Yesterday the DC transmission (ISS photodiode, North cavity) channel to acromag has been added and the viewer has been set on the Ubuntu machine. The channel has been also calibrated (against voltage externally injected). We are going to add other channels from the VME crates and work on the PDH board interface in order to remote control this unit.

Additionally the floor has been sweeped and mopped.

I cleaned the table with the computers and removed one of the monitors. I installed the Acromag units to the rack, powered them up and got them onto the network. They are:

XT1221 unit: 10.0.0.42

XT1521 unit: 10.0.0.41

I've pinged them successfully from other machines. I have an Ubuntu machine that I borrowed from the TCS lab to interface to them. I'll set up the EPICS control on Monday. In addition to adding the DC transmission channels to Acromag, we should be able to start migrating the PID controls away from the VME crates to these new units. 

Both lasers have been locked to the cavities for 24 hours. The slow control of the frequency is handed off to the PID loop. Antonio and I observed strange behaviour on the DC value of the cavity transmission.

As Evan had noted before, there are two polarizations that will resonate and they're about 3MHz apart (if I remember correctly). We can see these on the DC photodiodes on transmission (the ISS PD and the RF DC output). One peak is large and the other much smaller. However, when we have large transmission onto the RF photodiode we have small transmission onto the ISS PD and vice versa. It's likely we have a pick-off optic with strong polarization selectivity.

We couldn't find the beat yesterday or Thursday.

I had Facilities come and replace the dead light tube in the CTN lab antechamber. It's nice and bright in there right now.

We've noticed that the floor is pretty dusty - so we're implementing twice weekly mopping sessions starting tomorrow.

Saturday: 4 AM - 12 PM

Some necessities for implementing dual-quadrature RFAM suppression:

• Beam pickoffs:
  • Need to be in the forward-going direction of each beam.
  • Pickoff should be about 10% of incident power. With ~1 mW incident on each cavity, we then get about 100 µW on each RFAM monitor.
• Monitor PD:
  • Could be NF1811
  • Could be some in-house PD
• Electronics:
  • Need I&Q demodulator
• Voltage feedback:
  • Need to sum in dc/audio path into EOM drives along with rf

Compare with previous todo list.

To-do list (short term):

• Check health of PDH loops:
  • Check centering on RFPDs
  • Check slope and balancing of error signal
• Turn on chamber heating
• Re-establish beat
• Re-insert south EOAM

To-do list (medium term):

• Replace PBS/QWP reflection locking with Faraday isolators
  • Need two Faraday isolators with large apertures
  • Need four HWPs: one on each cavity input, one on each cavity output
• Fix power supply situation [Aidan already doing this]
• RFAM mitigation
• PMCs

To-do list (long term)

• Install crystal oscillators, retune RFPDs, retune EOMs
• Acromag

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.

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 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.

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'm practicing the procedure of aligning the PMC. The input beam isn't great (I might be clipping a little on the Faraday). I'm mostly trying to get the hang of aligning into the cavity again as it's been a while.

My technique, so far, is to adjust the alignment, in yaw, of the third mirror before the cavity and then sweep the final mirror before the cavity slowly in yaw. I'm looking for flashes of transmission on the trans-camera. The goal is to keep tweaking until the order of the transitted mode is reduced to a few or zero. I should probably be sweeping the PZT at the same time.

I'm going to adjust the alignment through the FI tomorrow - just as soon as I get my hands on a couple more mirrors.

https://nodus.ligo.caltech.edu:30889/ATFWiki/doku.php?id=main:resources:computing:acromag

Aidan.

Success!

I configured the Acromag XT1541 DAC to run with EPICS. This was a touch trickier than the ADC as there is a subtlety with the channel configuration in the EPICS database. The bottom line is that now I can change the value in an EPICS channel and a multimeter attached to the unit will show a corresponding change in voltage.

The attached files (ioctest2.cmd and IOCTEST2.db) are used to access the first output channel, OUT00, on the unit. Now that I've got the thing working I can debug the calibration. Once that's sorted I'll summarize the set-up procedure on a Wiki page with glorious detail for future reference.

The following command line is used to open the modbus EPICS server.

${EPICS_MODULES}/modbus/bin/${EPICS_HOST_ARCH}/modbusApp ioctest2.cmd

The ioctest1 files are for the ADC unit.

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.

Recall the horrible aftermath of using the bow-tie configuration for the green laser ALS setup at the 40m:

All of the supposedly HR mirrors leaked through enough light to make the table into a Christmas tree. The bow-tie should only be allowed when using the super HR mirrors from G&H which have T < 100 ppm over a wide range of angles.

I've been trying to lock the laser to the PMC since we adjusted the Faraday. It's basically badly out of alignment now. I can only see very higher order modes flash when I scan the cavity.

The problem, currently, is that the Faraday is too high and we don't have enough mirrors to control the beam going through it. I'm going to install a second mirror in a bow-tie configuration tomorrow and realign the beam through the Faraday, 21.5MHz EOM and, hopefully, the PMC.

I also spent a lot of today tracing out the control loop for the laser slow control. Once I have all the control loops understood, I'm going to draw a diagram for the Wiki.

P.S. Found out that the PSL PMC Servo board is D980352. I've updated the Electronics page on the Wiki to indicate this...

Aidan

Aidan rebooted the Sun machine and VME. It took a while to get the EPICS channels to work again. The following seemed to work:

1. Reboot Sun machine.
2. Reboot the VME crate by depressing the reboot button on the top of the crate.
3. Log into VME (psl1) at 10.0.0.2 from the Sun machine.
4. Check the existence of various channels with dbpr "C3:PSL-RCAV_RCPID_SETPOINT"
5. On the Sun, cd to /usr1/epics/psl/scripts and run "perl rcav_PID_2012_06_15.pl"
6. Confirmed that PID values started updating on the Sun screen.

