The ATF NPRO auxiliary laser has been moved on the PSL table. All the optics for beat note measurement are in place and alignment has been done.

The setup for this measurement is the same as described in elog 8333.

I've disabled the alarm for PEM_count_half, using the mask in the 40m.alhConfig file.  We can't do anything about it, and it's just annoying.

I turned the StripTool and ALARMS and BLRMS back on on op540m. Looks like it has been rebooted 5 days ago and no one turned these back on. Also, there was a bunch of junk strewn around its keyboard which I restrained myself from throwing in the trash.

The BLRMS trends should be active now.

Please!

Don't put laptops on the ISC Tables!

A small rat / large mouse just ran through the control room. Ugh.

Fire alarm went off several minutes ago. Talked to security and they said there was no fire. It beeped twice again just now. No one has been working on the IFO today.

The fire alarm came on around 15:05  for about 2-3 minutes. We all  left the lab and counted heads.  I called Paul Mackel x2646 (cell 626/ 890- 3259) at Fire Protection Services. He said that this alarm test was planned and we should of got an email notice. Perhaps I missed that notes.

It is posted at the 40m wiki with Gautam' help. Printed copies posted around doors also.

There was an alarm sound from the Smart-UPS 2200 sitting under the workstation. I see that the 'replace battery' light is red, and this elog tells me that these batteries are replaced every ~1-4 years; the last replacement was march 2016. Holding down the 'test' button for 2-3 seconds results in the alarm sound and does not clear the replace battery indicator.

Rana, Naomi

We dither locked the X arm and then aligned the green beam to it using the PZTs. Everything looks ready for us to do a mode scan tomorrow.

We got buildup for Red and Green, but saw no beat in the control room. Quick glance at the PSL seems OK, but needs more investigation. We did not try moving around the X-NPRO temperature.

Tomorrow: get the beat, scan the PhaseTracker, and get data using pyNDS.

Last week, we were talking about reviving the Fiber ALS box. Right now, it's not in great shape. Some changes to be made:

1. Supply power to the PDs (Menlo FPD310) via a power regulator board. The datasheet says the current consumption per PD is 250 mA. So we need 500mA. We have the D1000217 power regulator board available in the lab. It uses the LM2941 and LM2991 power regulator ICs, both of which are rated for 1A output current, so this seems suitable for our purposes. Thoughts?
2. Install power decoupling capacitors on the PDs.
3. Clean up the fiber arrangement inside the box.
4. Install better switches, plus LED indicators.
5. Cover the box.
6. Install it in a better way on the PSL table. Thoughts? e.g. can we mount the unit in some electronics rack and route the fibers to the rack? Perhaps the PSL IR and one of the arm fibers are long enough, but the other arm might be tricky.

Previous elog thread about work done on this box: elog11650

Summary:

Between frequent MC1 excursions, I worked on ALS recovery today. Attachment #1 shows the out-of-loop ALS noise as of today evening (taken with arms locked to IR) - I have yet to check loop shapes of the ALS servos, looks like there is some tuning to be done.

On the PSL table:

• First, I locked the arms to IR, ran the dither alignment servos to maximize transmission.
• I used the IR beat PDs to make sure a beat existed, at approximately.
• Then I used a scope to monitor the green beat, and tweaked steering mirror alignment until the beat amplitude was maximized. I was able to improve the X arm beat amplitude, which Koji and Naomi had tweaked last week, by ~factor of 2, and Y arm by ~factor of 10.
• I used the DC outputs of the BBPDs to center the beam onto the PD.
• Currently, the beat notes have amplitudes of ~-40dBm on the scopes in the control room (there are various couplers/amplifiers in the path so I am not sure what beatnote amplitude this translates to at the BBPD output). I have yet to do a thorough power budget, but I have in my mind that they used to be ~-30dBm. To be investigated.
• Removed the fiber beat PD 1U chassis unit from the PSL table for further work. The fibers have been capped and remain on the PSL table. Cleaned the NW corner of the PSL table up a bit.

To do:

• Optimization of the input pointing of the green beam for X (with PZTs) and Y (manual) arms.
• ALS PDH servo loop measurement. Attachment #1 suggests some loop gain adjustment is required for both arms (although the hump centered around ~70Hz seem to be coming from the IR lock).
• Power budgeting on the PSL table to compare to previous such efforts.

Note: Some of the ALS scripts are suffering from the recent inablilty of cdsutils to pull up testpoints (e.g. the script that is used to set the UGFs of the phase tracker servo). The workaround is to use DTT to open the test points first (just grab 0.1s time series for all channels of interest). Then the cdsutils scripts can read the required channels (but you have to keep the DTT open).

Today, I borrowed the fiber microscope from Johannes and took a look at the fibers coupled to the PDs. The PD labelled "BEAT PD AUX Y" has an end that seems scratched (Attachments #1 and #2). The scratch seems to be on (or at least very close to) the core. The other PD (Attachments #3 and #4) doesn't look very clean either, but at least the area near the core seems undamaged. The two attachments for each PD corresponds to the two available lighting settings on the fiber microscope.

I have not attempted to clean them yet, though I have also borrowed the cleaning supplies to facilitate this from Johannes. I also plan to inspect the ends of all other fiber connections before re-installing them.

Today, with Johannes' help, I cleaned the fiber tips of the photodiodes. The effect of the cleaning was dramatic - see Attachments #1-4, which are X Beat PD, axial illumination, X Beat PD, oblique illumination, Y beat PD, axial illumination, Y beat PD, oblique illumination. They look much cleaner now, and the feature that looked like a scratch has vanished.

The cleaning procedure followed was:

• Blow clean air over the fiber tip
• First, we tried cleaning with the Q-tip like tool, but the results weren't great. The way to use it is to dip the tip in the cleaning solvent for a few seconds, hold the tip to the fiber taking into account the angled cut, and apply 10 gentle quarter turns.
• Next, we tried cleaning with the wipes. We peeled out an approximately 5" section of the wipe, and laid it out on the table. We then applied cleaning solvent liberally on the central area where we were sure we hadn't touched the wipe. Then you just drag the fiber tip along the soaked part of the wipe. If you get the angle exactly right, the fiber glides smoothly along the surface, but if you are a little misaligned, you get a scratchy sensation. 
• Blow dry and inspect.

