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ID Date Author Type Category Subject
1994   Thu Nov 30 12:02:05 2017 awade, CraigDailyProgressscatterInitial buzz test and cleaning of periscope mirrors and final MM lens

--- This is a backpost from last week ---

The FSS and PLL are now pretty much locked all the time with the last reported settings (see PLL:1991). The PLL and FSS have all remained in locked state for periods of over 48 hours.

Craig and I had a first look at the scatter noise shelf with a white noise PZT buzzer to see where where the worst offending sources of parasitic interferometers would be.  The periscopes and around the PDs seem like the most likely points from our initial tests.

There was a lot of foam dust on the upward facing periscope mirrors (see pictures PSL:1980) and on the final MM mirrors before the ref cavs.  We gave these a clean as an initial test.  The north path 77.3 mm ROC mirror directly before the periscope has a permanent burn mark in the center: see picture below.  I tried a gentle wipe with methanol and then a little more of an aggressive scrub and it wont budge. Craig has cleaned another of the the same lenses and this is ready to switch in next time we need to do a serious realignment of the north cavity. We're leaving it for now as we want to get onto other issues like scatter in the transmission PLL beat path.

I took a reference PLL spectrum attached in the noise budget plot below. There is a new broad 2 kHz bump that we are trying to work out the source of. Previously tara had speculated that this was RFAM (PSL:1311) and Evan had speculated that it was FSS induced noise (PSL:1526). We re optimized the 14.75 MHz RF EOMs polariaziations in both paths reducing North to 80 dBm/sqrtHz and south to 90 dBm/sqrtHz. This didn't seem to be as good as some of my previous reported residual AM, its not clear why North was so difficult to reduce this time; previously North was optimizable down to the noise floor of the PD RF dark noise.  The 2 kHz hump is still there.  It might well be an artifact of the scatter.

2022   Tue Dec 19 22:44:48 2017 awadeDailyProgressscatterBuzz test of beat board

I used the buzzer to help locate sources of scatter in the transmission beat board.

Here I used the SR785 in swept sine mode: 10kHz to 20 Hz, 10 mV source, a 35 mV offset, amplified by a thorlabs HV amplifier (x15). I took the mechanical transfer function from the buzzer to the PLL actuation signal (the same used to measure the displacement spectrum).

Below I've noted the frequencies and the amplitude in units of dB (ratio of PLL signal to excitation 10 mV). The frequencies listed in the upper half of the labels are buzz applied horizontally on the mount, those on the lower half are resonances excited with vibrations coupled in the vertical direction.  I would have made this labeling clearer, but I saved accidentally closed as jpeg so its been flattened.

Vertically applied excitations seem to couple down into the board more easily and common frequencies may well be same the same scatter source being excite from different points in the board. There are some standard resonances from vertical excitations at 1.03 kHz, 1.13-1.15 kHz, 3.11-3.2 kHz. These can also be seen by exciting the elevated breadboard directly. Maybe we can apply some sorbothane dampeners to the underside of the board to kill these resonances.

Horizontal resonances were less likely to be common between mounts.

Note that since I took this picture the ND filters were removed from directly before the PD.

2023   Thu Dec 21 08:58:39 2017 CraigDailyProgressscatterScatter Shoulder Fit

I've been learning about scattering from here and here.  It seems most scattering equations are left in an arbitrary form of $x_{\text{scatter}}(t)$, and simply measure seismic noise then use it as $x_{\text{scatter}}(t)$ in the following FFT:

$S_f(\omega) = \int_{-\infty}^{\infty} \sin\left(\dfrac{4 \pi}{\lambda} x_{\text{scatter}}(t)\right) e^{i \, \omega \, t} dt$

If the sine wave were perfectly sinusoidal, say $x_{\text{scatter}}(t) = x_0\,\Omega \, t$, our FFT would yield delta functions at $\omega = \pm \Omega$.  However, scattering is rarely so clean.

If $x_{\text{scatter}}(t) \ll \lambda$ where lambda is the laser wavelength, then our sine wave is approximately linear, and we get a clean spectrum at frequencies above the seismic noise.

When $x_{\text{scatter}}(t) \approx \lambda$ , we get "upconversion" of scatter noise, i.e. the higher order modes of the sine wave start to matter, and this extends the scatter shelf into higher frequencies.

I fit a scattering shelf of the functional form $S_{\text{Hz}}(f) = A \, e^{-\pi \,\Gamma \,f}$, where $A$ is a scatter coupling coefficient in units of hertz, and $\Gamma$ is a half width half maximum (HWHM) of an underlying Lorentzian $L(t) = \dfrac{1}{\pi} \, \dfrac{\Gamma}{\left(t - t_0\right)^2+ \Gamma^2}$.

I found $A = 34.0 \, \text{Hz}$ and $\Gamma = 0.04 \, \text{s}$.

We can think of the HWHM as a function of overall scattering displacement and velocity: $\Gamma \approx \dfrac{x_{\text{scatter}}}{v_{\text{scatter}}}$

If $x_{\text{scatter}}(t) \approx \lambda$, this gives $v_{\text{scatter}} \approx 27 \, \dfrac{\mu \text{m}}{\text{s}}$

2026   Thu Dec 21 18:57:12 2017 awadeDailyProgressscatterSwitch out final MM lens in North path

As pictured in PSL:1994, there was a burn mark in the center of the final lens before the north reference cavity.  I've switch out this PLCX-25.4-77.3-UV-1064 for a new lens. Cavity is realigned with a Vmin,Vmax = 0.492V,1.56V, giving a visibility of 52% (about as good as it will get with this MM solution). Polarization is well aligned with the cavity basis.

Something is up with the PDH error signal (see attachment below), last time we checked this it was the textbook shape. Nothing has changed electronically, I need to double check if things are ok further up the path going through EOMs.  North has been locked nicely for about 3 weeks now with pretty much 100 % uptime.  Power into the cavity is 1.08 mW, so not a saturating thing. It could be AM modulation. I'll check systematically the EOM polarization alignment and that of the AEOM tomorrow. If that fails I'll look at the electronics.

I also blew some dust of optics around the periscope with the ion gun, this may need to be repeated a few times as there is a lot of dust on the table and will take a while to purge out of the tent after each blast.

2028   Fri Dec 22 14:38:24 2017 awadeDailyProgressscatterFixed bad PDH error signal

It turns out that the bad PDH signal caused by monitor daughter board (D040424) note being properly seated in its plug.  It wasn't screwed in and I must have just bumped it out when putting in some test leads.

I have screwed it in properly now.

