Since the beam waist after the Faraday had changed since the last time I measured it (maybe alignment changed a bit), I made a new mode matching calculation for green. I attached the results.
I'm going to align the beam into the Yarm.
RXA: JPG images deleted - replace with PDF please.
After working some more on the EY table, we are getting some TEM00 flashes for the Y arm green. We have had to raise the height of one of the MM lenses to prevent clipping.
We used a function generator to apply a ~300 mV 10 Hz triangle wave to scan the laser frequency while aligning.
We tried to use the C1:ALS-TRY_OUT channel to help us in our alignment but there are a couple problems:
1) It seems that there is an uncompensated whitening filter before the ADC - Annalisa is making a compensation filter now.
2) The data delay is too much to use this for fast alignment. We might need to get a coax cable down there or mount a wired ethernet computer on the wall.
3) We need to make DQ channels for the TRY and TRX OUT. We need long term data of these, not just test points.
I made the anti-whitening filter for the C1:ALS-TRY_OUT channel. But then I forgot to make an ELOG because I am bad.
[Annalisa, Gautam, Rana]
I made the anti-whitening filter for the C1:ALS-TRY_OUT channel.
zpk [,,1] Hz
Now we can look at the picks of this signal to align the green into the cavity.
We already had some 00 flash, but a better alignment has to be done.
- put the shutter along the beam path
- check the polarization (we have a new PBS for visible)
The green beam alignment has been improved, so we see much more 00 bright flashing. We checked the polarization and the Ygreen shutter is back in place.
A mirror is already in place to steer the rejected beam from the green Faraday into a PD, tomorrow morning we'll put a lens and the PD to take the signal for PDH locking.
DQ channels have been created in the C1ALS model for TRX and TRY. They are called TRX_OUT and TRY_OUT and the sampling rate is 2048 Hz.
The rejected beam from this Faraday comes out at a tiny, tiny angle and so its tough to pick it off without clipping the main beam.
Some care must be taken in setting this up - Steve may have some good ideas on what kind of mount can be placed so close to the beam.
Why did we ever order this terrible Faraday? Let's never get a Faraday with a tiny angle between the beams again.
The rejected beam from the Faraday is steered with a mirror into the PDA32A PD and a 75mm fl lens is used to focus the beam into it.
The main beam is a few millimeters away from the mirror mount (maybe 2mm), and I think it should be fine as long as the main beam is not supposed to move.
After connecting the PD with the reflection from the arm to the PDH box, theY arm has been locked on the 01 mode. Maximizing the alignment, we obtained a 00 mode locking, but we couldn't maximize the power.
The size of the reflected beam was different with respect to the size of the incoming beam, so probably a bad mode matching was one of the issues.
Moreover, the reflected beam is very low power. We need to figure out why it is so (bad alignment? related to mode matching?)
After measuring better all the distances, I did a new mode matching calculation. I put the lenses after measuring the beam waist, so the size of the beam on the lenses was the same as expected from the calculation. Nevertheless, the beam size on the beam splitter looks bigger than expected, and also in this case green flashes into the cavity at some HOM (again 01).
I also tried to lock again the cavity and maximize the alignment, but I didn't get any improvement with respect to the previous mode matching.
Still no good locking!
After making the reflected beam size closer to the injected one, I maximized alignment. I locked again in 00 mode, but I couldn't maximize the power.
I just realized that maybe I'm not using the correct radius of curvature for the ETMY in the simulation. Tomorrow I will start checking from that.
For the mode matching calculation I was using the ETMY focal length that I found on Kiwamu's plot on the wiki page.
Taking into account also the substrate, the focal length turns out to be
fl = ((n-1)*(1/R1 - 1/R2 + (n-1)d/(nR1R2)))^(-1) = -125.81 m
with n = 1.46071 (refraction index of fused silica at 532nm)
R1 = 5625 m (radius of curvature of the first surface)
R2 = 57.37 m (radius of curvature of the second surface)
d = 25mm (thickness)
The value of the focal length is sligthly different from the one I was using before in the calculation, but maybe it is enough to change the coupling.
The mode matching solution I found is very sensitive to the lenses position.
The beam waist position can vary up to 20m varying by 1cm the first lens position, while it is slightly less sensitive to the second lens displacement.
As shown in the picture, along the green beam path there is also a 1m focal length lens. It's position is fixed, because it is along the IR transmetted beam path also. I tried to get a better solution without it, but I found that the waist position was still strongly dependent on one of the two lenses position, so it would not solve the problem to remove this lens.
I think that the main issue of this mode matching is related to the "space contraints", because the two lenses' positions can vary in a very small space, even though the green beam path on the table is quite long.
