POX11 oscillation at 630 Hz was stopped by installing 630 Hz resonant gain to LSC_XARM.
After few hours, oscillation stopped. So I removed the resonant gain.
Our guess is that 630 Hz peak is some violin mode or something, and it was excited somehow, and didn't stopped for very long time because of its high Q. It coupled into POY11 somehow (scattering, electronics, etc).

  Don't use resonant gain - it can lead to a loop instability since it makes the loop have 3 UGFs.

Just use a elliptic bandstop filter at this harmonic frequency separately for each test mass. There are many detailed examples of this in elog entries from Rob and I over the past ~10 years. This bandstop should get clicked on automatically after lock acquisition.

I aligned the Xgreen and PSL green to overlap on the X beat PD, and reconnected the splitter which combines the X and Y beat signals and sends them to the control room.

I've been stepping the Xend laser temperature offset in steps of 20 counts, making sure the cavity unlocks and relocks on TEM00.  So far I have not seen any beat signals for the Xarm.  I've gone from 0 to 840.

I'll be back in a few hours to keep trying, although interested parties are invited to give it a whirl. 

We measured the Xend green laser PDH Open loop transfer function by following method:

• We first measured the feedback transfer function 'K' directly.
  • See attachment 2 for this measurement. We measured Out2/exc here.
• Then, we closed the loop as shown in attachment 1with SR560 as a summing juntion at error point.
  • We injected excitation through B channel in SR560 and measured transfer function Out1/Out2.
  • This measurement should give us by loop alegbra.
• Then we multiplied the two transfer function measurements to get open loop transfer function.

Result:

• Our measurement gives the same UGF of 10kHz and phase margin of 53.5 degrees as reported in 13238.
• The shape of measurement also follows 1/f above 10 Hz atleast.
• Our measurement might not be correct below 10 Hz but we did not see any saturation or loss of lock in 1Hz to 10 Hz measurement.
• This OLTF is different from the modelled OLTF here even though the UGF matches.
• The feedback gain is supposed to roll-off faster than 1/f in 30Hz to 1kHz region but it does not seem to in our measurement.
• This suggests that the actual uPDH box is shaping the loop different from what schematic suggests. This might mean that the gain is much lower in the low frequency region than we would like it to be.
• We will investigate the reason of difference between model and measurement unless someone has a better explaination for the descripancy.

Attachment 1 shows the closed loop of Xend Green laser Arm PDH lock loop. Free running laser noise gets injected at laser head after the PZT actuation as . The PDH error signal at output of miser is fed to a gain 1 SR560 used as summing junction here. Used in 'A-B mode', the B port is used for sending in excitation where .

We have access to three ports for measurement, marked at output of mixer, at output of SR560, and at PZT out monitor port in uPDH box. From loop algebra, we get following:

, where is the open loop transfer function of the loop.

So measurement of can be done in following two ways (not a complete set):

1. , if excitation amplitude is large enough such that over all frequencies.
   • In this method however, note that SR785 would be taking ratio of unsuppresed excitation at with suppressed excitation at
   • If the closed loop gain (suppression) is too much, the excitation signal might drop below noise floor of SR785 while measuring .
   • This would then appear as a flat response in the transfer function.
   • This happened with us when we tried to measure this transfer function using this method. Below few hundered Hz, the measurement will become flat at around 40 dB.
   • Increasing the excitation amplitude where suppression is large should ideally work. We even tried to use Auto level reference option in SR785.
   • But the PDH loop gets unlocked as soon as we put exciation above 35 mV at this point in this loop.
2. , if excitation amplitude is large enough such that over all frequencies.
   • In this method, channel 1 (denominator) on SR785 would remain high in amplitude throughout the measurement avoiding the above issue of suppression below noise floor.
   • We can easily measure the feedback transfer funciton with the loop open. Then multiplying the two measurements should give us estimate of open loop transfer function.
   • This is waht we did in 16194. But we still could not increase the excitation amplitude beyond 35 mV at injection point and got a noisy measurement.
   • We checked yesterday coherence of excitation signal with the three measurment points and it was 1 throughout the frequency region of measurement for excitation amplitudes above 20 mV.
   • So as of now, we are not sure why our signal to noise was so poor in lower frequency measurement.

Attachment 1 shows the case when excitation is sent at control point i.e. the PZT output. As before, free running laser noise  in units of Hz/rtHz is added after the actuator and I've also shown shot noise being added just before the detector.

Again, we have a access to three output points for measurement. right at the output of mixer (the PDH error signal), the feedback signal to be applied by uPDH box (PZT Mon) and the output of the summing box SR560.

