[Tara, Evan]

Tara also took a BRDF measurement of #114 after cleaning it.

After cleaning, TIS from 14° to 71° is 2.7(5) ppm. Much improved.

Data and code are on the SVN at CTNlab/measurements/2014_07_31.

Incident power: 20.0(1) mW

Exposure times used: 25 ms, 50 ms, 200 ms, 500 ms, 1000 ms

Transmitted power: 3.34(2) µW. This gives a transmission of 167(1) ppm for this mirror.

TIS from 16° to 73° is 18(1) ppm.

Data and code are on the SVN at CTNLab/measurements/2014_08_05.

 Basically the same story with 132.

I used the ThorLabs power meter to get the transmission coefficients for the five AlGaAs mirrors.

For each measurement, I wrote down the incident power (20 mW nominal), the transmitted power (≈3.5 µW, depending on the mirror and background light level), and the transmitted power with the beam blocked (to get the dark power).

In other news, Tara bonded mirror #114 to spacer #95. The contacting seems to be tough going because of some recalcitrant smudges on the substrate surfaces.

 New setup for fiber phase noise cancellation with one AOM

 

 

(Laser going to ACAV) I changed the angle of the half-waveplate before the PBS in order to increase the amount of power going into the fiber that goes to gyro lab.  Its original position was at 277 degrees.  I put a beam dump behind the lens (PLCX-25.4-38.6-UV-1064) so the higher power does not reach the photodiode.  The new position is at 248 degrees.  I will move it back before I leave.

 I filled in more values for a-Si at 120 K into the wiki that Matt Abernathy set up. Then I ran the optimization code for Brownian noise only:

The above plot shows the comparison between the optimized aLIGO coating (silica:tantala at 300K) v. the a-Si coating at 120 K.

Then, finally, I compared the TO and Brownian noise of the two designs using the plotTO120.m script:

The dashed curves are silica:tantala and the solid lines are a-Si:silica. The Brownian noise improvement is a factor of ~6. A factor of ~1.6 comes from the temperature and the remaining factor of ~3.9 comes from the low loss and the lower number of layers.

I think this is not yet the global optimum, but just what I got with a couple hours of fmincon. On the next iteration, we should make sure that we minimize the sensitvity to coating thickness variations. As it turns out, there was no need to do the thermo optic cancellation since the thermo-elastic is so low and the thermo-refractive is below the Brownian almost at all frequencies.

Current configuration:

• Target waist: 180 µm, z = 0 mm
• Lens 1: 140 mm focal length, z = −711 mm (24″ from center of vacuum chamber + 4″ through periscope)
• Lens 2: 84 mm focal length, z = −991 mm (11″ further behind lens 1)
• Seed waist = ??

Since we know we were mode-matched fairly well into the 180 µm waist of the silica/tantala cavity (>93% visibility), I asked alm to propagate this waist backward through the lenses in order to find a seed waist. It reports a waist of 161 µm at z = −1373 mm.

I asked alm for a new configuration using the same two lenses. The best configuration (mode overlap = 1) is as follows:

• Seed waist: 161 µm at z = −1373 mm
• Lens 1: 140 mm focal length, z = −743 mm
• Lens 2: 84 mm focal length, z = −1023 mm
• Target waist: 215 µm, z = 0 mm

So we should move lens 1 back by 32 mm (=1.3″), and move lens 2 back by the same amount.

I moved both lens mounts back by 1″, then adjusted the Vernier knobs and periscope mirrors to try to maximize the visibility as seen on north REFL DC.

The best I am able to do so far is a visibility of v = 1 − 0.57(1) V / 1.74(1) V = 0.672(6).

Tara and I took an SR785 measurement of the north photothermal transfer function.

Clearly there's something wrong with the measurement above 1 kHz.

Tara and I took another photothermal TF of the north cavity today. Relevant parameters:

• Power incident on cavity: 10 mW (up from the usual 1 mW)
• Beat frequency: 1.2 MHz, drifting to 650 kHz (we are hoping it will swing through 0 Hz overnight and settle above a few megahertz by tomorrow)
• DC voltage on north ISS PD: 2.39(5) V
• DC power transmitted through cavity: 3.77(2) mW
• PLL actuation coefficient: 50 kHz_pk / 1 V_rms
• PLL UGF: 80 kHz (measured)
• EOAM drive: 5 Vpp from 20 kHz to 300 Hz, then 3 Vpp from 300 Hz to 0.2 Hz

In the attached data I have already converted the raw data (in V/V) into hertz of beat frequency per watt of circulating power. For this I use the conversion factor (50 kHz / 2^1/2 V) × (2.39 V / 3.77 mW) × π / F, with F = 16 700. Since the TF (again) appears to be junk above 1 kHz, I haven't bothered undoing the CLTF of the PLL.