Eventually, I put the MM lenses found from this last simulation on the table, and it seems to work, since I've seen very strong 00 flashes. Unfortunately, while trying to maximize the alignment I broke it and I have to do it again, but I feel confident!
After restoring alignment I could see again strong 00 flashes (about 250-300 counts on ALS-TRY). So I locked the arm with IR and after enabling the PDH servo for the green locking, I also locked the green on the Y arm in 00 mode. Then I moved the two mode matching lenses to maximize the power into the 00 mode, but I didn't reach more than 30-35 counts.
Green power injected into the Y arm 0.680mW
Green power reflected back 0.090mW
Green power transmitted on the PSL few uW
I would expect more power on the PSL table (maybe 10x more).
> > Hmmm. You seem to be saying that more light is reflected than is injected. Is this a units problem? Or was some IR on the power meter during the 'reflected' measurement?
> > We should look at it with fresh eyes in the morning.
> Also, if you have been measuring the power of green refl at the rejection port of the green faraday, the polarization of the light entering the green faraday should be checked once again to make sure that you are measuring
> only the reflected power from the arm cavity.
Sorry Sorrry Sorry!!
It was 0.090 mW, I just forgot a zero!!!
Is this reflection measured with the cavity locked or unlocked?
So what's the actual designed reflectivity of the ETM for green? No one seems to be able to give me a straight answer about this.
Looking at the reflected beam when the beam is misaligned makes it look like it's << 0.9. Is that expected given the coating spec?
You say the cavity scan goes as high as 300cts but you can only lock to 30cts, are you locked on the sideband?
-The reflection is measured when the cavity is unlocked. I measured it with the power meter in front of the PD, so I interrupted the PDH loop.
- From the specs of ETM we have:
T(S1,HR,532nm)=5.0%+/-3% (+/-1% target), R(S2,AR,532nm)<1000ppm
It means that I should have about 600-550 uW in reflection, but I don't. I can say that there are many losses, and maybe some power is clipping inside the Faraday. Nonetheless, the reflected beam looks less strong than the injected one, so most of the losses should be on the ETM table.
(- The reflected power is 0.090 mW, I just wrote it wrong yesterday, sorry!)
- The last question is actually very interesting. Maybe I was locking on the sideband when I locked to 30 cts, but if it is the case I cannot really explain why today I locked on the carrier (I locked the cavity to about 200-250 cts), and everything I changed was the PD gain and the amplitude on signal generator connected to the PDH box. It seems like there should be some sign flip somewhere, but I need to think about.
It locked at almost 280 cts, and the transmitted power on the PSL table is about 40 uW.
To make it lock on the carrier I had to flip the sign of the error signal in the PDH loop, so I put a phase shifter (a Pomona box with a 23 uF capacitor) right before the LO input of the PDH box (on the model of the X arm).
Tomorrow I will put more details about the power budget and the phase shifter transfer function.
The ETMY Oplev servo didn't work properly, when it was activated the ETMY moved too much.
We measured the oplev TF for Pitch and Yaw and it turned out that the gain was too low by a factor 3, so we increased the gain from -.250 to -.750 on both.
We also locked the Y arm and we could see that the mirror's oscillations are actually suppressed.
I had some problem with the Oplev Servo today. I was working at the mode matching fine tuning and I left the Oplev servo enabled while aligning.
When I opened the Yend table lids, the Oplev beam started moving on the QPD and the Oplev servo didn't help in stopping the mirror movement, but it increased it.
So, the mirror was oscillating at a frequency of a few Hz
Koji suggested that maybe the shaking is due to the air conditioning moving the beam, so the servo tries to feed back the signal to the mirror, even if the mirror doesn't actually move.
I also measured the transfer function for the Oplev, but it didn't show any strange behavior.
Beat note setup
Y arm beat note found!
The green transmission on the PSL reads about 500 cts, and the transmitted power is about 50 uW.
(the second peak on the screen in the picture is the 29 MHz of the MC)
[Koji, Annalisa, Manasa]
Today we worked on the ALS servo stabilization for the Y arm.
First step: find the beat note
The beat note was found following the usual steps:
Beat note amplitude = -27 dBm @ 50 MHz
PSL temperature = 31.54 degC
Laser Offset on the slow servo2 = -11011
In the GREEN HORNET we did the following changes for the Y arm:
Input Signal Conditioning
On the C1ALS-BEATY_FINE screen the same antiwhitening filters of the C1ALS-BEATX_FINE have been reproduced. At moment, only the FM3 [10:1] is enabled.