Doing loop algebra as before, we get:

So measurement of can be done by

• For frequencies, where is large enough, to have an SNR of 100, we need that ratio of to integrated noise is 100.
• Assuming you are averaging for 'm' number of cycles in your swept sine measurement, time of integration for the noise signal would be where f is the frequency point of the seeping sine wave.
  • This means, the amplitude of integrated laser frequency noise at either or would be
  • Therefore, signal to laser free running noise ratio at f would be .
  • This means to keep a constant SNR of S, we need to shape the excitation amplitude as
  • Putting in numbers for X end Green PDH loop, laser free-running frequency noise ASD is 1e4/f Hz/rtHz, laser PZT actuation is 1MHz/V, then for 10 integration cycles and SNR of 100, we get:
• Assuming you are averaging for a constant time  in swept sine measurement, then the amplitude of integrated laser free noise would be
  • In this case, signal to laser free-running noise ratio at f would be
  • This means to keep a constant SNR of S, we need to shape the excitation amplitude as
  • Again putting in numbers as above and integration time of 1s, we need an excitation amplitude shape

This means at 100 Hz, with 10 integration cycles, we should have needed only 3 mV of excitation signal to get an SNR of 100. However, we have been unable to get good measurements with even 25 mV of excitation. We tried increasing the cycles, that did not work either.

This post is to summarize this analysis. We need more tests to get any conclusions.

We did the measurement of OLTF for Xend green laser PDH loop with excitation added at control point using a SR560 as shown in attachment 1 of 16202. We also measured coherence in our measurement, see attachment 1.

Measurement details:

• We took the measurement as per 16202.
• We did measurement in two pieces. First in High frequency region, from 1 kHz to 100 kHz.
  • In this setup, the excitation amplitude was kept constant to 5 mV.
  • In this region, the OLTF is small enough that signal to noise ratio is maintained in (SR560 sum output, measured on CH1). The coherence can be seen to be constant 1 throughout for CH1 in this region.
  • But for (PZT Mon, measured on CH2), the low OLTF actually starts damping both signal and noise and to elevate it above SR785 noise floor, we had a high pass (z:0Hz, p:100kHz, k:1000) SR560 amplifying before measurement (see attachment 2). This amplification has been corrected in Attachment 1. This allowed us to improve the coherence on CH2 to above 0.5 mostly.
• Second region is from 3 Hz to 1 kHz.
  • In this setup, the excitation was shaped with a low pass (p: 1Hz, k:5) SR560 filter with SR785 source amplitude as 1V.
  • We took 40 averaging cycles in this measurement to improve the coherence further.
  • In this freqeuency region, is mostly coherent as we shaped the excitation as  and due to constant cycle number averaging, the integrated noise goes as (see 16202 for math).
  • We still lost coherence in (CH1) for frequencyes below 100 Hz. the reason is that the excitation is suppressed by OLTF while the noise is not for this channel. So the shaping of excitation only helps fight against the suppression of OLTF somewhat and not against the noise.
    
  • We need shaping for this purpose but we were loosing lock with that shaping so we shifted back to shaping and captured whatever we could.
  • It is clear that the noise takes over below 100 Hz and coherence in CH1 is lost there.

Inferences:

• Yes, the OLTF does not look how it should look but:
• The green region in attachment 1 shows the data points where coherence on both CH1 and CH2 was higher than 0.75.  So the saturation measured below 1 kHz, particularly in 100 Hz to 500 Hz (where coherence on both channels is almost 1) is real.
• This brings the question, what is saturating. As has been suggested before, our excitation signal is probably saturating some internal stage in the uPDH box. We need to investigate this next.
  • It is however very non-intuitive to why this saturation is so non-uniform (zig-zaggy) in both magnitude and phase.
  • In past experiences, whenever I saw somehting saturating, it would cause a flat top response in transfer function.
• Another interesting thing to note is the reduced UGF in this measurement.
• UGF is about 40-45 kHz. This we believe is due to reduced mode matching of the green light to the XARM when temperature of the end increases too much. We took the measurement at 6 pm and Koji posted the Xend's temperature to be 30 C at 7 pm in 16206. It certainly becomes harder to lock at hot temperatures, probably due to reduced phase margin and loop gain.

X arm aligned to green.

Aligned the X arm to IR.
Used steering mirrors to align the X end green to the X arm while remaining locked for IR. X arm locks to green stably with GTRX at the PSL table measuring 235uW and corresponds to 2560counts in C!:ALS-TRX_OUT.

Next
1. PSL green alignment.
2. Search for beat note.
3. Resurrect ALS for X arm.

X-green and PSL green have been aligned so that they interfere at the beat PD for X.

I haven't scanned the X-end NPRO temperature to find the beat note. I found the earlier elog when this was done (elog 6851) and will use those temperatures to start with.

I was showing some green laser locking to Tega, I noticed that changing the PZT sliders of M1/M2 angular position on Xend had no effect on locked TEM01 or TEM00 mode. This is odd as changing these sliders should increase or decrease the mode-matching of these modes. I suspect that the controls are not working correctly and the PZTs are either not powered up or not connected. We'll investigate this in near future as per priority.

I locked the Xarm on green.  At the PSL table, I adjusted the steering mirror to get the beam centered on the GTRX DC PD.  We need a lens for this, and presumably for the GTRY as well. 

I then went down to the Xend, and adjusted the steering mirrors to maximize the transmitted green power.  I got as high as 2150 counts.

