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)
modematching = 0.58632
Optimized Path Component List:
label z (m) type parameters
----- ----- ---- ----------
L1 0.0923 lens focalLength: 0.0750
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).
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.
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.
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.
[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.
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.
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.
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
Beam trap for Pd refl is in place. Cabeling is ti·died up.
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.
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.
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.
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.
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.
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.
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:
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:
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.
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.
* Whitening for the transmission QPDs needs to be thought about more carefully. (Calculation, then hardware)
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 whitening board:
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 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.
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.
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.
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.
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
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.
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):
We measured the Xend green laser PDH Open loop transfer function by following method:
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.
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.
There is an oscillation in the Xarm at 631Hz, which is not in the Yarm. There is a small peak in POY11_I at this frequency, but only when the Xarm is locked. If the Xarm unlocks, the peak disappears from POY. The peak is 3 orders of magnitude larger in POX than in POY, and 4 orders of magnitude larger than the POY noise when this peak is not present. In the plot, I have turned off the POY whitening, so that its situation is the same as POX (we still need to fix POX whitening switching). Dark noise (MC unlocked) is the same for both PDs.
I don't know who left the X arm locked, but I just ran the Align Full IFO script, so everything is good in case Yoichi/someone comes in to lock the IFO this weekend.
Squishing cables at the ITMX satellite box seems to have fixed the wandering ITM that I observed yesterday - the sooner we are rid of these evil connectors the better.
I had changed the input pointing of the green injection from EX to mark a "good" alignment of the cavity axis, so I used the green beam to try and recover the X arm alignment. After some tweaking of the ITM and ETM angle bias voltages, I was able to get good GTRX values [Attachment #1], and also see clear evidence of (admittedly weak) IR resonances in TRX [Attachment #2]. I can't see the reflection from ITMX on the AS camera, but I suspect this is because the ITMY cage is in the way. This will likely have to be redone tomorrow after setting the input pointing for the Y arm cavity axis, but hopefully things will converge faster and we can close up sooner. Closing the PSL shutter for now...
I also rebooted the unresponsive c1susaux to facilitate the alignment work tomorrow.
Towards finding the x-arm beat note:
The green would not lock to a maximum GTRX this morning. In the course of aligning the green stably to the X arm, somewhere down the line, the input pointing got messed up (reasons unknown). To set this right, Koji tried to lock the Yarm with POY DC but it wouldn't work. The transmon for Y had to be set up temporarily and the Y arm was locked with TRY. This restored the input pointing and the arms locked with transmission TRX/TRY > 0.9 counts. The transmon path along the Y arm was then re-configured as mentioned in Annalisa's elog.
I still had trouble getting the X-green locked in TEM00 (similar situation mentioned by Jenne in elog). The arm cavity mirrors were tweaked to get the green to resonate in TEM00 but it wouldn't stay locked when the temperature of the x-end NPRO was changed. Koji helped recover missing links to filters for the ALS_X_SLOW servo from the archives. Enabling the filters helped keep the green locking stable for laser temperature changes (which corresponds to 'offset' change in ALS_X_SLOW servo screen).
PSL green alignment was checked once again and the X-end laser temperature was scanned trying to find the beatnote. RFMON from the beatbox was connected to the spectrum analyzer. I have scanned through the whole range of offset but have not been able to find the beat note yet.
The search will continue tomorrow
We should consider to hook up the temperature monitors of the NPROs to the ADCs.
Current Mode Matching and Gouy Phase Between Steering Mirrors
We found in 40m elog ID 3330 ( http://nodus.ligo.caltech.edu:8080/40m/3330) a documentation done by Kiwamu, where he measured the waist of the green. The waist of the green is about 35µm. Using a la mode, I was able to calculate the current mode matching, and the Gouy phase between the steering mirrors. In a la mode, I used the optical distances,which is just the distance measured times its index of refraction. I contacted someone from ThorLabs (which is the company that bought Optics For Research), and that person told that the Faraday IO-5-532-LP has a Terbium Gallium Garnet crystal of a length of 7mm and its index of refraction is 1.95. The current mode matching is 0.9343, and the current Gouy phase between steering mirrors is 0.2023 degrees. On Monday, Nick and I are planning to measure the actual mode matching. The attached below is the current X-arm optical layout.
