Every so often things just work out. You do the calculations, you put the lenses on the bench, you manually adjust the pointing and fiddle with the lenses a bit, you get massive chunks of assistance from Kiwamu to get the alignment controls and monitors set up and after quite a bit of fiddling and tweaking the cavity mirror alignment you might get some nice TEM_00 -like shapes showing up on your Y-arm video monitors.
So. We have resonating green light in the Y-arm. The beam is horribly off-axis and the mode-matching, while close enough to give decent looking spots, has in no way been optimised yet. Things to do tomorrow - fix the off-cavity-axis problem and tweak up the mode-matching... then start looking at the locking...
I think I had underestimated the responsivity of the Silicon PD at 1064nm. The previous value was based on a rough search online for the responsivity of Silicon (I couldn't find the product number of the actual PD we are using). For instance, the PDA100A Si detector from Thorlabs has a responsivity of 0.35-0.4A/W at 1064nm.
If we calculate the responsivity of the Hartmut PD from the measurements I made today (input power = 0.300mW, output voltage = 5.56mV, effective transimpedance = 80 Ohms), then the responsivity at 1064nm is 0.23 A/W which is not an unreasonable number given the response of the Thorlabs detector.
There is one other troubling point: using the estimate of responsivity on the Harmut PD * incident power * transimpedance at DC = (0.02A/W) * (0.28mW) * (80 V/A) = 0.45 mV.
But the measured DC voltage is 6.5mV = inconsistent.
Responsivity of SGD-444A
For instance, the PDA100A Si detector from Thorlabs has a responsivity of 0.35-0.4A/W at 1064nm.
Having convinced myself that the green Hartmut PD is giving an acceptable response at RF frequencies I decided to double-check the beatnote at IR (fiber transmission from the X-end beating with the PSL). This took a while because I had to realign the beam into the fiber at the X-end (I had a PD monitoring the output from the fiber on the PSL table and 40m of BNC cable giving me the signal from it at the X-end).
Eventually, I managed to get a beatnote on the PD. At first there was no signal at the temperature calculated using Koji and Suresh's calibration, but it turned out that the mode-overlap wasn't good enough on the PD. Now I can clearly see beats between a couple of modes, one of which is much stronger than the other. I think we should use a frequency discriminator on the output from the IR PD to servo the end laser and keep the strong beat note within <100MHz of DC.
I started to modify another green PD set.
It so far has the transimpedance of 240 Ohm on CLC409 for the RF output.
It shows the BB output upto ~100MHz.
The measurement shows the transimpedenca of ~90Ohm which is ~25% smaller than the expected gain of 120Ohm.
It is calibrated based on the transimpedances of Newfocus 1611 (10kOhm and 700Ohm for AF and RF).
The next step is to change the transimpedance resister to 2k and replace the PD to S3399 Si PD, which has the diameter of 3mm.
Then, the noise level will be measured. (and replace the RF opamp if necessary)
The Y-arm can now be locked with green light using the universal PDH servo. Modulation frequency is now 277kHz - chosen because it seems to produce smaller offsets due to AM effects
To lock, turn on the servo, align the system to give nice circular-looking TEM_00 resonances, and wait for a good one. It'll lock on a decent mode for a few seconds and then you can turn on the local boost and watch it lock for minutes and minutes and minutes.
The suspensions are bouncing around a bit on the Y-arm and the spot is quite low on the ETMY and a little low on ITMY, but from this point it can be tweaked and optimised.
OK… the Y-arm may be locked with green light, which was the goal, and this is all good but it's not yet awesome. Awesome would be locked and aligned properly and quiet and optimised. So... in order to assist in increasing the awesome-osity, here are a few stream-of-consciousness thoughts and stuff I've noticed and haven't had time to fix/investigate or have otherwise had pointed out to me that may help...