Either the alignment is particularly delicate, or something isn't quite right, but when I put the lid back on the optical table's box, the arm will no longer lock on the 00 mode.  It's pretty typical that the cavity will unlock while you put on the lid, but usually if you bang on the underside of the table, or toggle the green shutter, you'll get back to the 00 mode.  Tonight however, I can't get the 00 mode if the lid is on.  If I slide the lid off just enough to get my hand inside, then block the green beam with my hand, I immediately lock on the 00 mode.  Even if I gently slide the lid back on, I unlock the cavity, and with the lid on can't get better than a 01 mode in yaw.  I repeated this a few times, with the same result. 

A goal for the next few days:  Re-find the Xgreen beatnote.  Once we have the PRMI locking stably and reliably, we want to move on to PRFPMI.   

Note that I'm supposed to return one of the two green beat PDs and the power supply.
They are on the REFL path. I'll work on the restoration of the beat configuration.

 I have the X end transmission QPD, as well as the whitening board, out on the electronics bench.  Since the Thorlabs high-gain TRX PD also goes through this whitening board, we have no transmission signal for the Xarm at this time. The whitening board was in the left-most slot, of the top crate in the Xend rack.  The only cables that exist for it (like the Yend), are the ribbon from the QPD, the 4-pin lemo from the Thorlabs PD, and the ribbon going to the ADC.

I have taken photos, and want to make sure that I know what is going on on the circuits, before I put them back in. 

The QPD:

The whitening board:

[EricQ, Jenne]

We have put the Xend QPD back in place, and centered it.  The whitening board was replaced by me a few days ago.

We also went down to the Yend and centered the Yend QPD.

I used the offset.py script that Masayuki wrote to zero the offsets of the individual quadrants when the PSL shutter was closed, and then I averaged the output of the SUM filter banks, and made the gains 1/AvgSum, so that both the Thorlabs PD and the QPD are normalized to 1 at single-arm resonance, for each arm.

I don't know what the gain is of the QPD head off the top of my head, relative to the Thorlabs PD, but eventually we want them to be the same, so that 1=1 and 700=700 on each PD.

I have looked at the photo of the Xend QPD from elog 9367, as well as the schematic for the board (D990272). 

Things that will need swapping out:

• Thick film resistors in the signal path need to be changed to thin film.
• MAX333 needs to be replaced with MAX333A.  The 333 has "ON Resistance" of 140-175 ohms, whereas the 333A has "ON Resistance" of 20-35 ohms.
• AD797 needs to be replaced by OPA140.  The 797 is a low voltage noise op-amp, but for a diode we want low current noise.  AD797 has 2pA/rtHz at 1kHz, whereas the OP140 has 0.8fA/rtHz at 1kHz (see Zach's elog 8125 re: OPA140).

I have ordered from digikey via techmart 10 each of the MAX333A's and the OPA140's.  (4 per QPD times 2 QPDs plus 2 spares = 10).  Both of these new chips have the same footprint and pinout as the part that they are replacing, so it'll be a fairly easy task.

Next up, I need to make a LISO model for the circuit for one of the quadrants, to see what shape it'll turn out to be.  Part of this will include deciding what resistors and capacitors to put in the OPA140 gain stage. 

Right now, the AD797s say on the schematic that the gain options are different by a factor of 5, but the actual QPD has a different resistor than is on the schematic, and there is also a capacitor in parallel with each resistor, so I need to just pull those out, and pick some values that make sense to me. 

Rana and I have discussed ignoring the 2nd and 3rd gain switching options on each quadrant, as that is getting to be more fine control than we need. 

Other things on the board:

• The 50 ohm resistors to ground for the "QPD_rtn" have all shorted.  Rana says this is good, so leave it as-is.
• The positive input to the AD797's all have a 100 ohm resistor to ground, rather than just being connected to ground.  Why is this??

For now, I will probably just work on the QPD head, and not the whitening board.  For now, we can run with 1 stage of whitening, and if we need lower noise, we can revisit the whitening board (including replacing the thick film resistors with thin film).

When thinking about what gains I want on my gain stages, I want to have my full arm power (~700 TRX units) be ~20,000 counts from the ADC.  So, I want my single arm power (1 TRX unit) to be ~30 counts from the ADC.  This is not such a big number, so this may also require more thinking. 

 Since we use the TransMon QPD for triggering the high/low gain switching we need to run with the whitening OFF during lock acquisition and the turn it on after we have the arms locked with ALS. This should be put into the up/down scripts.

Okay, I have finished modifying the Xend QPD whitening board, although I will likely need to change the gain on Monday.

Rather than following my plan in elog 10782, I removed the AD602's entirely, and just use the AD620's as the amplifiers.  We don't need remotely adjustable gains, and the AD620s are a less noisy part.

I set the gain to be 30dB using a 1.65k resistor for R_G, which turns out to be too high.  After I installed the board and realized that my counts were much higher than they used to be, I realized that what we had been calling +30dB was in fact +13.2dB. ( I am assuming that the ADC for the gain sliders were putting out a maximum of +10V.  The AD620 used to have a 1/10 voltage divider at the input, and an overall gain of 1, so the output of the AD620 was 100mV.  This goes into pin 16 of the AD602, which has gain of 32*V_set + 10.  Which gives 32*0.1+10=13.2dB.  Ooops.  We've been lying to ourselves. )

Anyhow, before I made the gain realization, I was happily going along, setting the AD620s' gains all to 30dB. I also copied Koji's modification from April of this year, and permanently enabled the whitening filters.

