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ID Date Authorup Type Category Subject
  1468   Tue Aug 5 18:10:31 2014 EvanDailyProgressopticAlGaAs mirror transmissions; optical contacting

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).

Mirror
Transmission (ppm) Average (ppm)
#114 142(6) 142(6)
#132 162.4(1.4), 159.8(2.1), 163.0(2.1) 161.7(1.9)
#137 149.8(3.4), 149.5(2.0), 148.0(2.0) 149.1(2.5)
#141

154.9(2.0), 155.4(2.1), 155.4(2.1)

155.2(2.1)
#143 155.6(2.1), 154.7(2.1) 155.2(2.1)

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.

Attachment 1: almost.jpg
almost.jpg
Attachment 2: done.jpg
done.jpg
  1469   Wed Aug 6 17:29:30 2014 EvanDailyProgressRefCavOptical contacting

Tara has successfully formed the AlGaAs cavities. The configurations are as follows:

  • Spacer 95: to the left of the ATF logo is mirror 114, and to the right of the ATF logo is mirror 143.
  • Spacer 96: to the left of the ATF logo is mirror 141, and to the right of the ATF logo is mirror 132.

Mirror 137 has not been contacted.

Attachment 1: 114.jpg
114.jpg
  1470   Sun Aug 10 18:42:48 2014 EvanNotesPMCPMC heater, v2

I'm building this instead:

pmc_heater_2.jpg

  1473   Thu Aug 14 15:27:36 2014 EvanDailyProgressPMCPMC encap measurements

OD: 1.63"

Depth: ca. 0.9"

Minimum clearance between cap and mount: ca. 0.5"

  1475   Sat Aug 16 13:06:35 2014 EvanNotesRefCavExpected AlGaAs cavity parameters

Just so we have a concise table that we can refer to:

  North South Note
Length 1.45(1)" = 3.68(3) cm  
FSR 4070(28) MHz c/2L
Mirror ROCs 1.000(3) m Uncertainty is a guess
g factor 0.9632(3) 1 − L/R
TMS 353(3) MHz νFSR×arccos(g)/π
Transmission loss 297(6) ppm 317(3) ppm ctn:1468
Scatter loss 30 ppm 30 ppm Crude guess
Absorption loss 30(30) ppm 30(30) ppm Extremely crude guess
Finesse 16700(1400) 15100(340), 14600(1000) 2π/(losses); ctn:1480
Cavity pole
116(10) kHz 135(3) kHz, 139(10) kHz νFSR/(2F)

 

  1478   Tue Aug 19 08:55:24 2014 EvanDailyProgressLaserSouth NPRO PZT acutation coefficient

I used a function generator to drive the south NPRO PZT with a triangle wave. Then with the 14.75 MHz sidebands on, I used a PDA100A to watch the south cavity transmission.

Looking by eye at the carrier and sideband transmission peaks, I find an actuation coefficient of 4.4(2) MHz/V, which is higher than what Tara measured in 2010 (maybe the coefficient depends on which axial mode the NPRO is operating on?)

From the attached plot, we can also see that the mode splitting for the south cavity is 2.0(4) MHz.

Attachment 1: south_npro.pdf
south_npro.pdf
Attachment 2: southnpro.zip
  1479   Tue Aug 19 09:25:19 2014 EvanDailyProgressRefCavPolarization selectivity of south AlGaAs cavity

I temporarily removed the QWP immediately before the periscope. Then I added a HWP directly in front of the vacuum chamber window.

While sweeping the laser across the south TEM00 resonances, I monitored the peak voltage of each resonance.

For this particular HWP mount, rotating the mount to 23(1) degrees produces s polarization, in the sense that placing this HWP between two PBSs causes the second PBS to reflect 100% of the beam.

