[Manasa, Yuta]

Mechanical shutter for PSL green is installed right in front of PSL doubling crystal.
This is for blocking PSL green when we want to measure the power of green beam from the arms.

The shutter was previously sitting on AS table un-used. Channel name to control this shutter was C1:AUX-SPS_Shutter. This should be renamed as C1:AUX-GREEN_PSL_Shutter.

Next:
  We are going to restore both arm green in parallel to PRMI work.

  - Coarsely align IR input pointing and arms using A2L
  - Align X green
  - Install green DC PDs and cameras on PSL table

Both X and Y green are aligned such that the arm beams hit the broadband PD.  Also, the 4th port of the combining BS for each arm was used to put a camera and DC PD for each arm.  So, ALS-TRX and ALS-TRY are both active right now.  The camera currently labeled "GRNT" is the Ygreen transmission.  I have a camera installed for Xgreen transmission, but I have not run a cable to the video matrix.  For now, to speed things up, I'll just use the GRNT cable and move it back and forth between the cameras.

I dedicated my evening to trying to get the Ygreen beatnote (the idea being to then get the Xgreen beatnote).

First up was tweaking up the green alignment.  Per Yuta's suggestion, elog 8283, I increased the refl PD gain by 2 clicks (20dB) to keep the lock super stable while improving the alignment.  After I finished, I turned it back to its nominal value.  I discovered that I need lenses in front of the DC PD (for Ygreen, and I'm sure Xgreen will be the same).  The beam is just barely taking up the whole 2mm diode, so beam jitter translates directly to DC power change measured by the diode.  I ended up going just by the green transmission camera for the night, and achieved 225uW of Ygreen on the PSL table.  This was ~2,000 counts, but some of the beam is always falling off the diode, so my actual counts value should be higher after installing a lens. 

I then opened up the PSL green shutter, which is controlled by the button labeled "SPS" on the shutter screen - I will fix the label during some coffee break tomorrow.  Using my convenient new PSL green setup, removing the DC PD allows the beam to reflect all the way to the fuse box on the wall, so you can check beam overlap between the PSL green and the arm green at a range of distances.  I did this for Ygreen, and overlapped the Ygreen and PSL green. 

I checked the situation of the beat cabling, since Jamie has the beatbox out for whitening filter modifications tonight.  In order to get some signal into the control room, I connected the output of the BBPD amplifier (mounted on the front of the 1X2 rack) directly to the cable that goes to the control room.  (As part of my cleanup, I put all the cables back the way I found them, so that Jamie can hook everything back up like normal when he finishes the beatbox.) 

I then started watching the signal on the 8591E analyzer, but didn't magically see a peak (one can always hope....).

I decided that I should put the offset in the Y AUX laser slow servo back to the value that we had been using for a long time, ~29,000 counts.  This is where things started going south.  After letting that go for a minute or two, I thought to go check the actual temperature of the laser head.  The "T+" temperature on the controller read something like 42C, but the voltmeter which reads a voltage proportional to the temp (10C/V) was reading 5.6V.  I immediately turned off the offset, but it's going to take a while for it to cool down, so I'll come back in the morning.  I want the AUX laser to be something like 34C, so I just have to wait.  Ooops.

Still to do (for the short-term FPMI):

* Find Y beatnote.

* Align Xgreen to the arm - it's still off in pitch.

* Align Xgreen and PSL green to be overlapped, hitting the BBPD.

* Find the X beatnote.

* Reinstall the beatbox.

* Use ALS to stabilize both arms' lengths.

* Lock MICH with AS.

* Look at the noise spectrum of AS - is there more noise than we expect (Yuta and Koji saw extra noise last summer), and if so, where does it come from?  Yuta calculated (elog 6931) that the noise is much more than expected from just residual arm motion.

* Write a talk.

I ran a cable to the GTRX camera.  It is now input #2.  The videoswitch script input naming is modified to match this:  Input 2 used to be "IFOPO", and is now "GTRX".  Input 28 used to be "GRNT", and is now "GTRY".  Both green trans cameras are available from the video screen.

 This is happening again to the Yend laser.  It's been fine for the afternoon, and I've been playing with the temperature.  First I have been making big sweeps, to figure out what offset values do to the actual temperature, and more recently was starting to do a finer sweep.  Using the 'max hold' function on the 8591, I have seen the beat appear during my big sweeps.  Currently, the laser temperature measurement is at the Yend, and the RF analyzer is here in the control room, so I don't know what temp it was at when the peaks appeared.