Here is the schematic of what ended up happening.  The red modifications were already in place, and the greens are what I did today.

You can see the "before" picture in my elog Wednesday, elog 10774.  Here is an "after" photo:

Here is a spectrum comparing the dark noise of the Xend QPD after modification to the current Yend QPD (which is still using the AD602 as the main instrumentation amplifier).  I have given the Yend data an extra 16.8dB to make things match.

And, here is a set of spectra comparing both ends, dark noise versus single arm lock.  While I'll have to sacrifice a lot of it, there's oodles more SNR in the Xend now.  The Yend data still has the "gain fixing" extra 16.8dB.

The Xend quadrant input counts (before the de-whitening filters) now go up to peak values of about 1,000 at single arm lock.  If (optimistically) the we got full power recycling and the arms got to powers of 300, that would mean we would have 300,000 counts, which is obviously way more than we actually have ADC range for.  Currently, the Yend quadrant input counts go as high as 50, which with arm powers of 300 would give 15,000 counts.  I think I need to bring the Xend gain down to about the level of the Yend, so that we don't saturate at full arm powers.  I can't remember right now - are the ends 14-bit or 16-bit ADCs?  If they're 16-bit, then I can set the gain somewhere between the current X and Y values.

 Finally, I added a section of the 40m's DCC document tree for the QPD whitening:  E1400473, with a page for each end.  Xend = D1400414, Yend = D1400415.

 16 bit. There aren't any 14-bit ADCs anywhere in LIGO. The aLIGO suspensions have 18-bit DACs.

The Y-End gains seem reasonable to me. I think that we only use TRX/Y as error signals once we have arm powers of >5 so we should consider if the SNR is good enough at that point; i.e. what would be the actual arm motion if we are limited only by the dark noise?

I seem to remember that the estimate for the ultimate arm power is ~200, considering that we have such high losses in the arms.

In April, Koji logged that he had made some changes to the Yend QPD whitening board (elog 9854).  Today, I pulled the Xend board to see if it had the same modifications.  The filter shapes all seem to be the same (as in, the capacitors at the output filters were removed, etc.), and the final gain is the same.  I just realized that I didn't explicitly check if the whitening switches were pulled to ground to permanently turn on the whitenening, but hopefully I'll be able to see that in the photo. 

I have not made any changes today (yet) to the board, so the overall gain is still accessible via EPICS.  I wanted to do a quick check that we won't be saturating things at full power with the maximum gain, before I make a change.

EDIT:  After comparing the photos here and in elog 9854, the X end board has the filter shape modifications that were done some time ago, but the whitening is not permanently enabled.  For the Yend board, Koji added a jumper wire connecting (for example) R97 and R106 to the grounded side of C69.  This jumper wire is not in place on the X qpd board.

Before I re-pull the board and modify it, I want to make sure I know what I'm going to do for the gain slider override.

Here is a little PDF of what I plan to do to both of the transmission QPD whitening boards later today.  The idea is to take away the remote gain slider inputs, and force the gains to always be at +30dB.

The red and blue notes are from Koji's elog 9854, and the green are my plans for today. 

I will cut the traces from the gain slider inputs, and pull the negative input of the AD620 to ground.  The positive input will be connected to the +5 voltage line, with a divider so that the positive input to the AD620 is about 666mV. 

The AD602 will be maxed out at +30dB with anything over 625mV. 

Unless there are objections, I will start these modifications in an hour or so.  I will also make the Xarm whitening always-on, just like Koji has already done for the Yend.

EDIT, JCD, 12Dec2014:  These are not the modifications that were made.  Please see ____ for actual modifications.

I went down to the Xend table to look at it to understand Steve's proposal, and I noticed that the doubling crystal's heater's cable is mushed between the table's edge and the black table cover wall.  This made me sad, so I disabled the heater, turned it off, then unplugged the cable from the back of the controller.  I tried to re-route the cable through the hole in the black table cover wall, but going that way the cable is ~1 foot too short.  So I put it back the way it was, but used a totally hacky solution to prevent the cable from being mushed.  I put a dog clamp right at the edge of the table so it is pushing on the table cover wall a little bit, to give the cable space to get out.  This is very mickey mouse, and kind of lame.  But we either need to make a cable extension, or somehow get the heater controller to sit much, much higher under the table.

I plugged the heater controller back in, and turned it back on to the same setpoint that it was at (I think 37.5C).  It's probably warm by now, but when I turned it back on, the heater's actual temp was 33C.

[Rana Koji]

This is an elog about the activity on Friday night.

- The X arm green beam was aligned with assist of the ASX system.

- M1 PZT alignment was swept while M2 PZT was under the control of ASX.

- Everytime M1 was touched, M2 was restored by manual alignment so that the REFL beam hits the center of the REFL PD.
  This way we could recover the lock of TEM00. Once TEM00 is recovered, ASX took care of the alignment of M2

- The error signal used by the cavity dither did not give us a good indication where the optimal alignment is.