Attachment 1: pol.pdf
pol.pdf
Attachment 2: southpol.zip
  1480   Tue Aug 19 10:35:07 2014 EvanDailyProgressRefCavSouth cavity finesse

I swept the south NPRO PZT with a 2 Vpp, 5 Hz triangle wave and watched the transmission of the south cavity using a PDA100A. I saved three such transmission sweeps from the oscilloscope, and then performed Lorentzian fits on each of them in order to get the cavity pole.

From the Lorentzian fits along with the 4.4(2) MHz/V calibration found earlier [and FSR = 4070(30) MHz], I find the following:

  • Lower resonance: cavity pole of 135(3) kHz, corresponding to a finesse of 15100(340). This gives the total round-trip loss as 417(10) ppm.
  • Upper resonance: cavity pole of 139(9) kHz, corresponding to a finesse of 14600(1000). This gives the total round-trip loss as 429(28) ppm.
Attachment 1: southsweep.pdf
southsweep.pdf
Attachment 2: southsweep.zip
  1482   Wed Aug 20 19:00:03 2014 EvanNotesNoiseBudgetAlGaAs python noise budget

Quote:

I have started a python implementation of the AlGaAs noise budget. All parameters, functions, etc. are defined in a single notebook, and this same notebook generates the plot. The python uncertainties package facilitates estimation of uncertainties in material parameters, optical parameters, etc.

Currently, the coating thermo-optic trace is not an actual calculation; it is just a flat line culled from figure 5.9 of Tara's thesis.

The PDH shot noise trace is shown assuming an incident power of 1 mW on each cavity, a PDH modulation index of 0.2 rad, and a cavity visibility of 0.92.

To do:

  • Finish implementing true TO calculation
  • Add photothermal (requires RIN data)
  • Add seismic (requires seismic data, seismic stack TF data)
Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1485   Mon Aug 25 20:54:38 2014 EvanNotesNoiseBudgetAlGaAs python noise budget: TO implemented

Quote:

Quote:

I have started a python implementation of the AlGaAs noise budget. All parameters, functions, etc. are defined in a single notebook, and this same notebook generates the plot. The python uncertainties package facilitates estimation of uncertainties in material parameters, optical parameters, etc.

Currently, the coating thermo-optic trace is not an actual calculation; it is just a flat line culled from figure 5.9 of Tara's thesis.

The PDH shot noise trace is shown assuming an incident power of 1 mW on each cavity, a PDH modulation index of 0.2 rad, and a cavity visibility of 0.92.

To do:

  • Finish implementing true TO calculation
  • Add photothermal (requires RIN data)
  • Add seismic (requires seismic data, seismic stack TF data)

I've implemented the TO calculation following Evans et al. (2008), along with the so-called Yamamoto correction for the CTE.

These changes are on the SVN.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1487   Thu Aug 28 13:40:05 2014 EvanDailyProgressFSSPDH error signal on north cavity

North PZT sweep: 10 Vpp triangle wave, 3 Hz

North slow control voltage: 3.6805 V

Actuation on north broadband EOM removed

Phase tuning needed, mode-matching needed

Find TNC-SMA converters

Attachment 1: npdh_broad.pdf
npdh_broad.pdf
Attachment 2: npdh_fine.pdf
npdh_fine.pdf
Attachment 3: npdh_sweep.zip
  1488   Thu Aug 28 17:36:03 2014 EvanDailyProgressopticMode-matching solution for north cavity

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.

Attachment 1: ctn_algaas_alm.pdf
ctn_algaas_alm.pdf
Attachment 2: ctn_algaas.zip
  1489   Thu Aug 28 19:10:40 2014 EvanDailyProgressopticMode-matching solution for north cavity

Quote:

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).

  1490   Thu Aug 28 20:47:24 2014 EvanDailyProgressFSSPDH error signal on north cavity

Quote:

North PZT sweep: 10 Vpp triangle wave, 3 Hz

North slow control voltage: 3.6805 V

Actuation on north broadband EOM removed

Phase tuning needed, mode-matching needed

Find TNC-SMA converters

 Here's what I expect to happen given

  • perfect mode-matching,
  • critical coupling with 150 ppm transmissivity for each mirror,
  • p/s mode splitting of 2.0 MHz, and
  • perfect demod phase.