Anyhow, while trying to reaquire lock of the TEM00 mode after changing the temperature, I find that it is very difficult (the green seems misaligned in pitch), and every minute or so the light disappears, and I can no longer see the straight-through beam on the camera.  I went down to the end, and the same symptoms of LED on the laser head turning off, power out display goes to ~40mW, are happening.  I have turned off the laser as was the solution last time, in hopes that that will fix things.

Manasa has done some work to get the Xgreen aligned, so I'll switch to trying to find that beatnote for now.

[Jenne, Manasa]

Aligned X-arm green in TEM00.

It was difficult to get the X-arm to lock in TEM00 earlier. Even when it locked, it was just a TEM00 flash. The green was mainly bad in pitch.

I started aligning with the arms flashing in IR and it was still not possible to lock. The second trial was with the arms locked in IR; I lost the green lock when the arms were aligned for IR. I aligned by overlapping the ingoing green with the reflected green visible on the steering mirror. This got the green to lock in TEM00; but still it would only stay that way for 30 sec. Jenne pointed me to Yuta's suggestion of increasing the green refl PD gain. Once I increased the PD gain, the mode stabilized.

I went ahead and centered ALS TRX on the PD and GTRX camera (on the PSL table). ALS_TRX reads 2000+ counts.

EDIT by Jenne:  This corresponds to a power of 550uW on the PSL table, measured before the first out-of-vac steering mirror.

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. 

This work earlier today had required moving the harmonic separator back closer to its original position, so that the green could get through without clipping.  I locked the Xarm (overriding the trigger) and realigned TRX to the PD and camera.

I turned the laser back on around 1am.  This is still happening, although right now it is turning off more often than before, maybe every 15 seconds or so.  I am going to turn off the laser for the night.

The measured laser temperature is about 45C (I have a 25,000 count offset in the Y ALS Slow control right now....higher offset, lower temp), although the measured laser temp drops to ~43.5C when the power goes down.

I took a look at the laser. It is probably the LD TEC (DTEC) failure.
As the temperature of the LD (DTMP) gradually deviated from 25degCish,
the DTEC voltage also went up from 2Vish to 2.1, 2.2... 

When DTEC reaches 3V, it stopped lasing. This cools the diode a bit, and
it start lasing but repeat the above process.

I am not sure which of the head and controller has the issue.

The situation did not improve much by reducing the pumping current (ADJ: -15).

BTW, Turning on/off the noise eater did not change the situation.

I think the head/controller set should be sent out to JDSU and find how they will say.

 JDSU can repair the Lightwave M126-1064-700 NPRO, sn 415  They do not need the Controller sn 516 

 Posted in the 40m Wiki_ PSL_ NPRO  cost repair and/or option to buy Innolight laser as replacement

 NPRO shipped out for evaluation yesterday under RMA 18022707

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.

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.

Procedure

1. Aligned X-arm to IR.
2. Aligned green to the X-arm.
3. PSL green and X-green aligned to the X-green beat PD.
4. Scanned X-green laser temperature (sweep X slow servo offset through the whole range)

I did not succeed in finding the beat note; but noticed something I cannot explain.
With green very stably aligned to the X-arm, GTRX reads 3000 counts. But when the laser temperature is changed and the green unlocks and locks to the X-arm, it locks with GTRX counts over 5000. GTRX stays at 5000 counts as long as the temperature is changing but settles down to 3000 (over a time lapse of tens of seconds) when let to stay at any specific temperature.

X green beat note found!

Key points
1. Near-field and far-field alignment on the PSL table. The near-field alignment checked by looking at the camera and the far-field alignment checked by allowing the beams to propagate by removing the DC PD.
2. Check laser temperature and get a sense of how the offset translates to the actual laser temperature.
3. Get an idea of the expected temperature of laser using the plot in elog.

Data
PSL laser temperature = 31.45 deg C

X end laser temperature = 39.24 deg C
C1-ALS-X_SLOW_SEERVO2_OFFSET = 4810
Amplitude of beat note = -40dBm

I do not understand why
1. The amplitude of beatnote falls linearly with frequency (peak traced using 'hold' option of the spectrum analyzer).
2. I found the beat note at the RF output of the PD. Earlier, while I was trying to search for the beatnote from the RFmon output of the betabox, there was a strong peak at 29.6MHz that existed even when the green shutters were closed. It's source has to be traced.