The attached plot shows the expected photothermal TF in terms of hertz of beat frequency per watt of absorbed power per mirror. Therefore, the scaling factor that makes our measurement (given in hertz per watt of circulating power) overlap with the expected TF (given in hertz per watt of absorbed power per mirror) should be the average absorption of each mirror. I find that this scaling factor is 6 ppm, which seems surprisingly low, especially given our earlier finding that we have at least 120 ppm of scatter + absorption loss. So I will double check for missing factors of 2, 4, π, etc.

At any rate, the shape of the measured transfer function appears to be in good agreement with the expectation up to 100 Hz. If we believe that the coating/substrate photothermal crossover happens around 10 Hz, and we believe our measurement from 10 Hz to 100 Hz, then this seems to indicate that the thermo-optic cancellation has been somewhat successful.

Some minor maintenance/improvements to the optical setup:

• We replaced the existing lens before the beat PD (RoC = 51.5 mm) with a slightly faster lens (RoC = 38 mm) in order to reduce the spot size on the diode.
• Tara improved the clamping of mode-matching lenses before the south cavity (they weren't tightened down enough before)

I took a swept-sine measurement of the photothermal TF just as Tara and I did for the north cavity. To get a better measurement, I made some configuration changes:

• I turned the power incident on the south cavity up to 8.5 mW by adjusting the post-laser HWP from 318° to 286°.
• I placed an OD2.0 in front of the beat PD to prevent RF saturation.

Settings/values:

• The beat was at 13.8 MHz.
• The PLL Marconi was on 50 kHz FM deviation, and the SR560 gain was 100 V/V.
• South transmission PD was 460(5) mV dc.
• South transmission power (directly out of vacuum chamber) was 2.20(5) mW dc.

The results are attached. I'm not sure why there's a discrepancy around 200 Hz between the two traces. Below 100 Hz the measurement looks relatively clean.

The light rejected out of the post-EOAM PBS is only 2 mW (compared with 9 mW transmitted), which makes me suspicious that the post-EOAM QWP is not rotated properly, or else the input polarization into the EOAM is wrong. We should check this before redoing this measurement.

As with the north cavity, I find that an absorption of 6 ppm is needed make the measured curve lie on top of the theory curve.

For the time being, I have left the input power at 8 mW in case we want to take this again tomorrow. There's currently a dump upstream of the PMC to block the beam.

I didn't like the EOAM situation, so I rotated the post-EOAM QWP from 302° to 285°. With no voltage applied to the EOAM, this gives 6 mW of p and 6 mW of s. This may not be the true optimal setting, but the previous 2 mW / 9 mW situation seems too weird to be right. For commissioning the ISS I suspect we'll have to redo this EOAM setup to make sure the polarizations are behaving as we think.

The results are attached. I'm still seeing discrepancies at the points where the TFs are stitched together. Maybe it's because I'm using the SR785's auto source adjust feature.

I believe the factor of π / F here is an error. It should instead be the transmission T. That lowers the absorption estimates to more like 5 ppm.

I reran multidiel_rt with the as-built coating structure. The penetration depth is x₀ = 560 nm. With A = 5.6 ppm absorption on each mirror, the absorption coefficient is therefore α = 0.05 cm⁻¹.

Penetration depth x₀ is defined via E(x)/E(0) = exp[−x/(2x₀)]. Absorption coefficient is defined as α = A/(2x₀), since the effective distance traveled through the coating is 2x₀. [I belive this is the same definition that Garrett uses.]

The script for this is in the paper directory of the svn, under source files.

I reworked the beginning of the south optical path so that there are two steering mirrors before the beam goes into the FI.

Recall that previously we had no steering mirrors before the FI. Then in December, I just moved the FI sightly downstream, so that there was one mirror before the FI.

Today I added two steering mirrors  (Y1-1025-45P) in such a way that the total path length should be more or less unchanged. The first lens after the laser is now placed after the first steering mirror. (I tried to place it so that it has the same displacement from the laser head as it did previously.) The FI is placed after the second steering mirror, and it is immediately followed by a HWP.

Ideally we would maybe put down another HWP before the FI, since the steering mirrors are only HR for p-pol, and the beam on the first two steering mirrors is some combination of s-pol and p-pol (since we use a HWP + the FI to control the power after the FI).