I will repeat this procedure for all fiber connections once I start putting the box back together - I'm almost done with the new box, just waiting on some hardware to arrive.

[ericq, gautam]

• I was just getting the IFO aligned, and single arm lock going, when EricQ came in and asked if we could get some ALS data.
• ALS beats seemed fine, in particular the X-Arm. The broad hump around ~70Hz that was present in my previous ALS update was nowhere to be seen - reasons unknown.
• Copied over /opt/rtcds/caltech/c1/scripts/YARM/Lock_ALS_YARM.py to /opt/rtcds/caltech/c1/scripts/XARM/Lock_ALS_XARM.py. Could be useful when we want to do arm cavity scans.
• Made appropriate changes to allow ALS locking of Xarm - the testpoint inaccessibility makes things a little annoying but for tonight we just used DQ channels in place (or slow channels when DQ chans were not available)
• Calibration of X arm error signal seemed off - so we fixed it by driving a line in ETMX and matching up the peaks in the ALS error signal and POX11. We then updated the gain of the filter in the CINV filter bank accordingly.
• Got some decent data - X arm stayed locked on ALS for >60mins, during which time the Y arm stayed locked on POY11, and the Y green also reained locked . There was no evidence of the X arm 00 mode randomly dropping out of lock tonight.
• EQ will update with a sick comparison plot - today we looked at the ALS noise from the perspective of the Green Locking Izumi et. al. paper.
• Y arm ALS noise didn't look so hot tonight - to be investigated...

Leaving LSC mode OFF for now while CDS is still under investigation

Not really related to this work: We saw that the safe.snap file for c1oaf seems to have gotten overwritten at some point. I restored the EPICS values from a known good time, and over-wrote the safe.snap file.

My motivation tonight was to get an up-to-date spectrum of a calibrated measurement of the out-of-loop displacement of an arm locked on ALS (using the PDH signal as the out-of-loop sensor) to compare the performance of ALS control noise with the Izumi et al green locking paper. 

I was able to fish out the PSD from the paper from the 40m svn, but the comparison as plotted looks kind of fishy. I don't see why the noise from 10-60Hz should be so different/worse. We updated the POX counts to meters conversion by looking at the Hz-calibrated ALSX signal and a ~800Hz line injected on ETMX.

I worked a little bit on the Y arm ALS today. 

• Started by locking the Y arm to IR with POY, and then ran the dither alignment script to maximize Y arm transmission.
• Green TRY DC monitor was around 0.16, whereas I have seen ~0.45 when we were doing DRFPMI locking.
• So I went to the Y end table and tweaked the steering mirrors a little. I was able to get GTRY to ~0.42. I think this can be tweaked a little further but I decided to push on for tonight.
• The beat amplitude on the network analyzer in the control room is comparable to the X arm beat now.
• Adjusted the gain of the phase tracker servos, cleared phase history.
• Looking at the ALS beat noise with the arms locked to IR and the slow ALS temperature control loops ON (see Attachment #1), the current measurements line up quite well with the reference traces.

I am now going to measure the OLTFs of both green PDH loops to check that the overall loop gain is okay, and also check the measurement against EricQ's LISO model of the (modified) AUX green PDH servos. Results to follow.

Some weeks ago, I had moved some of the Green steering optics on the PSL table around, in order to flip some mirror mounts and try and get angles of incidence closer to ~45deg on some of the steering mirrors. As a result of this work, I can see some light on the GTRY CCD when the X green shutter is open. It is unclear if there is also some scattered light on the RFPDs. I will post pictures + a more detailed investigation of the situation on the PSL table later, there are multiple stray green beams on the PSL table which should probably be dumped.

As I was writing this elog, I saw the X green lock drop abruptly. During this time, the X arm stayed locked to the IR, and the Y arm beat on the control room network analyzer did not jump (at least not by an amount visible to the eye). Toggling the X end shutter a few times, the green TEM00 lock was re-acquired, but the beatnote has moved on the control room analyzer by ~40MHz. On Friday evening however, the X green lock held for >1 hour. Need to keep an eye on this.

Attachment #1 shows the results of my measurements tonight (SR785 data in Attachment #2). Both loops have a UGF of ~10kHz, with ~55 degrees of phase margin.

Excitation was injected via SR560 at the PDH error point, amplitude was 35mV. According to the LED indicators on these boxes, the low frequency boost stages were ON. Gain knob of the X end PDH box was at 6.5, that of the Y end PDH box was at 4.9. I need to check the schematics to interpret these numbers. GV Edit: According to this elog, these numbers mean that the overall gain of the X end PDH box is approx. 25dB, while that of the Y end PDH box is approx. 15dB. I believe the Y end Lightwave NPRO has an actuator discriminant ~5MHz/V, while the X end Innolight is more like 1MHz/V.

Not sure what to make of the X PDH loop measurement being so much noisier than the Y end, I need to think about this.

More detailed analysis to follow.

Didn't someone look at what the OLG req. should be for these servos at some point? I wonder if we can make a parallel digital path that we switch on after green lock. Then we could make this a simple 1/f box and just add in the digital path (take analog control signal into ADC, filter, and then sum into the control point further down the path to the laser) for the low frequency boost.

I completed the revamp of the box, and re-installed the box on the PSL table today. I think it would be ideal to install this on one of the electronic racks, perhaps 1X2 would be best. We would have to re-route the fibers from the PSL table to 1X2, but I think they have sufficient length, and this way, the whole arrangement is much cleaner.

Did a quick check to make sure I could see beat notes for both arms. I will now attempt to measure the ALS noise with this revamped box, to see if the improved power supply and grounding arrangement, as well as fiber cleaning, has had any effect.

Photos + power budget + plan of action for using this box to characterize the green PDH locking to follow. 

For quick reference: here is the AM/PM measurement done when we re-installed the repaired Innolight NPRO on the new X endtable.