 Quote: As pictured in PSL:1994, there was a burn mark in the center of the final lens before the north reference cavity.  I've switch out this PLCX-25.4-77.3-UV-1064 for a new lens. Cavity is realigned with a Vmin,Vmax = 0.492V,1.56V, giving a visibility of 52% (about as good as it will get with this MM solution). Polarization is well aligned with the cavity basis. Something is up with the PDH error signal (see attachment below), last time we checked this it was the textbook shape. Nothing has changed electronically, I need to double check if things are ok further up the path going through EOMs.  North has been locked nicely for about 3 weeks now with pretty much 100 % uptime.  Power into the cavity is 1.08 mW, so not a saturating thing. It could be AM modulation. I'll check systematically the EOM polarization alignment and that of the AEOM tomorrow. If that fails I'll look at the electronics. I also blew some dust of optics around the periscope with the ion gun, this may need to be repeated a few times as there is a lot of dust on the table and will take a while to purge out of the tent after each blast.

2033   Sun Jan 7 17:53:26 2018 Craig, awadeDailyProgressscatterScattering Sources Probably From Inside Vaccan

TL;DR We have (1) poor alignment of light into the cavities and (2) ghost beams interfering with our resonant light from inside the vaccanThis could be the cause of our scattering shelf.

We have been messing around on the transmission table, trying to determine where are 50 Hz scattering shelf is originating from. We have a pretty good candidate for the scatter source in the north path fringer, with what appears to be a ghost beam coming out of the north cavity window.  The photo below was taken when the cavities were unlocked and with half wave plates making the back table light p-polarized, generating these beams ghost beams which are always there and interfering with the main light.

We took off the vaccan's foam on the back, and looked in with the IR viewer.  I can count at least five spots of IR light, some bright and focused, some diffuse and clipped.

I'm going to play around with cavity alignment and try to eliminate the ghost beams.

2036   Mon Jan 8 14:46:09 2018 CraigDailyProgressscatterNorth Cavity Transmission Backreflection

Yesterday I optimized the North cavity visibility for a long time, and achieved around 60% visibility, which is pretty good for us.  This was all to realign the North cavity to eliminate ghost beams which are visibly interfering with our main beam.  After all of the optimization, however, the ghost beam is still visible when the cavity is unlocked, and fringing with the main beam when the cavity is locked.

I figured I would take out the post-cavity lens (PLCX-25.4-154.5-UV) to see if we could eliminate the ghost beam that way (Photo 1).  However, something very unusual happens when I take out the post-cavity lens.  North REFL starts oscillating in a square wave fashion (yellow oscilloscope trace in Photo 2).  These REFL oscillations also occurs if I put the lens back in, but adjust the alignment to be normal with the vaccan window.

This leads me to believe that there must be some direct backreflection from the transmission table into the north cavity, which affects the levels of reflected light on the North FSS RFPD.  This could explain why our scatterer is so robust, but it does not explain why we see the scatterer on transmission even when the cavity is unlocked.

I am now going to try to track down the source of the north cavity backscatterer.  We have looked for this type of scattering before, and no obvious candidates stood out.

Edit: The backscattering is coming from the NCAV Transmission DCPD.  Work is underway to eliminate this backscatterer.

Edit 2: The North cavity REFL oscillations were caused by the ISS still being on, not backscatter.  The post-cavity lens position only mattered because some positions allowed light on the North Trans DCPD and some did not, and if any light was on the DCPD, the ISS tried to stabilize it's intensity.  The ISS is now off.  The true North path scatterer is still present.  Work still focused on eliminating this ghost beam.

2064   Mon Jan 29 20:22:06 2018 CraigDailyProgressscatterPre-Vent Vaccan Tranmission Scattering Drawing

I drew the basic scatterers I see when viewing the inside of our vaccan from the transmission port with the IR viewer.  There are a couple of main points:

1) There are some definite ghost beams coming directly from reflections off the vaccan window.  They exist both on the REFL and TRANS side of the can, the TRANS ones are drawn below.  To make sure they were coming from the window, I took out our half wave plates and the lenses immediately behind the vaccan and blocked the main beam.  All points seen in the drawing remained steady.

2) The south cavity shield is in general brighter in IR than the north.  Both shields are bright relative to the rest of the vaccan interior.

3) It seems like the cavities are already angled slightly in the plane of the table.  Along the axis of the vaccan, if viewed from above, the cavities would be angled clockwise slightly, by maybe 3-4 degrees.  Perhaps Tara thought of scatterers when first assembling the can but was unable to completely mitigate them.

4) There is a distinct height difference in the main beam spots on the transmission window.  SCAV's beam is about 0.5 cm higher on the window than NCAV's.

5) In REFL, which is not shown in the drawing here, the south path has a visible scattered beam on the mirror itself, not on the cavity shield.  This is our most likely candidate for our main scattering noise.

In short, we have to vent.  We want to start pumping up tomorrow.  Our first order of business will be to investigate the wedge angle of the windows and make a geometric model of the beam paths in our vaccan.  Then we can precisely place the cavities, place our cleaned beam dumps, lower the laser power and test our new alignments in air, mode match, then pump back down.  This will be a week of effort, plus another week for unforeseen circumstances.

2084   Fri Feb 9 18:36:40 2018 awade, CraigDailyProgressscatterScatter dumping around FSS reflection PDs

I reinstalled the green glass dump in the north cavity FSS REFL PD prompt reflection path.  For some reason this was removed and a razor blade dump was in use instead.

The south FSS reflection PD was reangled the from 40 deg to 30 deg incidence.  The  polarization incident on the detector was flipped from s-pol to p-pol using a lambda/2 wave plate.  I installed a green glass dump to capture the prompt reflection from the PD.

Looking at the output of the south Faraday reflection port there are two additional dots that are not coming from the cavity.  They appear to be either coming from inside the Faraday or from on of the optics immediately after (maybe the half-wave plate). We need to check the source of these DC fields.

A stitched beat note spectrum will be posted presently.

2093   Thu Feb 15 18:45:35 2018 CraigDailyProgressscatterNeed more Black Glass Hexagonal Bases

Made nine black glass hex beam dumps today.  Only stopped because we ran out of hex bases.  Need to order more.
Cleaned the glass days ago with the Branson 8200 ultrasonic cleaner.  Wiped down with methanol to remove water evaporate residual.  Have enough black glass for at least three more hex beam dumps, plus some weird big ones or ones with holes in the middle.
Will now dump all beams in transmission with black glass dumps.  Hope to see some reduction in the scatter shelf, but not optimistic since we think the main scatterers are coming from the windows of the vaccan.