After steering through the FI, the beam looks pretty round on the IR card. I don't see any spray or stray beams.

I tuned the pre-FI HWP so that there is now 20.4 mW transmitted through the FI. The power transmitted through the 21.6 MHz EOM (which is after the third steering mirror) is 19.6 mW. I also don't see any spray on transmission.

Pointing into both cavities has been recovered.

I could not get the PMC on the south path to lock, so I have just taken it out for now. Then I resteered through the BB EOM and resonant EOM and into the south cavity.

The north path did not require much resteering. North seems to lock OK, although I have not checked the health of the PDH loop. On south we need to install an HV supply before locking.

I reinstalled the old, underpowered unipolar HV supply that we used to use for the south cavity. Since Aidan is going to redo the power distribution anyway, there's no point in fussing with it now. The south cavity locks fine. The digital temperature offloading seems to be working as well. Light incident on each cavity is about 5 mW.

The work that we need to do for the TCN experiment is at the state where we cannot easily go further without improving the lab organization.

I have collected the optics (1064nm) and made an inventory which is on dokuwiki: https://nodus.ligo.caltech.edu:30889/ATFWiki/doku.php?id=main:experiments:psl:optics. At moment is little but I hope we can keep this updated and we can follow the policy that is written there.

The inventory is shown in the picture.

• Yesterday and today some cleaning has been done by Eric and Aidan, removing big useless parts. The rack on the left is free now.

North Path

SUMMARY

Thigs done (Following the attached schematic):

1. The lens (77.3, f = 171.9mm) before the FI has been removed and placed after it;
2. The FI is closer and a bit tilted as the the back reflection was going straight to the laser;
3. lambda/4 and lambda/2 have been placed in the path before the FI and have been rotated in a way that at the output of the FI we have maximum power (~0.7W);
4. A lens (77.3) has been placed after the FI;
5. Lambda/2 has been placed in order to create P polarized light;
6. A high power PBS (PBSO borrowed from the 40m) has been implemented for power adjust;
7. Lambda/2 after the PBSO in order to create s-polarized light;
8. Beam has been measured in the area after the lambda/2 (distance reference) (as shown in the picture);
9. A BroadBand (BBEOM) has been placed with waist in the middle with resulting beam aligned in plus menus 10 micro meter range (This means that looking at a reference point with the ccd camera, after BBEOM was placed and aligned the beam was hitting the same point. ) The power of the incoming beam was 320uW while the power of the outcoming beam was 310uW.
10.  A lens (51.5, f = 114.5mm) after the BBEOM has been cleaned and placed (This lens has a little scratch);
11. A steering mirror (s) has been cleaned and implemented;
12.  A lambda/2 has placed after the mirror in order to have p-light;
13.  The AEOM has been implemented;
14.  Lambda/4 has been implemented and rotated at 10deg in order to have circular polarized light;
15.  PBS has been cleaned and implemented;
16.  Lambda/2 has been implemented in order to give s-polarized light (45deg);
17.  A lens (51.5, f = 114.5mm) has been cleaned and implemented in order to create the right beam for the resonant EOM to be used for the PMC sidebands;
18. Beam profile has been measured: the fit is not ok but the data are indicative enough (waist of ~ 100 um);
19.  A steering mirror (not labeled) has been power checked (“equal” for both s and p) and implemented;
20.  The resonant EOM (21.5MHz) to be used for the PMC sidebands has been implemented with alignment precision at the order of tens of microns (incoming power was 315 uW while the outcoming power was 306 uW);
21. Steering mirror implemented;
22.  Currently we are in the process of implementing two lenses to mode match the PMC;

Things to note:

1. The beam coming out of the laser is not perfectly at 3 inches height;
2. The resonant EOM used for the PMC will be removed while we take measurement without PMC;
3.  The beam coming out of the laser is very dirty;
4. There is a specific lens mount which fails to have the optics center at 1 inch;

Next step:

1. Mode matching for the PMC;

Attachment:

1. Schematic 

     2. Beam profile after l/2 (see point 8, zero is at l/2)

   3. Beam profile measured before EAOM (zero at the steering mirror)

4. Beam profile measure before EOM for PMC (zero reference at lens (51.5))

(to be updated: the fit is failing)

Summary

I have added two lenses in order to create a waist of 330um for when the PMC will be implemented.

Not satisfied about the size of the beam. This has to be improved, but we are close enough. I am thinking

to put the PMC in place and mode match the laser beam to the PMC, in order to have a better reference.