Next
Solve beatbox puzzle and lock arm using ALS.

I connected the X green beat PD output back to the beatbox, did the usual PSL alignment for green and searched for the beat note from the RFmon of the beatbox.

Yuta had used a power splitter which took Xbeat-RFmon and Ybeat-RFmon and used the SUM port to monitor the beat signals. I have removed this splitter and just used the X beat RFmon.

I found the beat note with:

Beat@58.7MHz : Amplitude -30dBm
C1:ALS-TRX_OUT16 = 3000 counts
C1:PSL-FSS_SLOWDC = 0.2250
PSL temperature = 31.52 degC
X- green temperature = 39.34 degC (OFFSET = 5140)

Next
Beatbox calibration

[Annalisa, Nic]

After connecting the PD with the reflection from the arm  to the PDH box, theY  arm has been locked on the 01 mode.  Maximizing the alignment, we obtained a 00 mode locking, but we couldn't maximize the power.

The size of the reflected beam was different with respect to the size of the incoming beam, so probably a bad mode matching was one of the issues.

Moreover, the reflected beam is very low power. We need to figure out why it is so (bad alignment? related to mode matching?)

After measuring better all the distances, I did a new mode matching calculation. I put the lenses after measuring the beam waist, so the size of the beam on the lenses was the same as expected from the calculation. Nevertheless, the beam size on the beam splitter looks bigger than expected, and also in this case green flashes into the cavity at some HOM (again 01).

I also tried to lock again the cavity and maximize the alignment, but I didn't get any improvement with respect to the previous mode matching.

Still no good locking!

After making the reflected beam size closer to the injected one, I maximized alignment. I locked again in 00 mode, but I couldn't maximize the power. 

I just realized that maybe I'm not using the correct radius of curvature for the ETMY in the simulation. Tomorrow I will start checking from that.

 Also make sure you are taking into account the substrate of the ETM.

I discussed with Yuta about the ALS servo and phase tracker and found that there was a lot of information lying around from last year but there aren't any clear elogs on how to enable ALS and obtain IR resonance.

Guide to enabling the ALS servo and find IR resonance:

The steps will explain in detail how to ressurrect the ALS servo for green X-arm and find IR resonance using ALS. The medm screens are very confusing right now.

(i) Finding the beat note

1. Get the IR to flash in TEM00 for the arm and lock it by enabling LSC (Locking the arm to IR keeps the arm cavity mirrors stable so that you can scan the temperature of the X-end laser to find the beat note).

2. Steer the X-green into the arm cavity such that the arm cavity locks in TEM00 for green as well. At this point you should also have the X-green reaching the PSL table.

3. Align the PSL doubled green (PSL-green) and the X-green in near-field (at the camera) and far-field (letting the beams to propagate beyond the Green-TRX PD).

4. Check cabling of the RF beat PD.

5. Change the X-laser temperature by sweeping the offset (C1: ALS-SLOW_SERVO2_OFFSET) in steps of 10.

6. Find the beat note and tune the alignment at the beat PD to maximize the beatnote amplitude. Disable LSC for X arm.

(ii) The GREEN HORNET explained

'Input signal conditioning' block takes I and Q signals after the delay frequency discriminator (DFD) in the beat box and these signals pass through C1ALS_BEATX_FINE filter banks. The output signal then enters the phase rotation matrix of the phase tracker. The phase tracker gives 'PHASE_OUT' which is the error signal that is fed to the ETM servo filter module (DOF filters)  through the 'Input matrix' in the medm. 

An offset can also be fed to the phase tracker which will scan the beat frequency (used to find IR resonance).

(iii) Scripts

1. easyALS.py - This runs from 'ON plus' or 'ON minus' buttons in the C1ALS_COMPACT. 

The script clears history of 'fine_phase' filter module and increases gain of the servo in steps ('ON plus' for positive gain and 'ON minus' for negative gain).

2. findIRresonance.py - This runs from 'IRres' button in the C1ALS_COMPACT.

It adds offset to the phase tracker in steps which scans the beat frequency to find IR resonance.

P.S. Check the scripts before enabling the servo so that the right filter modules are being turned ON. Using the wrong set of filter modules can kick the ETM.