It seems to match up qualitatively with the measurement. In particular, it does not appear possible to exceed 70% visibility for each mode.

Attachment 1: npdh_sim.pdf
npdh_sim.pdf
Attachment 2: npdh_sim_narrow.pdf
npdh_sim_narrow.pdf
Attachment 3: fpbirefringence.pdf
fpbirefringence.pdf
  1491   Fri Aug 29 09:21:26 2014 EvanDailyProgressFSSPDH shot noise estimate

Ignoring for the time being the issue of offsets in the PDH error signal, here's my prediction for the new PDH shot noise level, assuming a visibility of 0.92 × 0.7 = 0.64 and an incident power of 2 mW.

So our beat will be slightly worse around 1 kHz, but we aren't completely hosed by the shot noise. I'd think the true solution here is to find (or buy) two large-aperture Faraday isolators to replace the PBS+QWP setup (according to alm, the spot size in this region is about 1.1 mm). E.g., we might consider a large-aperture ThorLabs isolator.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1492   Fri Aug 29 12:35:54 2014 EvanDailyProgressBEATBeat breadboard in place

Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

  1493   Fri Aug 29 15:35:55 2014 EvanDailyProgressBEATMode-matching for beat

I predict (via alm) that the spot size on the diode (z = 991 mm) is 79 µm in the current configuration.

Attachment 1: ctnbeat_algaas.pdf
ctnbeat_algaas.pdf
Attachment 2: ctnbeat_algaas_alm.zip
  1494   Fri Aug 29 19:56:06 2014 EvanDailyProgressBEATBeat breadboard in place

Quote:

Beat breadboard is slid back into place. North transmission appears on north camera. Still need to do south transmission.

Tara has found south transmission on camera. I steered the transmitted beams onto the beat PD and then made the k-vectors as parallel as I could as seen on an IR card.

The DC voltage on the PD is okay (ca. 50 mV from each beam), but I cannot see a beat note on the AC path using the HP4395. Tara will give a temperature kick which hopefully will bring the beat note within the range of the 1811.

  1495   Sat Aug 30 19:10:11 2014 EvanDailyProgressNoiseBudgetAlGaAs python noise budget: seismic coupling added

Same data and same isolation model as for the silica/tantala noise budget. Since we have new table legs, we should retake this data (and make a spectral histogram).

The resonance frequencies of the stack are given as 10 Hz and 35 Hz in the noise budget. Are these for the old stack? I recall that with the new stack we measured resonances at 3, 7, and 10 Hz.

Also I want to double check the sequence of interpolation steps we've used on the silica/tantala noise budget. There are some seismic peaks and silica/tantala beat peaks that almost (but don't quite) match up in frequency, and I wonder whether this is an artifact of the interpolation.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1496   Tue Sep 2 11:06:54 2014 EvanDailyProgressBEATNo beat

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

  1497   Tue Sep 2 15:04:05 2014 EvanDailyProgressBEATNo beat

Quote:

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

On Koji's suggestion, I set up a second 1811 to monitor the beat on the input side of the cavities, so that we can see what the lasers are doing independent of the cavity resonances. For each path, I am using the s-polarized light that is rejected from a PBS, so that we don't need to add extra optics to the beam paths.

For example, for south slow at 1.206 V and north slow at 5.565 V, I get a 69 MHz beat (with both cavities unlocked).

We should use this in conjunction with the cavity locking optics to figure out what the correct axial cavity modes are, and whether the cavities need any temperature adjustment.

  1498   Tue Sep 2 17:11:27 2014 EvanDailyProgressBEATNo beat

Quote:

Quote:

Searched around over various axial modes in order to find a beat.

I fiddled a bit with the output QWPs in order to get the polarizations to match. Because of the birefringent coatings, light transmitted through the cavity is not circular, and the polarization state will depend on which of the two modes we lock to. In case, the original QWP angles were 202° for north and 19° for south.