-------------------------------------------------------------------------------------------------------------------------------------

The two lenses are 114.5mm and 401mm focal length rispectively, as shown in the attached schematic.

At moment the beam for the PMC (after the two lenses) is the following:

North Path

Summary
Following entry ID 1649, additional work along the North Path has been done

Conclusion
We have the light into the North cavity mode mateched and aligned to TEM00 
with a visibility {(Vis  = Vmax-Vmin) / Vmax} of ~71%.

Next Step
If FOR THE MOMENT we accept this mode matching I will hook up the electronics
and work on the North cavity locking, leaving modematching improvments for later.

Things done
1. PMC has been placed in the path and aligned. Visibility was about ~90%;
2. PMC has been removed;
3. A PLCX-51.5-UV lens has been cleaned and placed in the optical path soon after the PMC;
4. A resonant EOM (14.5MHz) for PDH sidebandshas been placed and aligned:

• Power at the input was ~3mW, at the output ~2.95mW

5. Two steering mirrors have been cleaned and placed in the path;
6. Two mode matching lenses have been placed:

• A PLCX-51.5-UV lens has been placed in a location that few inches above the nominal
  found in the design (ID 1661) ;
   
• A PLCX-103.0-UV lens has been placed in its nominal location;

7.  In between the two lenses a FI (IO-5-1064-HP) has been aligned:

• May be just for now, I have rotated the input polarizer in order to have light rejected from the cavity parallel to the table;
• The output polirizer has rotated while the FI was in reversal mode in order to optimize the isolation;
• A lambda/2 has been placed before the FI to maximize the output light;
• Input power ~ 2.98mW and output power~2.83mW;
• Some info about the FI: 
  https://www.thorlabs.us/newgrouppage9.cfm?objectgroup_id=4915&pn=IO-5-1064-HP
  https://spie.org/samples/PM200.pdf

8. The TEM00 has been found by scanning the PZT of the laser with a triangular wave (+-1V) at 10Hz through an HV amplifier set at ~40V;
    Additionally a voltage calibrator has been used to actuate on the crystal temperature of the laser in order to localize the are to be scanned with the PZT.
 

9. A Lambda/2 has been placed after the FI in order to have one linear polirized light:

• With light at ~45degree I have seen 2 TEM00 resonance not exactly the same;
• By rotating the waveplate we can have purely one or the other resonance (not sure yet at which angle);

10. A lens and a steering mirror have been placed in the path of the rejected light from FI to monitor the visibility/resonances with the DC output of the photodiodes;
    For now the visibility is at ~71%

11. This is the current setup:

Summary
Following ID 1649 we start the implementation of the South cavity path.
Some optics has been placed along the path.

Conclusions 
A quick beam profile measurement and the implementation of the BB EOM
is the next step.
 

Things done:

• Optics as shown in the setup have been placed;
• References of the beam along the path have been aligned  (Those should be kept in place)

Notes:

• The FI is mounted on a base which brings it NOT at 3" height; Two steering mirrors have been used to overcome this issue;
• There is a second beam coming from the FI; I did not have a chance to play with it, but Andrew did and the origin of that is not totally understood;

Setup

I was unable to remove a ~32 kHz ringing from the north path lock. I thought possibly that it might have to do with polarisation alignment into the EOM used to actuate in the FSS lock: this in the past had converted phase acutation into amplitude acutation from misalignment. However, the BB-EOM was placed so close to the prior waveplate that it was imposible to get a PBS in to check.  I've moved the BB-EOM 6 cm backward to give space for future diagnostics. Picture before and after attached. It seemed to be aligned ok.

After some realignment down the path I was able to get a lock again,  but there is still ringing, this time at 22 kHz.  The waveform and frequency change with some choices of gains. 

I've attached screen shots of what I seen on the osciloscope, not that that helps much. Not sure if a transfer function would help, if the loop is ringing that doesn't make the measurment easy. This is probably something obvious but I'm leaving it for today.

---

FYI side note, there is now an obvious small spray of light that I can see on the CCD cameras, like a flare.  Tuning up the alignment seemed to move it off a bit, I don't remember seeing it there before.  There doesn't seem to be an obvious scattering before the vacuum can.

[awade, Craig]

Yesterday we went hunting for residual AM from the PM phase modulators.  This is due to misalignment of the polarization going into the RF phase EOMs and is a potential source of lock point offset/noise in the PDH locks of the FSS.