Firstly, the beam is not aligned down the centre of the cavity. It's pretty good horizontally, but vertically it's too low by about 3/4->1cm on ETMY. The mirrors steering the beam into the cavity have no more vertical range left, so in order to get the beam higher the final two mirrors will have to be adjusted on the bench. Adding another mirror to create a square will give more range AND there will be less light lost due to off 45degree incident angles. When I tried this before I couldn't get the beam to return through the Faraday, but now the cavity is properly aligned this should not be a problem.
A side note on alignment - while setting cameras and viewports and things up, Steve noticed that one of the cables to one of the coils (UL) passes behind the ETMY. One of the biggest problems in getting the beam into the system to begin with was missing this cable. It doesn't fall directly into the beam path if the beam is well aligned to the cavity, but for initial alignment it obscures the beam - this may be a problem later for IR alignment.
Next, the final lambda/2 waveplate is not yet in the beam. This will only become a problem when it comes to beating the beams together at the vertex, but it WILL be a problem. Remember to put it in before trying to extract signals for full LSC cavity locking.
Speaking of components and suchlike things, the equipment for the green work was originally stored in 3 plastic boxes which were stored near the end of the X-arm. These boxes, minus the components now used to set up the Y-end, are now similarly stored near the end of the Y-arm.
Mechanical shutter - one needs to be installed on the Y-end just like the X-end. Wasn't necessary for initial locking, but necessary for remote control of the green light on/off.
Other control… the Universal PDH box isn't hooked up to the computers. Connections and such should be identical to the X-arm set-up, but someone who knows what they're doing should hook things up appropriately.
More control - haven't had a chance to optimise the locking and stability so the locking loop, while it appears to be fairly robust, isn't as quiet as we would like. There appears to be more AM coupling than we initially thought based on the Lightwave AM/PM measurements from before. It took a bit of fiddling with the modulation frequency to find a quiet point where the apparent AM effects don't prevent locking. 279kHz is the best point I've found so far. There is still a DC offset component in the feedback that prevents the gain being turned up - unity gain appears limited to about 1kHz maximum. Not sure whether this is due to an offset in the demod signal or from something in the electronics and haven't had time left to check it out properly yet. Again, be aware this may come back to bite you later.
Follow the bouncing spot - the Y-arm suspensions haven't been optimised for damping. I did a little bit of fiddling, but it definitely needs more work. I've roughly aligned the ETMY oplev since that seems to be the mass that's bouncing about most but a bit of work might not go amiss before trusting it to damp anything.
Think that's about all that springs to mind for now…
Thanks to everyone at the 40m lab for helping at various times and answering daft questions, like "Where do you keep your screwdrivers?" or "If I were a spectrum analyser, where would I be?" - it's been most enjoyable!
Y-end PDH electronics.
The transfer function of the Y-end universal PDH box:
I was investigating the beat note amplitude on the vertex PD again yesterday. The incident power on the PD was 150uW in the PSL green beam and 700uW in the X-ARM green beam. With perfect overlap and a transimpedance of 240, I expected to get a beat note signal of around 25mV or -19dBm. Instead, the size was -57dBm. Bryan and I adjusted the alignment of the green PSL beam to try and improve the mode overlap but we couldn't do much better than about -50dBm. (The noise floor of the PD is around -65dBm).
When we projected the beams to the wall of the enclosure, the xarm beam was 2 to 3x as large as the PSL green beam, indicating that the beam size and/or curvatures on the PD were less than ideal. There is a telescope that the XARM beam goes through just before it gets to the PD. I mounted the second lens in this telescope on a longitudinal translation stage. With some finagling of the position of that lens we were able to improve the beatnote signal strength to -41dBm.
Obviously the ideal solution would be to measure the beam size and RoC of the PSL beam and XARM beams and then design a telescope that would match them as precisely as possible because there's still another 20dB signal strength to be gained.
