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ID Date Author Type Category Subject
1831   Mon Nov 25 16:43:50 2019 DuoDailyProgress Time domain projection is consistent with LLO data (except LL channel, which saturates)

We see a report of noisemon problem from LLO: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=49892

The time domain data is projected with the transfer function measured here: https://alog.ligo-la.caltech.edu/aLOG/index.php?callRep=43212

We projected the output of the noisemon signal on time domain (attachment 1).

Attachment 2 is data from LLO posted in the alog above. Compare this with attachment 1, we can see our projection has roughly the same values with the other three channels. This means those channels have high number of counts because the drive signal is bigger. In other words, the other three channels should be working ok.

I attached the drive signal being projected in attachment 3.

Attachment 4 has all the data and code.

1832   Wed Nov 27 23:45:29 2019 DuoDailyProgress Noisemon Debug Progress

I found that two of the channels misbehaves after the board runs for a couple hours. Turning the power off and back on returns the circuit to normal functionality.

I sent 100 counts amplitude 10Hz sine into the board.

• I saw channel 4 has some bad behavior - about 3V of offset with a lot of noise and one spike goes up to saturation.

Then I switched power off and back on. It worked normally. I increase amplitude to 2500, same as Carl's 10000. It worked normally as well. However, when I came back after a couple hours,

• I saw two channels (1&4) saturates about 4 times a second. The other two still ok. (attachment 1)

Then I turned power off and on again. I got attachment 2 - normal behavior again.

1833   Mon Dec 2 15:28:03 2019 DuoMisc S1900296 board damaged

I think I shorted somewhere near the RTN testpoint on the board today while testing it. I saw some sparks. After that the board becomes non-responsive - it is not responding to whatever signal I send in. I will use another board and go on with testing.

1834   Tue Dec 3 09:50:18 2019 DuoDailyProgress Interesting observation on noisemon debugging

We have observed that the noisemon bug happens after it is powered on for about 1.5 hour. Noisemon has been powered on overnight and the following morning I came in and found

- Channel 1, 3, 4 bad (with signals like attachment 1. Green: normal behavior driven with 250 count, 10Hz signal. Brown: abnormal behavior with no drive)

- Then I used the oscilliscope and did the following on channel 4:

1. Connected channel 1 to measure the voltage between one side of C2 and GND.

2. Connected channel 2 to measure the voltage between another side of C2 and GND.

3. Use MATH on the oscilliscope to measure the voltage difference

- Then I found channel 4 is good! I did not turn the power off or do anything else.

- I repeated exactly the same procedure on channel 3 and it is repeatable.

- I left channel 1 as comparison and made attachment 2.

- Then I just use the probe of oscilliscope to connect one side of C2 on channel 1 to GND and got attachment 3, which channel 1 is good again.

I think this is a very strong hint that this whole problem is due to C2 charging up.

1835   Wed Dec 4 19:00:37 2019 Noisemon board modifiedDailyProgress Noisemon board modifications

According to previous post 1834, we think the noisemon problem is very likely caused by C2 charging. Hence we did the following modifications on a noisemon board:

1. Split R2 into two 400 Ohm resistors, with grounding between them.

2. Split C2 into two 3.3uF capacitors, with grounding between them.

We hope these modifications will provide a path for the bias current to go to ground, instead of charging up C2.

1836   Thu Dec 5 12:37:58 2019 Noisemon board modifiedDailyProgress Noisemon board modifications

The modification is successful. The attachment shows the result after the board runs for 12 hours. We modified channel 1 and 2 so we see FM0 and FM1 channels are still decent. We only split C2 but not R2 for channel 3 and 4 so FM2 and FM3 are bad.

 Quote: According to previous post 1834, we think the noisemon problem is very likely caused by C2 charging. Hence we did the following modifications on a noisemon board: 1. Split R2 into two 400 Ohm resistors, with grounding between them. 2. Split C2 into two 3.3uF capacitors, with grounding between them. We hope these modifications will provide a path for the bias current to go to ground, instead of charging up C2.

1839   Mon Dec 9 22:06:19 2019 DuoDailyProgress Two boards tested

I finished testing S1900294 and S1900297 and plan to ship them to the sites.

I got stuck at a couple things. I think it is good to make a note of these stuff.

1. Diaggui gives "unable to start excitation".

Solution: restart diaggui

2. Drive signal does not go in

Solution: check the status of the digital system - it crashed and needs restart.

3. FASTIMON gives too much noise that the transfer function looks like junk.

Solution: I use 50ohm resistors to replace the coils and 500 counts noise to measure the transfer function. If the resistance is large and the input signal is small, the current will be too small.

1840   Thu Dec 12 16:48:39 2019 DuoDailyProgress Modifying the board changes the transfer function

The noisemon board has been modified according to 1835. We do not expect any change in the transfer function but we see an increase in the gain above 20Hz.

Attachment 1 shows the comparison between modified board transfer function and the original board transfer function.

Attachment 3 shows the difference of the transfer functions in attachment 1. There is about 8-10dB increase at 100Hz after the modifications.

Attachment 4 is a comparison of LISO simulations. I calculated the transfer functions of the whole noisemon circuits before and after the modifications. I substracted them and found the difference.

Without putting attachment 3 and 4 in the same picture we can see that they are very different. LISO basically says there will not be any significant change in the transfer function but actually our measurement shows that there is.

1841   Mon Jan 13 16:48:52 2020 DuoDailyProgress Noisemon updates

I did some documentation work these days.

- Noisemon Test Plan: https://dcc.ligo.org/LIGO-E2000007

- Updated DCC, new schematics, PCB layout and BOM (due to changes logged in 1835)

- Two more boards are modified and will be tested.

1842   Tue Jan 14 17:07:49 2020 DuoDailyProgress Testing another two boards

I am testing another two boards to be shipped to Hanford today. I found and fixed some bad soldering. However, the frontend crashed and I needed to restart it. I could not log into cymac somehow. It says "no route to host" but the internet is still working. I cannot go on testing anymore. Let's wait and try again tomorrow.

Chris fixed the problem - I went on finishing the tests and the boards are ready to go now. They will be shipped tomorrow.

1843   Tue Jan 21 16:13:03 2020 DuoDailyProgress Distortion of noisemon

We have sent version 2 noisemon boards (modifications from version 1 noted in 1835) to Livingston and Hanford. Chris noticed that there might be some upconversion problems under 20Hz (attachment 1 and 2). These plots from Chris are noisemon output with drive subtracted. Attachment 1 is the spectrum after replacement of noisemon board (version 2 board used), compared to attachment 2 which is before the replacement (version 1 board used). There is something going on near 15Hz. We think it might be due to upconversions under 15Hz.

We still have a spare board here. I modified it, tested it and looked at this issue, trying to reproduce. I sent a 10Hz 100 count amplitude signal to the board and compared it to the output with no input. This produces attachment 3. We see that once the 10Hz sine signal is sent, there are  lines above 10Hz, which is a sign of distortion.

I changed the input frequency to 5Hz and got attachment 4.

Attachment 5 is a linear plot to see where exactly those lines are. It seems they are indeed harmonics of 5Hz when the input signal is 5Hz.

I also tried higher frequencies up to 100Hz and saw similar harmonics.

1847   Thu Feb 13 00:25:36 2020 DuoDailyProgress Noisemon at L1

I calculated the DAC noise for L1. Attachment 1 has all the plots and data. Attachment 2 is the result in strain.