____________________________________________________________________________________________________________________________________________________

X arm ALS progress:

I found the beat note and got ALS to work reasonably for the Xarm without kicking the ETM. I did this by manually toggling buttons and changing gains. The scripts need editing.

To do:

Modify the scripts to work as we want them to.

The ALS medm is SSSOOOO confusing. It definitely needs to be fixed (remove all unwanted parts of the screen that existed 'pre-phase tracker').

Find IR resonance.

 For the mode matching calculation I was using the ETMY focal length that I found on Kiwamu's plot on the wiki page. 

Taking into account also the substrate, the focal length turns out to be

fl = ((n-1)*(1/R1 - 1/R2 + (n-1)d/(nR1R2)))^(-1) = -125.81 m

with n = 1.46071 (refraction index of fused silica at 532nm)

R1 = 5625 m (radius of curvature of the first surface)

R2 = 57.37 m (radius of curvature of the second surface)

d = 25mm (thickness)

The value of the focal length is sligthly different from the one I was using before in the calculation, but maybe it is enough to change the coupling.

The mode matching solution I found is very sensitive to the lenses position. 

The beam waist position can vary up to 20m varying by 1cm the first lens position, while it is slightly less sensitive to the second lens displacement.

As shown in the picture, along the green beam path there is also a 1m focal length lens. It's position is fixed, because it is along the IR transmetted beam path also. I tried to get a better solution without it, but I found that the waist position was still strongly dependent on one of the two lenses position, so it would not solve the problem to remove this lens.

I think that the main issue of this mode matching is related to the "space contraints", because the two lenses' positions can vary in a very small space, even though the green beam path on the table is quite long.

Eventually, I put the MM lenses found from this last simulation on the table, and it seems to work, since I've seen very strong 00 flashes. Unfortunately, while trying to maximize the alignment I broke it  and I have to do it again, but I feel confident! 

After restoring alignment I could see again strong 00 flashes (about 250-300 counts on ALS-TRY). So I locked the arm with IR and after enabling the PDH servo for the green locking, I also locked the green on the Y arm in 00 mode. Then I moved the two mode matching lenses to maximize the power into the 00 mode, but I didn't reach more than 30-35 counts.

Green power injected into the Y arm                    0.680mW

Green power reflected back                                  0.090mW

Green power transmitted on the PSL                  few uW

I would expect more power on the PSL table (maybe 10x more).

Hmmm.  You seem to be saying that more light is reflected than is injected. Is this a units problem? Or was some IR on the power meter during the 'reflected' measurement? 
We should look at it with fresh eyes in the morning. 

> Hmmm.  You seem to be saying that more light is reflected than is injected. Is this a units problem? Or was some IR on the power meter during the 'reflected' measurement? 
> We should look at it with fresh eyes in the morning. 

Also, if you have been measuring the power of green refl at the rejection port of the green faraday, the polarization of the light entering the green faraday should be checked once again to make sure that you are measuring 
only the reflected power from the arm cavity.

> > Hmmm.  You seem to be saying that more light is reflected than is injected. Is this a units problem? Or was some IR on the power meter during the 'reflected' measurement? 
> > We should look at it with fresh eyes in the morning. 
> 
> Also, if you have been measuring the power of green refl at the rejection port of the green faraday, the polarization of the light entering the green faraday should be checked once again to make sure that you are measuring 
> only the reflected power from the arm cavity.

Sorry Sorrry Sorry!!
It was 0.090 mW, I just forgot a zero!!!
Sorry!

Is this reflection measured with the cavity locked or unlocked?

So what's the actual designed reflectivity of the ETM for green? No one seems to be able to give me a straight answer about this.

Looking at the reflected beam when the beam is misaligned makes it look like it's << 0.9. Is that expected given the coating spec?

You say the cavity scan goes as high as 300cts but you can only lock to 30cts, are you locked on the sideband?

Stabilized ALS and beat frequency sweep realized.

Procedure:

1. Enable appropriate filter modules and set appropriate servo gains.

2. Clear history of C1:ALS-BEATX_FINE_PHASE

3. Enable the servo loop. I had set limits on the servo loop and ramp time for gain switching so that I don't kick the ETMY hard.
Gains were decided such that the error signal C1:ALS-BEATX_FINE_PHASE_OUT was minimized.

4. Beat frequency sweep is realized by stepping up on C1:ALS-BEATX_FINE_OFFSET_OFFSET (from 0 to 2100 in this case).

Video1 shows the difference that can be seen at the RF spectrum analyzer when ALS is enabled.