With some assistance from Kiwamu and Koji, I've drawn up the electronics design for the Beat Box for the vertex green locking. The Omingraffle schematic is posted on the Green Locking Wiki page. It's also attached below. Some final touches are necessary before we can Altium this up.
Attachment 1: Schematic of beatbox
Attachment 2: Front and back panel designs.
- AC coupling for the comparator circuit of the green locking
In order to relieve the power consumption of the RF buffer, ac coupling circuits have been added.
The ac coupling before the buffer amp helps to relieve the power consumption in the chip.
But because of the distortion of the signal (and the limitation of the bandwidth), the output still has some DC (~0.6V).
Therefore, the output is also AC coupled.
Note that the BW pin of BUF634P should be directly connected to -15V in order to keep the bandwidth of the buffer.
The drawings are also uploaded on the green electronics wiki
1.The aim is the laser frequency stabilisation of PSL and AUX.
2.As a first step we want to couple some of the AUX laser beam into a single mode optical fibre and route the fibre to the PSL table.
3.The position of the optical fibre on the ETMY table is shown by the coupler in the attached picture. The yellow lines show the new scheme we want to implement.
4.WHAT WE DID TODAY.
The Lightwave NPRO power supply which is being shared between the AS table and the ETMY table has been shifted back to the ETMY table.
The current to the laser is set at 1.5A. The laser output is 200mW at this current level.
What I did today.
1. Collimation of a beam.
2. Coupling of the IR light at the ETMY table to a fibre.
June 22-June 24:
1.Coupling light into fibre at the ETMY.
2.Routing of the fibre to the PSL table.
June 27-June 30:
1.PSL optical table layout sketching.
2.Combining the PSL beam with fibre output onto a BS and then superpose them on a New Focus 1611 PD.
July 5-July 8:
1.Conversion of the PD output to voltage using MFD(Mixer Frequency Discriminator).
2. Report writing.
July 7: 5:00 pm: 1st Report Due.
July 11-July 22:
1.Locking Y-arm to PSL.
2.Setting up the feedback loop using the MFD output as the error signal and acting on the AUX laser frequency.
July 25-Aug 5:
1.Y-Arm cavity characterisation.
Measurement of the transmission of IR and green light through the cavity.
To obtain FSR, Finesse,Loss of the Cavity, Visibility, Transverse Modes(g-factor, astigmatism), Reflectivity, Q-factor.
3.Report and abstract writing.
Aug 1: 5:00 pm: 2nd Report and absract due.
Preparation for talk and seminar.
1. I tried to align the IR input beam by aligning the two mirrors, to couple input light into the fibre.
2.I was unsuccessful for a long time even though I tried a lot of tricks.
3. I also tried to use the optical fault locator to superpose the IR beam spot onto the beam spot of the other laser to facilitate effective coupling.
4.But the crucial point was to superpose the input beam path in the perfect direction of the output beam path and not just the beam spot.(the input cone and the output cone are perfectly aligned).
5.After one whole day of trial and thought, I managed to couple light into the fibre, and saw the output beam spot on the screen-camera-monitor set-up which we had arranged. Eurekka !!;)
6.I then used a power meter to measure the input beam power and the output beam power.
7.It was a disappointing 2% . I had read in project reports of many students of a 20% success.
8.After a lot of subtle tweaking of the mirrors using the knobs, I managed to increase the percentage of output beam to 12%.
9. This is a workable level.
10.A day of lot of new learning! Pictures of the setup are attached.:)
1. Suresh and I completed the alignment of the fibre and the three mirrors on the ETMY table.
2. We managed to get an output beam power of around 60% using the Ophir(Orion/PD) power meter to finetune the alignment. The power of the input beam is 74.4 mW and of the output beam is 38.5 mW.
3. The coupler on the output side of the fibre which had been put there to help in the alignment has been removed.
4. The picture of the ETMY layout as of now has been attached.
5. The labels A stands for the mirror used to turn the beam direction and B and C stand for the three mirrors used in the alignment of the beam into the coupler,D.(attachment 3).