[Kira, gautam]

Attachment #1 - Photo of the revamped beat setup. The top panel has to be installed. New features include:

• Regulated power supply via D1000217.
• Single power switch for both PDs.
• Power indicator LED.
• Chassis ground isolated from all other electronic grounds. For this purpose, I installed all the elctronics on a metal plate which is only connected to the chassis via nylon screws. The TO220 package power regulator ICs have been mounted with the TO220 mounting kits that provide a thin piece of plastic that electrically insulates its ground from the chassis ground.
• PD outputs routed through 20dB coupler on front panel for diagnostic purposes.
• Fiber routing has been cleaned up a little. I installed a winding fixture I got from Johannes, but perhaps we can install another one of these on top of the existing one to neaten up the fiber layout further.
• 90-10 light splitter (meant for diagnostic purposes) has been removed because of space constraints. 

Attachment #2 - Power budget inside the box. Some of these FC/APC connectors seem to not offer good coupling between the two fibers. Specifically, the one on the front panel meant to accept the PSL light input fiber seems particularly bad. Right now, the PSL light is entering the box through one of the front panel connectors marked "PSL + X out". I've also indicated the beat amplitude measured with an RF analyzer. Need to do the math now to confirm if these match the expected amplitudes based on the power levels measured.

Attachment #3 - We repeated the measurement detailed here. The X arm (locked to IR) was used for this test. The "X" delay line electronics were connected to the X green beat PD, while the "Y" delay line electronics were connected to the X IR beat PD. I divided the phase tracker Hz calibration factor by 2 to get IR Hz for the Y arm channels. IR beat was at ~38MHz, green beat was at ~76MHz. The broadband excess noise seen in the previous test is no longer present. Indeed, below ~20Hz, the IR beat seems less noisy. So seems like the cleaning / electronics revamp did some good. 

Further characterization needs to be done, but the results of this test are encouraging. If we are able to get this kind of out of loop ALS noise with the IR beat, perhaps we can avoid having to frequently fine-tune the green beat alignment on the PSL table. It would also be ideal to mount this whole 1U setup in an electronics rack instead of leaving it on the PSL table.

GV Edit: I've added better photos to the 40m Google Photos page. I've also started a wiki page for this box / the proposed IR ALS  system. For the moment, all that is there is the datasheet to the Fiber Couplers used, I will populate this more as I further characterize the setup.

Is it better to mount the box in the PSL under the existing shelf, or in a nearby PSL rack?

It seems like the main contribution to the RMS comes from the high frequency bump. When using the ALS loop to lock the arm to the beat, only the stuff below ~100 Hz will matter. Interesting to see what that noise budget will show. Perhaps the discrepancy between inloop and out of loop will go down.

I was having a chat with EricQ about this today, just noting some points from our discussion down here so that I remember to look into this tomorrow.

• I believe that currently, the channels C1:ALS-BEATX_FINE_PHASE_OUT_HZ_DQ and the Y arm analog read out the frequency of the green beat, in Hz.
• In the comparison I plotted, I WRONGLY divided the spectrum of the IR beat by 2, instead of multiplying in by 2, which is what should actually be done for an apples-to-apples comparison.
• The deeper question is, what should this channel actually readout?
• Looking at my codes from past arm scans etc, I see that I am dividing the downloaded data by 2 in order to convert the X-axis of these scans to "IR Hz". But this should really be all we care about.
• So I think I will have to re-do the cts-to-Hz calibration in the ALS models. It should be possible to do ~10FSR scans with the IR beat, and then we can use the sideband resonances (presumably the sideband frequencies are known with better precision than the arm length, and hence the FSR) to calibrate the phase tracker.
• I don't think this changes the fact that the Fiber ALS situation has been improved - but I will have to repeat the measurement to be sure. The improvement may not be as stellar as I tried to sell in my previous elog .

  ---

  Other thoughts: 

• Can we make use of the Jetstor raid array for some kind of consolidated 40m CDS backup system? Once we've gotten everything of interest out of it...

Summary:

I did some work today to see if I could use the IR beat for ALS control. Initial tests were encouraging.

I will now embark on the noise budgeting.

Details:

• For this test, I used the X arm
• I hooked up the X-arm + PSL IR beat to the X-arm DFD channel, and used the Y-arm DFD channels to simultaneously monitor the X-arm green beat.
• I then transitioned to ALS control and used POX as an out-of-loop sensor for the ALS noise.
• Attachment #1 shows a comparison of the measurements. In red is the IR beat, while the green traces are from the test EricQ and I did a couple of nights ago using the green beat.
• I also wanted to do some arm cavity scans with the arm under ALS control with the IR beat - but was unsucessful. The motivation was to fix the ALS model counts->Hz calibration factors.
• I did however manage to do a 10 FSR scan using the green beatnote - however, towards the end of this scan, the green beat frequency (read off the control room analyzer) was ~140MHz, which I believe is outside (or at least on the edge) of the bandwidth of the Green BBPDs. The fiber coupled IR beat photodiodes have a much larger (1GHz) spec'd bandwidth.

I am leaving the green beat electronics on the PSL table in the switched state for further testing...

[rana,gautam]

We set up a measurement of the AUX X laser AM today. Some notes:

• PDA 55 that was installed as a power monitor for the AUX X laser has been moved into the main green beam path - it is just upstream of the green shutter for this measurement.
• AUX X laser power into the doubling crystal was adjusted by rotating HWP upstream of IR Faraday (original angle was 100, now it is 120), until the DC level of the PDA 55 output was ~2.5V on a scope (high impedance).
• BNC-T was installed at the PZT input of the Innolight - one arm of the T is terminated to ground via 50 ohms. The purpose of this is to always have the output of the power splitter from the network analyzer RF source drive a 50 ohm load.
• The output of the Green PDH servo to the Innolight PZT was disconnected downstream of the summing Pomona box - it is now connected to one output of a power splitter (borrowed from SR function generator used to drive the PZT) connected to the RF source output of the AG4395.
• Other output of power splitter connected to input R of AG4395.
• PDA55 output has been disconnected from CH5 of the AA board. It is connected to input A of the AG4395 via DC block.