Video2 shows the beat frequency sweep as seen on the spectrum analyzer.

I could not get 'getdata' to work as I wanted. So I have attached the error signal trend before and after the ALS servo loop is enabled.

Thank you Jenne for helping retrieve more sensible data!

More info:
The beat note is very strong and we can clearly see its harmonics as well. Attached is the picture showing the several harmonics.
________________________________
Peak frequency(MHz)    Power(dBm)
________________________________
 47                                    -2.77
 93.5                                -27.56
 139                                 -32.75
 185.4                              -45.64
 231.9                              -57.10
 278.4                              -59.42
________________________________

To do:
1. Obtain IR resonance.
2. Check the digital anti-whitening filter after the beatbox.
3. The effect of the harmonics should be figured out.
4. Write scripts to enable ALS and findIRresonance.

-The reflection is measured when the cavity is unlocked. I measured it with the power meter in front of the PD, so I interrupted the PDH loop.

- From the specs of ETM we have:

T(S1,HR,532nm)=5.0%+/-3% (+/-1% target),  R(S2,AR,532nm)<1000ppm

It means that I should have about 600-550 uW in reflection, but I don't. I can say that there are many losses, and maybe some power is clipping inside the Faraday. Nonetheless, the reflected beam looks less strong than the injected one, so most of the losses should be on the ETM table.

(- The reflected power is 0.090 mW, I just wrote it wrong yesterday, sorry!)

- The last question is actually very interesting. Maybe I was locking on the sideband when I locked to 30 cts, but if it is the case I cannot really explain why today I locked on the carrier (I locked the cavity to about 200-250 cts), and everything I changed was the PD gain and the amplitude on signal generator connected to the PDH box. It seems like there should be some sign flip somewhere, but I need to think about.

It's nice that we are now able to scan the cavity again. We got close to PRMI+one arm one step further.
The calibration of the scan frequency and the evaluation of the in-loop/out-of-loop error signal in terms of (Hz/rtHz) would be necessary.

The beat amplitude looks actually huge aIthough I don't know where you are monitoring.
Talk to Jamie to figure out how much the signal should be at the monitoring point.
If it is more than we are supposed to have, put an attenuator somewhere.

I have edited the daq channels in c1als model.

Added: DQ channels for the error signal (phase tracker output)
Removed: DQ channels that existed for the beat_coarse signals

Installed and restarted the model on c1ioo.
Frame builder restarted.
Changes were committed to the svn. 

What I did: 

1. Followed the same procedure to enable ALS (http://nodus.ligo.caltech.edu:8080/40m/8703)
2. Enabling ALS servo stabilized the arm fluctuation and the beat frequency.
3. Beat frequency sweep was done (with ALS servo enabled) by changing offset C1:ALS-BEATX_FINE_OFFSET_OFFSET in steps.

Discussion:

Y arm locked on green carrier in 00 mode!

It locked at almost 280 cts, and the transmitted power on the PSL table is  about 40 uW.

To make it lock on the carrier I had to flip the sign of the error signal in the PDH loop, so I put a phase shifter (a Pomona box with a 23 uF capacitor) right before the LO input of the PDH box (on the model of the X arm).

Tomorrow I will put more details about the power budget and the phase shifter transfer function.

Measurements:

1. Calibrating offset :

I measured the shift in the beat frequency while scanning through the offset. Offset stepped by 50 resulted in 1MHz shift of the beat frequency.
 

2. Anti-whitening filter for beatbox output:

I made an anti-whitening filter for the beatbox output in the ALS_BEATX_FINE_I module by inverting the whitening filters that Jamie had installed in the beatbox earlier (elog).  I have kept the old anti-whitening filter in the module as well for the time-being because the new anti-whitening filter was not as good as the old one in stabilizing the servo (large error signals and unstable ALS).

3. Beat frequency scan for 3FSR:

With ALS loop enabled, I did an offset sweep corresponding to 3FSR (FSR = c/2L = 3.7MHz). The loop doesn't seem to be stable enough to reduce the arm fluctuation to get a resonance for IR. Time series of scan is shown below:

4. No-loop and in-loop spectrum:

I measured the spectrum of the error signal (C1:ALS-BEATX_FINE_I_IN1) with ALS loop enabled and disabled. To suppress the peaks at 3.2Hz and 16.5Hz, I turned ON the corresponding filters. I have recorded the error signal spectrum with only 16.5Hz res gain filter turned ON. Turning ON res gain 3.2Hz filter kicked ETM. 
Spectrum of error signal shown below:

To resolve:

1. What is wrong with the new anti-whiteing filter?

2. Why would the res gain filters kick ETM and show no noise suppression?

 I am working on the basic ALS servo model. The simulink model for the same is attached. The loop is not yet complete (I'm still debugging it) ; but this is just an update of where I am right now.