6. The fibre we used is 50m in length which was barely sufficient to reach the PSL table.
7. So, the fibre has been routed to the PSL table using the fibre tray running below the Y-arm tube as this was the shortest route possible(even though it is a rather acccident prone zone).
8. The fibre has been tied down at regular intervals so that it does not get snagged and pulled up inadvertently.
9. We will start with the preparation of the layout of the PSL table to superpose the two beams on Monday.
The ETMY laser was operating at 1.5 A current and 197 mW power.
For the efficient frequency doubling of the AUX laser beam at the ETMY table, a higher power is required.
Steve and I changed the current level of the laser from 1.5 A to 2.1 A in steps of 0.1 A and noted the corresponding power output . The graph is attached here.
The laser has been set to current 1.8 Amperes. At this current, the power of the output beam just near the laser output is measured to be 390 mW.
The power of the beam which is being coupled into the optical fibre is measured to be between 159 mW to 164 mW (The power meter was showing fluctuating readings).
The power out of the beam coming out of the fibre far-end at the PSL table is measured to be 72 mW. Here, I have attached a picture of the beam paths of the ETMY table with the beams labelled with their respective powers.
Next we are going to adjust the green alignment on the ETMY and then measure the power of the beam.
At the output end of the fibre on the PSL, a power meter has been put to dump the beam for now as well as to help with the alignment at the ETMY table.
For the phase locking or beat note measuring we only need ~1 mW. Its a bad idea to send so much power into the fiber because of SBS and safety. The power should be lowered until the output at the PSL is < 2 mW. In terms of SNR, there's no advantage to use such high powers.
Well,the plan is to put in a neutral density filter in the beam path before it enters the fibre. But before I could do that, I set up the camera on the PSL table to look at the fiber output . I will need it while I realign the beam after putting in the Neutral Density Filter. I have attached the ETMY layout with the Neutral Density filter in place herewith.
The fibres carrying the beams from the ETMX as well as the ETMY have been routed to the PSL table now.
A part of the PSL beam has to be superposed on the fibre-outputs to obtain a beat signal. We have located a stray beam on the PSL(which is currently being dumped) which we plan to redirect for the same. The layout of the plan is attached herewith.
While aligning the Y-end aux laser light into the fiber we noticed that the green power out of the doubling crystal was in microwatts. I checked to see what was the trouble and found that the oven was cold as the temperature controller had been disabled. I enabled it and scanned the temperature to maximise the green output. Yet the power is less than 10% of that at the X end (7mW).
To verify I checked the power of various beams on the Y-end table. They are listed below in the picture
The green beam power is proportional to the square of the IR incident power and this explains the drop in green power by a factor of (210/730)^2 thus making 7 mW --> 0.5 mW. However we may be able to double the power at the Y-arm oven if the uncoated lenses in the IR path are exchaned for coated ones.
The green beam injection into the Y-arm cavity also needs to be cleaned up as noted here. As seen in the picture below two of the mirrors which launch the beam into the arm cavity need to be fixed as well.
I finished wih the set-up at the ETMY table. Instead of the neutral Density Filter , I put in a mirror(Y1-1037-45S) which is reflective for IR , so that only 1% of the light is incident on the fibre as per Rana's suggestion.
Now, the power incident on the fibre is measured to be 6 mW and the power measured out of the fibre is 2.76 mW after the necessary alignments.
On the PSL able, I have routed the beam that is coming out of the back of the PMC(instead of the dumped light from the oven to prevent any light from reflecting back into the laser), to the area where I am putting the set-up for the superposition of the PSL and the ETMX and ETMY beams.
Today I will proceed with the layout.
1. I have used the PMC trans beam in my set-up as the required PSL beam.
2. I have superposed the ETMX-Fibre output with the PSL beam on the PSL table.
3. I have used suitable beam splitters and lens to match the power and the sizes of the overlapping beams and have aligned them to the optimum.
