I turned off fb40m2 and fb40m temporarily while we added an extra power strip to the (new) 1X6 rack at the bottom in the back. This is to allow for the addition of the 4600 computer given to us by Rolf (which needs a good name) into the rack above the fb machine. The fb40m2 was unfortunately plugged into the main power connectors, so we unplugged two of its cables, and put them into the new strip. While trying to undo some of the rats nest of cables in the back I also powered down and unpluged briefly the c0dcu1, the pem crate, and the myrinet bypass box.
I am in the process of bringing those machines back up and restoring the network.
Also this morning, Megatron was moved from the end station into the (new) 1X3 rack, along with its router. This is to allow for the installation of the new end computer and IO chassis.
I have uploaded to my directory a directory neural_plant. The most important file is reference_plant.c, which compiles with the command
We would appreciate some plots. Learning curves of recurrent NN working as a plant are interesting. For harmonic oscillator your RNN should not contain any hidden layers - only 1 input and 1 output node and 2 delays at each of them. Activation function should be linear. If your code is correct, this configuration will match oscillator perfectly. The question is how much time does it take to adapt.
Does FANN support regularization? I think this will make your controller more stable. Try to use more advanced algorithms then gradient descent for adaptation. They will increase convergence speed. For example, look at fminunc function at Matlab.
I've been on break this week, so in addition to working at my lab here, I've done some NN stuff. In response to Den's response to my last post, I've included learning curve plotting capabilities,
I've explored all of the currently documented capabilities of FANN (Fast Artificial Neural Network - it's a C library) (most likely, there are additions to the library floating around in open-source communities, but I have yet to look into those). There is extensive FANN documentation on the FANN website (http://leenissen.dk/fann/html/files/fann-h.html), but I'll cut it down to the basics here:
FANN Neural Network Architectures
standard: This creates a fully connected network, useful for small networks, as in the reference plant case
sparse: This creates a sparsely connected network (not all of the connections between all neurons exist at all times), useful for large networks, but not useful in the reference plant case, since the number of neurons is relatively small
shortcut: This creates some connections in the network which skip over various hidden layers. Not useful in the harmonic oscillator case since there are no hidden layers. Probably won't be useful in a better-modeled referrence plant since this reduces the non-linear capabilities of the model.
TRAIN_INCREMENTAL: updates the weights after every iteration, rather than after each epoch. This is faster than the other algorithms for the reference plant.
TRAIN_BATCH: updates the weights after training on the whole set. This should not be used on batches of data for the reference plant, seeing as the time history dependence of the plant is smaller than the size of the entire data set.
TRAIN_RPROP: batch training algorithm which updates the learning parameter.
TRAIN_QUICKPROP: updates the learning parameter, and uses second derivative information, instead of just first derivative, for backpropagation.
FANN Activation Functions
FANN offers a bunch of activation functions, including a function FANN_ELLIOT, which is essentially the "signmoid like" activation function Den and I used this summer, which runs in the order of multiplication and addition. The function parameters (steepness) can also be set.
As usual, the learning parameter can be set. While over the summer we worked with lower learning parameters, in the case of the harmonic oscillator reference plant, since the error is low after the first iteration, higher learning parameters (0.9, for example), work better. However, this is a very isolated case, and, in general, lower parameters, though convergence is slower, produce more optimal results.
The learning momentum is another parameter that can be set - the momentum factor is a coefficient in the weight adjustment equation which allows for the difference in weights beyond the previous weight to be factored in. In the case of the reference plant, a higher learning momentum (0.9) is optimal, although in most cases, a lower learning momentum is optimal so that the learning curve doesn't oscillate terribly.
FANN does not explicitly include regularization, but this can be implemented by checking the MSE at each iteration against the MSE at the n previous iterations, where n is the regularization parameter, and stopping training if there is no significant decrease (also determined by a parameter). The error bound I specified during training was 0.0001
The best result for the reference plant was obtained using FANN_TRAIN_INCREMENTAL, a "standard" architecture, a learning rate of 0.9 (as explained above) and a learning momentum of 0.9 (these values should NOT be used for highly non-linear and more complicated systems).
