I'll finish up the beat / frequency noise parts of the diagnosis tomorrow later, but I've done some investigation of the AUX X laser RIN. 

I placed a PDA255 at one of the rejected beams from the PBS on the downstream side of the IR faraday, making sure the power didn't saturate the PD. I measured the RIN on a SR785, and simultaneously looked at the signal on a 100MHz scope. 

The RIN has a very strong dependence on the laser diode current, and no noticable dependence on the crystal temperature or the presence of the PDH modulation / temperature control cables. Here are some traces, note that "nominal" current up until recently was 2.0A. 

When adjusting the diode current, a peak beings to appear in the tens of kHz, eventually noticible in the DC power trace on the scope. The point at which this occurs is not fixed.

At all times, I saw a strong intensity fluctuation at around 380-400kHz on the scope whose amplitude fluctuated a fair amount (at least 75mVrms over Vdc=6.5V, but would often be 2 or 3 times that).

I didn't look at the frequency noise while doing this, because the WiFi at the X end was too slow, I'll do more tomorrow in the daytime. 

  11908   Tue Jan 5 02:54:38 2016 ericqUpdateLSCAUX X Freq Noise attempt

[ericq, Gautam]

We set out to lock a marconi to the IR fiber beat of PSL + AUX X to measure some frequency noise, and failed.

In short, the Marconi's 1.6MHz max external FM isn't enough oomph to stabilize the PLL error signal. It's actually evident on the Agilent that the beat moves around a few times more than that, which I should've noticed sooner... We could briefly "lock" the PLL for a few tenths of a second, but weren't able to get a spectrum from this.

We also tried using the digital phase tracker temperature servo for some help at ~DC; this worked to the extent that we didn't have to twiddle the Marconi carrier frequency to stay on top of the fringes as the beat wandered, but it didn't otherwise stabilize the beat enough to make a difference in locking the PLL.

I suppose one more thing to try is to lock the PSL laser itself to each AUX laser in turn via PLL, and look for different / excess noise.

The Green and IR beat electronics are a in a little bit of disarray at the moment, but it's not like anyone else is going to be using them for the time being...

  11910   Tue Jan 5 13:17:06 2016 ranaUpdateLSCAUX X Freq Noise attempt

The problem here is that the MC displacement noise is leading to large frequency excursions of the PSL beam. Options

  1. Feed back the low frequency PLL control signal to the MC2 length to suppress the excursion required by the Marconi. This is better than driving the laser, since the drive to the laser would be squashed by the MC locking loop.
  2. Put the beat signal through a divider? Don't know if this makes the Marconi more able to handle it.
  3. Turn on the MCL path. this will make the low frequency MC error signal go to the MC length, thereby reducing the low frequency feedback to the NPRO.
  11912   Tue Jan 5 16:33:45 2016 ericqUpdateLSCAUX X Freq Noise attempt

Turning on the MCL path (in addition to the MCL FF we always have on) let me lock the PLL for multiple seconds, but low frequency excursions still break it in the end. I was able to briefly observe a level of ~50Hz/rtHz at 1kHz, which may or may not be real. Tomorrow we'll send the PLL control signal to MC2, which should lock it up just fine and give us time to twiddle laser diode current, measure the PLL loop shape, etc. 

  11917   Thu Jan 7 04:28:39 2016 ericqUpdateLSCAUX X Freq Noise measured

[ericq, Gautam]

Brief summary of tonights work:

  • Locked Marconi to AUX X vs PSL beat at around 320MHz, PSL shutter closed (i.e. both lasers free running)
  • Measured control signal spectrum at various laser diode currents, crystal temperatures. Oddly, spectra remained consistent across these variables. 
  • Measured OLG of PLL to calibrate into open-loop frequency noise of the beat, found UGF ~30kHz

Our "requirement" for the end laser is as follows: We expect to (and have in the past) achieved ALS sensitivity of 1Hz/rtHz at 100 Hz. If the end PDH loop is 1/f from 100Hz-10kHz, then we have 40dB of supression at 100Hz, meaning the free running AUX laser noise should be no more than 100Hz/rtHz at 100Hz.

So, if we expect both the PSL and AUX lasers to have this performance when free running, we would get the green curve below. We do not. frown


I'll post more details about the exact currents, temperatures and include calibrated plots for the >30kHz range later. Here's the OLG for kicks. 

  11919   Thu Jan 7 16:52:32 2016 ericqUpdateLSCAUX X Freq Noise measured

Here is some of the promised data. As mentioned, changing diode current and crystal temperature didn't have much effect on the frequency noise spectrum; but the spectrum itself does seem too high for our needs. 

At each temperature, we started measuring the spectrum at 1.8A, and stepped the current up, hoping to reach 2.0 A.

At 47.5 C, we were able to scan the current from 1.8 to 2.0 A without much problem. At 49.0C, the laser mode would hop away above 1.95A. At 50.4C it would hop away above 1.85A. The spectra were not seen to change when physically disconnecting the PZT actuation BNC from the rear of the laser. 

The flattening out at the upper end is likely due to the SR560 output noise. I foolishly neglected to record the output spectrum of it, but with the marconi external modulation set to 3.2MHz/V, the few Hz/rtHz above 20k translates to a signal on the order of uV/rtHz, which seems reasonable. 

Data and code attached. 

  11920   Thu Jan 7 19:04:25 2016 KojiUpdateLSCAUX X Freq Noise measured

The next step is to compare this data with the same measurement with the PSL and the AUX laser on the PSL table (or the end Y laser). If these show a lot lower noise level, we can say 1) the x-end laser is malfunctioning and 2) the y-end and AUX laser on the PSL are well low noise.

  11921   Fri Jan 8 14:47:33 2016 ericqUpdateLSCAUX Y Freq Noise measured

Here are some results from measuring the PSL / AUX Y beat. 

With the Y end laser, I was able to lock the PLL with a lower actuation range (1.6MHz/V), and with the PSL in both the free-running and MCL locked configurations. (In the latter, I had to do a bit of human-turning-knob servo to keep the control signal from running away). I also took a spectrum with the marconi detuned from the beat frequency, to estimate the noise from the PD+mixer+SR560. 

It looks like the AUX X laser is about 3 times noisier than the Y, though the Y laser looks more like a 10^5 noise-frequency product, whereas I thought we needed 10^4. 

Gautam is investigating the PSL / AUX PSL beat with Koji's setup now. 

  11922   Fri Jan 8 20:02:49 2016 ranaUpdateLSCAUX Y Freq Noise measured

Unless this is the limit from the way you guys set up the PLL, it seems like there's no difference between the two lasers that's of any import. So then the locking problem has been something else all along - perhaps its noise in the X-PDF lock somehow? PDH box oscillations?

ELOG V3.1.3-