4. A lens having f=7.6 cms is used to focus the beam into the PD.
5. Initially, I used the broadband 1611 NewFocus PD to find the IR beat signal by scanning the oven temperature. (using the digital sitemap controls.)
6. I checked the previous elog entries by Suresh and Koji on the green beat signal they had worked on and used their data to get an idea of the temperature range of the oven where I could obtain a beat.
7. I obtained peaks at three different temperatures as had been noted previously and set the temperature so that I am now sitting in the middle stable regime.
8. Then I switched to the 1811 100 MHz PD as it has a larger gain. It has a saturation power of 100 microWatts. The input power at the PD is measured to be 80 microWatts.
9. I was having trouble getting a clean peak due to presence of many harmonics as seen on the spectrum analyser. This happened because there was too much power incident on the PD which led to arising of non-linearity giving rise to harmonics.
10.To reduce the power entering the PD, I put in a ND 1.0 Filter just before the beam enters the PD and obtained a clean signal.
11. I will use the frequency counter tomorrow to check the resonant frequency and try to connect the output to acquire a digital signal.
12. Otherwise I will proceed to build a Mixer Frequency Discriminator.
13. After the feed-back loop is completed, I will proceed to compare the frequency-noises of the green-beat lock and the IR-beat lock.
Initially I was using RFPD-1611to get the IR beat frequency. Its gain was not very high, so I was getting a very low signal of power -37 dBm.
I used ZHL-32A-S with a gain of 25 dBm to amplify it before feeding it into the spectrum analyser.
I connected the ground of the amplifier circuit to the red of the power supply, which blew the amplifier.
I learnt that there is a small tab indicating the ground side of the BNC to banana connectors which I should have noticed.
I learnt to plug in the side with th little tab on it into the ground of the power supply. (Learnt it the hard way I guess!!)
The optics on the Y-end table which required to be moved have been repositioned. Please see the attached pic for details.
The green beam is not yet aligned to the cavity. That is my next task.
The beam axis of the Y green light has been aligned.
Now I can see TEM00 mode is flashing on the ETMY camera.
-- (What I will do tonight)
The next step is to refine some electronics in the PDH loops to get the green light locked to the Y arm cavity.
If the beam isn't locked, I guess the in-vac-work will be so difficult because of the low intensity of the green light.
According to a brief check on the circuits, a low pass filter after the demodulation mixer is in a sad situation.
It doesn't pass any signals and in fact it behaves more like an absorber.
On the other hand, the modulation system looks fine to me because I was able to see the 270 kHz sideband converted into AM due to the fringing.
(not yet) Alignment of the Y green beam (#5066)
I succeeded in locking the green light to the Y arm cavity, but it wasn't so robust. Something is unhealthy in the electronics.
I am leaving the Y green system as it is because I already can see a plenty of the green light flashing in the BS chamber.
So just a flashing of the green light is good enough for the in-vac-work.
The next step is to refine some electronics in the PDH loops to get the green light locked to the Y arm cavity.
[Jenne / Kiwamu]
The X green beam has been realigned to compensate the effect of the ETMX repositioning.
After the alignment we became able to lock the 00 mode with the X green beam.
For the alignment:
spot position on the ETMX mirror = within ~ 1 cm. This number is strictly constrained by a homemade aluminum iris that Jamie put last Friday.
spot position on the ITMX mirror = unknown, but looks pretty good on the CCD camera.
spot position on the PSL table = ~ 1 mm downward from what it used to be. The horizontal alignment is perfect.
The X green beam again became a reference of the beam axis.
The ETMX suspension tower is in a good place.
Kiwamu will work on the green alignment over the weekend. Assuming everything works out, we'll try the same procedure on ETMY on Monday.
The ETMY suspension tower is in a good place.
You meant ETMX, right? ETMY still hasn't been touched.
KI : sorry, I meant ETMX. I fixed the entry.