Aidan also added a sitemap (~/sitemap.adl), see attached image, for the CTN lab. Aidan added an alias to /home/controls/.bashrc

Aidan

The alias is:

alias sitemap="medm -x /home/controls/sitemap.adl"

Aidan

I've started removing a lot of the miscellaneous hardware from the lab (old pieces of Bosch framing, sheets of acrylic/plastic, etc). Some has gone into the ATF - we'll have to decide whether to keep it permanently or not. Right now, like Indiana Jones and the Last Crusade, I'm trying to see if there truly exists some space in this lab - or if it is a myth propagated though the ages. 

The Windows XP workstation in the lab died last week. After booting up to the "Windows XP" screen it reported a hardware problem. The subsequent reboot got to the BiOS and reported a corrupted memory problem. I'm going to pull the hard drive and replace the computer.

I ordered the following Acromag units for a new slow controls test setup. The idea is to replace the Sun workstation running the VME slow controls.

• XT1221-000: Modbus 8 channel differential analog input
• XT1541-000: Modbus 8 channel differential analog output
• PS5R-SC24: 24VDC power supply 

Keith Thorne has already used some code to interface to these controls at the site:

I spent this afternoon tracing out all the connections to and from all the chassis in the CTN lab.

We currently have 19 power supplies in use. Take a moment and think about that.

On the south path, we have placed a HWP so that the transmitted beams can have their polarizations matched. It is on a 1" post and held down with a fork.

In the longer term, this should probably be replaced with the solid metal blocks that were used to hold the QWPs. If these blocks are reinstalled, the waveplate mount should be twisted slightly in yaw in order to reduce the amount of backscatter into the cavities.

Evan, Kate

We made a few minor modifications to the optical breadboard in transmission of the cavities.

For one, we removed the QWPs which were the first optics in the transmission paths. These had been necessary for the prior cavities where the Silica Tantala mirror coatings were not birefringent. The circular polarization which was transmitted needed to be turned into linear polarization to get the beat note on the PD. Now, because the cavities with AlGaAs coatings are birefringent, the resonant and transmitted light is already linearly polarized and the QWPs unnecessary. Before removing them, the power on the main readout PD, a PD1811, was 208 mV. Afterwards, it was 194 mV. 

Second, we started to set up the fiber coupler to send some light to the ATF lab where it will later be used in a PLL to stabilize the laser used for the seismometer sensing. There's a 29.5 uW pick-off of the North cavity transmitted light which had been dumped. I found a high reflector mirror to put in its place to direct light to the fiber. I also made sure the fiber coupler at the other end is secured to the table and the output dumped. A first attempt to couple the light did not work, but I need to find a way either to monito remotely the power transmitted or just temporarily feed the fiber back into the CTN lab. 

Some tasks not included on the list:

• Temperature loops
  • Cavities
  • Can
  • PMC
• Noise candidates:
  • Scatter
  • RFAM
  • PDH loop noise

When Aidan and I turned on the south laser today, we found that the transmitted beam out of this Faraday was entirely crap. It was blindingly obvious on an IR card, and only 50 uW was making it to the input of the PMC. The rest was scattering at wide angles at the Faraday output port.

It is not clear to me how the pointing through the Faraday could have deteriorated, since it is on a solid metal mount and is only 10 cm from the output of the laser.

At any rate, I was able to "recover" the previous performance (i.e., crappy but workable) by placing the Faraday isolator slightly further down in the optical path. Before, the layout was:

Laser -> QWP -> HWP -> Faraday -> lens -> HWP -> steering mirror -> PMC EOM,

and the HWP angles were -1 deg and 167 deg, respectively.  Now the layout is

Laser -> QWP -> HWP -> lens -> steering mirror -> Faraday -> HWP -> PMC EOM,

and the HWP angles are 341 deg and 167 deg. The first HWP angle is chosen so that 20 mW is transmitted through the Faraday (the rest is dumped at the Faraday's various output ports). The second HWP angle is chosen to send s polarization through the PMC EOM. I then had to resteer through the PMC EOM and through the PMC. With 20 mW incident on the PMC, the transmission is 11 mW. Not great, but about the same as the previous situation.

I remark that the south optical path between the laser and the PMC should be reworked as soon as is feasible, because what I've done is a hack job to keep things moving. Either the Faraday mount needs to be remachined, or the optical path needs to be redesigned to allow for proper steering through the Faraday. Additionally, the table surface next to the laser mounts is noticeably warm to the touch, so I do not recommend trying to shim up the laser (as it may negatively impact the heatsinking).

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.

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.

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

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.

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.

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.

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.

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

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.

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.

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.

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.

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.

 We measured the transmission of the vacuum windows. The total transmission through two windows is 0.975 +/- 0.002.

•   This measurement is for checking how much the power is transmitted through the vacuum windows. It can be used for calibration of photo thermal measurement for better accuracy of absorption estimation.
•  The beam is incident normal to the two windows. The power before the vac tank and after the vac tank were measured. The incident level was set ~ 4 mW. I used a Thorlab power meter with ND filter off to measured the power.
•  

If we assume that both windows have the same transmission, the transmission for each window will be 0.988.

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

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

 RCAV transPD_DC :0.54 V

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

Perl scripts for controlling the vacuum tank and slow feedback to the two lasers are acting weird. Usually we can run three scripts simultaneously, but I just notice that only two can be run at the same time. When I restarted the script for acav feedback, the vac temp control stopped. When I restarted the vac temp control, acav slow feedback stopped.  I'll check this later.

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.

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.

We turned on both ISS loops today.

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

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 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.