2127   Fri Mar 9 16:10:41 2018 Craig, awadeDailyProgressscatter500 Hz resonant scatter hump in beatnote ASD

If you look at our beatnote ASD you can see a broad dirty hump at 500 Hz.
awade played a pure 500 Hz tone through our lab speakers, and you could see the resonance peak being driven.  Blue are the driven spectra, orange are the non-driven spectra.
If we turn up the speakers it causes our North path to lose lock.

Some sort of mechanical resonance in the North path is causing this.  Buzzing is underway.

2128   Sun Mar 11 16:50:59 2018 Craig, awadeDailyProgressscatter500 Hz resonant scatter hump in beatnote ASD

After buzzing the table with a probe at 500 Hz, the source of the 500 Hz resonance is the first steering mirror after the PMC.  The PMC itself also exhibits a smaller 500 Hz resonance, unclear how much of that is actually the PMC mount vs. coupling through the PMC to the first steering mirror.

 Quote: If you look at our beatnote ASD you can see a broad dirty hump at 500 Hz. awade played a pure 500 Hz tone through our lab speakers, and you could see the resonance peak being driven.  Blue are the driven spectra, orange are the non-driven spectra. If we turn up the speakers it causes our North path to lose lock. Some sort of mechanical resonance in the North path is causing this.  Buzzing is underway.

2135   Thu Mar 15 10:29:04 2018 awadeDailyProgressscatterAddressing 500 Hz scatter pickup

Going back the original issue of scattering, it appears that there is light being back reflected from somewhere in the post PMC path but before the reference cavities.

## Reducing number of optics after the North PMC

I had installed a bunch of polarization optics before the north 14.75 MHz EOM in an effort to reduce RFAM (see attachement 1).  It looks like stuffing so many optics in such a small space is a bad idea.  You can see weak retro reflected beams from the wave plates and, probably, the PBS as well.  The short propagation distance makes it difficult to angle optics enough to be able to separate them from the main beam laterally to dump.  The EOM can't really be moved because the mode matching solution is a little tight for the available space.

After talking with rana and Craig yesterday it seems like the Pre Mode Cleaner (PMC) should be filtering polarization well enough when locked that the PBS and quarter-wave plate (QWP) are unnecessary. I removed all but the half-wave plate (HWP) and checked the residual polarization on transmission with a diagnostic PBS in place. I found 2 µW of power out of 1.2 mW was remaining when tuned all the way to s-pol.: this is a 1:600 extinction ratio which is about what we would expect from such a beam cube.  This measurement may be biased by the lower limit of the power meter, PBS should be giving 1:1000.

I moved the PBS to before the PMC to clean up light out of the 21.5 MHz PMC phase modulator. The only optics in the post PMC-> EOM path are now a lens, a steering mirror and a half-wave plate (see attachment #2).  After realigning the PMC cavity and the north refcav I was able to reduce the RFAM to -55 dBm, which is good enough for now.  These slight changes in RFAM level mean that the FSS offset will need some adjustment.  I was unable to see any improvement in the beat spectrum as the beat note had drifted down to 2 MHz.  I turned the heating down a small amount and left it overnight to settle.

I didn't angle the HWP or lens by that much, this shouldn't be necessary because the PMC is a traveling wave cavity.  The elements should be pretty close to normal.  The glass beam dump should be checked to ensure it is not clipping any retro-reflected beams on the rough edge of the glass.

## Clamping down the PMC

I never clamped down the PMC. It is just sitting on the ball baring points. This isn't great.

When I realized the tapped holes on the side of the base I went looking for clamps.  They are pictured in attachment #3 but they do not fit.  It turns out there were some issues with the choice of ball bearings on which the PMC sits.  The ball barrings sit over holes so that the PMC when placed will realigned exactly with its previous position on the base.  Antonio had found that the holes drilled for the ball barrings were spec'ed a little too big.  For standard increments of bearings size the closest size fits nicely over the hole but under force they actually slip down into the hole and are almost impossible to get out.  He bought the next ball bearing size up. However, this means that the clamps no longer reach the full height PMC assembly.  The assumed tolerances were made too tight on all these components, the next edition of drawings should allow for some wiggle room.

The drawings should be updated with at least 1-3 mm of range on slot cut side pieces for the clamps so there is room for changes in height due to ball bearing size.  Possibly even more, if future people want to put Viton or Sorbothane dampening into the clamping. The non-tapped holes should also be changed to through-all. Or at least drill with a narrower diameter through-all. This will help future users poke out objects that get stuck in the holes.

For that matter the design of the clamps seems wrong.  There is a bar that goes over the top that is fixed with a slot-cut piece affixed to each side. This is intuitively wrong as the bolts all go in horizontally when the clamping force needs to be applied is downwards!  It means that the clamps are locking a vertically applied force from the sides; to bolt the PMC down you need to apply force to the bar and tighten the bolts at the same time for two different clamping bars.  The screws should have at least one vertical pair on each clamp so that tension can be applied in the same direction as the clamping force.

## PMC documents on the DCC

For future reference, here is a list of all the PMC documents on the DCC:

Evan's technical note for PMC design considerations: LIGO-T1600071.

I can't find assembly procedures on the DCC.  There was a report from one of Kate Dooley's summer students, LIGO-T1600503, that shows a jig for gluing the PZT.

 Quote: Today I buzzed the table and determined there was a strong 500 Hz dirty resonance on the first steering mirror after the PMC.  This caused me to go around tightening bolts everywhere, including the offending steering mirror and the optics around it.  This did not reduce the resonance. I tightening the PMC REFL steering mirror as well, and this caused a misalignment onto the PMC REFL PD.  This took me a little while to figure out why the North path refused to lock.  I realigned the PMC REFL steering mirror into the PD. After I got the North PMC locking again, the North path itself was not locking anymore.  I reranged the autolocker slow volts, but this did not help.  Turns out the North Trans PD threshold voltage was not high enough.  This is likely because of the bolt tightening, causing some slight misalignment into the North cavity, lowering the overall circulating power in the cavity.  I lowered the autolocker threshold from 1.1 volts to 1.0 volts, and aligned the North Trans PD.  We need to rescan the North cavity to get better alignment/mode matching, but I'm gonna put this off until we replace this offending 500 Hz post-PMC steering mirror. While I was realigning the Trans PD, I noticed that even touching the trans optics tables causes large ~1Hz oscillations in the trans voltage.  This is definitely exacerbating any scattering problem we have.  Also, the Trans PD output for both paths is "breathing", going up and down with a period of about a minute.  This is bad for our autolocker's threshold.  It's possible that we should build two periscopes for the north and south paths to eliminate these elevated tables which cause coherent oscillations on all trans optics.  We could copy Tara's front periscope design.