On Koji's suggestion, I set up a second 1811 to monitor the beat on the input side of the cavities, so that we can see what the lasers are doing independent of the cavity resonances. For each path, I am using the s-polarized light that is rejected from a PBS, so that we don't need to add extra optics to the beam paths.

For example, for south slow at 1.206 V and north slow at 5.565 V, I get a 69 MHz beat (with both cavities unlocked).

We should use this in conjunction with the cavity locking optics to figure out what the correct axial cavity modes are, and whether the cavities need any temperature adjustment.

With this setup, I find that a sub-100-MHz beat occurs for north slow at 5.015 V and south slow at 0.725 V. This south slow voltage corresponds to a south cavity TEM00 mode, but the nearest north slow voltage is at 5.298 V.

  1501   Thu Sep 4 11:47:42 2014 EvanDailyProgressNoiseBudgetAlGaAs python noise budget: photothermal TFs added

The first attachment shows the photothermal TFs which take absorbed power (in watts) to the mirror displacement (in meters) as sensed by our 215-µm beam. Since last night, I've fixed the coating TE part and committed the updated ipynb to the SVN.

The second attachment shows the noise budget, with the photothermal shot noise contribution.

Attachment 1: photothermalTF.pdf
photothermalTF.pdf
Attachment 2: noiseBudget.pdf
noiseBudget.pdf
  1504   Sun Sep 7 11:54:21 2014 EvanDailyProgressFSSTTFSS OLTFs, residual frequency noise

Acutally it does look like it's a 50 Ω loading issue. I find that when 50 Ω inline terminators are added to OUT1 and OUT2, the measured OLTF is reduced by a factor of 1.6. This explains the discrepancy between the SR785 and HP4395 measurements. I've attached the corrected OLTF plots, along with plots of a vector fit, and the expected residual frequency noise [assuming a free-running NPRO noise of 104 Hz/Hz1/2 × (1 Hz / f)].

South UGF is at 200 kHz with almost no phase margin. We need to fix this.

Attachment 1: pdhTFs.pdf
pdhTFs.pdf
Attachment 2: oltfNorth.pdf
oltfNorth.pdf
Attachment 3: oltfSouth.pdf
oltfSouth.pdf
Attachment 4: frnoise.pdf
frnoise.pdf
Attachment 5: noiseBudget.pdf
noiseBudget.pdf
  1508   Mon Sep 8 12:41:32 2014 EvanDailyProgressTempCtrlChamber temp change

The beat is currently at 450 kHz. So I've changed the setpoint on the chamber temperature from 31.200 °C to 31.300 °C. We'll see if this pushes the beat to a higher frequency.

Edit: actually I decided to leave the chamber temperature alone and instead adjust the north cavity heater again. With a DVM, I measured the initial heater voltage as 10.64 V. I turned the power supply knob ever so slightly to get 10.60 V instead.

  1509   Tue Sep 9 12:22:31 2014 EvanDailyProgressBEATReduced south scatter shelf

Last night, I looked at the TTFSS OUT2 on the spectrum analyzer, with the cavity unlocked and the laser PZT and broadband EOM unplugged. I believe this should give the demodulated spectrum of the RAM.

For north, the spectrum was white. For south, the spectrum was white down to 10 Hz, and below 10 Hz there was an apparent scattering shelf that rose several orders of magnitude above the white spectrum.

Tara and I went through the south path and placed an OD0.3 ND filter between various optics. We tried twisting one of the mode-matching lenses to reduce the scatter shelf, but it didn't seem to work.

This morning I went through the south path again with an OD0.5 filter, and eventually focused on the lens right after the FI. I found that I could greatly reduce the height of the scattering shelf (relative to the height of the white noise) by placing the filter downstream of lens, but not upstream of it. So I twisted this lens slightly, re-modematched into the cavity (minimal adjustment was required), and I found that the scatter shelf was reduced to less than 1 order of magnitude above the white noise.