We have noisemon at all four stations: ITMX, ITMY, ETMX, ETMY. DTT gives me the CSD, the coherences and the ASD of all the channels at all the four stations. I use this to calculate the transfer functions of the coil driver and the noisemon.

$H(f) = CSD_{ab}(f)/ D^2(f)$

where D(f) is the drive and CSD is the CSD between the drive and the output of noisemon. The absolute value of H(f) will be the gain of the circuit, in ADC counts / DAC counts. Then I use the coherence to calculate the total noise

$N_{drive}(f)=D(f)\sqrt{1 - C_{ab}(f)}$

This noise has DAC noise, ADC noise, noisemon noise in it. This noise is in DAC counts. I will call this "DriveNoise".

Then I picked another time when the interferomter is not running and the drive is zero. I measured the noisemon spectrum N(f) at that time. The plots and data of these spectrum can be found in attachment 1. The plots are considered to be a result of the noisemon noise and ADC noise, which I will call "NoDriveNoise" (in ADC cts). Since the drive is zero, there is no DAC noise in it - just ADC noise and noisemon noise. I use the transfer function to covert it to DAC counts

$N_{noDrive}(f)=N(f)/|H(f)|$

Then I subtract the noise drive noise from the drive noise to get the DAC noise to get the DAC noise, which is then converted to DAC volts
$N_{DAC}(f) = \sqrt{N_{drive}^2(f)-N_{noDrive}^2(f)}\frac{20V}{2^{18}cts}$

Then I find the current on the coil using the transfer functions of the coil driver, assuming the coil driver is in LP OFF and ACQ OFF state. The transfer function can be found in attachment 1.

$I(f)=N_{DAC}(f)*H_{I}(f)$

where the transfer function H is a voltage-to-current transfer function.

Then we have the force

$F(f)=I(f) \times 0.0309N/A$

Lastly we have the displacement and strain

$h(f)=\frac{F(f) (10Hz/f)^43\times10^{-8}}{4000}\sqrt{Hz}$

For each station I summed all the four channels, LL, LR, UL, UR and then I calculated for all the four stations and summed as

$h_{tot}(f)=\sqrt{h_{ITMX}^2(f)+h_{ITMY}^2(f)+h_{ETMX}^2(f)+h_{ETMY}^2(f))}$

I tried to compare this with the GWINC model - it is much higher. I do not have real L1 noise at the moment. I will see once we have real noise data.

1883   Wed Dec 23 15:43:01 2020 PacoDailyProgress OPO cavity assembly

Laseroptik optics (4x pairs of cavity mirrors) arrived earlier this week, so I began assembling the input mirror with Noliac (NAC2124) PZT. The (15 mm OD) pzt will sit between a 1" post spacer and the mirror. I applied a thin layer of BT-120-50 (bondatherm) adhesive, which I found in EE shop. From what I gather this adhesive doesn't have softeners (almost doesn't smell) and is a good electrical insulator. The PZT + spacer is sitting below a metallic weight block on the left corner of the table (by the electronics test bench corner), and should finish hardening in a little over 24 hours at room temperature. The PZT was labeled 520 nF (spec. 510 nF).

1914   Wed Jul 7 13:05:07 2021 PacoDailyProgress1418 nm AUX ECDLNew aspheric flexures

Today we fired up the 1418 nm ECDL and attempted initial adjustment of the aspheric lens. The design follows D2100115 which is a copy of the 2 um ECDL so we just changed the diode, the grating flexure angle, and the aspheric + flexure assembly and we are good to go. Radhika removed the 1900 nm aspheric flexure and we mounted the new collimating assembly which uses a f=3.1 mm (NA = 0.69) lens. At the beginning we had to feed over 300 mA of current to be able to see a beam (which was still diverging) so we had to free the flexure completely and align by hand to find the nominal positioning for a collimated beam. We lost a 2-56 screw in the process, but the final assembly is still in progress. The plan to follow is:

• Finalize flexure alignment
• Insert grating
• Characterize ECDL emission
1915   Fri Jul 9 11:40:42 2021 PacoDailyProgress1418 nm AUX ECDLNew aspheric flexures

We tweaked the flexure alignment until we had a nominally collimated beam (~2 mW @ 250 mA of diode current) through the output aperture in the ECDL housing. We noted that the collimated beam is off-centered on that circular aperture along the horizontal (yaw) angle. After this, Radhika installed the ECDL grating and we hooked up the fiber output onto a InGaAs PD to monitor the power output. We tweaked the alignment of the grating (mostly yaw) to try and see a change in the power output to indicate optical gain in the diode, but saw no changes. We observed a change in the PD photocurrent as a function of the diode current in the absence of the grating (no optical feedback) which is indicative of ASE. We measured this level to be ~ 140 mV at 200 mA of current; with no observed threshold. In conclusion, we still need to refine our grating alignment to provide gain on the diode and observe lasing at the nominal 1450 nm wavelength.

1916   Wed Jul 14 12:21:08 2021 PacoDailyProgress1418 nm AUX ECDLECDL lases... and MZ locked mid-fringe

[Paco]

Worked for a few hours to get the aspheric properly aligned. The procedure is quite finnicky, as the four 2-56 flexure screws have too much game and the fine thread setscrew that adds tension is too constrained. Anyways, it generally goes like this:

1. With the grating out of the way, and the 2-56 screws slightly loose, move the aspheric flexure until a collimated beam (as round as possible) exits the centered round aperture in front of the SAF chip.
2. Very carefully tighten the fine threaded setscrew in place to register the aspheric alignment.
3. Check that the desired beam hasn't changed
4. Insert grating careful not to touch the aspheric flexure (again, the mechanical registration is not great!)
5. Manually rotate the grating while monitoring the power at the fiber output until non-ASE light appears. This is quite sensitive to alignment/angle and the better the mode is matched back into the fiber, the easier it is to find the right position.
6. Fix the grating flexure.
7. Slightly tweak the grating mirror knobs by hand to maximize said power (careful to avoid saturating PD)

After this, I installed a second amplified InGaAs detector, hooked up the unbalanced MZ beamsplitter output into the two PDs, adjusted the gains to equalize the output voltages and then hooked the two signals to the A and B inputs of an SR560 in "A-B" mode. The output (gain 1) was good enough to feed back in the HV PZT amplifier input modulation which allowed the MZ to lock mid-fringe. The lock is rough, as the balanced homodyne signal retains a tiny offset due to imperfect balancing... Attachment 1 shows the setup, including a typical scope trace after coarse current tuning (Ch1 and Ch2 in yellow and blue represent the photocurrents in the two MZ ports in the absence of feedback).

Indeed, scanning the nominal PZT voltage broke the lock, potentially after crossing a mode hopping region.

• Power characterization, including RIN, emission vs current.
• Emission spectrum characterization; using the homemade spectrograph (grating + lens + camera) as the PZT is scanned
• MZ feedback loop characterization, including optimizing the balancing stage
• Self-homodyne phase noise spectrum

Next, as was suggested during yesterday's group meeting, we will transition into a self-heterodyne setup (with an AOM which I have yet to check out in the QIL).