Calculation For the New Optical Layout
Since the current Gouy phase between the steering mirror is essentially zero, we need to find a way how to increase the Gouy Phase. We tried to add two more lenses after the second steering mirror, and we found that increasing the Gouy phase result in a dramatically decrease in mode matching. For instance, a Gouy phase of about 50 degrees results in a mode matching of about .2, which is awful. We removed the first lens after the faraday, and we added two more mirrors and two more lenses after the second steering mirror. I modified the photo that I took and I place where the new lenses and new mirrors should go as shown in the second pictures attached below. Using a la mode, we found the following solution:
label z (m) type parameters
----- ----- ---- ----------
lens 1 0.0800 lens focalLength: 0.1000
First mirror 0.1550 flat mirror none:
Second mirror 0.2800 flat mirror none:
lens 2 0.4275 lens focalLength: Inf
lens 3 0.6549 lens focalLength: 0.3000
lens 4 0.8968 lens focalLength: -0.250
Third mirror 1.0675 flat mirror none:
Fourth mirror 1.4183 flat mirror none:
lens 5 1.6384 lens focalLength: -0.100
Fifth mirror 1.7351 flat mirror none:
Sixth mirror 2.0859 flat mirror none:
lens 6 2.1621 lens focalLength: 0.6000
ETM 2.7407 lens focalLength: -129.7
ITM 40.5307 flat mirror none:
The mode matching is 0.9786. The different Gouy phase different between Third Mirror and Fourth Mirror is 69.59 degrees, Gouy Phase between Fourth and Fifth 18.80 degrees, Gouy phase between Fifth and Sixth mirrors is 1.28 degrees, Gouy phase between Third and Fifth 88.38 degrees, and the Gouy phase between Fourth and Sixth is 20.08 degrees. Bellow attached the a la Mode code and the Plots.
Plan for this week
I don't think we have the lenses that we need for this new setup. Mostly, we will need to order the lenses on Monday. As I mention, Nick and I are going to measure the actual mode matching on Monday. If everything look good, then we will move on and do the Upgrade.
% In this code we are using a la mode to optimatize the mode matching and
% to optimatize the Gouy phase between mirror 1 and mirror 2. All the units
% are in meter
w0=(50*1e-6)/sqrt(2); % The Waist of the laser measured after SHG
z0_laser=-0.0083; % position measured where the waist is located
lamb= 532*10^-9; % wavelength of green light in mm
lFaraday=.0638; % Length of the faraday
[Rana, Jenne, EricQ]
We did several things tonight. First, a list (so I can remember them all), and then some details.
(1) Jiggled ETMY SUS cables, removed kicks.
(2) Locked X and Y ALS, looked at POX, POY as out of loop sensors.
(3) Measured stuff (?) at the Yend.
(4) Reconnected REFL DC to SR560.
(5) Attempted CARM offset reduction.
When Rana and I started locking this evening, we saw (as Q has been witnessing for a while now) the ETMY kick a lot. However, it seemed to be kicking even more than usual. Since Q had been down at the end station recabling things, we wondered if a SUS-related cable got bumped. Rana went down to the end and pushed all the cables into their receptacles. One of the last sets that he pushed was the satellite box. We didn't have walkie-talkie communication, but the DC offset of the ETMY oplevs changed just a minute or two before he returned to the control room. So, we guess that it was the satellite box cables that were loose. Unfortunately, there is no clear way to strain relieve them, which is why they can so often be troublesome. Anyhow, the ETMY hasn't kicked since.
We locked the arms with ALS. We saw that the POX signal was about 20% of the full pk-pk height of the PDH signal, so it's mostly within the linear range, but not entirely. It is what it is, however, and we took measurements assuming that it's okay. I calibrated POX by putting an excitation onto ETMX, and matching the height of the peak in POX and BEATX_FINE_PHASE_OUT_HZ.
Q and Rana had also [remembered / put in / something] a digital readback for the end green PDH error point. Q went down to the end and gave me a number of 2600 Hz/V for the err mon port of the PDH board, which is what is connected to the ADC. With that and 20/2^16 V/cts, I had a calibration of 0.8 Hz/ct.