Attachment #1 shows a preliminary scan from tonight - we looked at the region 10kHz-10MHz, with an IF bandwidth of 100Hz, 16 averages, and 801 log-spaced frequencies. The idea was to get an idea of where some promising notches in the AM lie, and do more fine-bandwidth scans around those points. Data + code used to generate this plot in Attachment #2.

Rana points out that some of the AM could also be coming from beam jitter - so to put this hypothesis to test, we will put a lens to focus the spot more tightly onto the PD, repeat the measurement, and see if we get different results.

There were a whole bunch of little illegal things Rana spotted on the EX table which he will make a separate post about.

I am running 40 more scans with the same params for some statistics - should be done by the morning.

Update 12:00 21 Sep: Attachment #3 shows schematically the arrangement we use for the AM measurement. A similar sketch for the proposed PM measurement strategy to follow. After lunch, Steve and I will lay out a longish BNC cable from the LSC rack to the IOO rack, from where there is already a long cable running to the X end. This is to facilitate the PM measurement.

Update 18:30 21 Sep: Attachment #4 was generated using Craig's nice plotting utility. The TF magnitude plot was converted to RIN/V by dividing by the DC voltage of the PDA 55 of ~2.3V (assumption is that there isn't significant difference between the DC gain and RF transimpedance gain of the PDA 55 in the measurement band) The right-hand columns are generated by calculating the deviation of individual measurements from the mean value. We're working on improving this utility and aesthetics - specifically use these statistics to compute coherence, this is a work in progress. Git repo details to follow.

There are only 23 measurements (I was aiming for 40) because of some network connectivity issue due to which the script stalled - this is also something to look into. But this sample already suggests that these measurement parameters give consistent results on repeated measurements above 100kHz.

TO CHECK: PDA 55 is in 0dB gain setting, at which it has a BW of 10MHz (claimed in datasheet).

Some math about relation between coherence  and standard deviation of transfer function measurements:

  --- relation to variance in TF magnitude. We estimate the variance using the usual variance estimator, and can then back out the coherence using this relation.

 --- relation to variance in TF phase. Should give a coherence profile that is consistent with that obtained using the preceeding equation.

It remains to code all of this up into Craig's plotting utility.

Image #1: No - we do not use magnetic mounts for beam dumps. Use a real clamp. It has to be rigid. "its not going anywhere" is a nonsense statement; this is about vibration amplitude of nanometers.

Image #2: No - we do not use sticky tape to put black glass beam dumps in place ever, anywhere. Rigid dumps only.

Image #3: Please do not ruin our nice black glass with double sticky tape. We want to keep the surfaces clean. This one and a few of the other Mickey Mouse black glass dumps on this table were dirty with fingerprints and so very useless.

Image #4: This one was worst of all: a piece of black glass was sticky taped to the wall. Shameful.

Please do not do any work on this table without elogging. Please never again do any of these type of beam dumping - they are all illegal. Better to not dump beams than to do this kind of thing.

All dumps have to be rigidly mounted. There is no finger contacting black glass or razor dumps - if you do, you might as well throw it in the garbage.

[steve,gautam]

We laid out a 45m long BNC cable from the LSC rack to the IOO rack via overhead cable trays. There is ~5m excess length on either side, which have been coiled up and cable-tied for now. The ends are labelled "TO LSC RACK" and "TO IOO RACK" on the appropriate ends. This is to facilitate hooking up the output of the DFD for making a PM measurement of the AUX X laser. There is already a long cable that runs from the IOO rack to the X end.

[steve, gautam]

The Fiber ALS box has been installed on the existing shelf on the PSL table. We had to re-arrange some existing cabling to make this possible, but the end result seems okay (to me). The box lid was also re-installed.

Some stuff that still needs to be fixed:

1. Power supply to ZHL amplifiers - it is coming from a table-top DC supply currently, we should hook these up to the Sorensens.
2. We should probably extend the corrugated fiber protection tubing for the three fibers all the way up to the shelf. 

Beat spectrum post changes to follow.

Attachment #1: Result of AM sweeps with EX laser crystal at nominal operating temperature ~ 31.75 C.

Attachment #2: Tarball of data for Attachment #1.

Attachment #3: Result of AM sweeps with EX laser crystal at higher operating temperature ~ 40.95 C.

Attachment #4: Tarball of data for Attachment #2.

Remarks:

• Confirmed that PDA 55 is in the "0dB" setting - the actual dial is unmarked, and has 5 states. I guessed that the left-most one is 0dB, and checked that if I twiddled the dial by one state to the right, the DC level on the scope increased by 10dB as advertized. Didn't check all the states.
• DC level is ~2.3V on a high-impedance scope. So it will be ~1.15V to a 50ohm load, which is what the DC block is. The inverse of this value is used to calibrate the vertical axis of the TF measurement to RIN/V.
• Input R (split RF source signal) attenuation: 20dB. Input A (PDA55 output) attenuation: 0dB.
• Main problem is still network hangups when trying to do many sweeps.
• Seems to persist even when I connect the GPIB box to one of the network switches - so don't think we can blame the WiFi.
• Need to explore possibility of speedup - takes >2hours to run ~50scans!

To-do:

• Overlay median and uncertainty plots for the two temp. settings. There is a visible diference in both the locations and depths/heights of various notches/peaks in the AM profile.
• Repeat test with a fast focusing lens to focus the beam more tightly on the PD active area to confirm that the measured AM is indeed due to the PZT drive and not from beam-jitter (presently, spot diameter is ~0.5x active area diameter, to eye).
• Get the PM data.
• Depending on what the PM data looks like, do a more fine-grained scan around some promising AM notches / PM peaks.

Attachment #1 is a sketch of the proposed setup to measure the PM response of the EX NPRO. Previously, this measurement was done via PLL. In this approach, we will need to calibrate the DFD output into units of phase, in order to calibrate the transfer function measurement into rad/V. The idea is to repeat the same measurement technique used for the AM - take ~50 1 average measurements with the AG4395, and look at the statistics. 