1917   Wed Aug 4 11:36:30 2021 RadhikaDailyProgress1418 nm AUX ECDL1419 nm ECDL with 2um AOM tests

In order to transition the ECDL laser noise characterization to a heterodyne setup, we needed to test the AOM (acousto-optic modulator). We wanted to drive the AOM at 80MHz using the Marconi signal generator. Since the AOM has a max driving power of 600 mW, we determined that if we run the Marconi at max output power (13dBm), we saturate the AOM through a variable attenuator and a 5W amplifier. The detailed setup is in Attachment 1.

As we scanned the AOM RF input power, we monitored the mean of the 0th and 1st order power outputs using 2 amplified photodiodes on the scope. Attachment 2 plots the results of the scan; although we noticed the 0th order dropping, we did not see evidence of diffraction in the 1st order. Our suspected theory is that the lost power from the 0th order is due to thermally-driven attenuation inside the AOM (we do not know what is inside the AOM, so this is purely speculative). The next thing we want to try is to add a DC power level to the AOM RF input, but we will double check with Aidan.

1918   Tue Aug 10 09:51:44 2021 PacoDailyProgress1418 nm AUX ECDL1419 nm ECDL AOM diffraction at 95 MHz

We changed the setup to use a low power amplifier rather than the 5W amp from last time. The updated schematic is in Attachment 2. This is in part because 5W is an overkill to drive a fiber AOM which is known to saturate at 0.6 mW of RF input, but also because working with lower power active elements is easier and considerably safer. We dropped the 5W amp. in Rana's office last Friday, and got a ZHL-3A-sma. This little guy gives a max power output of 29.5 dBm (~890 mW) which should be more than enough while using the Marconi as our source (max output +13 dBm).

We hooked the amplifier to the load (AOM) without any couplers or attenuators in between, powered it with +24 VDC and quickly repeated a scan of the source power level while to see any sign of diffraction in the PDs. The result is in Attachment 2. We were a little bit disappointed that there appeared to be no diffraction, so next we tried scanning the RF frequency (it was nominally at 80 MHz) around and we finally succeeded in seeing some diffraction at 95 MHz! Paco thinks the internal fiber coupling made for the design wavelength (2004 nm) is suboptimal at 80 MHz and ~1.4 um wavelength. Therefore, to couple the 1st order back into the fiber, we need to shift the RF frequency to restore the diffraction angle at the cost of potentially not driving the optimal efficiency. An interesting observation made at the same time we saw 1st order light was that the power seemed to drift very slowly (-1%/min), which may have to do with some thermal drift inside the crystal... Our plan is to make a complete characterization of the diffraction efficiency at 1.4 um, and also investigate the slow intensity drifts as a function of RF input. The goal is not so much to understand and fix this last one, but to be able to operate the setup at a point where things are stable for a low frequency, frequency noise measurement.

1920   Thu Aug 12 11:49:59 2021 RadhikaDailyProgress1418 nm AUX ECDL1419 nm ECDL AOM diffraction at 95 MHz

When previously trying to characterize the AOM, we had noticed no 1st order diffraction when operating at 80 MHz, but significant diffraction at 95 MHz. This motivated us to take measurements while sweeping across both RF drive frequency and Marconi drive power. For frequency, we swept from 80-120 MHz in steps of 1 MHz. For power, we swept across [3, 0, -3] dBm (3 dBm is max power before saturating AOM). We took our measurements of 0th and 1st order signal using an oscilloscope.

Contour plots of the 0th and 1st order signals can be seen in Attachments 1 and 2, respectively. Peak 1st order diffraction seems to occur at ~106 MHz. Using this AOM for a beat note measurement, the frequency difference would be greater than intended, which could lead to a weaker beat note signal.

*Bonus: Today we moved the ECDL setup off the cryostat table and onto the other table. These measurements were taken after the move.

1921   Tue Aug 17 11:09:29 2021 ranaDailyProgress1418 nm AUX ECDL1419 nm ECDL AOM diffraction at 95 MHz

Should measure the S-matrix using a bi-directional coupler.

1922   Wed Sep 1 13:12:02 2021 RadhikaDailyProgress1418 nm AUX ECDL1419 nm ECDL AOM diffraction at 95 MHz

Today we tried to pick up from [1920] by repeating the sweep measurements across RF frequency, at 3 dBm (max power). We noticed that the 0th order signal would dip around the expected value, consistent with the plot in [1920]. However, there was no signal from the 1st order. Clearly diffraction was occurring as seen by the dip in 0th order, but nothing was coming out of the 1st order port. We spent some time debugging by swapping the photodetector inputs / playing with the PD gains / performing power cycles, but got no insight into the issue.

We suspected the 1st order fiber coming out of the AOM might be damaged, since it loops around fairly tightly. After giving it more slack, we still saw no signal. We wanted to test the fiber, so we took an unused output of the 50-50 beamsplitter and fed it into the 1st order port, effectively running the AOM in reverse. We hooked up the input and 0th order ports to the photodiodes and did not observe any signal. From here we were more convinced that the 1st order fiber may have seen some damage.

For next steps, we can still use the existing fiber setup to take measurements of relative intensity noise (RIN), using the 0th order output of the AOM. I plan to do this in the next few days. Meanwhile, Paco is looking into ordering parts for a free space setup. We found a free-space AOM at 1064nm that seems promising, and we will work to transition the setup accordingly.

1923   Thu Sep 2 17:31:38 2021 RadhikaDailyProgress1418 nm AUX ECDL1418 nm ECDL Relative Intensity Noise

I took a relative intensity noise (RIN) measurement of the ECDL, by feeding the 0th order output of the AOM to the SR785. The RF power driving the AOM was set to 0 dBm. The RIN at 1 Hz is about 3x10-5, which is consistent with informal measurements we took on 08/13. From my understanding this noise looks pretty low, which is good. I will consult with Paco and add more discussion or conclusions, if any.

1924   Thu Sep 16 15:21:21 2021 PacoDailyProgress1418 nm AUX ECDLFree space AOM

Uninstalled the fiber AOM and temporarily removed the third fiber 2x2 port beamsplitter. We are now using this free-space AOM. Then, I managed to launch one of the outputs of the second fiber beamsplitter into free space using a F220APC-1550 fixed collimator. The beam clears the  AOM aperture nicely and lands on the other side.

This AOM operates at a RF frequency of 35 MHz, so we set up a sweep on the Marconi to cover a window of 35 MHz +- 15 MHz. Using an IR detector card, we looked for evidence of 1st-order diffraction (from the setup geometry, the 1st order beam should have been visibly discernable). We first scanned the AOM across yaw but did not notice diffraction. Then, Paco lowered the height of the fixed collimator and we repeated scanning across yaw. We eventually saw the beam "jump" - diffraction! We adjusted yaw until we recovered both 0th and 1st order beams, at 50/50 intensity.

In summary, the free-space AOM works and we have managed to see 1st order diffraction. Next steps will be to quantitatively measure this diffraction while sweeping across RF frequency and power.

1925   Wed Sep 22 16:44:34 2021 RadhikaDailyProgress1418 nm AUX ECDLFree space AOM

We had previously noticed that the ECDL laser power seemed weaker compared to when we originally set it up and tested it. Today Paco opened it up and tweaked the grating inside to obtain a max power of 3 mW. This way, we could better resolve the 0th and 1st order beams coming out of the AOM.