[Jenne, with ample supervision by Kiwamu and Suresh]
Y-green was aligned, and is now flashing. The ETMY trans camera was very helpful for this alignment. I didn't end up needing to use a foil aperture.
Kiwamu and Suresh had just closed up the IOO doors, so we don't know yet where it's hitting on the PSL table (if the beam is making it that far). Tomorrow we'll look at ITMY to see if the green beam is centered there, and if it's coming out to the PSL table.
Uniblitz mechanical shutter installed in the green beam path at ETMY-ISCT The remote control cable has not been connected.
Another Hamasutu S3399 photodiode was tested with the electronic circuit as described in LIGO-D-1002969-v.
RF transimpedance is 1k although the DC transimpedance is 2k.
The noise level is 25pA/sqrt(Hz) which corresponds to a dark current of 1.9mA or 1.7mA in the independent measurement.
At all frequencies the noise is larger compared to Koji's measurement (see labbook page 4778).
In file idet_S3399.pdf the first point is not within its error bars on the fitted curved. This point corresponds to the dark noise measurement
I made this measurement again. Now it is on the fitted curve. In the previous measurement I pushed the save button a bit too early. The
averaging process has not been ready while I pushed the 'save' button.
Dark current is 1.05mA and noise is lower than in the previous measurement.
New file are the XXX_v2.pdf files
[Kiwamu / Katrin]
On Wednesday, the green light was locked to the Y arm cavity.
Modulation frequency was changed from 279kHz to 178875Hz. The amplitude was changed from 10Vpp to 0.01Vpp to achieve a modulation index of 0.38. The modulation frequency was changed to minimize AM. With the new modulation frequency the laser light could still locked to the cavity.
The signal of the LO and the photodiode are multiplied by a ZAD-8 mini circuit mixer (Level 7). This mixer requires LO input is +7dBm = 1.4Vpp. Thus we put a 36dB attenuator between the LO and the PZT at the laser. PDH error signal shows lots of peaks that are most likely higher order sidebands. Thus, the next step is to work on the low-pass filter. However the SNR of the error signal has improved with the new modulation frequency. With the old mod. frequency the PDH signal was 4mVpp and the noise floor was 2mVpp.
Phase between the photodiode signal and LO is shifted by about 10 degrees. Step two is to work on a phase shifter.
I inserted several beam blocks and iris on the Y arm Green table. There was/is lots of stray light because a lot of the mirrors are not under an angle of incident of 45°. Some stray light is left since couldn't find an appropriate beam block/dump due to lack of space on the table.
This is a kind of summary of what I have worked on this week.
After all the changes made last week, I could not manage to lock the green light to the cavity, but the PDH error signal looks nicer.....at least something.
Alignment of the light to the cavity:
Low-pass filter (LPF)
PDH error signal
Phase shift between green PD signal and LO
Thank you for the summary.
I think now you are getting into a phase where you should start doing some quantitative and careful checks.
1. Calculate how much light will be reflected from the cavity if the alignment is perfect.
2. Investigate where those kHz oscillations are coming from.
3. Estimate how much the 1.1 kHz corner frequency in LPF will reduce the phase margin around 10 kHz.
4. Calculate the estimated amplitude of the PDH signal.
5. Compute how big the gain of the PDH box should be.
This is a kind of summary of what I have worked on this week.
As reported earlier an oscillation around 7kHz is an the PDH error signal. The lower spectrum show that there is a peak from 6-7kHz.
This peak is somehow dependent on the modulation frequency. This means the peak can be shifted to a higher frequency when the modulation frequency is increased (see for comparsion f_mod=279kHz).
If the power supply for the green PD is switched of the peak vanishes. The same happens if the LO is switched of.
On the Green YARM end table the second mirror behind the laser has been exchanged.
Reason: The light is impinging on the mirror under an angle of about 10 degrees, but the old mirror was coated for angle of incidence (aoi) of 45°.