Some more notes:

• Delay line box is passive, just contains a length of cable.
• IQ Demodulation is done using an aLIGO 1U chassis unit, with the actual demod board electronics being D0902745
• The RF beatnote amplitude out of the IR beat PD is ~ -8dBm.
• The ZHL-3A amplifiers have gain of 24dB, so the amplified beat should be ~16dBm
• At the LSC rack, the amplified beat is split into two - one path goes to the LO input of D0902745 (so at most 13dBm), the other goes through the delay line.
• On the demod board, the LO signal is amplified with a AP1053, rated at 10dB gain, max output of 26dBm, so the signal levels should be fine for us, even though the schematic says the nominal LO level is 10dBm - moreover, I've ignored cable losses, insertion losses etc so we should be well within spec.
• The mixer is PE4140. The datasheet quotes LO levels of 17dBm for all the "nominal" tests, we should be within a couple of dBm of this number.
• There is no maximum value specified for the RF input signal level to the mixer on the datasheet, but I expect it to be <10dBm.
• We should park the beatnote around 30MHz as this should be well within the operational ranges for the various components in the signal chain.

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

There are no more double sided tape on this table.

Attachment #1 is the updated diagram of the Fiber ALS setup. I've indicated part numbers, power levels (optical and electrical). For the light power levels, numbers in green are for the AUX lasers, numbers in red are for the PSL.

I confirmed that the output of the power splitter is going to the "RF input" and the output of the delay line is going to the "LO input" of the demodulator box. Shouldn't this be the other way around? Unless the labels are misleading and the actual signal routing inside the 1U chassis is correctly done :/

• Mode-matching into the fibers is rather abysmal everywhere.
• In this diagram, only the power levels measured at the lasers and inputs of the fiber couplers are from today's measurements. I just reproduced numbers for inside the beat mouth from elog13254.
• Inside the beat mouth, the PD output actually goes through a 20dB coupler which is included in this diagram for brevity. Both the direct and coupled outputs are available at the front panel of the beat mouth. The latter is meant for diagnostic purposes. The number of -8dBm of beat @30MHz is quoted using the direct output, and not the coupled output.

Still facing some CDS troubles, will start ALS recovery once I address them.

Attachment #2 is the svg file of Attachment #1, which we can update as we improve things. I'll put it on the DCC 40m tree eventually.

• Aligned IFO to IR.
  • Ran dither alignment to maximize arm transmission.
  • Centered Oplev reflections onto their respective QPDs for ITMs, ETMs and BS, as DC alignment reference. Also updated all the DC alignment save/restore files with current alignment. 
• Undid the first 5 bullets of elog13325. The AUX laser power monitor PD remains to be re-installed and re-integrated with the DAQ.
  • I stupidly did not refer to my previous elog of the changes made to the X end table, and so spent ages trying to convince Johannes that the X end green alignment had shifted, and turned out that the green locking wasn't going because of the 50ohm terminator added to the X end NPRO PZT input. I am sorry for the hours wasted 
  • GTRY and GTRX at levels I am used to seeing (i.e. ~0.25 and ~0.5) now. I tweaked input pointing of green and also movable MM lenses at both ends to try and maximize this. 
  • Input green power into X arm after re-adjusting previously rotated HWP to ~100 degrees on the dial is ~2.2mW. Seems consistent with what I reported here.
  • Adjusted both GTR cameras on the PSL table to have the spots roughly centered on the monitors.
  • Will update shortly with measured OLTFs for both end PDH loops.
  • X end PDH seems to have UGF ~9kHz, Y end has ~4.5kHz. Phase margin ~60 degrees in both cases. Data + plotting code attached. During the measurement, GTRY ~0.22, GTRX~0.45.

Next, I will work on commissioning the BEAT MOUTH for ALS beat generation. 

Note: In the ~40mins that I've been typing out these elogs, the IR lock has been stable for both the X and Y arms. But the X green has dropped lock twice, and the Y green has been fluctuating rather more, but has mangaged to stay locked. I think the low frequency Y-arm GTRY fluctuations are correlated with the arm cavity alignment drifting around. But the frequent X arm green lock dropouts - not sure what's up with that. Need to look at IR arm control signals and ALS signals at lock drop times to see if there is some info there.

I did a cursory check of the ALS signal chain in preparation for commissioning the IR ALS system. The main elements of this system are shown in my diagram in the previous elog in this thread.

Questions I have:

1. Does anyone know what exactly is inside the "Delay Line" box? I can't find a diagram anywhere.
   • Jessica's SURF report would suggest that there are just 2 50m cables in there.
   • There are two power splitters taped to the top of this box.
   • It is unclear to me if there are any active components in the box.
   • It is unclear to me if there is any thermal/acoustic insulation in there.
   • For completeness, I'd like to temporarily pull the box out of the LSC rack, open it up, take photos, and make a diagram unless there are any objections.
2. If you believe the front panel labeling, then currently, the "LO" input of the mixer is being driven by the part of the ALS beat signal that goes through the delay line. The direct (i.e. non delayed) output of the power splitter goes to the "RF" input of the mixer. The mixer used, according to the DCC diagram, is a PE4140. Datasheet suggests the LO power can range from -7dBm to +20dBm. For a -8dBm beat from the IR beat PDs, with +24dB gain from the ZHL3A but -3dB from the power splitter, and assuming 9dB loss in the cable (I don't know what the actual loss is, but according to a Frank Seifert elog, the optimal loss is 8.7dB and I assume our delay line is close to optimal), this means that we have ~4dBm at the "LO" input of the demod board. The schematic says the nominal level the circuit expects is 10dBm. If we use the non-delayed output of the power splitter, we would have, for a -8dBm beat, (-8+24-3)dBm ~13dBm, plus probably some cabling loss along the way which would be closer to 10dBm. So should we use the non-delayed version for the LO signal? Is there any reason why the current wiring is done in this way?

After labeling cables I would disconnect, I pulled the box out of the LSC rack. Attachment #1 is a picture of the insides of the box - looks like it is indeed just two lengths of cabling. There was also some foam haphazardly stuck around inside - presumably an attempt at insulation/isolation.