I have included plots of the learning curves - each title includes the architecture, the learning algorithm, the learning parameter, and the learning momentum if I modified it explicitly.
All of my code (and more plots!) can be found in /users/masha/neural_plant
On the whole, FANN has rather limited capabilities, especially in terms of learning algorithms, where it only has 4 (+ all of the changes one can make to parameters and rates). It is, however, much more intuitive to code with and faster han the Matlab NN library, although the later has more algorithms. I'll browse around for more open-source packages.
Red and blue curves: frequency fluctuation of the beat node between PSL and YARM laser.
Green and broen curves: Actuation on ETMY. In ALS_CONTROL.adl ETMY filter bank 4 and 5 were switched on. Gain was 0.3
Nice reduction of the frequency fluctuation.
Y axis is in volts^2 per counts. In order to go to MHz/sqrt(Hz) you have to take the square root and then times [20Volts/(2^16)counts]*[10Hz/0.04V].
Started to scan the cavity, but this didn't work. Green light all out of lock. IR beam was badly aligned to cavity. Now, my time is over and I have to leave you.
Thanks, for your help and the nice time.
Below is an inventory of the signal feedthroughs that need to be installed on the vacuum Acromag crate this week.
**The original documentation lists five satellite boxes (one for each test mass chamber and one for the beamsplitter chamber), but Chub reports not all of them are in use. We may remove the ones not used.
- New feedthrus [4xDB25 Qty 4 / 8xDB25 Qty 1] are placed on the wire shelf at the entrance -> Jordan, please clean them.
- There are plenty of 2" DLC mounts. There are also many 1.5" mounts but they are less useful.
We need at least 3 1" moounts and 1 1" or 2" lens mount (and the lens). Let's purchase them on Thorlabs. I'll work on the order.
[Koji, Annalisa, Manasa]
NPRO with controller from ATF joins the 40m. We have put it on the POY table where we plan to use it for ABSL.
I've added 4 proposed schemes for implementing ALS in voyager. Major thing to figure out is what AUX laser would be and how we would compare the different PSL and AUX lasers to create an error signal for ALS. Everywhere below, 2um would mean wavelengths near 2 um including the proposed 2128nm. Since it is not fixed, I'm using a categorical name. Same is the case for 1um which actually would mean half of whatever 2 um carries.
After discussing with Koji, I decided to try and align the input beam polarization at the PSL fiber coupler to one of the special axes of the PM fiber. The motivation is to try and narrow down the source of the large RF beatnote amplitude drift I noticed and reported last night.
The setup for doing so is shown in Attachment #1 - essentially, I setup one of the newly purchased couplers in a mount, set up a PBS, and placed two photodiodes at the S and P ports of the PBS. The idea is to rotate the input coupler in its mount, thereby maximizing the PER (monitored on two Thorlabs PDA520s - I didn't check the gain balance of them).
I spent ~30mins doing some preliminary trials just now, and, I was able to achieve a PER of ~1/20. But I think much better numbers were reported in this SURF project (although I'm not entirely sure I understand that measurement). I will spend a little more time tweaking the alignment. The procedure is tricky as at some point, simply rotating the mount reduces the mode-matching efficiency into the fiber so much that it is not possible to get a meaningful PER measurement from the photodiodes. I'm adjouring for now, more to follow...
Current configuration of PSL free-space to fiber coupling is:
I had noticed that the RF beat amplitude was fluctuating by up to 20dBm as viewed on the control room analyzer. As detailed in my earlier elog, I suspected this to be because of random polarization drift between the PSL and EX fields incident on the Fiber coupled PDs. Since I am confident the problem is optical (as opposed to something funny in the electronics), we'd like to be able to isolate which of the many fiber segments is dominating the contribution to this random polarization drift.
Some useful references:
Procedure and details:
Last week, we were talking about reviving the Fiber ALS box. Right now, it's not in great shape. Some changes to be made:
Previous elog thread about work done on this box: elog11650
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:
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.