On the C1ALS-BEATY_FINE_PHASE screen the gain was set at 3600, since the amplitude of the Q signal after the Phase rotator (BEATY_FINE_Q_ERR) was about 30. To set this value we made a proportion with respect to a previous optimized value, where the amplitude was 100 and the gain was set to 1200.
In order to stabilize the beat frequency, we started enabling the FM5 [1000:1] filter in the C1ALS_YARM panel, and then we started increasing the gain first in small steps (0.1), in order to understand which sign the gain should have without kicking the mirror.
We measured the Power Spectrum of the C1:ALS-BEATY_FINE_PHASE_OUT in-loop signal while varying the gain of the C1ALS_YARM servo filter.
Eventually, we enabled the following filters:
Gain = -30.
Koji expects the UGF of the loop to be around 100-ish Hz, and he also expected the small bump around 300-400 Hz.
Then we realized that the channel we were measuring was not calibrated in unit of Hz, so we took again the measurement looking at the channel C1:ALS-BEATY_FINE_PHASE_OUT_HZ. In this case, we didn't observe any bump. Maybe the beat frequency was slightly changed from the previous measurement and the all servo shape was also different. The final value of the gain was set at -8.
The Y axis unit is missing (bad me!). It's in deg/sqrt(Hz) for the first plot and Hz/sqrt(Hz) for the second one.
I realized that I cannot open the attached plots. I'll fix them tomorrow.
[Manasa, Jenne, Annalisa]
I was going to find the beat note to start the cavity scan, but I couldn't.
These are the steps I followed:
After spanning the temperature by approximately 4degC, we started be suspicious that I couldn't find the beat in the range of temperature where it was supposed to be found, and we started making several trials:
The same trials were done also for the X arm, but we didn't succeed in finding the beat for the X neither.
I found the beat note for the Y arm. Nothing was changed with respect to yesterday night, but the beat is back!
Yesterday I did a cavity scan with IR while holding the Yarm with green.
ALS servo tuning:
The gain of the loop is set such that BEATY_FINE_Q_ERR x GAIN = 120k. This is a kind of "empirical low" in order to have the UGF around 1kHz.
Start with FM5 [1000:1] enabled, determine the sign of the gain increasing it in small steps and making sure that the mirror doesn't get a kick. Then gradually raise it while looking at the BEATY_PHASE_OUT power spectrum.
Enable FM7 [RG16.5], FM6 [RG3.2], FM3 [1:5], FM2[0:1], FM10 [40:7].
Plot 1 shows the power spectrum of BEATY_PHASE_OUT (calibrated in Hz).
Offset setting and cavity scan
The C1ALS_OFFSETTER2 was used to set an offset for ALS scan.
Many scans have been done to find the optimal offset conditions, I only attached one (Plot 2).
I also misaligned the END mirror in pitch to enhance the HOMs peaks, but it turned out that it was not enough, because I didn't see a very big difference between the "aligned" and the "slightly misaligned" measurements (Plot 3).
Increase the cavity misalignment both in pitch and in yaw and repeat the measurement.
I started doing a scan of the Y arm cavity with IR with ALS enabled.
The servo tuning procedure is basically the same as described in elog 8831.
This time I had a stronger beat note(-14 dBm instead of -24 dBm of the last measurement) thanks to a better alignment.
Plot1 shows the Power spectrum of the BEATY_PHASE_OUT. The RMS is smaller by a factor of 2 (400Hz), corresponding to a residual motion of about 25 pm.
Offset setting avity scan
In order to give an offset linearly growing in time, I used the ezcastep script instead of giving the offset in OFFSETTER2. If the ramp time is long enough, it is not necessary to enable the 30mHz filter.
To span 2 FSR, I started from an offset of 450 and I gave a maximum value of 1600 with a delay of 0.2s between two consecutive steps.
I did a first scan with the cavity well aligned, basically to know the position of the 00 peaks and choose the best offset range (Plot2)
Then I misaligned the TT2, first in PITCH and yhen in YAW, in order to enhance the HOMs. (Plot3 and Plot4)
More investigation and measurements needed.
Yesterday evening Nic and me were in the lab. The Mode Cleaner was unlocked, but after many attempt we could fix it and we did many scans of the Y arm cavity.
Today I was not able to keep the MC locked. Koji helped me remotely, and eventually the MC locked back, but after half an hour of measurements I had to stop.
I made some more scan of the Y arm though. I also tried to do the same for the X arm, but the MC unlocked before the measurement was finished. I'll try to come back in the night.
Yesterday and today I was in the lab doing many cavity scan.
First I did many measurement with the cavity aligned in order to get the position of the 00 modes, then I misaligned the beam in many different ways to enhance the higher order modes.