Then I took a beat spectrum (carrier at 11.5 MHz, seems stable). The scatter shelf in the beat is not reduced by much, so I'll have to think about where to look next.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1511   Thu Sep 11 00:47:14 2014 EvanDailyProgressopticSouth photothermal TF

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.

Attachment 1: spt.pdf
spt.pdf
Attachment 2: southPT.zip
Attachment 3: photothermalTF.pdf
photothermalTF.pdf
  1512   Thu Sep 11 11:40:41 2014 EvanDailyProgressopticSouth photothermal TF

Quote:

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.

 

Attachment 1: spt.pdf
spt.pdf
Attachment 2: southPT.zip
Attachment 3: photothermalTF.pdf
photothermalTF.pdf
  1514   Sun Sep 14 10:37:57 2014 EvanDailyProgressBEATNew beat

Quote:

We turned on both ISS loops today.

Here is an in-loop characterization of the south RIN with and without the ISS.

Here is the beat measurement from Thursday, with the ISS loops on and the table floated.

I've fudged the photothermal noise slightly by just using twice the south cavity's PT measurement, rather than south and north. I need to take RIN data from north, and then I can add south + north PT.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1515   Sun Sep 14 14:46:44 2014 EvanDailyProgressBEATReinstalled aux 1811

I reinstalled the auxiliary 1811 on the input side of the table.

I tried to get the free-running noise of the north cavity by locking south and then using the PLL to read out the beat from the 1811. However, even on a FM deviation of 400 kHz / Vrms, I could not get the PLL to lock. So I suspect the free-running noise is just too high to use this method.

I also used this aux 1811 to optimize the PDH EOM alignments. I saw no change in the beat spectrum after doing this. I would like to demodulate the 1811 signal using the PDH LO, but this will require some reconfiguration of the RF distribution.

  1518   Mon Sep 15 07:18:38 2014 EvanNotesBEATnote for tonight beat

Quote:

 RCAV transPD_DC :0.54 V

ACAV transPD_DC: 0.16 V (loop might oscillate when DC level was measured, need to double check)

 In this noise budget I corrected the PDH shot noise level (incident power is 1 mW, visibility is 0.5).

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1519   Mon Sep 15 18:29:47 2014 EvanDailyProgressBEATAttempts at scatter reduction

I went through the table today looking for ghost beams. Most were already dumped. For those that weren't, I put down a dump or an iris.

I again looked at TTFSS OUT2 with the cavities unlocked (i.e., the open-loop error signals) and found that the low-frequency seismic/scatter wall appears only on south. So I hunted around south for a while. I found a series of ghost beams reflecting off the EOAM input and hitting dangerously close to the EOM output aperture. So I moved the EOAM forward a few inches, then adjusted its kinematic mount to offset these beams a bit. The EOAM should be realigned, and we should check to make sure the ghost beams are not entering the EOM again.

With the increased space between the EOM and EOAM, I installed a flipper mirror that takes the beam to the 1811. Then I minimized the RAM (from –54 dBm to –72 dBm with 85 mV dc).

FM dev: 10 kHz

Averages: 10, 50, 100, 500

  1521   Tue Sep 16 15:08:57 2014 EvanDailyProgressBEATAttempts at scatter reduction

Quote:

I went through the table today looking for ghost beams. Most were already dumped. For those that weren't, I put down a dump or an iris.

I again looked at TTFSS OUT2 with the cavities unlocked (i.e., the open-loop error signals) and found that the low-frequency seismic/scatter wall appears only on south. So I hunted around south for a while. I found a series of ghost beams reflecting off the EOAM input and hitting dangerously close to the EOM output aperture. So I moved the EOAM forward a few inches, then adjusted its kinematic mount to offset these beams a bit. The EOAM should be realigned, and we should check to make sure the ghost beams are not entering the EOM again.

With the increased space between the EOM and EOAM, I installed a flipper mirror that takes the beam to the 1811. Then I minimized the RAM (from –54 dBm to –72 dBm with 85 mV dc).