Since we don't yet have a lens to send the collimated 1st-order beam to fiber, we connected a power meter to detect the beam and hooked it up to the oscilloscope. We noted peak diffraction at around 38.5 MHz (rough estimate). Using the inverse relationship between laser wavelength and the RF frequency $f \lambda = constant$, and the fact that the AOM is designed to operate at 1550 nm at 35 MHz, we calculated that the ECDL wavelength should be ~1409 nm. Of course this is a rough estimation, but it is a quick validation that we are indeed operating near 1418 nm.

1926   Mon Oct 4 17:44:34 2021 RadhikaDailyProgress1418 nm AUX ECDLFree space AOM

Last Friday we received a new lens to direct the AOM 1st-order beam from free space into a fiber cable. We mounted the lens and connected a fiber cable into the photodiode, and tried to align the lens and see a jump in the oscilloscope. We were not able to do so and wrapped up for the day.

Today we continued aligning the lens with the fine adjustment on the mount, and eventually saw signal on the scope! Hooray, done with free space. We then prepared for eventually taking a heterodyne beat note measurement and hooked up the appropriate inputs/outputs to the beamsplitters. We added in the 50-50 beamsplitter that takes in the 1st order diffracted beam along with the beam from the delay line as inputs. We passed one of the outputs to the photodiode and had to retweak the freespace-to-fiber lens until we recovered signal on the scope, and we saw the beatnote signal.

Next, while Paco is out of town I will continue to work towards making a frequency noise measurement. We made a roadmap today:

I will demodulate the beat note using a mixer and a 35 MHz LO sourced from the Marconi. The result will be a 2f cosine term, along with a much lower frequency term which encloses the frequency noise information. This will be passed through a low-pass filter to get rid of the first high-frequency term. The remaining time-domain signal will be passed to the SR785 to obtain a spectra of the frequency noise. Calibration will need to be performed to obtain the right units for the spectra, Hz2/Hz (or Hz/rtHz).

1927   Tue Oct 19 13:52:03 2021 RadhikaDailyProgress1418 nm AUX ECDL1418nm ECDL Frequency noise

Attachment 1 is a diagram of the current setup for measuring ECDL frequency noise. Since the last update, I have fed the beat note signal to a mixer, using a 35 MHz LO sourced from the Marconi. The resulting demodulated signal is passed to a low-pass filter, removing the 2f sinusoidal term (any trace of the frequency difference) and leaving behind a low-frequency term containing frequency noise information of the original beam (accumulated over the length of delay line).

I took spectra of the resulting signal using the SR785 (Attachment 2). Note that these units are still in V/rtHz, since the signal has not been calibrated to the appropriate units for frequency noise, Hz/rtHz. Finding the calibration term will involve study of delay line frequency discrimination.

1928   Tue Mar 8 09:32:56 2022 PacoDailyProgress1418 nm AUX ECDL1418nm ECDL Frequency noise

## Beatnote recovery

Restarted ECDL characterization last Friday. After some lab cleanup, and beatnote amplitude optimization we borrowed Moku Lab from Cryo lab to fast-track phase noise measurements. Attachment #1 shows a sketch of our delayed self-heterodyne interferometer. The Marconi 2023A feeds +7 dBm to a  ZHA-3A amplfier which shifts the frequency of the laser in one of the arms using a free space AOM. The first order is coupled back into a fiber beamsplitter to interfere with a 10 m delay line beam.

## Improved beatnote detection

The 38.5 MHz beatnote was barely detectable before when using PDA20CS2 because at unity (lowest) gain stage, the bandwidth was only 11 MHz... We instead switched to an FPD310-FC-NIR type which has a more adequate high-frequency response. Attachment #2 shows the beatnote power spectrum taken with Moku Lab spectrum analyzer. The two vertical lines indicate (1) the heterodyne beatnote frequency and (2) the "free spectral range" indicating the actual delay in the MZ arms, which is calibrated to $c\tau/n$ = 9.73 m (using 1.46 for n, the fused silica fiber index).

## Phase meter and freq noise calibration

We then tried using the phase meter application on the Moku. The internal PLL automatically detected the 38.499 MHz center frequency and produced an unwrapped RF phase timeseries (e.g. shown in Attachment #3). The MZ interferometer gives an AC signal

$I_{\rm AC} = I_0 \cos(\Omega_0t + \phi(t + \tau) - \phi(t))$

oscillating at $\Omega_0$ , i.e. the angular beatnote frequency. The delay (calibrated above) characterizes the response of the MZ relating the RF phase noise spectrum to the optical phase noise spectrum. The RF phase obtained through the phase meter has a fourier transform

$\tilde{\phi}_{\rm RF}(\omega) = \tilde{\phi}(\omega) e^{-i \omega \tau} - \tilde{\phi}(\omega)$

So the optical phase spectral density is related to the rf phase spectral density by a transfer function $H(\omega) = e^{-i \omega \tau} - 1$  Then, the RF & optical phase power spectral densities are related by $S_{\phi_{\rm RF}}(\omega) = |1 - e^{-i \omega \tau}|^2 S_{\phi}(\omega)$  or

$S_{\phi}(\omega) = \frac{S_{\phi_{\rm RF}}(\omega) }{ 4 \sin^2(\omega \tau /2) }$

Then, because the instantaneous laser frequency is $2 \pi \nu = \dot{\phi}$,  in fourier domain $\tilde{\nu} = \frac{i\omega}{2 \pi} \tilde{\phi}$ the frequency and phase PSDs are related by the magnitude square of this transfer function like

$S_{\nu}(\omega) = f^2 S_{\phi}(\omega)$

Following this prescription, we compute an estimate for the frequency noise ASD (square root of the PSD) shown in Attachment #4. The frequency noise estimated by this method has several contributions and *does not* necessarily represent the free-running ECDL frequency noise.

## Next steps

• Noise budgeting (experiment)
• Control loop (open/closed) models
161   Tue Oct 12 10:51:36 2010 AlastairMiscBladesUseful document

We're going to use this elog to store some of our lab work on blades that will be going on in the SUS lab.

As a first entry here is a useful document on the ALIGO blade design on the DCC:  LIGO T030107  by M.V.Plissi

We made a drawing for a structure hat will hold the maraging blade. The details aren't complete yet. The holes for the clamping will be  identified,  but the sketch shows the rough idea.

We want to clamp the blade to a structure. The drawing for the clamp will be provided by Ryan (he found it in the dcc.) The structure is consisted of the base and the pillar. Although a monolithic structure is better, it might be to expensive to carve out a big piece of Al block, so Koji suggested that we do it like this. The base will be mounted on the table, and the pillar will be mounted on the base by 4 screws. The height of the pillar is not decided yet. It depends on how big the Al mass block we need to pull down the blade by its weight, and how the mirror for reflecting the beam up will be mounted, but it should be around 6 - 8 inches.

The mass block will be used for mounting the end mirror of the interferometer + a translational stage. This way we can steer the beam with 2 mirrors and adjust the arm length. We will determine the weight, so we can estimate the size of the mass block, assuming we will use Al.

We made a sketch for the weight clamp that will carry the mass block on the end of the blades. This will be done in Solidwork tomorrow.

We plan  to load a block of mass under the tip of the blade by using a pair of knife edge pieces so that the rubbing between the mass block and the blade is minimized.

The edge of the blade cannot be too large, or it will be noisy when the blade is driven. On the other hands, if the blade angle is too small (sharper blade), the stress on the blade due to the weight will be too large and cause plastic deformation on the blade, which we don't want. We plan to make it flat ~ 1mm wide, with 120degree open angle.