Attached is the simulink and matlab file. 

[Koji, Annalisa]

Alignment improving

• The alignment of the green beam injected into the Yarm has been improved, and when the green laser is locked on the cavity in 00 mode we moved from 500 cnts to almost  650 cnts in transmission on the PSL table. Now we have about 70 uW transmitted.
• Since the beam size on the GRTY PD was too big, I put a lens to focus it better.

Beat note setup

• The Ybeat RFmon was connected back to the power splitter that Yuta put and Manasa temporarily removed (as described in elog 8666), so that now we are using again the SUM port to monitor the beat signal.

TO DO

• Align better the beams on the BeatPD, in order to get a stronger beat note
• Calibrate the ALS screen to tune remotely the laser temperature
• Find the beat note!

I have added more DAQ channels to the c1als model. Installed and restarted the model on c1ioo. Frame builder restarted.

DAQ channels added:
C1:ALS-XARM_IN1
C1:ALS-YARM_IN1
C1:ALS-OFFSETTER1_OUT
C1:ALS-OFFSETTER2_OUT

I have modified the ALS model to now include PHASE_OUT calibration for both X and Y. MEDM screen has not been edited to include these yet.

 Could be that this is OK, but it doesn't yet make sense to me. Can you please explain in words how this manages to apply the calibration rather than just add an extra gain to the phase tracking loop?

 Y arm beat note found!

Procedure

• Arm lock on IR to align the mirrors
• Green Laser locked on the arm
• Green Transmission on the PSL and PSL green beam aligned into the BeatPD: they have been aligned both in the near field (looking at the beam on the camera) and in the far field (removing the DC PD and looking at the two beams on the wall)
• Checked the PSL temperature and, following the plot of the beat note measurement between "Alberto" laser and PSL reported in elog 8396, I got an idea of the range of temperature where the beat note could be found (I used the values of the second curve)
• Scanned the Y-green laser temperature using the slow servo on the ALS command window

Data

• PSL temperature = 31.58°C
• PSL slow servo temperature offset = 0
• "Alberto" laser temperature = 40.35 °C (ADJ = 0)
• Thermal output offset on the ALS screen = -11140
• Beat note frequency = 22 MHz
• Beat note amplitude = -31.7 dBm

The green transmission on the PSL reads about 500 cts, and the transmitted power is about 50 uW.

(the second peak on the screen in the picture is the 29 MHz of the MC)

The calibration is applied by adding an extra gain. But, I missed the point that I should be doing this outside the phase-tracking loop....my BAD .

So I modified the model such that the calibration is done without disturbing the phase tracking loop.

Right now, epics input 'PHASE_OUT_CALIB' accepts the calibration and we get the calibrated phase tracker output converted from deg to Hz at 'PHASE_OUT_HZ'.  I have also made it a DAQ channel to be used with dataviewer and dtt.

medm screens have been modified to accommodate these additions to the phase tracker screen. I used Yuta's phase tracker calibration data in elog to set PHASE_OUT_CALIB in the medm screens.

 [Koji, Annalisa, Gautam]

Annalisa noticed that over the weekend the Y-arm green PDH was locked to a sideband, despite not having changed anything on the PDH box (the sign switch was left as it was). On friday, we tried turning on and off some of the filters on the slow servo (C1ALS_Y_SLOW) which may have changed something but this warranted further investigation. We initially thought that the demodulation phase was not at the optimal value, and decided to try introducing some capacitances in the path from the function generator to the LO input on the universal PDH box. We modelled the circuit and determined that significant phase change was introduced by capacitances between 1nF and 100nF, so we picked out some capacitors (WIMA FKP) and set up a breadboard on which to try these out.