FM dev: 10 kHz

Averages: 10, 50, 100, 500

I added a flipper mirror before the north EOAM, as well as a HWP before the resonant EOM (so that we can independently control the polarization entering the two EOMs). I optimized the RAM, but saw no improvement in the beat.

  1522   Wed Sep 17 17:47:39 2014 EvanDailyProgressFSSPDH block diagram

Since the straightforward tabletop optimizations (mode-matching, RAM minimization) have not been able to make the high-frequency excess beat noise disappear, perhaps it is time to undertake a more systematic study of the PDH loop noise and add these traces to the noise budget.

Here's my interpretation of the PDH block diagram for one of the two cavities.

Attachment 1: ctnBlock.pdf
ctnBlock.pdf
  1523   Thu Sep 18 13:57:00 2014 EvanNotesISSEOAM phenomenology

I've been unsure of how the EOAMs are affecting the state of the light impinging on the cavity.

So far we've been rotating the post-EOAM QWPs so as to maximize the strength of the amplitude modulation. I'm still not sure what this does. I'd like to instead fix the QWP at ±45° and then insert the EOAM, regardless of whether this introduces a DC power offset. At the very least this will give us a polarization state that we think we understand.

Attachment 1: eoam.pdf
eoam.pdf eoam.pdf
  1524   Thu Sep 18 22:13:35 2014 EvanDailyProgressRFAMRAM coherence

I used the auxiliary 1811 as an out-of-loop RAM monitor. The RF from the 1811 is mixed with the PDH LO, and then low-passed at 1.9 MHz.

I'm not sure about the RAM calibration here. I took the raw spectrum (in V/rtHz), multiplied by 10^(4/20) (assuming 4 dB conversion loss in the mixer), then divided by the measured dc voltage (about 20 mV), then divided by 40 (because of the different dc/ac tranimpedances).

Anyway, the point is that the 200 Hz hump we see in the beat seems to be from the north RAM.

Attachment 1: ramCoherence.zip
Attachment 2: ramCoherence.pdf
ramCoherence.pdf
  1525   Fri Sep 19 12:01:13 2014 EvanDailyProgressEnvironmentNorth EOM heater

I added a 48 Ω kapton heater to the north resonant EOM. It's got 40 mA going through it right now; no loop yet.

  1526   Tue Sep 23 18:40:08 2014 EvanDailyProgressEOMNo more 2 kHz hump in beat

Background

Yesterday I think I narrowed down the source of the 2 kHz frequency noise hump: it is voltage noise from the TTFSS being injected into the broadband EOM.

With the north cavity unlocked (and the TTFSS set to "test"), I monitored the (undemodulated) RAM using the auxiliary 1811 and the HP4395A. There were clear, broad 600 Hz humps on either side of the 14.75 MHz carrier. It disappeared when I unplugged the drive to the broadband EOM.

Then I looked at various test points on the TTFSS HV board with the SR785. On the COM → EOM path, the TF shaping takes the COM noise and produces (what I think is) the same 600 Hz bump, which is then sent to the EOM. In the beat, the bump appears at 2 kHz because of the north TTFSS boost; with the boost off, it reverts to 600 Hz.

This is the case on both TTFSS boards, but it only leaked into the beat on the north cavity. So I suspected it was an issue with how the EOMs are aligned on the north path. On north, the BB EOM was immediately followed by the resonant PDH EOM; on south, between the BB EOM and PDH EOM there is a PMC, an FI, and some other optics.

Today's work

I moved the resonant EOM so that it follows the EOAM. After the post-EOAM PBS, I did the following:

  • I set down a HWP, and then used a temporary PBS to ensure s-polarization of the beam.
  • A few inches after the first HWP, I set down a second HWP and used a temporary PBS to ensure p-polarization of the beam.
  • Between the HWPs, I placed the resonant EOM, screwed it down, and then aligned the beam through it.