Since I have the box out, I plan to measure the delay in each path, and also the signal attenuation. I'll also try and neaten the foam padding arrangement - Steve was showing me some foam we have, I'll use that. If anyone has comments on other changes that should be made / additional tests that should be done, please let me know.

20180111_2200: I'm running some TF measurements on the delay line box with the Agilent in the control room area (script running in tmux sesh on pianosa). Results will be uploaded later.

With Johannes' help, I re-installed the box in the LSC electronics rack. In the end, I couldn't find a good solution to thermally insulate the inside of the box with foam - the 2U box is already pretty crowded with ~100m of cabling inside of it. So I just removed all the haphazardly placed foam and closed the box up for now. We can evaluate if thermal stability of the delay line is limiting us anywhere we care about and then think about what to do in this respect. This box is actually rather heavy with ~100m of cabling inside, and is right now mounted just by using the ears on the front - probably should try and implement a more robust mounting solution for the box with some rails for it to sit on.

I then restored all the cabling - but now, the delayed part of the split RF beat signal goes to the "RF in" input of the demod board, and the non-delayed part goes to the back-panel "LO" input. I also re-did the cabling at the PSL table, to connect the two ZHL3-A amplifier inputs to the IR beat PDs in the BeatMouth instead of the green BBPDs.

I didn't measure any power levels today, my plan was to try and get a quick ALS error signal spectrum - but looks like there is too much beat signal power available at the moment, the ADC inputs for both arm beat signals are overflowing often. The flat gain on the AS165 (=ALS X) and POP55 (=ALS Y) channels have been set to 0dB, but still the input signals seem way too large. The signals on the control room spectrum analyzer come from the "RF mon" ports on the demod board, and are marked as -23dBm. I looked at these peak heights with the end green beams locked to the arm cavities, as per the proposed new ALS scheme. Not sure how much cable loss we have from the LSC rack to the network analyzer, but assuming 3dB (which is the Google value for 100ft of RG58), and reading off the peak heights from the control room analyzer, I figure that we have ~0dBm of RF signal in the X arm. => I would expect ~3dBm of signal to the LO input. Both these numbers seem well within range of what the demod board is designed to handle so I'm not sure why we are saturating.

Note that the nominal (differential) I and Q demodulated outputs from the demod board come out of a backplane connector - but we seem to be using the front panel (single-ended) "MON" channels to acquire these signals. I also need to update my Fiber ALS diagram to indicate the power loss in cabling from the PSL table to the LSC electronics rack, expect it to be a couple of dB.

Summary:

I am facing two problems:

1. The X arm beat seems to be broadband noisier than the Y arm beat - see Attachment #1. The Y-axis calibration is uncertain, but at least the Y beat has the same profile as the reference traces, which are for the green beat from a time when we had ALS running. There is also a rather huge ~5kHz peak, which I confirmed isn't present in the PDH error/control signal spectra (with SR785).
2. The Y-arm beat amplitude, at times, "breathes" in amplitude (as judged by control room analyzer). Attachment #2 is a time-lapse of this behaviour (left beat is X arm beat, right peak is the Y arm peak) - I caught only part of it, the the beat note basically vanishes into the control room noise floor and then comes back up to almost the same level as the X beat. The scale is 10dB/div. During this time, the green (and IR for that matter) stay stably locked to the arm - you'll have to take my word for it as I have no way to sync my video with StripTool Traces, but I was watching the DC transmission levels the whole time. The whole process happens over a few (1< <5) minutes - I didn't time it exactly. I can't really say this behaviour is periodic either - after the level comes back up, it sometimes stays at a given level almost indefinitely.

More details:

• Spent some time today trying to figure out losses in various parts of the signal chain, to make sure I wasn't in danger of saturating RF amplifiers. Cabling from PSL table -> LSC rack results in ~2dB loss.
• I will upload the updated schematic of the Beat-Mouth based ALS - I didn't get a chance to re-measure the optical powers into the Beat Mouth, as someone had left the Fiber Power Meter unplugged, and it had lost all of its charge .
• The Demod boards have a nice "RF/LO power monitor" available at the backplane of the chassis - we should hook these channels up to the DAQ for long term monitoring.
  • The schematic claims "120mV/dBm" into 50ohms at these monitoring pins.
  • I measured the signal levels with a DMM (Teed with 50ohm), but couldn't really make the numbers jive - converting the measured backplane voltage into dBm of input power gives me an inferred power level that is ~5dBm higher than the actual measured power levels (measured with Agilent analyzer in Spectrum Analyzer mode).
• Looking at the time series of the ALS I and Q inputs, the signals are large, but we are well clear of saturating our 16-bit ADCs.
• In the brief periods when both beats were stable in amplitude (as judged by control room analyzer), the output of the Q quadrature of the phase tracker servo was ~12,000 cts - the number I am familiar with for the green days is ~2000cts - so naively, I would say we have ~6x the RF beat power from the Beat Mouth compared to green ALS.
• I didn't characterize the conversion efficiency of the demod boards so I don't have a V (IF)/V (RF) number at the moment.
• I confirmed that the various peaks seen in the X arm beat spectrum aren't seen in the control signal of the EX Green PDH, by looking at the spectrum on an SR785 (it is also supposedly recorded in the DAQ system, but I can't find the channel and the cable is labelled "GCX-PZT_OUT", which doesn't match any of our current channels).
  Note to self from the future: the relevant channels are: C1:ALS-X_ERR_MON_IN1 (green PDH error signal with x10 gain from an SR560) and C1:ALS-X_SLOW_SERVO_IN1 (green PDH control signal from monitor point - I believe this is DC coupled as this is the error signal to the slow EX laser PZT temp control). I've changed the cable labels at the X end to reflect this reality. At some point I will calibrate these to Hz.
• The control room analyzer signals come from the "RF mon" outputs on the demod board, which supposedly couple the RF input with gain of -23dBm. These are then routed reverse through a power splitter to combine the X and Y signals, which is then plugged into the HP analyzer. The problem is not local to this path, as during the "breathing" of the Y beat RF amplitude, I can see the Q output of the phase tracker also breathing.