After some trial and error, Koji dropped by and felt that the loop was optimized for the old laser, the various loop parameters had not been tweaked since the new laser was installed. The following parameters had to be optimized for the new laser;

• Servo gain
• LO frequency
• LO modulation depth
• Demodulation phase

The setup was as follows: 

• PDH box error signal to Oscilloscope CH1
• Green PD output to Oscilloscope CH2
• No capacitor between Function Generator and the PDH box
• 0.1Hz triangle wave (30 counts amplitude) applied to ETMY via awggui (so as to sweep the cavity and see stronger, more regular TEM00 flashes)

The PDH error signal did not have very well-defined features, so Koji tweaked the LO frequency and the modulation depth till we got a reasonably well-defined PDH signal. Then we turned the excitation off and locked the cavity to green. The servo gain was then optimized by reducing oscillations in the error signal. Eventually, we settled on values for the Servo Gain, LO frequency and modulation depth such that the UGF was ~20kHz (determined by looking at the frequency of oscillation of the error signal on an Oscilloscope), and the PDH signal had well-defined features (while the cavity was unlocked). The current parameters are

• LO frequency: 205.020 kHz
• modulation depth: 0.032 Vpp

We then proceeded to find the optimal demodulation phase by simulating the circuit with various capacitances between the function generator and the PDH box (circuit diagram and plots attached). The simulation seemed to suggest that there was no need to introduce any additional capacitance in this path (introducing a 1nF capacitance added a phase-lag of ~90 degrees-this was confirmed as the error-signal amplitude decreased drastically when we hooked up a 1nF capacitor on our makeshift breadboard). In the current configuration, the LO is connected directly to the PDH box.

Misc Points:

• The phase shifter in the PDH box is not connected: the IC in the box, JSPHS-26, is designed for operation in the range 18-26MHz. If necessary, an all-pass-filter could be introduced, with a tuneable rheostat to adjust the phase for our frequency range. Right now, turning the knob marked "LO phase angle" on the front panel doesn't do anything. The mixer on the PDH board is also not used for the same reason.
• PSL shutter was closed sometime earlier this evening, because we suspected some IR light was reaching the Green PD on the y-endtable, and was influencing the error signal. Its back open now.
• Useful information about the old y-end laser relevant to selecting the right LO frequency, modulation depth, and servo gain can be found here and in elog 2746 and subsequent replies, though the details of how the measurement were made aren't entirely clear. The idea is that the characteristics of the piezoelectric element in the laser has some characteristics which will determine the optimal LO frequency, modulation depth and servo gain.

To Do:

Now that we are reasonably confident that the loop parameters are optimal, we need to stabilise the C1ALS_Y_SLOW loop to stabilise the beat note itself. Appropriate filters need to be added to this servo.

Circuit Diagram: 50 ohm input impedance on the source, 50 ohm output impedance seen on the PDH box, capacitance varied between 1nF and 100nF in steps.

Plots for various capacitances: Gold-green trace (largest amplitude) direct from LO, other traces at input to PDH box.

[Koji, Annalisa, Manasa]

Today we worked on the ALS servo stabilization for the Y arm.

First step: find the beat note

The beat note was found following the usual steps:

• Y arm cavity locked on IR to have a good alignment
• Y arm cavity locked on green (eventually unlocked on IR)
• beat note alignment maximized on the PSL table

Beat note amplitude = -27 dBm @ 50 MHz

PSL temperature = 31.54 degC

Laser Offset on the slow servo2 = -11011

In the GREEN HORNET we did the following changes for the Y arm:

Input Signal Conditioning

On the C1ALS-BEATY_FINE  screen the same antiwhitening filters of the C1ALS-BEATX_FINE have been reproduced. At moment, only the FM3 [10:1] is enabled.

On the C1ALS-BEATY_FINE_PHASE screen the gain was set at 3600, since the amplitude of the Q signal after the Phase rotator (BEATY_FINE_Q_ERR) was about 30. To set this value we made a proportion with respect to a previous optimized value, where the amplitude was 100 and the gain was set to 1200.

DOF filters

In order to stabilize the beat frequency, we started enabling the FM5 [1000:1] filter in the C1ALS_YARM panel, and then we started increasing the gain first in small steps (0.1), in order to understand which sign the gain should have without kicking the mirror.

We measured the Power Spectrum of the C1:ALS-BEATY_FINE_PHASE_OUT in-loop signal while varying the gain of the C1ALS_YARM servo filter.

Eventually, we enabled the following filters:

FM2 [0:1]

FM3 [1:5]

FM4 [1:50]

FM5 [1000:1]

FM6 [RG3.2]

FM7 [RG16.5]

Gain = -30.