What we see in this plot is that the green end PDH is not the limiting noise for the POX out of loop measurement of the residual arm motion. Also, in the multi-color metrology paper, Fig 7 (which is posted in the control room), we see at about a little over 1 Hz a ratio of about 4.5 between the residual motion and the AUX PDH error signal. In today's plot, I see a ratio of about 20. I infer from this that the green PDH for the Xarm is fine, and that we may want to re-look at the ALS digital loop, but we should leave the X PDH alone.
Here is the Xarm plot:
Q took the data for the Yarm plot, so hopefully he can give it to us in the morning. What we did notice was that the noise was much worse for the Yarm. This prompted Item 3, measuring the loop.
Q and Rana went down to the Yend and measured some things. They came back, and said that they hadn't changed anything in analog while they were down there. One thing that Q did note was that we have almost 90 degrees of phase margin (since it's a 1/f loop), and about 10 dB of gain margin, above the UGF. So, we're in good shape for being able to try triggering the boost on the PDH box. Q will give us more notes on this work, as well as plots, in the morning.
At some point, I remembered that Q and Gabriele had repurposed the SR560 that we had been using for the REFLDC input to the common mode board. So, Q went and put it back, so that REFL DC goes into the SR560, and so does a DAC channel so that we can remotely set the offset. The A-B output goes to the REFL11I whitening channel, since real REFL11I goes into the input of the CM board. I think that today, the SR 560 was left at a gain of 1.
We decided to carry on and try to reduce the CARM offset some. An annoyance is that the Yarm still has pretty significant low-frequency noise, but the idea is that if we can get over to the sqrtInvTrans signals, it will be fine.
So, we didn't get much farther than we had in the past, but it was nice to get there at all again. I ran the carm_cm_up script (many times). One of the times, all I wanted to do was see how much I could reduce the CARM offset. CARM was on sqrtInvTrans, DARM was on ALS diff, and I was able to get the arm powers up to about 2.5. I don't know why I lost lock. The sqrtInv signals should be good until at least arm powers of 20 or so.
I was able to see the REFL DC dip, but only a teensy tiny bit. It went down by maybe 1 count. Q suggested looking at how deep it could get while leaving CARM and DARM both on ALS, and setting both offsets to 0. We were seeing arm flashes of about 50 counts, and REFL DC went from 0 to -800. So, I wasn't seeing much of a REFL dip, but it was definitely there when I went to arm powers of 2ish.
We tried looking at different sqrtInv options for DARM, and haven't come to any real conclusion. In the plot below, we are looking at a swept sine between DARM_IN1 (ALSdiff) and either MC_IN1 0.3*(sqrtInvX - sqrtInvY) or SRCL_IN1 (TRX - TRY / sqrt(TRX + TRY) ):
We have a few things to add to the to-do list:
* Put UGF servos for LSC loops in place.
* Implement UGF "servos" (per Koji's suggested method) for phase trackers.
* Write a lockloss script that is run by the ALS watch scripts - print a PDF of error and control signals for every lockloss, and save it somewhere.
* Fix up Ygreen modematching on the PSL table. The X green spot is quite similar on the camera to the corresponding PSL green spot. However the Y green spot is not at all the same as its PSL green spot.
Nick and I did the upgrade for the green steering mirror today. We locked in the TEM00 mode.
We placed the shutter and everything. We move the OL, but we placed it back. Tonight, I'll be doing a more complete elog with more details.
That was super fast! Great job, Andres and Nic!
Reasonable amounts of time were spent bending the AG4395 to my will; i.e. figuring out the calibration things Jenne and Rana did, finding the right excitation amplitude and profile that would leave the light steadily locked, and finding the right GPIB incantation for getting spectra in PSD units instead of power units. I'm nearing completion of a newer version of AG4395 scripts that have proper units, and pseudo-log spectra (i.e. logarithmically spaced linear sweeps)
Here is too many traces on one plot showing parts of the OLTF for the x green PDH. One notable omission is the PD response (note to self:check model and bandwidth). The servo oddly seems to have a notch around 100k. My calibration for the CLG injection may not have been perfect, instead of flattening out at 0dB, I had 2dB residual. I tried to correct for it after the fact, assuming that certain regions were truly flat at 0dB, but I want to revisit it to be thorough. I found some old measurements of the Innolight PZT PM response, which claims to be in rad/V, and have included that on the plot.