2149   Wed Mar 21 00:40:01 2018 CraigDailyProgressscatterThe Sentinels: Transmission Table Black Glass Beam Dumps

Scattering is a huge problem in our setup, and we aren't sure where exactly the offending scattering is coming from.  The most basic thing to do is to go through our entire optics table, find all stray beams, dump them, then see what kind of spectrum we're left with.  At that point we can try more advanced techniques, like buzzing and damping resonant optics or upconverting the scatter source out of our band.

Many stray beams are coming directly from our cavities and polluting our transmission table.  Also, some beams are trying to make their way back into the can.  These beams tend to be close to the main beam, making dumping difficult.
To aid with dumping these mutant beams, I have created what I call the Sentinels.  The Sentinels stand guard at the transmission window of the vaccan, daring any puny beams to interfere with the main beam.

2150   Wed Mar 21 11:07:17 2018 awadeDailyProgressscatterThe Sentinels: Transmission Table Black (Green) Glass Beam Dumps

Which direction are you trying to dump? Into or out-of the can?

The scatter going inward is being deflected rather than trapped. It is still attenutated which is better. That which is reflected off the cavities/window is normally reflected back off the black (green) glass. Would we be better with just two pieces of glass (in a Vee)? Or a double trap with a Vee pointing forward and backward (an X-dump?)

 Quote: Scattering is a huge problem in our setup, and we aren't sure where exactly the offending scattering is coming from.  The most basic thing to do is to go through our entire optics table, find all stray beams, dump them, then see what kind of spectrum we're left with.  At that point we can try more advanced techniques, like buzzing and damping resonant optics or upconverting the scatter source out of our band. Many stray beams are coming directly from our cavities and polluting our transmission table.  Also, some beams are trying to make their way back into the can.  These beams tend to be close to the main beam, making dumping difficult. To aid with dumping these mutant beams, I have created what I call the Sentinels.  The Sentinels stand guard at the transmission window of the vaccan, daring any puny beams to interfere with the main beam.

2152   Fri Mar 23 15:12:00 2018 CraigDailyProgressscatterPowerpoint Trans Table Diagram

This diagram will be for quickly keeping track of scattering beams/scattering resonances on the trans table.  Need to improve by making some drawing in SolidWorks or something.

2155   Wed Mar 28 11:29:19 2018 awadeNotesscatterHigher quality vaccan windows: 40m stock of wedged windows

I just checked the 40m's stock of wedged, AR coated, optics in the pull out draws.

It looks like there is about nine CVI W2-LW-1-1025-UV-1064-45P windows: these are 1° wedged and coated on both sides for 45 incident p-pol.  Don't think this is what we want (i.e. 45 degree polarized). Also, 1 inch might be inconveniently small to use in practice.

There is one CVI W2-LW-1-2050-C-1064-0, this is the not UV grade fused silica so should probably not be used. Also we need four.

Everything else is either coated only on one side, the wrong type of glass, wedge or coating.

2156   Wed Mar 28 15:29:42 2018 KojiNotesscatterHigher quality vaccan windows: 40m stock of wedged windows

That was unfortunate. But why does BK7 uncompatible with the purpose? We need UV fused silica only for the high power reason, I thought.

2158   Thu Mar 29 11:14:52 2018 awadeNotesscatterHigher quality vaccan windows: 40m stock of wedged windows

I guess BK7 is fine, we're not going to be putting high power in.  I just though UV fused silica would be better practice if someone wanted to repurpose the flange in a few years.  Seems like a minor extra cost.

Bottom line is that I don't think we have four identical of anything we can use. Should order 1.5" W2s from CVI or somewhere else?

 Quote: That was unfortunate. But why does BK7 uncompatible with the purpose? We need UV fused silica only for the high power reason, I thought.

2172   Mon Apr 16 16:23:09 2018 awadeDailyProgressscatterBeam size on trans BN detector

I thought I'd have a look at how big the beam is on the current 1811 New Focus detector. Over focusing here might be a source of scatter so this is a number we should probably know.

## Razor blade measurement of beam on NF1811 Trans BN detector

I borrowed one of the translation mounts mounted with razor blades from the 40m and did a quick measurement this afternoon.

Because of the tightness of space on the transmission beat breadboard and the shape of the mount, the closest I could get the blade to the PD was about 1.0 cm.  I took a series of measurements cutting the beam and noting the transmitted DC power (in units of Volts).

# Data: vertical sweep of razor blade 1 cm in front of post cav BN detector
ypos = np.array([6.,7.,8.,9.,10.,11.,12.,13.,14.,15.,16.,17.,18.,19.,20.]) / 1000. *25.4e-3  # In units of 1/1000s of inch converted to [m]
yPDVolt = np.array([1.74,1.86,2.64,5.10,12.9,28.2,53.4,82.8,112,132,143,148,149,150,150])  # [mV]

I fitted the integral of the Gaussian profile and plotted (see plot below).  This is a quick diagnostic measurement. Iused least squares fit, so no error analyses. Here are the fitted values:

Fitted beam center relative to zero of measurement 0.3240 mm
Fitted peak power 148.2308 mV
Fitted detector dark DC reading 1.6333 mV
Fitted beam width wz 97.3314 um

## Time to make a switch?

This beam is quite small although the NF1811 detector diameter is only 0.3 mm.  Not sure how scatter scales with beam size here, is there a good reference I can look up on this?

Now might be a good time to switch to Koji's new PD.  I've managed to stabilize the beat note to 20 MHz it seems to stay within a <1 kHz (3.2 µK) range over a periods of sometime more than 6 hours.  Although, it can take 12 hours to settle down over night after a large disturbance.

2174   Thu Apr 19 18:28:11 2018 ranaDailyProgressscatterBeam size on trans BN detector

This beam is WAY too big for the PD. If the beam radius (wz) is 100 microns and the PD active area diameter is 300 microns, than you're always scattering a lot of beam off of the metal of the can. For new focus 1811, the beam radius should be ~30-50 microns.

2176   Sun Apr 22 22:00:15 2018 awadeDailyProgressscatterSwitching out NF1811 detector for KA25MHz

Rather than beating the NF1811 dead horse any more I've switched it out for the new KA 26 MHz detector.

## Installing detector and initial test

I found that after removing the focusing lens that was previously the last element before the BN detector, the beam size was roughly 300 µm (radius) about where the PD needed to go.  Here the photodiode size is 2 mm diameter. At this size the beam fits in about 1/3 of the diameter of the new detector area, so this looks like a perfect fit.  As a bonus we have one less optic in this critical part of the optical path.