- Thus the best alignment of M1 had to be manually scanned. The resulting maximum green transmission was ~0.88

- Once the beam was aligned, the out-of-loop stability of the Xarm was measured.
  There has been no indication of the improvement compared to Manasa's measurement taken before our beam alignment.

ASX scripts for PZT dither have been fixed appropriately. Script resides in scripts/ASX.

You can run the scripts from the ASX medm screen now.

 Shutter moved, no more clipping.

Pick-off mirror 2" replaced by 1" one. Laseroptik HR 532nm, incident angle 30-45 degrees, AR 532 nm

Green REFL PD moved to 4" close to pick-off mirror. Pd being close to pick-off does not separate multiple reflections on it. I'll replace Laseroptic mirror with Al one. It is not easy to find.

 Hole cut into side wall for doubler oven cable to exit.

- An Aluminum mirror instead of 2" unknown mirror for the pick-off for the rejected beam from the green faraday isolator (Steve)
=> Replaced. To be reviewed

- Faraday mount replacement. Check what we have for the replacement. (Steve)

- The green REFL PD should be closer to the pick-off mirror. (Steve)
=> Moved. To be reviewed

- A beam dump should be placed for the green REFL PD

- Move the green shutter to the place where the spot is small (Steve)
=> Moved. To be reviewed.

- The pole of the PZT mounting should be replaced with a reasonable one. (Steve with Manasa's supervision)

- Tidying up doubling oven cable. Make a hole on the wall. (Steve)
=> Done. To be reviewed.

- Tidying up the PZT cabling (Steve)

- The optics are dirty. To be drag wiped. (Manasa, Masayuki)

 Beam trap for Pd refl is in place. Cabeling is ti·died up.

 Laseroptic 1" mirror is replaced by Al 1" mirror. Problem remains the same. This diffraction patter has to be coming from the Faraday.

  Atm1, good separation when Pd is far 

  Atm2, bad separation when Pd is close 

 The extra high post 3.375"  for PZT is ready. We also have 2 more 2" green Laseroptik mirrors. I'm ready to swap them in.

The  75 mm  focal length  lens  was placed in front of the green REFL PD yesterday.

Working in Xend with mask on has become unbearable. It is very hot there and I would really like if we fix this issue.

Today, the Xend Green laser was just unable to hold lock for longer than 10's of seconds. The longest I could see it hold lock was for about 2 minutes. I couldn't find anything obviously wrong with it. Attached are noise spectrums of error and control points. The control point spectrum shows good matching with typical free running laser noise.

Are the few peaks above 10 kHz in error point spectrum worrysome? I need to think more about it in a cooler place to make sure.

I wanted to take a high frequency spectrum of error point to make sure that higher harmonics of 250 kHz modulation frequency are not leaking into the PDH box after demodulation. However, the lock could not be maintained long enough to take this final measurement. I'll try again tomorrow morning. It is generally cooler in the mornings.

This post is just an update on what's happening. I need to work more to get some meaningful inferences about this loop.

The Xgreen PD now has a cable going over to the beatbox. Once beatbox characterization is done I can re-find the beat, and we can do some stuff with the beatbox.

[Yuta, Koji, Jenne]

Lots of small things happened tonight, in preparation for having both arms' ALS working simultaneously.

1. Xarm aligned in IR

     1.1 ETMX oplev centered

2. Xgreen coarsely aligned to Xarm

3. X beat setup on PSL table resurrected. 

     3.1 Steering optics for both X and Y green (before PBS) were touched to fix clipping Xgreen on some of the first mirrors after the light exits the chambers.

     3.2 Xgreen aligned to beat PD

     3.3 PSL green waveplate rotated so ~half of the light goes to X beat, other ~half goes to Y beat (recall we had rotated the polarization so we had max light on the Y beat PD a few weeks ago).

          3.3.1 Now we have ~80uW of PSL green going to each beat PD.

     3.4 PSL green aligned to X beat PD

          3.4.1 Replaced mount for mirror between PBS (which splits PSL green light) and BS (which combines PSL green and X green) so that I could get the alignment correct without having to use the full range of the knobs on the mount.

     3.5 Realigned (coarsely) Ygreen to Y beat PD - the mirrors just after the chambers had been touched, so Y green was no longer directly on the PD.  This will need to be done more finely when we're ready to lock the Yarm again.

     3.6 Dedicated cables for the DC of each beat PD were put in place, so we have those in addition to the DC transmission PDs which we are putting in temporarily each time we align the green to the cavities.  Some mystery unused cables that were running under the PSL table were removed.  The power for the X beat PD was rerouted so that it's much closer to the actual diode, and out of the way. 

4. Better alignment of X green to X arm.

     4.1 Put Green Transmission camera into place

     4.2 Noticed that the X green spot on the transmission camera is not nearly as steady as the Y green.  Increased the gain of the X green refl PD on the end table to see if it helped the spot be more steady, but it's still very wiggly.  We reverted the gain to what it was.  We need to fix this!!!!

    4.3 Removed camera, looked at X transmission DC (PD is temporarily in front of the beat PD), tried to increase the transmission.