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.
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 :/
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.
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:
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.
For completeness, I'd like to temporarily pull the box out of the rack, open it up, take photos, and make a diagram unless there are any objections.
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.
I am facing two problems:
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:
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
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:
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?
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).
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 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...
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
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.
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.
Attachment #1 - Photo of the revamped beat setup. The top panel has to be installed. New features include:
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?
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
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.
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...
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.
I am leaving the green beat electronics on the PSL table in the switched state for further testing...
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:
Beat spectrum post changes to follow.
To couple the spare NPRO into our Panda PM980 fibers, in order to carry out tests to characterize the fibers, in order to use them in FOL.
Manasa and I spent this morning building the setup to couple NPRO light into the fibers. We used two steering mirrors to precisely guide the beam into the coupler (collimator).
We also attached the lens to a moveable stage (in the z axis), so the setup could be fine tuned to put the beam waist precisely at the photodiode.
The fiber was attached to a fiber-coupled powermeter, so I would be able to tell the coupling efficiency.
During alignment, the NPRO was operating at 1.0 amps, roughly half of nominal current (2.1A).
I first placed the coupler at the distance that I believed the target waist of 231um to be.
Using the steering mirrors and the stage that holds the couple, I aligned the axes of the coupler and the beam.
Finally, I used the variable stage that the lens is attached to to fine tune the location of the target waist.
Once I was getting readings on the powermeter (~0.5nW), the laser was turned up to nominal current of 2.1A.
At this point, I and getting 120nW through the fiber.
While far from "good" coupling, it is enough to start measuring some fiber characteristics.
Tomorrow, I hope to borrow the beam profiler once again so as to measure the fiber mode.
Beyond this, I will be taking further measurements of the Polarization Extinction Ratio, the Frequency Noise within the fiber, and the effects of a temperature gradient upon the fiber.
Once these measurements are completed, the fiber will have been characterized, and will be ready for implementation in FOL.
We wanted to measure the mode coming out of the fibers, so we can later couple it to experimental setups for measuring different noise sources within the fiber. i.e. Polarization Extinction Ratio, Frequency Noise, Temperature Effects.
I used the beamscan mounted on a micrometer stage in order to measure the spot sizes of the fiber coupled light at different points along the optical axis, in much the same way as in the razorblade setup I used earlier in the summer.
I entered my data (z coordinates, spot size in x, spot size in y) into a la mode to obtain the beam profile (waist size, location)
Code is attached in .zip file.
After I took these measurements, Manasa pointed out that I need points over a longer distance. (These were taken over the range of the micrometer stage, which is 0.5 inches.)
I will be coming in to the 40m early on Monday to make these measurements, since precious beamscan time is so elusive.
Eventually, we will use this measurement to design optical setups to characterize Polarization Extinction Ratio, Frequency Noise, and temperature effects of the fibers, for further use in FOL.
The idea was to measure the profile of the light coming out of the fiber, so we could have knowledge of it for further design of measurement apparatuses, for characterization of the fibers' properties.
The method was the same as the last time I tried to measure the fiber mode.
This time I moved the beam profiler in a wider range along the z-axis.
Additionally, I adjusted the coupling until it gave ~1mW through the fiber, so the signal was high enough to be reliably detectable.
Measurements were taken in both X and Y transections of the beam.
The range of movement was limited by the aperture of the beam profiler, which cuts off at 9mm. My measurements stop at 8.3mm, as the next possible measurement was beyond the beam profiler's range.
I entered my data into A La Mode, which gave me a waist of 5um, at a location of z = -0.0071 m, that is to say, 7.1mm inside the fiber.
Note that in the plot, data points and fits overlap, and so are sometimes hard to distinguish from each other.
Code is attached.
Using this data, I will begin designing setups to measure fiber characteristics, the first of which being Polarization Extinction Ratio.
Eventually, the data collected from these measurements will be put to use in the frequency offset locking setup.
The previous data were flawed, in that they were taken in groups of three, as I had to move the micrometer stage which held the beamscan between holes in the optical table.