The new detector was mounted at 3" on two 1" diameter posts.  There are bolt holes at 1" and 2" spacing on the base of the detector but I didn't have a more solid base in stock.  This mounting should be good enough for now, but can be improved on.

The the incident beam was angled at 30 deg and when centered gave a DC output voltage of about 115 mV from a DC power of 170 µW.  This is about right for the DC path where from Koji's schematic (see PSL:2162) the DC path has transresistance of 10 Ω followed by gain of 101 (total G=1010): we should expect 129 mV, the small discrepancy could be my sloppy power measurement.  This power is down from the usual value of about 800 µW mostly because I refloated the table and didn't realign the refcav input beams. Hopefully I'll get to fixing that issue tomorrow.

The activity on the table unlocked the cavities a few times which caused the beat note PID to kick the frequency around quite a bit.  I was unable to relock the PLL tonight but the peak power as the beat note slewed across 26 MHz was about 55.2 mVrms (-12.2 dBm).  We should expect that with about 85 µW in from each path, 100% overlap, 0.75 A/W responsivity, and 1.27 kΩ gain that the beat note would be on order -2.8dBm (0.162 mVrms) in this case.  I'd say there is some optimizing to do with alignment.  Could have also missed the exact point where the BN crossed over 26 MHz.

Its going to take all night for the BN frequency PID to settle (bring on the intelligent NL controls). This will have to wait till tomorrow.

## Max beat note power permissible ?

Not sure what the largest permissible beat note power can be here.  Koji went with a MAX4107 which has a slew rate of  500 V/µs. There were comments from in previous posts about slew rate (see PSL:2161). What is a good rule of thumb for slew rate of signal vs the rated response of an op amp in situations like this?

Edit Mon Apr 23 13:47:34 2018 (awade): added detector diameter.

 Quote: This beam is WAY too big for the PD. If the beam radius (wz) is 100 microns and the PD active area diameter is 300 microns, than you're always scattering a lot of beam off of the metal of the can. For new focus 1811, the beam radius should be ~30-50 microns.

2403   Wed Aug 28 09:49:44 2019 anchalSummaryscatterBeam Dump Status

We need to switch out normal flat-faced beam dumps with triangular cavity beam dumps in all places where they are not present. Following is a summary of beam dump status

Total Normal Beam Dumps behind PMCs: 12

Triangular Cavity Beam Dump Mount Requirements
Position South Path Present? North Path Present? Needed more?
PMC RFPD reflection 1 1 0
FSS RFPD reflection 1 1 0
PMC Back-reflection 0 1 1
BS Discard before PMC EOM 0 1 1
Faraday Isolator discard before cavities 0 0 2
Trans CCD (Common) 1 0 1
Trans ISS PD Reflection 0 0 2
Trans BS discard before PD 0 0 2
Beatnote NF1811 (Common) 1 0 0
Beatnote SN101 (Common) 1 0 0
Total     9

Present Inventory for triangular beam dumps and requirements
Present Quantity in CTN Needed more according to the above table
Triangular Cavity Mounts 3 6
Square 1"x1" Black Glass 16 11
Rectangular 1"x1.5" (Estimate) Black Glass 4 0
Square 1"x1" Black Glass with Hole 4 0

2528   Mon Feb 10 17:41:25 2020 anchalDailyProgressscatterCleaned up table; Installed hex beam dumps

Today I cleaned up the table, removed Scott's RFAM measurement setup and installed hex beam dumps on the input rejection of faraday isolators.

### Recalibration

• I measured the laser power at (50, 26) and (50, 29) when the lasers were detuned from the cavities using a power meter. Then I updated the ratio of DC readout of SN010 and SN009 with these power levels to infer reflected power in mW correctly.
• Similarly, I measured the laser power at (9, 26) and (9, 29) right after the cavities and updated ratio with dc readout of cavity transmission photodiodes at (4, 37) and (9, 34) to infer transmitted power correctly.
• I also adjusted horizontal tilt of periscope top mirrors at (46, 26) and (46, 29) to improve modematching to about 68% on North side and 75% on South side (numbers calculated by ratio of transmission to reflected power).

### Table cleanup and new beam dumps

• Not complete, but I removed a lot of unused optics, beam dumps, clamps etc from the table.
• I removed the photodiodes I put in before and after PMC to measure the intensity noise in the north path and its origin (CTN:2502).
• I dismantled Scott's RFAM measurement set up (CTN:2377) on the South path which used pick off from the input rejection of the faraday isolator at (58, 19).
• I have now installed new hex beam dumps at the input rejection of the faraday isolators on path path at (62, 16) and (65, 27).

2533   Thu Feb 13 18:00:30 2020 anchalDailyProgressscatterblocked NF1811 with hex beam dump

I have blocked the unused output port of the beam splitter before our beatnote detector with a hex beam dump at (13, 19). This was being used for broadband detector NF1811. We don't need it now.

403   Fri Nov 19 01:24:03 2010 taraDailyProgressopticchanging Faraday Isolator's mount/ TF from ACAV path

I switched the post to V-block for Faraday Isolator mount, for better stability, and adjusted the Faraday isolator to minimize back reflection to the laser.

I also measure the TF from ACAV path,  The UGF is ~65 kHz.

The faraday isolator was installed on a standard pillar post, so I use a V block to mount it instead.

After adjusting the FI, I remeasured the beat note frequency, and the signal did not change from yesterday measurement.

(no differences between red and green plots)

blue: beat signal before fixing the ACAV opening

red: beat signal after fixing the ACAV opening

green : beat signal after re installing the FI

ACAV TF: I connect the signal output after the PDH servo box to SR560 A and SR785 B (resp)

Then source out from SR785 is connected to SR560 B

The output of SR560 is connected to SR785 B (ref) and to the VCO

The setup for SR560 is DC coupling for A and B, select A - B, gain 1, no filter.

437   Mon Dec 20 19:55:11 2010 taraDailyProgressopticrearraging optics for beat measurement

I designed the layout for optics behind the cavities for beat measurement, and calculated the mode matching for the beam.

Since the current optics height for beat is quite high (7 inches), we want to lower it to 3 inches, make it more symmetric, and more compact.

The PD's diameter is 300 mm, so the beam spot on it will be ~50um.

All the lenses I need are prepared.

440   Thu Dec 23 22:41:28 2010 taraDailyProgressopticrearraging optics for beat measurement

Beat measurement optics' height is changed to 3". I cleaned all optics already, but I couldn't really clean 1/2 and 1/4 wave plates, one of the f =200 mm lens is quite hard to clean.