     4.4 Aligning the green to the X arm has been really tough - there were a few more iterations of camera then DC PD.

     4.5 Measured X green power on the PSL table - 02 mode was ~150uW.  The 00 mode is still not very stable, which is frustrating, although we have a reasonable amount of power transmitted.   

     4.6 The X end green shutter was moved out of the beam path since the green beam was clipping while going through the shutter.  We need to put it back now that the beam is pretty much aligned.  The beam size and the aperture are roughly the same, so we should look to see if there is a different place on the table where the beam is a little smaller, where we can put the shutter.

5. Whitening filters (Pomona box-style) made for the Xarm I and Q channels - these are the same as the whitening for the Y arm.

6. 30m SMA cable made to be used for 2nd delay line.

     6.1 Steve reminded me this morning that we returned one of the fancy spools of cable that was purchased for the delay lines, since it was defective.  We didn't get it replaced because there was debate as to what is the best kind of cable to use.  We need to come to a conclusion, but for now we have a regular RG-405 cable.

7.  Jamie has started work on modifying the beatbox so that we can have 2-arm ALS.  Hopefully that will be done soon-ish, because we're otherwise pretty close to being ready.

[Jenne, Jamie]

We took a look at the Xend green, and we weren't able to make it lock.  We improved the alignment a little bit, and when we looked at the error signal, it looked nice and PDH-y, but for whatever reason, the cavity won't catch lock.

While aligning the green to the arm, Jamie noticed that the reflection from the intracavity power (not the prompt reflection) was not overlapping with the input beam or prompt reflection.  This means that the cavity axis and the input green beam were not co-linear.  I adjusted the BS and ITMX to get the IR transmitted beam (which had been near clipping on the top edge of the first (2 inch) optic it sees out of the vacuum) back near the input green beam spot on the combining beam splitter.  Then we continued tweaking the green alignment until we saw nice TEM00 flashes in the cavity.  The SNR of the error signal increased significantly after this work, since the cavity buildup was much higher.  But alas, still no lock. 

 I tweaked the alignment of ITMX and ETMX a teeny bit to get the TEM00 flashes back (the work in the previous elog was pre-dinner, so it had been a few hours), then took a screenshot of the error signal and refl dc power on the photodiode for the green xend setup.

The error signal is certainly noisy, although I think when Jamie and I were looking at it earlier this evening, the SNR was a little better.

I need to look at the modulation depth, to see if it's correct, ... maybe lock the Xarm on IR and scan the green laser PZT to check the sideband heights.

I should also check to make sure that the PD is powered, and the gain is high enough (currently the PD gain is set to 20dB).  Earlier today, when I set the gain to 30dB, Jamie said that it was saturating, so I put it back down to the 20dB where we found it.

Still no lock of the green though :(

Edit: realized I was bad and didn't label the traces on the plot:  green is refl dc power, blue is demodulated error signal.

The Y end aux laser light leaking after the doubling crystal has been coupled into the 70m long PM fiber. 

Input power = 250mW;  Output after 70m = 20mW

The poor efficiency is partially due to the ellipticity of the beam itself and partially due to the compromise I had to make using a single lens to couple the light into the fiber (given the limitations in space). But 20mW should be more than sufficient for a beat note setup.

Light propagates as follows after the doubling crystal:

Doubler ---> Harmonic Separator (45deg) ---> Lens (f=12.5cm) --> Steering mirror (Y1) --> Fiber collimator ( Thor labs CFC-2X-C) --> FIber end

I will update photos of the setup shortly.

I have left the 70m fiber in its spool sitting at the Y end and blocked the light before the last Y1 steering mirror in the above setup. So it should be safe.

Other:

Through the course of the work, I disabled the ETMY oplev and enabled it before closing the enclosure. I also reduced the AUX laser power and brought it back up after the work.
I did a check to see if the arms are locking in both IR and green and they did.
 

 10% seems like a pretty bad coupling efficiency, even for a single lens. I know that the NPRO itself isn't so elliptical as that. Where is the other 230 mW going? random scattering?

Given that this is such an invasive process and, since its so painful to lose a whole night of locking due to end table business, I suggest that you always measure the out-of-loop ALS noise at the end of the end table work. Just checking that the green laser is locked to the arm is not sufficient to prove that the end table work won't prevent us from locking the interferometer.

We should insist on this anytime someone works on the optics or electronics at EX or EY. Don't have enough time to do an out-of-loop ALS spectrum? Then don't work at the end tables at all that day. We've got PZT alignment and mode matching work to do, as well as the rebuild of the EX table enclosure, so this is a good discipline to pick up now.

I used the above configuration and was able to obtain ~52% coupling.

Input power = 250mW
Output power with absorptive ND 1.0 = 13 mW

I used the absorptive ND filter before the lens to keep the coupled output power within the range of fiber power meter and also avoid scattering of enormous amount of uncoupled light all over the table.

I have attached the screenshot of the out of loop ALS noise before opening the table (BLUE) and after closing down (MAGENTA). The beat note frequency and amplitude before and after were (14.4MHz/-9.3dBm) and (20.9MHz/-10 dBm).