Next steps (that I can think of, ideas welcome!):

1. For Problem #1 - usual debugging tactic of switching X and Y electronics paths to see if the problem lies in the light or in the electronics. If it is in the electronics, we can swap around at various points in the signal chain to try and isolate the problematic component.
2. For Problem #2 - hook up the backplane monitor channels to monitor RF amplitudes over time and see if the drifts are correlated with other channels.
3. There is evidence of some acoustic peaks, which are possibly originating from the fibers - need to track these down, but I think for a first pass to try and get the red ALS going, we shouldn't be bothered by these.

I swapped the inputs to the ZHL-3A at the PSL table - so now the X beat RF signals from the beat mouth are going through what was previously the Y arm ALS electronics. From Attachment #1, you can see that the Y arm beat is now noisier than the X. The ~5kHz peak has also vanished.

So I will pursue this strategy of switching to try and isolate where the problem lies...

Somebody had forgotten to turn the HEPA variac on the PSL table down. It was set at 70. I set it at 20, and there is already a huge difference in the ALS spectra

[rana, kevin, udit, gautam]

quick notes of some discussions we had today:

1. Earlier in the day, Udit and I measured (with a 20dB coupler and AG4395) ~20dBm of RF beat power at input to power splitter (just before delay line box) at the LSC rack. This means that we have ~17dBm going into the LO input of the demod board. The AP1053 can only really handle a max of 16dBm at the input. After discussion with Rana, I put a 3dB attenuator at the input to the power splitter so as to preserve the LO/RF ratio in the demod circuit.
2. Need to make a detailed optical and RF power budget for both arms.
3. The demod circuit board is configured to have gain of x100 post demod (conversion loss of the mixer is ~-8dB). This works well for the PDH cavity locking type of demod scheme, where the loop squishes the error signal in lock, so most of the time, the RF signal is tiny, and so a gain of x100 is good. For ALS, the application needs are rather different. So we lowered the gain of the "Audio IF amplifier" stage of the circuit from x100 to x10, by effecting the resistor swaps 10ohms->50ohms, 1kohm->500ohms (more details about this later).
4. There is some subtlety regarding the usage of the whitening interface boards - I need to look at the circuit again and understand this better, but Rana advised against running with the whitening gain at low values. Point #3 above should have helped with this regard.
5. I wanted to test the new signal chain (with 3dB attenuation and modified IF gain) but ETMX is not happy now, and is making it impossible to keep the X arm locked. Will try again tomorrow.
6. Eventually: need to measure the mode of the fiber, and up the MM efficiency to at least 80%, which should be doable without using any fancy lenses/collimators.
7. Udit and I felt that the back panel RF power monitor wasn't working as expected - I will re-investigate this when I have the board out again to make the IF gain change permanent with the right footprint SMD resistors.

RXA: 0805 size SMD thin film resistors have been ordered from Mouser, to be shipped on Monday. **note that these thin film resistors are black; i.e. it is NOT true that all black SMD resistors are thick film**

I did some work on the PSL table today. Main motivations were to get a pickoff for the BeatMouth PSL beam before any RF modulations are imposed on it, and to improve the mode-matching into the fiber. Currently, we use the IR light reflected by the post doubling oven harmonic separator. This has the PMC modulation sideband on it, and also some green leakage. 

So I picked off ~8.5mW of PSL light from the first PBS (pre Faraday rotator), out of the ~40 mW available here, using a BS-80-1064-S. I dumped the 80% reflected light into the large beam dump that was previously being used to dump this PBS reflection. Initially, I used a R=10% BS for S-pol that I found on the SP table, but Koji tipped me off on the fact that these produce multiple reflected beams, so I changed strategy to use the R=80% BS instead.

The transmitted 20% is routed to the West edge of the PSL table via 2 1" Y1-1037-45S optics, towards the rough vicinity of the fiber coupler. For now it is just dumped, tomorrow I will work on the mode matching. We may want to cut the power further - ideally, we want ~2.5mW of power in the fiber - this is then divided by 4 inside the beat mouth before reaching the beat PD, and with other losses, I expect ~500mW of PSL power and comparable AUX light, we will have a strong >0dBm beat.

Attachment #1 is a picture of my modifications. For this work, I

• Closed PSL shutter, turned HEPA up 
• Moved HP GHz spec analyzer to the side for ease of access to the table.
• Moved several optics that look to me as to have once been part of the RefCav setup - I don't think this would have been a useful alignment reference in any case as we moved the RefCav in a non-deterministic way for the PSL secondary shelf install.
• Used one 1" 45 deg S-pol optic from the optics cabinet - remaining optics were scavenged from PSL table and SP table.
• Removed an SMA cable connected to an EOM, whose other end wasn't connected to anything.
• Turned HEPA back down, IMC locks fine now.

On Friday, while Udit and I were doing some characterization of the EX+PSL IR beat at the LSC rack, I noticed that there were sidebands around the main beat peak at 20dBm lower level. These were offset from the main peak by ~200kHz - I didn't do a careful characterization but because of the symmetric nature of these sidebands and the fact that they appeared with the same offset from the main peak for various values of the central beat frequency, I hypothesize that these are from the modulation sidebands we use for PDH locking the EX laser to the arm cavity. So we can estimate the modulation depth from the relative powers of the main beat peak and the ~200kHz offset sidebands.

Since the IR light is used for the beat and we directly couple it to the fiber to make the beat, there is no green or IR cavity pole involved here. 20dBm in power means . And so the modulation depth, . I will do a more careful meaurement of this, but this method of measuring the modulation depth can give us a precise estimate - for what it's worth, this number is in the same ballpark as the measurement I quote in elog12105.

What is the implication of having these sidebands on our ALS noise? I need to think about this, effectively the phase noise of the SR function generators we use to do the phase modulation of the EX laser is getting imprinted on the ALS noise? Is this hurting us in any frequency range that matters?