I found that both North and Souths paths had quite a bit of residual AM.  I dropped the locks on both cavities and offset the laser frequency to well below the TEM00 mode. Craig and I also installed a flipper on the north path (just before the periscopes) to return light without interacting with the cavity at all. We can see the AM by looking at the 14.75 MHz beat coming out of the RF port of each PD.  

Noise power from both detectors was in excess of 85 dBm/Hz (0.4 mV) on both detectors.  Maybe I should have just used units of dBm here, but this is the setting I chose at the time: we just need to see to minimize for now. 

---

Reducing AM

To reduce the AM, I put a PBS in before the EOM and walked the lambda/2 and lambda/4 to minimize transmitted light. This gave maximum linearization of the signal.  To minimize the AM the PBS was removed and the beat on the reflection PDs (with lasers off resonance) was observed while only the lambda/2 was adjusted.

One problem with this optimization is that the output of the EOM then goes through a FI to separate linear ingoing/outgoing fields.  I found that by walking the pre EOM lambda/2 WP and correcting the reflected cavity DC level with the lambda/2 before the FI it was possible to find a sweet spot where power through the FI. The adjustments of WP angle only needs to be very small, but it can be a fine tune to get it very close to totally minimized.

North path was reduced to level of -96 dBm/Hz above a SA dark noise background of -130 dBm/Hz. South path was reduced to a residual AM of -97.5 dBm/Hz on a dark noise background of -122 dBm/Hz.  It should be noted that the noise floors of the two detectors are about an order of magnitude different looking on the Aglient SA.  Something we need to check is the relative noise of the two 14.75 PDs as they are not the same build.

---

Optimizing polarization through BBEOMs

To ensure that the BBEOMs used for very fast actuation were aligned properly I put a ±10 V @ 1234 Hz sine wave into them and looked at the reflected BN out of the RF of the PDs.  It was very fast to align the BBEOM polarization to reduce to below the noise floor of the Aglient.

---

Changes to North optical path

I found that there was no space in front of the BBEOM, AEOM and resonant resonant/(amplified) EOM in the north path to put a PBS.  It is not super critical that these modulators sit exactly at the waist of the beam so I moved them back 3 cm so that a PBS on a narrow mount can be fitted into the gap for diagnostic purposes. I also temporarily removed a lambda/4 wave plate from after the AEOM as we are not using this for any power modulation at the moment.

---

Powers incident of cavities

After making various adjustments for the two paths I set the power going into the north path refcav to 1.27 mW. For the south path the power incident was 1.22 mW

For some reason after the weekend, I found that the laser power on the south path decreased by about 300 uW. On checking power with a laser power meter, I found that a PBS, right after South EAOM and a half waveplate, is dumping more than half of the power as it is vertically polarized. The half waveplate is supposed to rotate the vertical polarization from EOAM into horizontal for cleaning by the PBS. I have rotated this waveplate to get maximum output from PBS.

Still, the loss at this stage seems much more in comparison to North Path.
Loss at North Path after EAOM  due to horizontal polarization at output = (354 uW)/(2431 uW) x 100 = 14.56%
Loss at South Path after EAOM due to horizontal polarization at output = (545 uW)/(1957 Uw) x 100 = 27.85%
I have checked that input polarization is correctly vertical for both EAOMs.(Only about ~1% light horizontally polarized for both paths at this point). Also, both EAOMs (New Focus 4104) are currently terminated with 50 Ohm terminators at their modulation ports.

--- 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.

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.

I've been learning about scattering from here and here.  It seems most scattering equations are left in an arbitrary form of , and simply measure seismic noise then use it as in the following FFT:

If the sine wave were perfectly sinusoidal, say , our FFT would yield delta functions at .  However, scattering is rarely so clean.

If 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 , 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 , where is a scatter coupling coefficient in units of hertz, and is a half width half maximum (HWHM) of an underlying Lorentzian .

I found and .

We can think of the HWHM as a function of overall scattering displacement and velocity: . 

If , this gives

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.

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.

TL;DR We have (1) poor alignment of light into the cavities and (2) ghost beams interfering with our resonant light from inside the vaccan.  This 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.

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.

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.

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.

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.

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.
 

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.

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:

• LIGO-E1400332 (PMC body and end cap drawings)

• LIGO-D1600172 (PMC clamp side piece clamp Stud)

• LIGO-D1600171 (PMC bar clamp)

• LIGO-D1600170 (PMC base) 

• LIGO-E1400241 (PMC flat beam splitter specification)

• LIGO-E1400241 (PMC curved mirror specification)

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. 

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.

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?)

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.

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.

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

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?

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. 

 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.

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.

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