The yield tensile strength of maraging steel is ~ 1 -2 GPa. With the contact area at the knife edge we can calculate the maximum clamping force.

The width of the edge is ~ 5cm

The thickness of the edge ~ 1mm.

so the maximum force should not exceed ~ 1 GPa x 0.05 m x 0.001 m ~10^4 newton.

We will use spring washers to make sure that we do not tighten the clamps together with too much force and cause plastic deformation on the blade.

We finalized the drawing for blade clamping system. The drawings are posted here and in Crackle ATF Wiki. We will submit the drawings to the machine shop tomorrow.

For each blade, the clamping system will consist of: 1)Steel base, 2)Steel pillar, 3) Steel top clamp, 4) Al knife edge top piece,5)Al knife edge bottom piece,and 6) Al end piece.

1) Steel base x1:   The steel base is 3"x3"x0.5" . It has 4 counter sunk holes that allow us to mount the steel pillar on it. It has 3" rails on both sides, so we can mount it on the table. Extra clamps can be used to hold the base on the table.

2) Steel pillar x1:   It is 5.5" height with 2"x2" square cross section.  There are 4 tapped 1/4-20 holes , 1" in depth, on the bottom for mounting it on the base. There are 2 tapped 3/8 , 1" in depth, on top for clamping two clamps along with the blade.

3) Steel top clamping piece x1, This will clamp the blade on the pillar.

4) Aluminum knife edge, top piece x1,

5) Aluminum knife edge, bottom piece x1: (4&5) The two knife edge pieces will be used for loading the mass block on the maraging blade tip. The explanation is written in this entry.

6) Aluminum end piece that holds the mirror mount on the blade tip x1: We want to have a steerable mirror for the IFO. So we need a mirror mount. The block will hold the mount and the blade tip together through screws. This piece is uploaded in the above entry.

The assembly (without the blade and the mirror mount) is shown below.

We submitted the drawing to the machine shop today. The works should be done before May 23rd.

The base/ pillar/ blade clamp will be made from stainless steel. The knife edge pieces and mirror mount at the blade tip will be made from aluminum.

368   Mon Oct 24 16:18:45 2011 Giordon StarkHowToCOMSOL+MatlabHow to set up COMSOL to communicate with Matlab (Mac OSX Lion)

Mac OSX Lion came out pretty recently and COMSOL 4.2 [at the time of the writing] does not successfully install on these OSes. There's two issues in general - the first issue is that Lion changed the way some of the paths work (that COMSOL depends on) and COMSOL will throw seemingly unrelated errors in trying to start up.

install COMSOL 4.2 Update 3 (or better): http://www.comsol.com/support/updates/comsol42p3/

After installing, we need to update the MATLAB paths. Matlab normally installs in a directory under /Applications called 'MATLAB_R2010b'. Rename it by changing the underscore to a dash [COMSOL interprets the underscore as a 'space' which means it will never find it]. The new name should be 'MATLAB-R2010b'. Next, navigate to the COMSOL42/bin folder in /Applications. Depending on which mac build you're running (32-bit or 64-bit) - select the appropriate 'maci##' folder.

Inside are 4 initialization files to change:

• comsolserver.ini
• comsol.ini
• comsolbatch.ini
• comsolcompile.ini

On the very last line of each file - it should read:

-Dcs.mlroot=/Applications/MATLAB-R2010b.app

Once you've made the changes - you should be able to launch COMSOL+Matlab without (m)any issues.
385   Thu Jan 26 11:49:45 2012 ZachDailyProgressCoating QParts gathered, etc.

[Giordon, Zach]

Yesterday, we went to the 40m and stole most of the parts we will need for the Q-measurement polarimeter. I have asked Giordon to put a list of these parts on the elog, as training for good elog routine.

We brought them to the SUS lab and started placing things on the table. The plan is to build a barebones setup, in air, to get a feel for the readout scheme. As a first pass, we will find any old piece of transparent material and drive it mechanically (read: flick it) to see if we see a low-Q ringdown. In parallel, Giordon is supposed to be working on his SNR calculations (and I am supposed to be answering some questions he has).

We set up a HeNe and some polarizing elements, but were somehow unable to linearize the polarization. This is somewhat puzzling:

• We are using quarter- and half-wave plates that are ThorLabs-labelled (etched) for 633 nm.
• We are using a 1-in PBS from the 40m that was unlabeled. There were three such cubes on the SP table near other visible optics, so I assumed that they could be from an old SURF HeNe setup that Rana mentioned. Using a red laser pointer, I verified that I could get reasonable visibility (say ~80-90% by eye) by rotating it with respect to the PBS. The effect was much smaller with a green pointer, so I concluded that it was a 633-nm PBS.
• Setting the elements up in the usual way, I was only able to get contrast of ~30% or so by adjusting the waveplates. Frank looked up the HeNe head part number and found that its output was circular, but of course this is the point of the QWP.
• We replaced the head with another one (on the off chance that the head was broken and somehow putting out a time-varying polarization), and this had no effect.
• Frank, Vladimir and I did some independent testing of the polarization optics. We couldn't find a red laser pointer (which was the only source with which I could get the PBS working), so instead we tested them by eye with the (polarized) LCD computer monitors in the lab.
• All the optics behaved as expected:
• Putting the PBS between our eyes and the screen, we could achieve very high visibility by rotating it.
• Leaving the PBS where it was, and adding the HWP between it and the screen, we could achieve very similar visibility by rotating the HWP alone.
• Switching out the HWP for the QWP, we could set the QWP such that no orientation of the PBS blocked light from the LCD (i.e., the polarization was circularized).
• It should be noted that we focused our attention to RED images on the screen, and that these trials did not work equally for all colors
• So, the question is: if every component seems to be working, and switching the laser has no effect, WHAT THE HELL IS GOING ON?

386   Thu Jan 26 15:48:16 2012 Giordon StarkDailyProgressCoating QScrounging of Parts and slight issues

Zach and Giordon went to the 40m yesterday afternoon and rounded up a whole slew of parts. These include:

• 3 metallic/silver mirrors
• 1 633nm Polarizing Beamsplitter (verified wavelength using a pen laser) [PBS]
• 1 PLCX-25.4-33.7-UV-1064 (planar convex mirror)
• 1 PLCX-25.4-46.4-UV (planar convex mirror)
• 1 633nm L/4 (waveplate: quarter-wavelength) [L/4]
• 1 633nm L/2 (waveplate: half-wavelength) [L/2]

8 items/parts total.

We went back over to the lab in Bridge and took out the parts. There is a laser set-up in there at around 633nm and so we did the following:

1. Set up the wave plates (L/4 then L/2) and then the PBS
2. Changed the rotation on L/4 and L/2 and saw that we changed the laser output intensity (it varied) but it never zeroed out
3. Removed the wave plates, put in L/4 only
4. Repeated step 2 - saw the output never zeroed out
5. Removed L/4, put in L/2 only
6. Repeated step 2 - saw the output never zeroed out

Our conclusion of what's wrong is unknown. Some possible ideas include the laser is not "fixed" (it changes polarizations, it's continuously rotating, etc...). Further testing will be done Friday morning to determine whether the wave plates are not working the way we expect them to or if the laser is the issue.

An updated version of the Experimental Set up with more details will be added after Friday's conclusion.