Then I redid the mode-matching into the north cavity and measured the beat. I kept it locked for about 90 minutes and didn't see the 2 kHz hump appear, so I'm guessing this solved the issue.

To do

  • Minimize RAM on north cavity
  • RIN data is stale and needs to be retaken
  • Need to fix a nominal operating power for beat PD (I pick 7 dBm, because we're using a ZRPD-1 phase detector)
  • Marconi noise data is stale and needs to be retaken
  • PLL readout data is stale and needs to be retaken
  • Seismic data is stale and needs to be retaken
Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1527   Thu Sep 25 16:50:09 2014 EvanDailyProgressFSSShould probably move south BB EOM

We've noticed for a while now that we cannot turn up the gain on the south TTFSS as high as on the north TTFSS, despite having similar optical power levels, similar mode-matching, etc. (See the OLTFs in ctn:1504.) The north gain can be set to 900/900 on the common/fast knobpots, but on south it's more like 600/600.

Because the BB EOM is placed before the PMC, I suspect the cavity pole of the PMC (1.8 MHz, measured in elog: in 2010) is giving us extra phase which prevents us from turning the loop gain up higher. Indeed, when I remove the PMC from the south optical path (and realign into the south cavity) I find I can turn the south TTFSS knobpots up to 800/800. A new OLTF is probably in order.

The easy thing to do for now is to leave the PMC out. The better thing is probably to move the BB EOM to come after the PMC. Since there's no room, this probably means putting the BB EOM where the resonant EOM currently is, put the resonant EOM where the EOAM currently is, and then put the EOAM elsewhere. The EOAM could just as well come before the PMC, since we're only attempting intensity stabilization well below the PMC cavity pole.

  1528   Mon Sep 29 23:28:37 2014 EvanDailyProgressPMCRe-inserted PMC, moved BB EOM

I swept the south laser with a triangle wave and optimized the mode-matching as best I could using the periscope mirrors and the translation stages. I got to a visibility of 0.3, which stinks (the maximum is 0.7 with these birefringent coatings).

I took a beat spectrum (attached) and noticed that the noise around 0.1–1 kHz is improved. Indeed, by reducing the visibility south I find the beat gets worse.

I decided some more involved mode-matching (involving beam profiling and alm simulation) is needed.

Before setting up the beam profiler, I noticed that the beam entering the cavity does not appear Gaussian, as seen on an IR card.

The laser beam entering the first Faraday isolator appears to be 1–2 mm too low. It is clipping on the input aperture, and the transmitted beam looks like crap.

Neither the Faraday nor the laser itself have any alignment adjustment knobs. I therefore had to choose between two evils: shim up the laser mount (and thereby risk having to realign the entire optical path, as well as possibly reducing the heatsinking of the mount to the table) or reinsert the PMC and move the BB EOM.

I opted for the latter: I reinserted the PMC, removed the EOAM (+QWP+PBS), placed the resonant EOM where the EOAM used to be, and then placed the BB EOM where the resonant EOM used to be.

I will optimize the alignment through these components, check the polarization, and then take a new beat.

Attachment 1: noiseBudget.pdf
noiseBudget.pdf
  1529   Tue Sep 30 21:45:14 2014 EvanDailyProgressPMCRe-inserted PMC, moved BB EOM

Quote:

I will optimize the alignment through these components, check the polarization, and then take a new beat.

I found that the beat above 100 Hz was about 10 times worse than before, with a 2 kHz hump similar to what I saw on the north cavity before I separated its BB and resonant EOMs.

I suspect this is some kind of effect involving light bouncing back and forth several times between the two EOMs.

To remedy this, I took out the post-PMC Faraday isolator and put the BB EOM in its place. This gives a longer path length between the EOMs. Then I realigned through the EOMs and took a beat. Now I've recovered the 0.03–0.05 Hz/rtHz level that I had yesterday morning.

I turned up the incident powers on the cavities to 3 mW, and then optimized the mode-matching. However, I do not seem to be able to push down the beat any further. So perhaps it is now limited by something else.