Koji expects the UGF of the loop to be around 100-ish Hz, and he also expected the small bump around 300-400 Hz.

Then we realized that the channel we were measuring was not calibrated in unit of Hz, so we took again the measurement looking at the channel C1:ALS-BEATY_FINE_PHASE_OUT_HZ. In this case, we didn't observe any bump. Maybe the beat frequency was slightly changed from the previous measurement and the all servo shape was also different. The final value of the gain was set at -8.

The Y axis unit is missing (bad me!). It's in deg/sqrt(Hz) for the first plot and Hz/sqrt(Hz) for the second one.

I realized that I cannot open the attached plots. I'll fix them tomorrow.

[Koji, Manasa]

I wanted to investigate on the ALS electronics(in particular the beatbox and the phase tracker) and find out if the beatbox is showing a linear behavior
as we expect it to and as to why we have been seeing sudden jumps at the phase tracker output.

I have been using the Xarm part of the beabox.
I used Marconi as well as signal generator to do frequency sweep/modulation at the RF input of the beatbox and looked at the I_MON output of the beatbox.

We observed sudden jumps in the beatbox output from time to time while we either varied the carrier frequency or the RF amplitude.
Also the beatbox output shows high frequency oscillations at ~95MHz (source unknown). It is for sure that the beatbox is not behaving the way it should
but we could not tell more or troubleshoot with the beatbox mounted on the rack.

I am going to let Annalisa do her Y arm ALS scan tonight and pull out the beatbox tomorrow to fix it.

Alex, Gautam and Steve,

Single mode fiber 50m long is layed out into cable tray that is attached to the beam tube of the Y arm.

It goes from ETMY to PSL enclosure. It is protected at both ends with " clear- pvc, slit corrugated loom tubing " 1.5" ID

The fiber is not protected between 1Y1 and 1Y4

 [Annalisa, gautam]

Summary:

We have measured the open-loop transfer function of the Y-end green PDH loop. From the measurement, the loop UGF is ~12kHz.

Details:

We have been trying to measure this transfer function for some time now, and playing around with various points of injecting the excitation and measuring the output. Koji helped arrive at one that actually worked, and the scheme used to make this measurement is shown in the sketch below. The SR785 signal analyzer was used to make the measurement, while an SR560 preamp was used to sum the output from the PDH box (PZT-OUT) and the excitation, with this sum being delivered to the auxiliary laser PZT via a pomona box that sums the servo output and the signal from the LO. The transfer function measurement made was a1/a2 w.r.t the sketch attached.

• The swept-sine measurement was done from high to low frequencies, as the open-loop gain was expected to be high at low frequencies.
• After some trial and error, we realised that the excitation amplitude on the SR785 can be varied continuously during the course of a swept sine measurement using the dial on the front panel. We started out with a 1mVpp signal at the high end of the frequency sweep (~102kHz, the upper limit on the SR785) and went up to 17mVpp at ~30Hz). These values were determined by trial and error, and were approximately the maximum that did not kick the loop out of lock/into a higher order mode.

Remarks:

• As per this paper, the expected bandwidth of this loop is expected to be ~30kHz, while the measured UGF was more like 11.7kHz. Perhaps we can get this closer to the expected 30kHz by increasing the servo gain. The measurement shown was done with the servo gain knob on the Universal PDH box set to ~7.86. We tried two other values, ~8.2 and 10 (this was the limit on the knob), but the UGF first increased to ~13kHz (for the 8.2 gain), and then decreased to ~5kHz with a gain of 10. Not sure why this was, but it can be looked into further. 
   

Set-up to measure Y-end Green PDH transfer function:

Measured Open Loop Transfer Function:

 The beatbox output showed high frequency oscillations during the troubleshooting process yesterday. I removed the beatbox from the rack. With no RF inputs, just powering the beatbox showed these high frequency oscillations at the beatbox output. This confirms that these oscillations are from the op-amp AD829JR. I replaced these with low noise OP27G. Also I removed the AD829JR that were soldered to the frequency divider and comparator which are not being used. Output buffer U10 was also removed.

After replacing with OP27G, I rechecked the beatbox with and without the RF input. There were no more high frequency contaminations and beatbox seemed to behave as it is supposed to when a frequency modulated RF input is fed. I put the beatbox back on the rack and did  a quick recheck.

Before (top) and after (bottom) pictures