In particular, I first misaligned the mode cleaner to make the beam clipping into the Faraday. To do this, I set to 0 the WFS gain, but I left the autolocker still enabled. In this way, the autolocker couldn't bring the mirrors back to the aligned position.
Then I misaligned also the TT2 to get even more HOMs.
Eventually, Rana came and we misaligned TT1 to clip the beam, and using TT2 we aligned back the beam to the arm.
To increase the SNR, we changed the gain of the TRY PD, setting it to 20dB (which corresponds to a factor 100 in digital scale)
I attached one scan that I did with Rana on Sunday night. I could not upload a better resolution image because the file size was too big, but here's the path to find all of the scans:
There are many folders, one per each day I measured. In each folder there are measurements relative to aligned cavity, Pitch and Yaw misalignment.
The PDA520 used for TRY was set to 0 dB analog gain. This corresponds to ~500 counts out of 32768. The change to 20 dB actually increases the gain by 100. This makes the single arm lock saturate at ~25000 counts (obviously in analog before the ADC). The right setting for our usual running is probably 10 dB.
For the IMC WFS, we had disabled the turn on in the autolocker to use the IMC to steer the beam in the FI, but that was a flop (not enough range, not enough lever arm). In the end, I think we didn't get any clipping.
The spot on the IPANG QPD was checked. The spot is higher than the center and South side of the lens.
Some photos are found below.
The spot on the IPANG steering mirrors in the ETMY chamber was also checked.
It is clipped at the top of the steering mirror. (See attachment 4)
So basically the spot is about 1" above the center of the mirror.
After the vent, the IPang spot position on the steering mirrors on the Yend table moved approximately by 1 inch down.
Inside the chamber, the spot position is in the center of the steering mirror. (difficult to take a picture because the PSL beam power has been reduced)
Yend table picture updated on the wiki page
I did a calibration measurement for the Y part of the BeatBox using a Marconi. This is in order to get a more accurate calibration for the arm cavity scan measurement.
The calibration factor I found is:
C1:ALS-BEATX_FINE_PHASE_OUT 50.801 +/- 0.009 deg/MHz
During my cavity scan measurement, I had recorded the beat frequency and amplitude from the Spectrum Analyzer at each zero crossing.
I connected the Marconi to the RF in of the Y part of the BeatBox, and I set the Marconi carrier frequency at one of this zero-crossing frequency that I had recorded, while I set the amplitude in way to have on the spectrum analyzer the same beat amplitude that I read during the measurements or, equivalently, in order to have C1:ALS_BEATY_FINE_Q of the order of 1200 (which is the same value I had during my measurements).
I started with
Then I monitored the C1:ALS_BEATY_FINE_I on the oscilloscope and I adjusted the carrier frequency so that I had zero signal on the oscilloscope. Eventually the frequency corresponding to the zero crossing was 79.989 MHz.
I resetted the phase (clear history in the BEATY_FINE_PHASE panel) and I started changing the frequency by steps of 0.2 MHz, and I spanned about 70 MHz (from 32 to 102 MHz).
The calibration coefficient I found is not so different from the one that Yuta measured (elog 8199).
Here are the fit parameters:
y = a + bx
a = -4239.7 +/- 0.6 deg
b = 50.801 +/- 0.009 deg/MHz
I tried to put together a rudimentary heater setup.
As a heating element, I used the soldering iron tip heated up to ~800°C.
To make a reflector, I used the small basket which holds the cork of champains battles (see figure 1), and I covered it with alumnum foil. Of course, it cannot be really considered as a parabolic reflector, but it's something close (see figure 2).
Then, I put a ZnSe 1 inch lens, 3.5 inch FL (borrowed from TCS lab) right after the reflector, in order to collect as much as possible the radiation and focus it onto an image (figure 3). In principle, if the heat is collimated by the reflector, the lens should focus it in a pretty small image. Finally, in order to see the image, I put a screen and a small piece of packaging sponge (because it shouldn't diffuse too much), and I tried to see the projected pattern with a thermal camera (also borrowed from Aidan). However, putting the screen in the lens focal plane didn't really give a sharp image, maybe because the reflector is not exactly parabolic and the heater not in its focus. However, light is still focused on the focal plane, although the image appears still blurred. Perahps I should find a better material (with less dispersion) to project the thermal image onto. (figure 4)
Finally, I measured the transmitted power with a broadband power meter, which resulted to be around 10mW in the focal plane.
I made some simulation to study the change that the heater setup can induce on the Radius of Curvature of the ETM.