Since I obtained a poor coupling efficiency from the earlier setup, I went back to calculate the coupling efficiency of the current setup.

For the current setup, I took the average of the x and y waist of the input beam and calculated the distance at which the input beam diameter would match the (fiber +collimator) beam diameter.

Average waist = 40.2um @-3.3mm from face of doubling crystal

(Fiber PM980 + Collimator f=2.0mm) beam waist = 205um

Distance(z) at which the input beam waist is 205um = 11.9cm

The closest available lens was f = 12.5cm. So I used it to couple the input beam by placing it at z ~12.5cm on a micrometer stage.

Since this gave only 10% coupling, I went back to calculate (using 'a la mode') the best possible coupling that can be obtained taking into consideration the ellipticity of the beam.

The maximum obtainable coupling (mode overlap) is 14.5%  which is still poor.

Redesign

Taking into account the ellipticity of the input beam, the available lenses and the space restrictions (lens can be placed only between z= 8 to 28cm), I calculated the best possible coupling efficiency (using 'a la mode').

The maximum possible mode overlap that can be obtained is 58.6% (matlab code and plot attached)

>>auxmode

modematching = 0.58632 

Optimized Path Component List:

    label    z (m)     type    parameters         

    -----    -----     ----    ----------         

    L1       0.0923    lens    focalLength: 0.0750

As for the X Arm, this the transfer function I measured for the Y arm cavity.

This time I'm using a different photodiode than the PDA255 on the Y end table.

The diode I'm using is the PDA520 from where TRY comes from.

I'm going to repeat the measurement with PDA255.

 Measurement repeated with the PDA255 PD at the end but not big changes

 I was getting the Y Arm ready for Eric Q's loss measurements and so I looked at the noise and loop shape. The loop shape was strange:

You can see that the gain margin is too low at high frequencies. That's why we have >15 dB of gain peaking. Way too much! I think this is from Masayuki and Manasa increasing the phase margin at some point in the past. I lowered the gain by 3 dB from 0.1 to 0.07 and now the awful gain peaking is less. But what about the low frequency gain? Is there enough?

I calibrated the OUT channel with 14 nm/count (1/f^2) with a Q = 10 pole pair at 1 Hz. The error signal is done to cross over at 180 Hz. It looks like the resonant gain at 25 Hz is a little too much and the in-loop RMS is 10 pm. Jenne says the linewidth is ~1 nm, so this seems sort of OK. Except that the LIGO-I DARM RMS had to be <0.1 pm for ~the same linewidth. Do we need to do better before trying to bring the arms into resonance?

I've remove FM1 and FM8. I put the RollRG of FM8 into the BounceRG and renamed it BounceRoll. Also changed the Y-arm restore so that RollRG and the 5,5:0,0 are no longer triggered automatically since the double integrator was overkill and we already have a 1:0 in FM2. I also lowered the peak gain for the roll mode RG from 30 to 10 dB because it was also overkill. We've gained a few more degrees at the UGF.

Yesterday and today I was in the lab doing many cavity scan.

First I did many measurement with the cavity aligned in order to get the position of the 00 modes, then I misaligned the beam in many different ways to enhance the higher order modes.

In particular, I first misaligned the mode cleaner to make the beam clipping into the Faraday. To do this, I set to 0 the WFS gain, but I left the autolocker still enabled. In this way, the autolocker couldn't bring the mirrors back to the aligned position.

Then I misaligned also the TT2 to get even more HOMs.

Eventually, Rana came and we misaligned TT1 to clip the beam, and using TT2 we aligned back the beam to the arm.

To increase the SNR, we changed the gain of the TRY PD, setting it to 20dB (which corresponds to a factor 100 in digital scale)

I attached one scan that I did with Rana on Sunday night. I could not upload a better resolution image because the file size was too big, but here's the path to find all of the scans:

../users/annalisa/sweep/Yarm

There are many folders, one per each day I measured. In each folder there are measurements relative to aligned cavity, Pitch and Yaw misalignment.

 RXA EDIT:

The PDA520 used for TRY was set to 0 dB analog gain. This corresponds to ~500 counts out of 32768. The change to 20 dB actually increases the gain by 100. This makes the single arm lock saturate at ~25000 counts (obviously in analog before the ADC). The right setting for our usual running is probably 10 dB.

For the IMC WFS, we had disabled the turn on in the autolocker to use the IMC to steer the beam in the FI, but that was a flop (not enough range, not enough lever arm). In the end, I think we didn't get any clipping.

I lined up the Y Arm for locking and then centered the oplevs for ETMY and ITMY.

* The ITMY OL has still got the old style laser. Steve, pleaes swap this one for a HeNe. Also the optical layout seems strange: there are two copies of the laser beam going into the chamber (??). Also, the QPD transimpedance needs to be increase by a factor of ~10. We're only getting ~500 counts per quadrant. Its worth it for someone to re-examine the whole ITMY OL beam layout.

* The ETMY OL beam was coming out but clipping on the mount for the ETMY OL HeNe. This indicates a failure on our part to do the ETMY closeout alignment properly. In fact, I get the feeling from looking around that we overlooked aligning the OL and IPPOS/ANG beams this time. If we're unlucky this could cause us to vent again. I undid part of the laser mount and changed the height on the receiving mirror to get the beam back onto the QPD.