In order to correct for this, I clamped a straightedge (ruler) to the table, so I could more consistently align the profiler with the beam axis.
These data gave a waist w_o = 4um, located 6mm inside the fiber. While these figures are very close to what I would expect (3.3um at the end of the fiber) the fitting still isn't as good as I would like.
The fit given by ALM is below.
I would like to get a stage//rail so I can align the axes of the beam and profiler more consistently.
I would also like to use an aperture the more precisely align the profiler aperture with the beam axis.
Once these measurements have been made, I can begin assembling the setup to measure the Polarization Extinction Ratio of the fiber.
I repeated this process once more, this time using the computer controlled stage that the beam profiler is designed to be mounted to.
These data//fitting appears to be within error bars. The range of my measurements was limited when the beam width was near the effective aperture of the profiler.
This latest trial yielded a waist of 4um, located 2.9 mm inside the fiber for the X profile, and 3.0mm inside the fiber for the Y profile.
Code is attached in fiberModeMeasurement4.zip. Note that the z=0 point is defined as the end of the fiber.
We want a measurement of the fiber modes at either end, with the collimators, because these will be the modes that we'll be trying to match in order to couple light into the fibers, for FOL and/or future projects.
In order to measure these modes, I used the beam profiler (Thorlabs BP 209-VIS) to take measurements of the beam diameter (cut off at 13.5% of the amplitude) along the optical axis, for each of the fiber ends.
The ends are arbitrarily labelled End 1 and End 2.
For each measurement, the fibers were coupled to roughly 30%, or 25mW at the output.
Regarding the issue of free rotation in the collimator stages: while End 1 was relatively stable, End 2 tended to move away from its optimal coupling position. In order to correct for this, I chose a position where coupling was good, and repositioned the stage to that coordinate (124 degrees) before taking each measurement.
The data were then entered into A La Mode, which gave waist measurements as follows:
End 1--- X Waist: 197um at Z = 4.8mm Y Waist: 190um at Z = 13.6mm
End 2--- X Waist: 192um at Z = 7.4mm Y Waist: 190um at Z = 6.0mm
A La Mode code is attached in .zip file
These are the types of profiles that we will hopefully be matching the PSL and AUX lasers to, for use in frequency offset locking.
More characterization of the fibers is to follow, including Polarization Extinction Ratio.
We also hope to be testing the overall setup soon.
I pulled out the Fiber Optic Module for FOL from the rack inside the PSL table enclosure and modified it. The beat PDs were moved into the box to avoid breaking the fiber pigtail input to the PD.
The box has 3 input FC/APC connectors (PSL and AUX lasers) and 2 output FC/APC connectors (10% of the beatnote for the AUX lasers).
Attachment shows what is inside the box. The box will again go back on the rack inside the PSL enclosure.
Earlier today Q and I somewhat resurrected my old PER measurement setup so I could run the temperature characterization experiment.
Unfortunately, when I tried to use the fiber illuminator, no light came from the other end, causing me to fail my primary goal for the summer of "don't break anything." The fiber has been re-spooled and labeled appropriately. Also sorry.
In addition to this, Q and I scavenged parts from the telescopes on the PSL and Y End tables, which were either not functional, or needed to have their mode matching adjusted, since we're using the non-PM fibers for FOL, which have a different numerical aperture, and thus slightly different output modes.
Specifically, this is involved removing the rotational mounts, and appropriate beam dumping.
My "calorimeter" still remains intact, in case anyone wants to make this measurement in the future, as this is my last day in the lab.
It's also effective at keeping drinks cold, if you'd rather use it for that.
We want to characterize the sort of response the fibers have to temperature gradients along them (potentially altering indices of refraction, etc.)
I have constructed a sort of two chambered "calorimeter" (by which I mean some coolers and other assorted pieces of recycling.)
The idea is that half of the length of PM fiber resides in one chamber, and the other in the other.
One chamber will remain at an uncontrolled, stable temperature (as measured by thermocouple probe) while the other's temperature is varied using a heat gun.