First, I used a non-sequential ray tracing software (Zemax) to calculate the heat pattern. I made a CAD of the elliptical reflector and I put a radiative element inside it (similar to the rod-heater 30mm long, 3.8mm diameter that we ordered), placing it in such a way that the heater tip is as close as possible to the ellipse first focus. (figure 1)
Then, by putting a screen at the second focus of the ellipse (where we suppose to place the mirror HR surface), I could find the projected heat pattern, as shown in figure 2 and 3 (section). Notice that the scale is in INCH, even if the label says mm. As you can see, the heat pattern is pretty broad, but still enough to induce a RoC change.
In order to compute the mirror deformation induced by this kind of pattern, I used this map produced with Zemax as absorption map in COMSOL. I considered ~1W total power absorbed by the mirror (just to have a unitary number).
The mirror temperature and deformation maps induced by this heat pattern are shown in figures 4 and 5.
RoC change evaluation
Then I had to evaluate the RoC change. In particular, I did it by fitting the Radius of Curvature over a circle of radius:
where is the waist of tha Gaussian mode on the ETMY (5mm) and n is the mode order. This is a way to approximately know which is the Radius of Curvature as "seen" by each HOM, and is shown in figure 6 (the RoC of the cold mirror is set to be 57.37m). Of course, besides being very tiny, the difference in RoC strongly depends on the heat pattern.
Gouy phase variation
Considering this absorbed power, the cavity Gouy phase variation between hot and cold state is roughly 15kHz (I leave to the SURFs the details of the calculation).
So the still unaswered questions are:
- which is the minimum variation we are able to resolve with our measurement
- how much heating power do we expect to be projected onto the mirror surface (I'll make another entry on that)
Today both the heater and the reflector were delivered, and we set down the setup to make some first test.
The schematic is the usual: the rod heater (30mm long, 3.8 mm diameter) is set inside the elliptical reflector, as close as possible to the first focus. In the second focus we put the power meter in order to measure the radiated power. The broadband power meter wavelength calibration has been set at 4µm: indeed, the heater emits all over the spectrum with the Black Body radiation distribution, and the broadband power meter measures all of them, but only starting from 4µm they will be actually absorbed my the mirror, that's why that calibration was chosen.
We measured the cold resistance of the heater, and it was about 3.5 Ohm. The heater was powered with the BK precision DC power supply 1735, and we took measurements at different input current.
We also aimed at measuring the heater temperature at each step, but the Fluke thermal camera is sensitive up to 300°C and also the FLIR seems to have a very limited temperature range (150°C?). We thought about using a thermocouple, but we tested its response and it seems definitely too slow.
Some pictures of the setup are shown in figures 1 and 6.
Then we put an absorbing screen in the suspension mount to see the heat pattern, in such a way to get an idea of the heat spot position and size on the ETMY. (figure 2)
The projected pattern is shown in figures 3-4-5
The optimal position of the heater which minimizes the heat beam spot seems when the heater inserted by 2/3 in the reflector (1/3 out). However, this is just a qualitative evaluation.
Finally, two more pictures showing the DB connector on the flange and the in-vacuum cables.
In order to power the heater setup to be installed in the ETMY chamber, we took the Sorensen DSC33-33E power supply from the Xend rack which was supposed to power the heater for the seismometer setup.
We modified the J3 connector behind in such a way to allow a remote control (unsoldered pins 9 and 8).
Now pins 9 and 12 need to be connected to a BNC cable running to the EPICS.
RXA update: the Sorensen's have the capability to be controlled by an external current source, voltage source, or resistive load. We have configured it so that 0-5V moves the output from 0-33 V. There is also the possibility to make it a current source and have the output current (rather than voltage) follow the control voltage. This might be useful since out heater resistance is changing with temperature.
[Gautam, Johannes, Koji, Annalisa]
Tonight we increased the power of the PSL laser and we achieved the lock of both arms with high power.
The AUX beam alignment to the Y arm was recovered and the PLL restored (using the Marconi as LO).
We made a quick measurement of the phase noise and the results will be posted tomorrow.
The beam on the PSL has been blocked, as well as the AUX beam on the AS table. The Marconi has been switched off.
The schematic of the homodyne configuration is shown below.
Following are the list of components
One set of fiber is now kept along the arm of the interferometer
Fiber coupled (3 No's)
Free space ( 2 No's)
Attachment #1 shows the schematic of the test setup. Signal generator (Marconi) was used to supply the RF input. We observed the IF output in the following three test conditions.
We characterized the power splitter ( Minicircuit- ZAPD-2-252-S+). The schematic of the measurement setup is shown in attachment #1. The network/spectrum/impedance analyzer (Agilent 4395A) was used in the network analyzer mode for the characterisation. The RF output is enabled in the network analyser mode. We used an other spliiter (Power splitter #1) to splitt the RF power such that one part goes to the network analzer and the other part goes to the power spliiter (Power splitter #2) . We are characterising power splitter #2 in this test. The characterisation results and comparison with the data sheet values are shown in Attachment # 2-4.