In the end, the mixer and PZT response make it look like getting over 10kHz bandwidth may be tough. Even finding a good higher modulation frequency to be able to scoot the LP up would leave us with the sharp slope in the PZT phase loss, and could cause bad gain peaking. Maybe it's worth thinking about a faster way of modulating the green light?
Tomorrow morning, I'll calibrate all the noise spectra I have into real units. These include:
However, looking at the floors, it occurs to me that I may have left the attenuation on the input too high, in an effort to protect the input the PDH box, which rails all the time when not locked to a 00 mode, sometimes even with the input terminated or open. It's kind of a pain that the agilent makes it really hard to see the data when you're in V/rtHz mode, because I should've caught this while measuring :/
I used a scope to capture a pdh signal happening, which will let me transform the mixer output into cavity motion. The control signal goes to the innolight PZT with a ~1MHz/V factor. Here are the uncalibrated plots, for now.
A MIST simulation tells me that the green pdh horn-to-horn displacement is about 1.2nm, or ~18kHz. I used this, along with the scope trace attached to the previous post, to calibrate the mixer output at 193419 Hz per V. (EDIT: I was a little too hasty here. What I'm really after is the slope of the zero crossing, which turns out to be almost exactly twice my earlier naïve estimate. See later post for correct spectra)
For the control signal, I assumed a flat Innolight PZT PM response of 1MHz/V. ( Under 10kHz, it is indeed flat, and this is the region where the control signal is above the servo output noise in yesterday's measurements)
Here are all of the same spectra from last night, with the above calibrations.
Going off Jenne's earlier plot, it looks like the in-loop error signal RMS is ten times bigger than the CARM linewidth.
I remeasured all of the noise spectra again today, making sure the input attenuation was as low as it could safely be. I also got a snap of the y green PDH signal; it's fairly larger than I saw the other day, which is good. I used this to calibrate the error signal voltage spectra.
Here are the noise traces for each arm. During these measurements GTRX was about .6, GTRY about 1.0 The Yarm noise doesn't look so good: the error signal is just barely above the mixer+lowpass output noise, and the RMS is plauged by 60Hz lines. (Is this related to what we see in IR TRY sometimes?)
Here are the arms error signals compared directly:
While Gautam is working the restoration of Yarm ASS, I worked on Xarm.
Basically, I have changed the oscillator freqs and amps so as to have linear signals to the misalignment of the mirrors.
Also reduced the complexity of the input/output matrices to avoid any confusion.
Now the ITM dither takes care of the ITM alignment, and the ETM dither takes care of the ETM alignment.
The cavity alignment servos (4dofs) are running fine although the control band widths are still low (<0.1Hz).
The ETM spot positions should be controlled by the BS alignment, but it seems that these loops have suspicion about the signal quality.
While Gautam wa stouching the input TTs, we occasionally saw anomalously high transmission of the arm cavities (~1.2).
We decided to use this beam as this could have indicated partial clipping of the beam somewhere in the input optics chain.
Then the arm cavity was aligned to have reasonably high transmission for the green beam. i.e. Use the green power mon PD as a part of the alignment reference.
This resulted very stable transmission of both the IR and green beams. We liked them. We decide to use this a reference beam at least for now.
Attachment1: GTRX image at the end of the work.
Attachment2: ASSX screen shot
Attachment3: ASSX servo screen shot
Attachment4: Green ASX servo screen shot
Attachment 5: Screen shot of the ASS X strip tool
Attachment 6: Screen shot of the ASS X input matrix
Attachment 7: Screen shot of the ASS X output matrix
X-Arm ASS was fixed.
ASS_DITHER_ON.snap was updated so that the new setting can be loaded from the ASS screen.
The input and output matrices and the servo gains were adjusted as found in the attached image.
The output matrix was adjusted by looking at the static response of the error signals when a DC offset
was applied to each actuator.
The servo was tested with misalignment of the ITM, ETM, and BS. In fact, the servo restored transmission
from 0.15 to 1.
The resulting contrast after ASSing was ~99% level. (I forgot to record the measurement but the dark fringe level of ASDC was 4~5count.)