I noticed that there is significant green light now getting into some of the IR PDs; beacuse of this there are weird offsets in the TRY QPD and perhaps elsewhere. We had better purchase some filters to tape over the front of the sensitive IR sensors to prevent the couplling from the green laser.

* There is a beam on IPPOS, but its too big for the detector (this has always been the case). We need to put a 2" lens with a weak focusing power on this path so as to halve the beam size on the detector. Right now its clipping and misleading. There is also a 0.9V offset on the SUM signal. I'm not sure if this readout is working at all.

* I couldn't find any beam on IPANG at all. Not sure what's changed since Kiwamu saw it.

 The 2nd beam from this laser is for the SRM's OpLev, so that shouldn't be changed.

For better or worse, we didn't do anything to the ETM OpLevs, because they don't have any in-vac steering optics.  We did however go through and check on all the corner OpLevs.

I went to the Y end. The AUX laser was on Standby. I pushed the Standby button. The laser turned on and there was some green light. However, the controller displayed the message "CABLE?" which according to the manual means that the laser head is powered but there is no control over the laser (e.g. the control cable is disconnected). I turned off the controller and disconnected both the power and control cables. I put them back and turned the controller back on.

I pushed the Standby button, the laser turned on and this time the controller displayed the laserhead's state. I was able to change the current/temperature. The problem seems to be resolved.

The first Faraday isolater rejected beam path from the NPRO is fixed.

I borrowed the GTRY BBPD  for the REFL165 trial before.

Now the PD is back on the PSL table.

The PD is intentionally misaligned so that anyone can find it is not aligned.

I did a calibration measurement for the Y part of the BeatBox using a Marconi. This is in order to get a more accurate calibration for the arm cavity scan measurement.

The calibration factor I found is:

C1:ALS-BEATX_FINE_PHASE_OUT   50.801 +/- 0.009 deg/MHz

Procedure

During my cavity scan measurement, I had recorded the beat frequency and amplitude from the Spectrum Analyzer at each zero crossing.

I connected the Marconi to the RF in of the Y part of the BeatBox, and I set the Marconi carrier frequency at one of this zero-crossing frequency that I had recorded, while I set the amplitude in way to have on the spectrum analyzer the same beat amplitude that I read during the measurements or, equivalently, in order to have C1:ALS_BEATY_FINE_Q of the order of 1200 (which is the same value I had during my measurements).

I started with

• Carrier frequency = 80.2 MHz
• Amplitude = -3dBm

Then I monitored the C1:ALS_BEATY_FINE_I on the oscilloscope and I adjusted the carrier frequency so that I had zero signal on the oscilloscope. Eventually the frequency corresponding to the zero crossing was 79.989 MHz.

I resetted the phase (clear history in the BEATY_FINE_PHASE panel) and I started changing the frequency by steps of 0.2 MHz, and I spanned about 70 MHz (from 32 to 102 MHz).

Resutls

The calibration coefficient I found is not so different from the one that Yuta measured (elog 8199).

Here are the fit parameters:

y = a + bx

a = -4239.7 +/- 0.6 deg

b = 50.801 +/- 0.009 deg/MHz

I tried the "Yarm + PRMI" configuration to see what happens.
The Y arm was locked at a resonance and held with the ALS technique.
On the other hand, the X arm was freely swinging.

I briefly tried severl demod signals to calm down the central part, but didn't succeed.
Now I feel I really want to have the X arm locked with the ALS technique too.
Give me the beat-box !

The attached screen shot shows the transmitted light of both arms as a function of time.
TRY is always above 1, since it was kept at a resonance.
Sometimes TRY went to 50 or so.

I calculated how the DC signals should look like in the Y arm PRMI configuration.

The expected signals are overlaid in the same plot as that of shown in #6313.

You can see there are disagreements between the observed and expected signals in the plot below at around the time when the arm is brought to the resonance.

(expected behaviors)

•  TRY: At the end it should be at 1 (remember TRY is normarlized) and should not go more than that, since the power-recycling is in a weird situation and it is not fully recycling the power.
•  ASDC: It should become brighter at the end because the arm cavity flips the sign of the reflected light and hence the dark port must be on a bright fringe.
•  REFLDC: It will decrease a little bit because the arm cavity and MICH try to suck some amount of the power into the interferometer.

I tried the Yarm + PRMI configuration again.

The PRMI part was locked, but it didn't stay locked during the Y arm was brought to the resonance point.

I will post the time series data later.

(locking of the PRMI part)

Tonight I was able lock the PRMI when the arm was off from the resonance by 10 nm (#6306).

This time I used REFL11Q to lock the MICH instead of the usual AS55Q because the MICH didn't stay locked with AS55Q for some reason.

The PRCL was held by REFL33I as usual.

Also I disabled the power normalization for the error signals because it could do something bad during the Y arm is borough to the resonance.

In order to reduce the number of the glitches, PRM was slightly misaligned because I knew that the lower finesse gives fewer glitches.