I was looking into the physics of polarization maintaining fibers, and then I was trying to remember whether the fibers we use are actually polarization maintaining. Looking up the photos I put in the elog of the fibers when I cleaned them some months ago, at least the short length of fiber attached to the PD doesn't show any stress elements that I did see in the Thorlabs fibers. I'm pretty sure the fiber beam splitters also don't have any stress elements (see Attached photo). So at least ~1m of fiber length before the PD sensing element is probably not PM - just something to keep in mind when thinking about mode overlap and how much beat we actually get.  

I was looking at this a little more closely. As I understand it, the purpose of the audio differential IF amplifier is:

1. To provide desired amplification at DC-audio frequencies
2. To low pass the 2f component of the mixer output

Attachment #1 shows, the changes to the TF of this stage as a result of changing R19->50ohm, R17->500ohm. For the ALS application, we expect the beat signal to be in the range 20-100MHz, so the 2f frequency component of the mixer output will be between 40-200MHz, where the proposed change preserves >50dB attenuation. The Q of the ~500kHz resonance because of the series LCR at the input is increased as a result of reducing R17, so we have slightly more gain there. 

At the meeting yesterday, Koji suggested incorporating some whitening in the preamp itself, but I don't see a non-hacky way to use the existing PCB footprint and just replace components to get whitening at audio frequencies. I'm going to try and measure the spectrum of the I and Q demodulated outputs with the actual beat signal to see if the lack of whitening is going to limit the ALS noise in some frequency band of interest.

Does this look okay?

I tried to couple the PSL pickoff into the fiber today for several hours, but got nowhere really, achieved a maximum coupling efficiency of ~10%. TBC tomorrow... Work done yesterday and today:

• I changed the collimator from the fixed focal length but adjustable lens position CFC-2X-C to the truly fixed F220-APC-1064 recommended by johannes.
• Used a pair of irises to level the beam out at 4" with two steering mirrors.
• Used a connector on the PSL table to couple the EX laser light to the PSL fiber - then measured the mode using the beam-scanner (beam is ~300uW) 
• Measured the mode of the PSL pickoff beam, also using the beam scanner. 
• Per specs on the Thorlabs website, the F220-APC-1064 has a divergence angle of 0.032 degrees. So expected waist is ~1200um, and the Rayleigh range is ~4.3m, so this is not a very easy beam to measure and fit. I may be thinking about this wrong?
• Measured beam 1/e^2 dia over ~0.65m, and found it to be fairly constant around 1800um (so waist of 900um) - beam is also pretty symmetric in x and y directions, but I didn't attempt an M^2 measurement.
• The pickoff from the PSL also did not yield a very clean beam profile measurement, even though I measured over ~1m z-propagation distance. Nevertheless, this looked more like a Gaussian beam, and I confirmed the fitted waist size/location approximately by placing the beam profiler at the predicted waist location and checking the spot size.
• Used jammt to calculate a candidate mode-matching solution - the best option seemed to be to use a combination of a f=150mm and f=-75mm lens in front of the collimator. 
• Despite my best efforts, I couldn't get more than ~500uW of light coupled into the fiber - out of the 8mW available, this is a paltry 12.5% 
• Because the mode coming out of the fiber is relatively large, and because I have tons of space available on the PSL table, this shouldn't be a hard mode-matching problem, should be doable without any fast lenses - perhaps I'm doing something stupid and not realizing it. I'm giving up for tonight and will try a fresh assault tomorrow. 

I think part of the problem was that the rejected beam from the PBS was not really very Gaussian - looking at the spot on the beam profiler, I saw at least 3 local maxima in the intensity profile. So I'm now switching strategies to use a leakage beam from one of the PMC input steering optics- this isn't ideal as it already has the PMC modulation sideband on it, and this field won't be attenuated by the PMC transmission - but at least we can use a pre-doubler pickoff. This beam looks beautifully Gaussian with the beam profiler. Pics to follow shortly...

Attachment #1 shows the current situation of the PSL table IR pickoff. It isn't the greatest photo but it's hard to get a good one of this setup. Now there is no need to open the Green PSL shutter for there to be an IR beat note.

• The key to improving the mode-matching was to abandon my "measurements" of the input mode and the mode from the collimator.
• The best I could do with these measurements was ~25% coupling, whereas now I have ~78% (all powers measured with Ophir power meter).
• Focusing was done using two f=300mm lenses (see attachment).
• By moving the second (closer to collimator) lens through ~1inch of its current position, I was able to see a clear maximum of the coupled power.
• By moving the second lens by ~5mm, and touching up the alignment, I couldn't see any improvement.

All this lead me to conclude that I have reached at least some sort of local maximum. The AR coating of the lens has ~0.5% reflection at 8 degrees AOI according to spec, and EricG mentioned today that the fiber itself probably has ~4% reflection at the interface due to there not being any special AR coating. There is also the fact that the mode of the collimator isn't exactly Gaussian. Anyways I think this is a big improvement from what was the situation before, and I am moving on to debugging the ALS electronics.

There is 3.65mW of power coupled into the fiber - our fiber coupled PDs have a damage threshold of 2mW, and this 3.65mW does get split by 4 before reaching the PDs, but good to keep this number in mind. For a quick measurement of the PMC and X end PDH modulation depth measurements, I used an ND=0.5 filter in the beam path.

I used the Beat Mouth to make a quick measurement of the PMC and EX modulation depths. They are, respectively, 60mrad and 90mrad. See Attachments #1 and #2 for spectra from the beat photodiode outputs, monitored using the Agilent analyzer, 16 averages, IF bandwidth set to resolve peaks offset from the main beat frequency peak by 33.5MHz for the PMC and by ~230kHz for the EX green PDH.

For this work, I had to re-align the IFO so as to lock the arms to IR. c1susaux was unresponsive and had to be power-cycled. As mentioned in the earlier elog, to avoid saturating the Fiber Coupled beat PDs, I placed a ND=0.5 filter in the fiber collimator path, such that the coupled power was ~1mW, which is well inside the safe regime.

For the EX modulation depth, I could have gotten multiple estimates of the modulation depth using the higher order products that are visible in the spectrum, but I didn't.