I'll wait and ask someone before trying to clean again. I cannot lock both cavities at the same time, once I can, I'll align the beam on the PD.

Also ACAV's PD for ACAV_trans_PD is broken. It gives out 11 V regardless of the beam falling on the PD, so I replace it with a PD that is used for NPRO_PWRMON.

 Quote: I designed the layout for optics behind the cavities for beat measurement, and calculated the mode matching for the beam. Since the current optics height for beat is quite high (7 inches), we want to lower it to 3 inches, make it more symmetric, and more compact. The PD's diameter is 300 mm, so the beam spot on it will be ~50um. All the lenses I need are prepared.

441   Sun Dec 26 02:42:47 2010 taraDailyProgressopticrearraging optics for beat measurement

Both cavities are locked at the same time. The temperature setting are, RCAV = 34.95, ACAV = 37.2.

I realigned the beam onto the PD to get maximum contrast. I'll readjust the setting back to the original value

and see if the beat noise is improved.

I just notice that one of the beam on the mirror on ACAV's path behind the cavity is almost clipped. I'll readjust it tomorrow.

Quote:

Beat measurement optics' height is changed to 3". I cleaned all optics already, but I couldn't really clean 1/2 and 1/4 wave plates, one of the f =200 mm lens is quite hard to clean.

I'll wait and ask someone before trying to clean again. I cannot lock both cavities at the same time, once I can, I'll align the beam on the PD.

Also ACAV's PD for ACAV_trans_PD is broken. It gives out 11 V regardless of the beam falling on the PD, so I replace it with a PD that is used for NPRO_PWRMON.

 Quote: I designed the layout for optics behind the cavities for beat measurement, and calculated the mode matching for the beam. Since the current optics height for beat is quite high (7 inches), we want to lower it to 3 inches, make it more symmetric, and more compact. The PD's diameter is 300 mm, so the beam spot on it will be ~50um. All the lenses I need are prepared.

442   Mon Dec 27 02:51:33 2010 taraDailyProgressopticrearraging optics for beat measurement

I measured the beat noise after I realigned all optics behind the cavities. The power has not been reduced to 1 mW yet.

This is just a quick measurement to see where we stand (red curve). The noise gets worse compared to the best measurement (green) before the optics behind the

cavities are rearranged, but the mechanical peaks around 1kHz are suppressed significantly.

599   Wed May 4 01:20:00 2011 taraDailyProgressopticminimizing RFAM/ aligning 35.5 MHz EOM

I minimized the RFAM by aligning the 35.5 MHz EOM and remeasured the RIN coupling coefficient.

The upper limit is 5 [Hz/uW (fluctuation of input power = RIN x Pin) ]@ 10 Hz

(This entry is approved by Kiwamu and is written in his style)Tue May 10 19:20:22 2011

As pointed out in the LIGO-X meeting that my setup might suffer a lot from RFAM, so I came back to:

• 1. minimize the RFAM by aligning the 35.5 MHz EOM,
• 2. determine how much Vmod I can apply to amplitude modulation without exciting the RFAM noise above the background, and
• 3. remeasure RIN coupling coefficient again with the allowed maximum Vmod.

[Setup]

The power input was 1mW as usual.

The frequency of Vmod is 10Hz. The amplitude of Vmod to EAOM for amplitude modulation was varied from 2 to 10 Vpkpk. Common/Fast gain was 500/900. I had to reduce it so the signal is not too large. I measured the spectrum of FASTMON and tried to observe the peak at 10Hz  with 12.5 mHz linewidth. The background level was ~10mV.

I do this to determine what is the maximum driving voltage where the effect from RFAM is still small compared to the background.

____________________________________

Drive Vpkpk     FASTMON peak(Vrms/rtHz)

10                   74.7

8                     48.17

5                     29.6

3                    23.02

2                  ~comparable to BG level ~ 10mV/rtHz

____________________________________

[ 1. aligning EOM ]

I picked up the beam after EOM on RCAV path and sent it to a PD (Thorlabs PDA10A.)  There were 35.5 MHz pick up on the table, so I had to choose where the peak from pickup was minimum. Then I adjusted the half wave plate before the EOM and EOM's pitch/yaw position to minimize the peak.

[ 2. determine max Vmod ]

Although we want to modulate the power as small as possible to have a good linear approximation, we also need the signal to be large enough to be able to see the effect. However, the alignment of the EOM is not perfect, there will be RFAM effect adds into the signal. If the modulation is too large, the RFAM will mask the real signal.  I need to determine what is the maximum Vmod I can use without having the RFAM effect excited above the background.

To see the effect of RFAM, I kept the setup similar to what I did with RIN coupling coefficient measurement, but without locking the cavity, and the laser frequency off from the resonance. This will tell us how much "fake signal" is produced by RFAM.

When the cavity is not locked, all the carrier and sidebands will be incident on the RFPD. The signal should be flat (beat between the carrier and both sidebands cancel each another,) and after it is demodulated by 35.5 MHz from LO, the level should be zero. However, if the amplitude is modulated at 35.5 MHz due to misalignment of the EOM, this will appear as DC signal at the error point. Hence, any power modulation at f0 (for this case, 10 Hz) will multiply up the error signal and cause offset fluctuation and slope change at f0. Slope change is not a problem, but the offset is. It will change the point where the laser will be locked, as the error signal moves up and down. Thus the system will interpret it as frequency noise of the laser and try to fight against it. This will appear as a peak in the FASTOUT spectrum at the modulation frequency, f0.

I measured the spectrum of FASTOUT (MIXER OUT is another option) to see the effect of RFAM

[ 3. remeasure RIN coupling coefficient ]

So I used 2Vpkpk drive, locked the cavity, and measured FASTMON again to see if I can measured the RIN coupling or not. The gain was set back to optimum value (common = 970 fast= 900.)  However, there was no observable peak at 10Hz from FASTMON signal. It was quite flat ~100 uVrms/rtHz.

I made sure that the amplitude was really modulated by checking RCTRANSPD. It had a 5.37 Vrms/rtHz peak at 10Hz with 200mV DC level. Therefore, the laser noise is higher than the thermo-optic effect at this modulation level. I cannot increase modulation depth because the RFAM will mask the signal.

If I use this number to calculate the coupling coefficient, (flat level of FASTOUT, and peak from RCTRANSPD)

it will be ~ 8 [Hz/ uW of fluctuation of the input power into the cavity] still larger than 1[Hz/uW] as measured at 40m, but it's getting smaller than the last entry (60 [Hz/uW] of input power)

I still can change the power input, but I think the RFAM will scale up by the same amount and mask the signal again. I'll try that later.