Attachment #2 : Comparison of total loss in port 1 and 2
Attachment #3 : Comparison of amplitude unbalance
Attachment #4 : Comparison of phase unbalance
The goal was to characterise the new amplifier (AP1053). For a practice, I did the characterisation of the old amplifier.This test is similar to that reported in Elog ID 13602.
Attachment #1 shows the schematic of the experimental setup for the frequency noise measurement of 1 um laser source.
AUX laser will be used as the seed source and it is already coupled to a 60 m fiber (PM980). The other end of the fiber was at the AS table and we have now removed it and placed in the PSL table.
Attachment # 2 shows the photograph of the experimental setup. The orange line shows the beam that is coupled to the delayed arm of MZI and the red dotted line shows the undelayed path.
As mentioned, AUX is already coupled to the 60 m fiber and the other end of the fiber is now moved to the PSL table. This end needs to be collimated. We are planning to take the same collimator from AS table where it was coupled into before. The position where the collimator to be installed is shown in attachment #2. Also, we need to rotate the mirror (as indicated in attachment #2) to get the delayed beam along with the undelayed beam and then to combine them. As indicated in attachment #2, we can install one more photo diode to perform balanced detection.
We need to decide on which photodetector to be used. It could be NF1801 or PDA255.
We also performed the power measurement at different locations in the beam path. The different locations at which power measurement is done is shown attachment #3
There is an AOM in the beam path that coupled to the delayed arm of MZI. The output beam after AOM was coupled to the zero-order port during this measurement. That is the input voltage to the AOM was at 0 V, which essentially says that the beam after the AOM is not deflected and it is coupled to the zero-order port. The power levels measured at different locations in this condition are as follows. A)282 mW B)276 mW C)274 mW D)274 mW E)273 mW F)278 mW G)278 mW H)261 mW I)263 mW J)260 mW K)131 mW L)128 mW M)127 mW N)130 mW
It can be seen that the power is halved from J to K. This because of a neutral density filter in the path of the beam
In this case, we measured a power of 55 mW at the output of the delayed fiber. We then adjusted the input voltage to the AOM driver to 1 V such that the output of AOM is coupled to the first order port. This reduced the power level in the zero-order port of AOM that is coupled to the delayed arm of the MZI. In this case we measured a power of 0.8 mW at the output of delayed fiber.
We must be careful about the power level that is reaching the photodetector such that it should not exceed the damage threshold of the detector.
The power measured at the output of undelayed path is 0.8 mW.
We also must place the QWP and HWP in the beam path to align the polarisation.
To facilitate the 1um MZ frequency stabilization project, I decided that the AUX laser was a better candidate than any of the other 3 active NPROs in the lab as (i) it is already coupled into a ~60m long fiber, (ii) the PSL table has the most room available to set up the readout optics for the delayed/non-delayed beams and (iii) this way I can keep working on the IR ALS system in parallel. So we moved the end of the fiber from the AS table to the SE corner of the PSL table. None of the optics mode-matching the AUX beam to the interferometer were touched, and we do not anticipate disturbing the input coupling into the fiber either, so it should be possible to recover the AUX beam injection into the IFO relatively easily.
Anjali is going to post detailed photos, beam layout, and her proposed layout/MM solutions later today. The plan is to use free space components for everything except the fiber delay line, as we have these available readily. It is not necessarily the most low-noise option, but for a first pass, maybe this is sufficient and we can start building up a noise budget and identify possible improvements.
The AUX laser remians in STANDBY mode for now. HEPA was turned up while working at the PSL table, and remains on high while Anjali works on the layout.
One of the main draw backs of the measurement was the polarisation was not aligned properly in the setup. So, then the next step was to identify the polarisation at different locations in the beam path and to maximise the polarisation to either S or P component.
So, we introduced HWP at the input beam path after isolator as shown in attachment #1. Also, the polarisation was tested at positions P1, P2, P3, and P4 shown in attachment #1 by placing a polarisation beam splitter at these locations and then by observing the transmitted (P component) and reflected light (S component) using power meter.
The observations at different locations are as the follows
These observations show that the P and S components are almost equal, and this is not a good polarisation arrangement. At this point, we also had to check whether the incoming beam is linearly polarised or not.
To test the same, the PBS was placed at position P1 and the P and S components were observed with power meter as the HWP is rotated.Attachment # 2 shows the results of the same, that is the variation in P and S component as the HWP is rotated.