Using this setup, one can measure losses in power, and effects on polarization within the fiber.
This is currently living on the electronics bench until tomorrow morning, and is a little fragile, just in case it needs to be moved.
To maintain PM fibers all the way through to the photodiode, I had ordered some PM versions of the 50/50 fiber beamsplitters from AFW technologies. They arrived some days ago, and today I installed them in the BeatMouth. Before installation, I checked that the ends of the fibers were clean with the fiber microscope. I also did a little cleanup of the NW corner of the PSL table, where the 1um MZ setup was completely disassembled. We now have 4 non-PM fiber beamsplitters which may be useful for non polarizaiton sensitive applications - they are stored in the glass-door cabinet slightly east of the IY chamber along the Y arm, together with all the other fiber-related hardware.
Anjali had changed the coupling of the beam to the slow axis for her experiment but I ordered beamsplitters which have the slow axis blocked (because that was the original config). I need to revert to this config, and then make a measurement of the ALS noise - if things look good, I'll also patch up the Y arm ALS. We made several changes to the proposed timeline for the summer but I'd like to see this ALS thing through to the end while I still have some momentum before embarking on the BHD project. More to follow later in the eve.
Get a fiber BS that is capable of maintaining the beam polarization all the way through to the beat photodiode. I've asked AFW technologies (the company that made our existing fiber BS parts) if they supply such a device, and Andrew is looking into a similar component from Thorlabs.
I looked into this a little more today.
This is a problem - such large shifts in the signal level means we have to leave sufficient headroom in the choice of RF amplifier gain to prevent saturation, whereas we want to boost the signal as much as possible. Moreover, this kind of operation of tweaking the fiber seating to increase the RF signal level is not repeatable/reliable. Options as I see it:
The fibers around the PSL table were shielded to avoid any tampering.
[Aidan, Tara, Joe]
We pulled out what used to be the LSC/ASC fiber from the 1Y3 arm rack, and then redirected it to the 1X1 rack. This will be used as the c1ioo 1PPS timing signal. So c1ioo is using the old c1iovme fiber for RFM communications back to the bypass switch, and the old LSC fiber for 1PPS.
The c1sus machine will be using the former sosvme fiber for communications to the RFM bypass switch. It already had a 1 PPS timing fiber.
The c1iscex machine had a new timing fiber already put in, and will be using the c1iscey vme crate's RFM for communication.
We still need to pull up the extra blue fiber which was used to connect c1iscex directly to c1sus, and reuse it as the 1PPS signal to the new front end on the Y arm.
Alex has said he'll come in tomorrow morning to install the new FB code.
I swapped the EX fiber for the PSL fiber in the polarization monitoring setup. There is a lot more power in this fiber, and one of the PDs was saturated. I should really have put a PBS to cut the power, but I opted for putting an absorptive ND1.0 filter on the PD instead for this test. I want to monitor the stability in this beam and compare it to the EX beam's polarization wandering.
It looks like the drift in polarization content in the PSL pickoff is actually much higher than that in the EX pickoff. Note that to prevent the P-pol diode from saturating, I put an ND filter in front of the PD, so the Y axis actually has to be multiplied by 10 to compare power in S and P polarizations. If this drift is because of the input (linear) polarization being poorly matched to one of the fiber's special axes, then we can improve the situation relatively easily. But if the polarization drift is happening as a result of time-varying stress (due to temp. fluctuations, acoustics etc) on the (PM) fiber from the PSL fiber coupler to the BeatMouth, then I think this is a much more challenging problem to solve.
I'll attempt to quantify the contribution (in Hz/rtHz) of beat amplitude RIN to phase tracker output noise, which will tell us how much of a problem this really is and in which frequency bands. In particular, I'm interested in seeing if the excess noise around 100 Hz is because of beat amplitude fluctuations. But on the evidence thus far, I've seen the beat amplitude drift by ~15 dB (over long timescales) on the control room network analyzer, and this drift seems to be dominated by PSL light amplitude fluctuations.