387   Fri Jan 27 11:40:05 2012 Giordon StarkDailyProgressCoating QLaser Specifications

We have two lasers in the lab which have (quoted) "random polarization". These are the uniphase lasers labeled 1103. Details found here:

http://www.edmundoptics.com/products/displayproduct.cfm?productID=3053&dc&PageNum=1#products

Under the stock number NT64-103 (reproduced below):

 Model Number 1103 Beam Diameter, 1/e2 (mm) 0.63 Beam Divergence (mrad) 1.30 Minimum Output Power, TEM00 (mW) 2.00 Longitudinal Mode Spacing, Nominal (MHz) 730 RMS Noise, 30Hz - 10MHz (%) 0.10 Polarization Random Laser Class - CDRH IIIa Diameter of Laser Head (inches) 1.245 Length of Laser Head (inches) 9.50 Weight (lbs) 0.9 RoHS Exempt CE Certified Yes

There is also a PDF for the uniphase lasers in particular found here which gives more specific details about the structure: http://www.photonicshop.co.uk/fe/Pdf/JUHeNeLaserHeads_1100Series.pdf.

To remedy this, Zach found a laser with 1103P (the P specification is plane-polarized) and seems to be working as we expect now.

388   Fri Jan 27 17:22:34 2012 Giordon StarkDailyProgressCoating QLaser Issues (part 2)

The 1103P we found was pretty crappy and wasn't staying on. We broke for lunch, Zach had a meeting, and met up again around 3:30pm. Zach found another working one from Koji's office in Bridge and we set-up roughly what the experiment is and just checked out the equipment and the signals from the photodetectors, lock-in amplifier, pre-amps, and so on.

We ran a noise measurement on the photodiodes with and without the laser (they're the same, so it's just one measurement really, but we'll include both in a later ELOG post to show that they are the same). Quick observations (number crunching to be done in near future): we ran it from 0-100kHz and notice a rather broad (50kHz) hump around 60kHz which is most likely due to specs about the photodetectors themselves.

The noise of the pre-amplifier seems much higher than we expected - and it happens to be the cause of the bump - we grounded the input of the pre-amp and noticed the hump is there. We also increase the amplification by a factor of 10 and noticed that the noise floor increased to be at the same level as the hump - which tells us that the noise is introduced into the pre-amp after the amplification of the signal. We replaced the cable and noticed no change. It appears that by switching from power supply to battery power fixes this.

389   Mon Jan 30 04:38:14 2012 Giordon StarkDailyProgressCoating QCalculated Noise Measurements of Photodiodes, Preamp Setup

Attached a lot of things - the PDF contains an updated list of parts with links to part specifications. We're still missing information (or would like more information) about the following parts that we currently have:

• vacuum
• polarizing beam splitter
• QWP and HWP

The PDF also describes the calculated noise shown in ComparisonOfNoise.png. We found that the noise expected from each photodiode is 7.6e-8 -- coming out of the preamp would have a noise of 7.6e-8 * sqrt(2) [factor from the combination of the input photodiode signals]. Everything seems to be on the up and up - but I will talk with Zach (and Rana if he's around) about what we've got here.

Talked with Zach after everything on Friday - here's a brief discussion of what we talked about on the whiteboard using markers that contained all the colors of the rainbow.

• Step 1: Once we get the ESD and the sample, we're going to look at the output and compare it with the drive frequency so that we expect to see oscillations
• Step 2: Then, we'll excite the sample and then let it decay and observe a decaying sinusoidal on the oscilloscope
• Step 3: Then, we'll add in the lock-in amplifier and tune it so that we effectively chop off most of the signal (in a perfect case, we see no more sinusoidals, and simply just a decay curve)
• Step 4: ???
• Step 5: Win a senior thesis

390   Mon Jan 30 12:28:03 2012 Giordon StarkDailyProgressCoating QLock-in Amplifier Fun

[Giordon, and Zach came along]

So, we grabbed (nabbed) a chopper from the 40m and stuck it on our set-up in between the linear polarizer and the PBS.

• Zeroed out the signal on the oscilloscope and then turned on chopper to 1kHz
• Verified that the square signal we see on the oscilloscope had a frequency of 1kHz corresponding to the chopper freq (attached picture shows Ch1 being the photodetector output, Ch2 being the chopper output - shows they have the same freq)
• Plug it into the lock-in amplifier and do a calibration at 1 Volt sensitivity to get a factor of 4.375
• Verified that this factor is linear by changing the signal power and noticing predictable changes in the LIA output
• this was done by unbalancing the signal power (so we look at the output of one over the other) - calibrating on a balanced output (theoretically zero) makes no sense
• then we changed the phase of the chopper output to maximize the LIA output
• then we measured the LIA output versus the photodetector output (e.g. 1.4 V LIA against 320 mV PD)

I apparently have a Bioethics class which got rescheduled for today at 1pm - so that kinda cuts into plans for the rest of the day. Wednesday morning - I'll come into lab and do the test runs mentioned within the manual for the LIA SR830.

391   Mon Jan 30 15:10:42 2012 ZachDailyProgressCoating QESD preliminary design

I have made a first draft of the design for the ESD (see PDF below). The electrode comb spacing is 0.2", which should be roughly what we want for objects ~1-3" in size.

Sunstone offers boards printed on Rogers 4350 material as part of their fast and cheap QuickTurn service. This is a glass- and ceramic-based material that is designed for RF applications, but I have seen some examples of it being used at UHV in some ion trapping experiments. Since our vacuum doesn't have to be outrageously good (and the lab isn't clean enough for that anyway), this ought to work fine.

The finish will be silver (gold would be preferable for oxidation purposes, but Sunstone only offers silver with this service---otherwise we'd have to submit a much costlier and time-delaying custom order).

I have designed the ESD such that there are two holes offset horizontally from the center of the plate. These are for the passage of the measurement beam. I chose off-center because most modes' signals should be weaker at the center from symmetry. I chose to put two in for no particular reason other than symmetry, again. NOTE: we will have to drill these holes ourselves.

The electrodes will be connected to the HV supply by soldering to relatively large plates on the back side. One will be connected to the (positive, single-sided) HV amp output, while the other will most likely be connected to a wire that is bolted to the chamber (earth) at the other end.

It was not clear from the quote page, but apparently the RO4350 material is a little pricier than the standard FR4. For two of these boards, the cost is $493.35 ($246.68 /ea.). I think this is reasonable---assuming it works---considering how fast we can get it.

If no one has any objections or comments, I will put the order in.