Let's check what does this value give us in the noise budget @10Hz. The input power is 1mW, RIN = 10^-4. Frequency noise will be

8 [Hz/uW] x 1000 [uW] x 10^-4 [RIN] = 0.8 [Hz/rtHz] which is higher than coating noise (10 [mHz/rtHz]@10Hz) So we still cannot ignore the effect.

[Take II]

I tried 16 mW input power, there was signal from RFAM when I measured FASTOUT with unlocked cavity, the peak was 46 m[Vrms/rtHz] above 10m[Vrms/rtHz] background. Vmod = 1Vpkpk.

When I lock the cavity and measure the coupling:

FASTMON peak = 278.6 uVrms/rtHz

RCTRANSPD peak = 30.82 mV, DC level = 2.57 V, Pin = 16mW. Linewidth = 12.5mHz.

Common/Fast gain = 480/906

Use the calculation from here.

The upper limit for coupling coeffiicient is ~ 5 [Hz/uW]. It is only the upper limit because RFAM effect is still present.

602   Wed May 18 20:51:35 2011 taraDailyProgressopticoptic layout for new fss setup

I planned the layout for new fss setup.

The new setup has 1) both cavities placed in the same vacuum chamber, 2) two AOMs used in both RCAV and ACAV paths, 3) more compact beat path.

In the layout, I assumed that

• Two cavities in the chamber are 3 inches apart.
• Two AOMs are of the same model, have the same setup
• There is no change of plan for the layout between PMC and the laser

This is just a plan, no mode matching has been calculated yet.

I am concerned  that the mode matching lens might block the beam in ACAV path where the incoming beam and reflected beam cross, but this can be adjust later.

The outer foam box will be smaller, but it should have enough space to keep some electronics inside like we have now.

I should find two similar sets of beam splitters/ mirrors for beams in the beat path behind the cavity. So the pick up beams from two cavities can have same power.

Right now the power going into two PDs for RCTRANSPD are not the same because the splitter are not the similar.

Note that we might install a platform  behind the cavities so that we don't need the periscopes to lower the beam, and get rid of their associated mechanical peaks.

603   Thu May 19 11:32:46 2011 taraDailyProgressopticoptic layout for new fss setup

I added more details on the layout, and necessary half wave plates in the beam path.

605   Wed May 25 18:29:09 2011 taraNotesopticpurchases

I ordered a few opto mechanical components to replace the current shaky periscopes.

The new  periscope is shown here, elog:574. Currently we have only one set, so I ordered a post clamp to complete another set.

I also ordered 4 mirror mounts that can be mounted on 45 degree mounting adapters. The thickness of these mounts are thinner than a regular mirror mount, so it can be fit on the adapter. I plan to use these in Crackle experiment as well.

606   Mon May 30 15:23:53 2011 ranaNotesopticpurchases

The periscopes for the refcav ought to be made custom. None of the store bought type are stiff enough. Koji has a design from the 40m green that Daisuke made.

607   Tue May 31 11:31:18 2011 taraNotesopticpurchases

I looked up 40m elog and found Daisuke's design for periscope. I'll make a sketch FSS' periscopes.

The design for 40m pericopes by Daisuke can be found here .

 Quote: The periscopes for the refcav ought to be made custom. None of the store bought type are stiff enough. Koji has a design from the 40m green that Daisuke made.

609   Wed Jun 1 01:50:35 2011 KojiNotesopticpurchases

They are found in DCC. Some comments

- You can not steer the beam. The beam should be steered before or after the periscope.

- The side plates were too thick. It can be 1/2" thickness to reduce the total weight.

 D1001446-v1 40m Vertex Green Locking Periscope A Base Daisuke Tatsumi et al. Auxiliary Optics Basic R&D 15 Jul 2010 D1001447-v1 40m Vertex Green Locking Periscope A Sidebar Daisuke Tatsumi et al. Auxiliary Optics Basic R&D 15 Jul 2010 D1001448-v1 40m Vertex Green Locking Periscope A Mirror Holder Daisuke Tatsumi et al. Auxiliary Optics Basic R&D 15 Jul 2010 D1001613-v1 40m Vertex Green Locking Periscope A PTFE Post Koji Arai Auxiliary Optics Basic R&D 15 Jul 2010

Quote:

I looked up 40m elog and found Daisuke's design for periscope. I'll make a sketch FSS' periscopes.

The design for 40m pericopes by Daisuke can be found here .

 Quote: The periscopes for the refcav ought to be made custom. None of the store bought type are stiff enough. Koji has a design from the 40m green that Daisuke made.

610   Wed Jun 1 21:00:05 2011 FrankNotesopticpurchases

we have to design our own. The 40m one has 2" mirrors (too large, we don't have the space), wrong height for incoming/outgoing beam and is clamped to the table, which i think is bad in terms of stability.
The design principle does not look much different compared to the original refcav periscope design, except for the mirror holder itself. That was bad designed for the old one.

Quote:

They are found in DCC. Some comments

- You can not steer the beam. The beam should be steered before or after the periscope.

- The side plates were too thick. It can be 1/2" thickness to reduce the total weight.

 D1001446-v1 40m Vertex Green Locking Periscope A Base Daisuke Tatsumi et al. Auxiliary Optics Basic R&D 15 Jul 2010 D1001447-v1 40m Vertex Green Locking Periscope A Sidebar Daisuke Tatsumi et al. Auxiliary Optics Basic R&D 15 Jul 2010 D1001448-v1 40m Vertex Green Locking Periscope A Mirror Holder Daisuke Tatsumi et al. Auxiliary Optics Basic R&D 15 Jul 2010 D1001613-v1 40m Vertex Green Locking Periscope A PTFE Post Koji Arai Auxiliary Optics Basic R&D 15 Jul 2010

Quote:

I looked up 40m elog and found Daisuke's design for periscope. I'll make a sketch FSS' periscopes.

The design for 40m pericopes by Daisuke can be found here .

 Quote: The periscopes for the refcav ought to be made custom. None of the store bought type are stiff enough. Koji has a design from the 40m green that Daisuke made.

614   Tue Jun 14 23:07:03 2011 taraDailyProgressopticoptic layout for new fss setup

The mode matching for new FSS is calculated. The plan is shown below.

Note for the setup:

1) the spotsize in the AOM is 200um, the specsheet says 550 um (I might have to correct this).

2) Two AOMs are of the same model.