This result clearly shows that the input beam is linearly polarised. The HWP was then adjusted such that the P component is maximum and coupled to the MZI. With this orientation of HWP, the polarisation observed at different positions P1, P2, P3, and P4 are as follows.
This shows that the polarisation is linearly polarised as well as it is oriented along the P direction (parallel to the optical table).
We have the polarisation maintaining fiber (PM 980) as the delay fiber. The polarisation of the light as it propagates through a PM fiber depends on how well the input beam is coupled to the axis (slow or fast) of the fiber. So, the next task was to couple the light to one of the axes of the fiber.
The alignment key on the fiber is a good indication of the axis of the fiber. In our case, the alignment key lines up with the slow axis of the fiber. We decided to couple the light to the fast axis of the fiber. Since the incoming beam is P polarised, the output fiber coupler was aligned such that the fast axis is parallel to optical table as possible.
A PBS was then introduced after the fiber output collimator . There is a HWP (marked as HWP2 in attachment 1) in front of the input coupler of the fiber as well. This HWP was then rotated and observed the P and S component from the PBS that is now placed after the output coupler with a power meter.The idea was , when the light is coupled to the fast axis of the fiber, we will see the maximum at the P componet at the output
Attachment # 3 shows the observation.
In this way I tried to find the orientation of the HWP2 such that the P component is maximum at the output. But I was not succeeded in this method and observed that the output was fluctuating when the fiber was disturbed. One doubt we had was whether the fiber is PM or not . Thus we checked the fiber end with fiber microscope and confirmed that it is PM fiber.
Thus, we modifed the setup as shown in attachement # 4.The photodetector (PDA55) was monitoring the S component and the output of the detector was observed on an oscilloscope. We rotated the HWP2 such that the S component is almost minimum. At the same time, we were disturbing the fiber and was observing whether the output is fluctuating. The HWP2 angle was tweaked around the minimum of S component and observed the output with disturbing the fiber. This way we found the orientation of HWP2 such that the light is coupled to the fast axis of the fiber and the output was not fluctuating while we disturb the fiber. We tested it by heating the fiber with a heat gun as well and confirmed that the output is not fluctuating and thus the light is coupled to the fast axis of the fiber.
Steve had showed me some stock of long fibers a while back - they are from Oz Optics, and are 50m long, and are already spooled - so barring objections, we will try the MZ setup with the spooled fiber and see if there is any improvement in the fringing rate of the MZ. Then we can evaluate what additional stabilization of the fiber length is required. Anjali will upload a photo of the spooled fiber.
The alignement was disturbed after the replcement of the beam splitter. We tried to get the alignment back . But we are not succeeded yet in getting good interfernce pattern. This is mainly because of poor mode matching of two beams. We will also try with the spooled fiber.
At some point I'd like to reclaim this setup for ALS, but meantime, Anjali can work on characterization/noise budgeting. Since we have some CDS signals, we can even think of temperature control of the NPRO using pythonPID to keep the fringe in the linear regime for an extended period of time.
My understanding is that the main advantage in going to the heterodyne scheme is that we can extract the frequecy noise information without worrying about locking to the linear region of MZI. Arctan of the ratio of the inphase and quadrature component will give us phase as a function of time, with a frequency offset. We need to to correct for this frequency offset. Then the frequency noise can be deduced. But still the frequency noise value extracted would have the contribution from both the frequency noise of the laser as well as from fiber length fluctuation. I have not understood the method of giving temperature feedback to the NPRO.I would like to discuss the same.
The functional form used for the curve labeled as theory is 5x104/f. The power spectral density (V2/Hz) of the the data in attachment #1 is found using the pwelch function in Matlab and square root of the same gives y axis in V/rtHz. From the experimental data, we get the value of Vmax and Vmin. To ride from Vmax to Vmin , the corrsponding phase change is pi. From this information, V/rad can be calculated. This value is then multiplied with 2*pi*time dealy to get the quantity in V/Hz. Dividing V/rtHz value with V/Hz value gives y axis in Hz/rtHz. The calculated value of shot noise and dark current noise are way below (of the order of 10-4 Hz/rtHz) in this frequency range.
I forgor to take the picture of the setup at that time. Now Andrew has taken the fiber beam splitter back for his experiment. Attachment #1 shows the current view of the setup. The data from the previous trial is saved in /users/anjali/MZ/MZdata_20190417.hdf5
If I understand correctly, the Mach-Zehnder readout port power is only a function of the differential phase accumulated between the two interfering light beams. In the homodyne setup, this phase difference can come about because of either fiber length change OR laser frequency change. We cannot directly separate the two effects. Can you help me understand what advantage, if any, the heterodyne setup offers in this regard? Or is the point of going to heterodyne mainly for the feedback control, as there is presumably some easy way to combine the I and Q outputs of the heterodyne measurement to always produce an error signal that is a linear function of the differential phase, as opposed to the sin^2 in the free-running homodyne setup? What is the scheme for doing this operation in a high bandwidth way (i.e. what is supposed to happen to the demodulated outputs in Attachment #3 of your elog)? What is the advantage of the heterodyne scheme over applying temperature feedback to the NPRO with 0.5 Hz tracking bandwidth so that we always stay in the linear regime of the homodyne readout?