393   Wed Feb 1 00:11:16 2012 ZachDailyProgressCoating QESD preliminary design

With only minor changes to the actual electrode pattern, I have made the ESD design much more compact, which will reduce the cost by over 33%. The total cost for two boards is now $327. I am going to purchase them with Steve's card tomorrow.  Quote: I have made a first draft of the design for the ESD (see PDF below). The electrode comb spacing is 0.2", which should be roughly what we want for objects ~1-3" in size. Sunstone offers boards printed on Rogers 4350 material as part of their fast and cheap QuickTurn service. This is a glass- and ceramic-based material that is designed for RF applications, but I have seen some examples of it being used at UHV in some ion trapping experiments. Since our vacuum doesn't have to be outrageously good (and the lab isn't clean enough for that anyway), this ought to work fine. The finish will be silver (gold would be preferable for oxidation purposes, but Sunstone only offers silver with this service---otherwise we'd have to submit a much costlier and time-delaying custom order). I have designed the ESD such that there are two holes offset horizontally from the center of the plate. These are for the passage of the measurement beam. I chose off-center because most modes' signals should be weaker at the center from symmetry. I chose to put two in for no particular reason other than symmetry, again. NOTE: we will have to drill these holes ourselves. The electrodes will be connected to the HV supply by soldering to relatively large plates on the back side. One will be connected to the (positive, single-sided) HV amp output, while the other will most likely be connected to a wire that is bolted to the chamber (earth) at the other end. It was not clear from the quote page, but apparently the RO4350 material is a little pricier than the standard FR4. For two of these boards, the cost is$493.35 ($246.68 /ea.). I think this is reasonable---assuming it works---considering how fast we can get it. If no one has any objections or comments, I will put the order in. 394 Wed Feb 1 13:48:24 2012 Giordon StarkDailyProgressCoating QLock-in Amplifier Testing (Good Results) [Giordon] I ran the testing for the lock-in amplifier specified on pages 6-1 through 6-22 of the attached PDF (data sheet / lab manual for LIA). All systems seem to be running well within normal operating bounds. Should be seeing Zach and Alastair after lunch around 2pm to work on the vacuum system. The most important result appears to be the input noise which was measured at 4 nV/root(Hz). 395 Wed Feb 1 19:05:34 2012 ZachDailyProgressCoating QESDs purchased I made some final changes to the ESD design and then purchased them. The only changes from yesterday were: • Add 4 holes for mounting: • 2 x 8-32 on the bottom, as we are planning on only holding from one end at the moment • 2 x 4-40 on the top, in case we decide we need to hold it there too. The reason for the size discrepancy is because I wanted to have at least two that were relatively beefy and I didn't have room for 4 in this board area range. • Indicate a 5-kV maximum voltage rating, based on the ~0.5-mm thickness of the board and the ~30 kV/mm dielectric strength of RO4350, with a factor-of-3 safety margin. • Make the board size as large as I could while staying in the same board area price range After talking with Alastair about it, I also decided to order 4 boards instead of only two. The price goes down from$163/ea to $102/ea, and Alastair figured we might burn one out in testing and/or want to modify one later, etc. So, the total shipped cost was$410.

Here is the final layout:

396   Wed Feb 1 19:37:00 2012 ZachDailyProgressCoating Qvacuum system inspected

[Alastair, Giordon, Zach]

Alastair showed us how to safely remove the lid of the bell jar with the hoist. We inspected the inside to see how we might set up our first measurement. His fiber setup is still in there, and we decided that as a first go, we will just weld the first sample we received from Gregg to the end of the fiber (after we've shortened it appropriately). Part of the reasoning is that this (3" x 0.25") sample looks as though it's already been welded before:

The setup inside the vacuum system will be:

• Sample suspension
• Intermediate fused silica mass suspended via fiber from the steel plates toward the top of the chamber
• Our disc suspended from the intermediate mass via fiber
• ESD
• There is a network of teflon bars on threaded rods, which was designed to be adjustable
• We will drill one of these rods so that I can screw the ESD onto it and adjust it to the right height/horizontal position
• HV connection
• There is already a proper HV feedthrough from the HV amplifier, and we have in-vac HV wire on spools
• I will talk to Margot or someone to find out the best way to solder this wire to the (+) end of the ESD
• Using another piece of wire, we will connect the (-) end of the ESD to the vacuum (earth) via a bolt or something.

This is very nice because we will have a minimal amount of equipment inside the vacuum chamber. We may decide that we want to build the fancy cat's cradle (nodal) support, or something else, but this is the fastest way to start measuring something.

It should also be mentioned that this single-point welded support has been a big problem for interferometric setups because of wild torsional motion. We are hoping that this will be a higher-order effect with our transmissive setup. We'll see.

397   Thu Feb 2 14:32:30 2012 Giordon StarkDailyProgressCoating QLock In Amplifier Testing Results

Here are the results of the SR830 Test Run I mentioned in an eLog post yesterday. Everything seem to be correct and in working order within the defined bounds.

398   Thu Feb 2 15:11:44 2012 Giordon StarkDailyProgressCoating QNEP of Experimental Set up [FIXED]

[Giordon, Zach]

The last post we discussed the noise measurements with a noticeable peak around the 50kHz. This time, we've set the preamp gain to 500 and retook the measurements. The attached image shows the result of said measurements.

One thing we noticed is that the photodetector noise is roughly two orders of magnitude smaller than our best estimates for this. Zach thinks it's not real, I think it's pretty awesome. Here some explanations of the measurements.

• A-B output means that the output comes from the preamplifier on an AC coupled setting amplified at 500. This either came with the laser on or off. We fixed the range here and then ran a noise floor measurement where we grounded the input.
• PDA and PDB mean Photodetector A and Photodetector B as listed in the experimental set up drawing in an eLog post some days ago. PDA has a Noise Floor (NFLR) measurement like before as we noticed that the range of it was significantly different from the A-B output range [this is not true of PDB].
• Theoretical PD and Theoretical Preamp are values highlighted from the rough noise calculations made about a week ago. (Same place as the inclusion of the experimental set up).

I've also noticed that the plot doesn't make complete sense to me. For example, the noise floor of PD1 seems to be higher than PD1 itself - but maybe Zach can re-explain to me the difference between fixed range and auto range here.

For lazy people, I've reattached the experimental set up and noise calculations.

399   Thu Feb 2 18:59:20 2012 ZachDailyProgressCoating Qfront-end noise budget

I took the liberty of making my own plot. This was both in the interest of time and also to better illustrate how the plot should be made by example:

• "NEP" is a relatively useless term. Manufacturers use it so they can report the smallest possible intensity noise level for a given electronic noise level. Also, it would be in W/rHz, not V/rHz.
• It is useful to plot the intensity noise level for our particular setup (i.e., wavelength) alongside the electronic noise level in volts. This is best accomplished by putting another axis on the right side with the proper scale factor with respect to the other one. For this, you use the PD's transimpedance gain [V/A], its responsivity [A/W], and your brain [?].
• The only tricky part, really, comes when you have to plot the manufacturer's quoted noise level. For this, you have to go from their NEP [W/rHz], to the corresponding electronic noise level [V/rHz], using the proper responsivity at the peak wavelength. In this case, the peak responsivity (@ lambda = 970 nm) is 0.65 A/W, while for us (@ lambda = 633 nm), it's ~0.41 A/W. A good check to make sure you've done all the calibrating correctly is to make sure that the manufacturer's spec that you've plotted---in W/rHz on the right side---is equal to their NEP number times the ratio of the two responsivities.
• I'm not sure how you got things to end up out of whack in yours, but the calibration should be simple: take the raw voltage noise level we measured with the spectrum analyzer, then divide by our preamp gain of 500. This is the input-referred noise level, as we have discussed. That means that even though the input noise level of the spectrum analyzer is actually higher than the dark PD noise in an absolute sense, it actually looks much lower, since we've amplified our signal with the SR560.
• The trace names should be clear and make sense:
• PD1 dark: The electronic noise measured out of the first PD with no laser light on it
• PD2 dark: The electronic noise measured out of the second PD with no laser light on it
• PD1 - PD2, dark: The differential noise of the two PDs, with no laser light on them.
• Notice how this is roughly sqrt(2) times the level of the previous two traces. This is what we expect from the incoherent sum of the two random signals.
• Dark noise floor: The self-noise (shorted input) of the spectrum analyzer, while on the input range setting we used for the dark measurements.
• We saved this as "PD1NFL", but since the input range never changed throughout the first 3 measurements, and the noise spectrum of the analyzer is stationary, this is a valid noise floor measurement for all three.
• PD1 - PD2, balanced: The differential noise of the two PDs, with the laser on and the DC difference zeroed out (hence "balanced").
• Notice how this measurement is significantly noisier than all the others. This means that, despite the large common-mode rejection we get of intensity noise from the laser (since the PDs are balanced), there is still a strong enough differential component that we see noise from the laser. This is a crucial thing to take home.
• As of right now, this level (~25 nV/rHz --OR-- ~50 pW/rHz) sets the ultimate noise floor of the measurement in this configuration. Once we have figured out how to calculate an achievable signal size, we can calculate the sort of SNRs we can expect. This requires the following knowledge:
• Using our ESD, to how large of a strain amplitude can we drive up the samples' modes?
• How do we get from a strain amplitude to a rotation of polarization?
• How do we get from a polarization rotation to a differential power signal in W at the PDs? (This last one is easy, and you should work it out ASAP)
• Bright noise floor: Measured in the same way as the dark noise floor, but using the input range setting we used for the measurements with the laser on
• Preamp noise (SR560): The well known---and visually re-measured---high-gain-setting input noise of the SR560 preamp (4 nV/rHz above ~10 Hz)
• PD manufacturer spec (PDA36A): Calculated in the above-described way using the "NEP" figure. It seems strange that our actual measurement should be so far below this (~10x, not 100x), but it turns out that this is an over-estimated number by the manufacturer to save their aes. Depending on the quality of the 3rd-party components that go in, some units might be closer to this level than others. Considering the transimpedance gain of 1510 V/A and knowing that the opamp used is an AD829, the measured output noise value of ~7.5 nV/rHz is totally reasonable.