I tried reinserting the Faraday isolator between the two EOMs, but could not place it in such a way to get the beam to transmit through. Since it's not an essential component, I think I'm going to leave it out for the time being rather than undertake a huge realignment marathon.

  1530   Fri Oct 3 12:32:00 2014 EvanDailyProgressFSSSouth TTFSS input-referred noise

Yesterday I took some TFs and noise spectra on the south TTFSS with the loop open and the beam blocked. Relevant information:

  • Gains were 800 common, 800 fast
  • Excitations were injected into COM EXC
  • The relevant test points I monitored were COM TP 4 (henceforth "com"), FAST OUT 2 (henceforth "fast"), and HV TP 4 (henceforth "HV").
  • At each test point I took a TF with the excitation on, and a noise spectrum with the excitation off.
  • I also took a TF from COM EXC to COM OUT 2, so that I can use the known gain of the first amplifier (−4 V/V) to refer everything to COM OUT 1.

Then using the noise spectra, I divided by the relevant TF to arrive at an input noise referred to COM OUT 1.

The results are attached. The low/high frequency upswings on the HV trace are due to the input noise of the SR785.

Since the common, fast, and HV traces all lie on top of each other, I interpret this to mean that the noise of all of them is dominated by sources occurring upstream of COM TP 4. So the TTFSS is limited by the noise of its input stages, with a spectrum (at COM OUT 1) of 25(5) nV/rtHz.

I also measured the slope of the south PDH error signal (with 3 mW incident, and with fresh mode-matching), and found a slope of 8.4 V/MHz, as measured at COM OUT 1. This gives a frequency noise of 3.0(6) mHz/rtHz, which is well below the current beat level.

Attachment 1: inputNoise.pdf
inputNoise.pdf
Attachment 2: southInputNoise.zip
  1531   Sun Nov 9 18:54:15 2014 EvanSummaryDocumentationHow to run the CTN experiment

A manual for running the CTN experiment is attached. I'll update and expand as needed.

Attachment 1: Manual.pdf
Manual.pdf
  1532   Sun Nov 16 23:32:49 2014 EvanDailyProgressopticNorth photothermal TF

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.

 

  1533   Wed Dec 10 23:59:54 2014 EvanNotesNoiseBudgetThe sense in which phi_c is a "coating loss angle"

I thought I had posted this several months ago, but I cannot find it now.

I believe this document explains how the "coating loss angle" ϕ_c (as measured in an optical experiment) is related to the true mechanical loss angles of the coating materials (as measured by a ringdown). In general they aren't the same, if I'm understanding the formalisms of Nakgawa and Hong correctly.

Attachment 1: hongbreakout.pdf
hongbreakout.pdf hongbreakout.pdf
  1546   Fri Apr 10 09:33:43 2015 EvanSummaryComputersAcromag ADC set up
Quote:

I set up an Acromag slow controls based on the procedure that Keith wrote in T1400200. It's really pretty easy. It took an hour and 15 minutes from installing Ubuntu on a machine to having a functioning ADC channel from the Acromag unit. I haven't yet set up a DAC unit - this will require some tweaking of some of the EPICS parameters. Once I've done that I'll upload a complete procedure to the Wiki.

This is relatively promising for supporting/replacing VME slow channels.

yesyes

  1550   Sat Jun 20 10:14:50 2015 EvanNotesopticcoating optimization for AlGaAs:electric field in coating layer

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.

Attachment 1: Efieldtrans.pdf
Efieldtrans.pdf
  1551   Sat Jun 20 22:59:58 2015 EvanDailyProgressopticSouth Faraday isolator path reworked

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.

Attachment 1: southfi.jpg
southfi.jpg
  1552   Tue Jun 23 20:37:22 2015 EvanDailyProgressopticPointing recovered

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.

  1553   Wed Jun 24 16:19:37 2015 EvanDailyProgressopticLocking recovered

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.

ELOG V3.1.3-