3) For mode matching to the AOM in acav path, I used only a single lens.

4) focal lengths of the lenses are in mm, We have to order the one with * (f = 57.4 mm)

5) Both cavities are 1" apart (3" from center to center)

6) Mistake in the drawing: the x2 QWPs just before the beams enter the vacuum chamber should be placed before the periscopes, not after.

628   Mon Jul 18 02:20:01 2011 taraDailyProgressopticoptic layout for new fss setup

The new mode matching for optics in front of the cavities is done.  The rest (for beat measurement) will be finished soon.

A few changes in this layout are:

1) spotsize for AOM is 500 um, as specified by the datasheet.

2) Mirrors behind the AOMs will be changed to R= 2.0 m instead of 0.3 m.

3) Spot size in the 35.5 MHz EOM is ~300um which is good for the model.

4) More mirrors (for steering the beam) for the AOMs are added.

I'm a bit worried about using f=57.4mm lenses because they are quite sensitive when we have to move the lenses around, but the space is very limited this time.

I'll let Raphael double check my calculation so he can learn how to do mode matching.

629   Mon Jul 18 11:41:22 2011 ranaDailyProgressopticoptic layout for new fss setup

There's no need to use such a large spot size on either the AOM or the EOM.

When using high power this could be an issue, but you can use a beam radius of more like 100-200 microns for the AOM to get fast response time.

631   Wed Jul 20 15:54:21 2011 taraDailyProgressopticoptic layout for new fss setup

I edited the layout so that the spots in both AOMs are 200 um. I'll list what optics we might have to buy.

Most of the optics are already used on the table. I need to find:

•  a lens with f = 343.6 m (plcx R =154.5mm)
•  one more curve mirror with R = 0.3m for the second AOM.
• aom adaptor plate (need to submit this to the work shop to have it done
• periscope sets for both ACAV and RCAV (we need 4 in total, but we have only 2 sets)
• second VCO

The optics on ACAV path have been removed, I left the optics on RCAV path for now because Raphael might want to remeasure EOM TF.

Once the measurement is done, all optics will be removed. We will clean the table, clean the optics before put them back on the table.

632   Wed Jul 20 23:00:30 2011 FrankDailyProgressopticoptic layout for new fss setup

the lens and mirror are in the ATF, a second VCO is in the left cabinet.

636   Fri Jul 22 20:51:13 2011 frank, taraDailyProgressopticcleaning opto mechanical parts

As we removed some optics on the table, we use pressurized air to blow away dust/dirt on the mechanical parts (mount/ post/ lens holder) Optics have not been cleaned yet. We will clean it before we put everything back on the table. The cleaned parts are kept in a plastic box.

640   Tue Jul 26 18:42:16 2011 raphael, taraDailyProgressopticcleaning opto mechanical parts

Today we removed the optics behind the PMC, ACAV, and cleaned the table.

•   Optic mounts and posts are cleaned by pressurized air, and kept in a plastic box.
•   Lens and mirrors are kept in optic cases.
•   ACAV is moved to the end of the table, ion pump is unplugged.
• Table is cleaned with methanol, but some grease( under acav) is still on it.

NOTE: I just realized that the HEPA filter above the table close to the entrance ( the one that has the laser) is unplugged.

I could not find any available outlet to plug it back yet. We should turn it on soon.

644   Thu Jul 28 01:54:21 2011 frank, taraDailyProgressopticcleaning opto mechanical parts

Today we started working on the layout. There is one mistake in the layout, the mirror behind AOM for REFCAV is too close to the insulation box, so we have to fix the layout.

The oil on the table actually comes from holes on the table. About 5-6 screw holes had lot of oil, so I flushed them with methanol a few times.

The HEPA filter is plugged in and turned on. I unplugged one of the monitors and used the outlet for the filter.

The window is measured to be ~ 6 inches in diameter. Thus, the assumption in the design that the centers between two cavities are 3 inches is ok. If necessary, it can go up to 4 or 5 inches.

The layout is updated. The spot size in both AOMs are adjusted to 220 um.

647   Fri Jul 29 00:42:19 2011 frank, taraDailyProgressopticcleaning opto mechanical parts

We preparing optics for the new layout. To reduce scattering noise, most of the Y1-1064 mirrors we have been using will be replaced by super polished mirrors.

We think Y1-1064 mirrors can cause scattering noise in the setup because the coating surfaces look very milky.

fig1: Y1-1064 mirror.

We have ~ 10-20 super polished mirrors. Some of them are good, some of them are rejected from the site. The good one will be used for periscope/ beat setup.  I tested a couples of the rejected mirrors, but they can reflect both p and s beams with high efficiency. We will ask Peter to find out what is wrong with them.

fig2: Left and right, super polished mirror.

fig3: left, mirrors' case, right, certificate.

I have cleaned about half of the required optics, I think we should be able to lock the first cavity before next Wednesday.

648   Tue Aug 2 23:58:40 2011 taraDailyProgressopticNew RCAV setup is locked

RCAV is locked, I have not optimized the mode matching yet, the coupling efficiency is ~ 67%.

This new setup has a double passed AOM. The frequency is shifted by 160 MHz.

I will try to optimize the mode matching tomorrow, then I can check the loop performance that it works as before.

653   Tue Aug 9 00:39:30 2011 taraDailyProgressopticRCAV modematching optimized

I optimized mode matching for RCAV. The coupling is ~75%.  I also minimized RFAM from the 35.5MHz EOM.

For RCAV mode matching, I moved only two lenses in front of RCAV (R=51.5 and 20.6 mm) to optimize the mode matching.

I have not tried moving other lenses or the mirror behind AOM yet, because I think 75% is enough for now.

The reflected beam will cause the shot noise level to be higher, but it should not be critical for our current situation.

658   Mon Aug 15 23:13:53 2011 taraDailyProgressopticRCAV modematching optimized

I recalculated the mode matching so that the spot radius in AOMs is 100 um. Now the visibility of RCAV is 90%.

From the previous mode matching calculation, the spot radius in AOM is 220 um. This was too large for ISOMET AOM and caused beam distortion. The AOM was designed for much smaller spot radius (50 - 110 um). So I recalculated to make the spot radius inside the AOM to be 100 um. This spotsize is small enough for ISOMET and not too small for Crystal Tech AOM.

Rise time is 35 ns (28.5MHz) for 100 um radius in ISOMET AOM, diffraction eff ~80%. This should be sufficient for our less than 1MHz bandwidth loop.

For the new layout, I have to remove the Faraday isolator behind the EOM for another lens. I'll try to intall it back later.

ELOG V3.1.3-