Thus it was more like a beam splitter. The new mirror is a 1" Goock & Housego mirror which has a better performance for an aoi of 10 degree.
Realignment through Faraday Isolator and SHG cristall as well as 532nm isolator is almost finished.
We were poking around and tried to make a button for the Y-green shutter, just like the X-green already has. I don't know where the X-green shutter button goes to in model-land, so I can't figure out if there is already a channel set up for the Y end. Just switching the X for a Y didn't work. Someone (maybe me) should fix this in the next soon.
After all realignment is finished, here are the powers at several positions:
The angle of incidence of light is for some mirrors on the YARM end table different from 45° even though the mirrors are coated for 45°.
The mirrors below are useful if there are plans to replace these mirrors by properly coated ones.
* This is the new mirror as decribed on http://nodus.ligo.caltech.edu:8080/40m/5623
There was a 60 Hz and 120 Hz oscillation on the green PDH photo diode output. After a long search, I could identify that
the source was a broken BNC cable which was connected to the photo diode. I exchanged that BNC cable and the 60 Hz
and 120 Hz are gone :-)
With the new cable the PD output was less noisy so that it was easier to achieve a better alignment of the light to the cavity.
The reflected power could be reduced from 40% to 30%. For perfect alignment the reflected power would be 20%.
The mini circuit power splitter ZFRSC-42S+ used at the YARM has no balanced output as it should have according to the data sheet.
@ 0.05MHz the amplitude unbalance should be 0.03 dB
A quick measurement shows that there is a LO amplitude dependent unbalance:
So my question is, shall I replace the power splitter just in case it is further degrading?
It is a 4th order filter with cut of frequency of 120 kHz.
The ETMY shutter can be remotely switched from medm screen POY of mechanical shutters.
The new cable from ETMY controller goes to east vertex EV-ISCT where it is connected to POY shutter hook up BNC cable.
The mechanical shutter on the Y end is now fully functional.
The mechanical shutter on the Y end is now fully functional.
It is newly named to 'C1AUX_GREEN_Y_Shutter' in the EPICS world.
It uses the same binary output channel which had been served for the POY shutter.
To change the EPICS name I edited a db file called ShutterInterLock.db, which resides in /cvs/cds/caltech/target/c1aux.
After editing the file I rebooted the c1aux machine, by telnet and the reboot command in order to make the change effective.
Also I added this new shutter on the ALS overview screen (see the attachment below)
Yesterday, I measured the transfer function of the YARM PDH box.
I tested the electronic board and couldn't find a frequency dependent behaviour. So I measured the TF again and it looked nice.
Today's nice measurement could is/was reproducible. I suppose yesterday's measurement is just an artefact.
The electronic board is modified according to Kiwamu's wiki entry http://blue.ligo-wa.caltech.edu:8000/40m/Electronics/PDH_Universal_Box
Btw. The light could be locked to the cavity for ~3min.
I could not improve the locking. So, I checked the transfer function of the PDH box again. The transfer function looks okay if the gain knob is <=2.0.
If the gain knob is >2.0 the 20dB step appears in the transfer function (see elog page 5713). This step is shifted to higher frequencies if the gain is
increased. The PZT drive out was not saturated at any time. Yesterday, I checked the electronic circuit with a gain of 2.0. Thus, I couldn't find the broken
gain amplifier (AD8336). The amplifier is ordered in will arrive on Monday.
From time to time the 20 dB jump in the transfer function still occurs. The new AD8336 op amp did not change that issue. I am sure that the op amp was broken,
because the amplitude of the sine did not change when I turned the gain knob.
The above two curves were measured with different input amplitude of the sine from the spectrum analyzer. Nothing changed in between except that there was no
jump when Kiwamu was around. Very strange. Testing the electronic board led to no clue what is happening.
For now, I will just use the PDH box as it is, but one should keep this odd behaviour in mind.