Also, some general plotting tips:

• Use a linewidth of 2 for your plots. It gives them an air of confidence
• The same applies for the fonts. The axis ticks, etc, should be ~12-14pt, the labels should be 14-16pt, and the title should be closer to 20pt. Otherwise, you just can't read anything. Legend font sizes depend on what you can fit in.
• Use a grid so that you don't have to break out a ruler to interpolate
• Set your axis limits so that you make good use of the plot space. All that whiteness is just a waste. Dynamic range rules.

Try to let this stuff soak in, as we'll probably have to make this whole measurement again. Next time, you will plot it!

Another thing: our whole noise budget will get a little more complicated once we add in the lockin, but this analysis of the front-end (the very place where physics meets measurement apparatus) is extremely important.

 Quote: [Giordon, Zach]  The last post we discussed the noise measurements with a noticeable peak around the 50kHz. This time, we've set the preamp gain to 500 and retook the measurements. The attached image shows the result of said measurements. One thing we noticed is that the photodetector noise is roughly two orders of magnitude smaller than our best estimates for this. Zach thinks it's not real, I think it's pretty awesome. Here some explanations of the measurements. A-B output means that the output comes from the preamplifier on an AC coupled setting amplified at 500. This either came with the laser on or off. We fixed the range here and then ran a noise floor measurement where we grounded the input. PDA and PDB mean Photodetector A and Photodetector B as listed in the experimental set up drawing in an eLog post some days ago. PDA has a Noise Floor (NFLR) measurement like before as we noticed that the range of it was significantly different from the A-B output range [this is not true of PDB]. Theoretical PD and Theoretical Preamp are values highlighted from the rough noise calculations made about a week ago. (Same place as the inclusion of the experimental set up). I've also noticed that the plot doesn't make complete sense to me. For example, the noise floor of PD1 seems to be higher than PD1 itself - but maybe Zach can re-explain to me the difference between fixed range and auto range here. For lazy people, I've reattached the experimental set up and noise calculations.

400   Fri Feb 3 16:50:02 2012 Giordon StarkDailyProgressCoating QNew Setup (closer to what we planned out)

Lab set up update. Pictures and panorama attached.

We've taken apart the previous set up and included the vacuum chamber in it. This was done by slightly shifting/rotating the vacuum chamber in a way that the HV cables weren't super taut (or stretched) while allowing the beam to get through the whole thing. This part was tricky as the supports in the chamber block opposite halves of the windows such that, given a straight beam, the whole chamber has to be at an angle. The chamber also de-focused our beam so we added in a focusing lens at the far end of the set up before the polarizing beam splitter. Finally, we checked the output levels of the photodiodes and set up the wave plates so that we're at a balanced state (equal outputs out of both ends).

We should have the ESD by Monday, which means we'll have Alastair in the morning to help us quickly with the first set up so we're pretty confident in what to do.

401   Fri Feb 3 23:32:51 2012 ZachDailyProgressCoating QStuff we're getting

The ESDs have shipped, though for some reason I chose UPS Ground and so they are scheduled to get here on Wednesday. Oops.

I spoke with Margot today and she thinks she can provide us with as much in-vac solder as we're likely to need for them. She was going to stop by the SUS lab to check out the setup this afternoon, but I think we were both too busy. I'll see if she can come by on Monday.

I also spoke with Rich about the in-vac HV connectors. The idea is that we want to be able to quickly attach and detach the ESD from the HV supply wires so that we can get the drive and the sample set up nicely before lowering it back into the vacuum chamber. For illustration, I think Giordon should make a little diagram of what the in-vac setup looks like (i.e., suspended from the stainless steel disk at the top, with metal-bar-and-teflon-block frame hanging downward. The ESD will be screwed into one of the teflon blocks.

Rich pointed me to this relatively cheap PEEK in-vac connector. We can attach this to the chamber and have the +HV and ground connected to it semi-permanently. Then, we can have long cables soldered to the ESD on one end that we screw into this connector each time we re-lower the suspension into the chamber. Voilà.

I'll order this ASAP and ship it quickly.

402   Mon Feb 6 12:14:09 2012 ZachDailyProgressCoating QStuff we're getting

I ordered the PEEK connector and it should be in by Wed. Worst case, we can set up the ESD and weld the sample without having the connector, and then hook it up once it's in.

I went for the 4-terminal one (instead of the 8-terminal one linked below), since it was half the price and we shouldn't need more than 2 terminals anyway. I chose 4 instead of 2 in case we wanted to expand later.

 Quote: The ESDs have shipped, though for some reason I chose UPS Ground and so they are scheduled to get here on Wednesday. Oops. I spoke with Margot today and she thinks she can provide us with as much in-vac solder as we're likely to need for them. She was going to stop by the SUS lab to check out the setup this afternoon, but I think we were both too busy. I'll see if she can come by on Monday. I also spoke with Rich about the in-vac HV connectors. The idea is that we want to be able to quickly attach and detach the ESD from the HV supply wires so that we can get the drive and the sample set up nicely before lowering it back into the vacuum chamber. For illustration, I think Giordon should make a little diagram of what the in-vac setup looks like (i.e., suspended from the stainless steel disk at the top, with metal-bar-and-teflon-block frame hanging downward. The ESD will be screwed into one of the teflon blocks. Rich pointed me to this relatively cheap PEEK in-vac connector. We can attach this to the chamber and have the +HV and ground connected to it semi-permanently. Then, we can have long cables soldered to the ESD on one end that we screw into this connector each time we re-lower the suspension into the chamber. Voilà. I'll order this ASAP and ship it quickly.

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