Also, what is the functional form of the curve labelled "Theory" in Attachment #2? How did you convert from voltage units in Attachment #1 to frequency units in Attachment #2? Does it make sense that you're apparently measuring laser frequency noise above 10 Hz? i.e. where do the "Dark Current Noise" and "Shot Noise" traces for the experiment lie relative to the blue curve in Attachment #2? Can you point to where the data is stored, and also add a photo of the setup?
It is noticed that one of the doors (door # 2 ) of the PSL table is broken. Attachement #1 shows the image
From the earlier results with homodyne measurement,the Vmax and Vmin values observed were comparable with the expected results . So in the time interval between these two points, the MZI is assumed to be in the linear region and I tried to find the frequency noise based on data available in this region.This results is not significantly different from that we got before when we took the complete time series to calculate the frequency noise. Attachment #1 shows the time domain data considered and attachment #2 shows the frequecy noise extracted from that.
As discussed, we will be trying the heterodyne method next. Initialy, we will be trying to save the data with two channel ADC with 16 kHz sampling rate. With this setup, we can get the information only upto 8 kHz.
We repeated the homodyne measurement to check whether we are measuring the actual frequency noise of the laser. The idea was to repeat the experiment when the laser is not locked and when the laser is locked to IMC.The frequency noise of the laser is expected to be reduced at higher frequency (the expected value is about 0.1 Hz/rtHz at 100 Hz ) when it is locked to IMC . In this measurement, the fiber beam splitter used is Non PM. Following are the observations
1. Time domain output_laser unlocked.pdf : Time domain output when the laser is not locked. The frequency noise is estimated from data corresponds to the linear regime. Following time intervals are considered to calculate the frequency noise (a) 104-116 s (b) 164-167 s (c) 285-289 s
2. Frequency_noise_laser_unlocked.pdf: Frequency noise when the laser is not locked. The model used has the functional form of 5x104/f as we did before. Compared to our previous results, the closeness of the experimental results to the model is less from this measurement. In both the cases, we have the uncertainty because of the fiber length fluctuation. Moreover, this measurement could have effect of polarisation fluctuation as well.
3.Time domain output_laser locked.pdf :Time domain output when the laser is locked. Following time intervals are considered to calculate the frequency noise (a) 70-73 s (b) 142-145 s (c) 266-269 s.
4. Frequency_noise_laser_locked.pdf : Frequency noise when the laser is locked
5. Frequency noise_comparison.pdf : Comparison of frequency noise in two cases. The two values are not significantly different above 10 Hz. We would expect reduction in frequency noise at higher frequency once the laser is locked to IMC. But this result may indicate that we are not really measuring the actual frequency noise of the laser.
I borrowed one Marconi (2023 B) from 40 m lab to QIL lab.
I am currently working on an optical arrangement consisting of a QPD that measures the fluctuations of an incoming HeNe laser beam that is reflected by a mirror. The goal is to add a second QPD to the optical arrangement to form a linear combination that effectively cancels out the (angular) fluctuations from the laser beam itself so that we can only focus on the fluctuations produced by the mirror.
In order to solve this problem, I have written a program for calculating the different contributions of the fluctuations of the HeNe laser and fluctuations from the mirror, for each QPD (program script attached). The goal of the program is to find the optimal combination of L0, L1, L2, and f2 that cancels the fluctuations from the laser beam (while retaining solely the fluctuations from the mirror) when adding the fluctuations of QPD 1 and QPD 2 together.
By running this program for different combinations of distances and focal lengths, I have found that the following values should work to cancel out the effects of the oscillations from the HeNe laser beam (assuming a focal length of 0.2 m for the lens in front of the original QPD):
L0 = 1.0000 m (distance from laser tube to mirror)
L1 = 0.8510 m (distance from mirror to lens in front of QPD 1)
L2 = 0.9319 m (distance from beamsplitter to lens in front of QPD 2)
f2 = 0.3011 m (focal length of lens in front of QPD 2)
Based on these calculations, I propose to try the following lens for QPD 2:
1’’ UV Fused Silica Plano-Convex Lens, AR-Coated: 350 - 700 nm (focal length 0.3011 m). https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=6508