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1238   Sat Apr 11 23:42:03 2015 ZachNotesMechanicsPost and base vs. pedestal and fork

Tonight, I got to do an experiment that I've wanted to do for some time now.

For years, I've heard in conversations with people who shall remain nameless (unless they care to contest this work) that the 3/4"-post-on-rectangular-1/4"-thick-base optic support method (used at the 40m and adopted into LIGO) is better than the standard 1"-pedestal-and-fork method (used by many experiments in our own labs and elsewhere). After many attempts, I have never succeeded in getting any hard data to support that claim. So, I decided to make a measurement myself.

I set up a simple michelson using one of the SiFi beams, once using each support scheme for the beamsplitter and end mirrors:

There is a HWP to find the polarization for which the "50:50" BS is closest to balanced, a lens to focus into the IFO, a second lens to focus the AS beam onto the PD, and the PD itself, which is a PDA255. There is an ND=0.7 filter on the PD.

Below is a scope screenshot of some fringing action when pushing on an end mirror. The contrast defect in each case was pretty low at ~2.5 x 10-3.

Once the IFO was aligned in each case, I pushed on one mirror a bit to creep it into a half-fringe state. This took some time, since I had to push and wait a few seconds for it to settle. After doing that, I took a spectrum (actually 3 at different spans from 1 kHz to 100 kHz). The results are below, with a zoomed plot to the right.

Conclusion

As you can see, the difference is pretty minimal. The post-and-base setup has slightly higher RMS below ~1 kHz, owing to two high-Q resonances (at 340 Hz and, to a lesser extent, 920 Hz). My detractors will accuse me of bias (e.g., in tightening, etc.), but I invite anyone to come test this with their post-and-base clamping chops.

With this, I'd like to put to rest the notion that the post-and-base method is somehow fundamentally superior. I DO acknowledge that there are definitely wrong ways to use the pedestal-and-fork, and this can lead to the non-idealities noted in the folklore. The post-and-base method is foolproof in a way, since the proper procedure is somewhat manifest (use two screws, use washers, etc.), while the pedestal-and-fork requires some diligence to get just right. However, with just a little bit of care up front, the pedestal-and-fork offers huge advantages:

• Arbitrary placement (most importantly, the ability to always work along table hole axes, which makes alignment incredibly easier).
• Free-hand alignment, since the fork can be placed without jostling of the optic (see below).
• Space, since it takes up way less of it.

How to fork

To mitigate the potential recklessness of this post, I offer the Zach-Approved™ Forking Method.

First of all, this is the only fork you should be using, the Newport PS-F (maybe there actually are other acceptable ones, but none that I've found that don't apply horizontal forces on the pedestal upon clamping):

Now, the forking method (accompanied by the GIF below):

1. Locate where you want to place the optic.
2. Place the optic and align by hand. The pedestal is heavy enough to hold itself in place with friction.
3. Locate the appropriate screwhole and set the fork gently onto the pedestal, slightly un-engaged. Of crucial importance is that you pick a hole that will be somewhere close to the middle of the slot. Working along the table hole axes ensures there should be several options in every case.
4. Using your finger, engage the fork on the pedestal. If you use just enough force to move the fork, it will come to a stop when fully engaged and you won't have moved the pedestal (and optic) at all.
5. Insert the screw and hand-tighten a few threadlengths.
6. Use a ball driver to finish screwing almost until tight.
7. Just before tightening, give the fork one final nudge against the pedestal. Sometimes I use the ball driver tip, but this can also be done with your hand so that you can keep the driver in the cap.
8. Tighten as desired.

At no point after initial alignment should you have to touch the optic, and, if you follow the procedure above, the optic orientation should not have shifted by more than a mrad or so. You can see how little the optic moves over the operation in the GIF.

1237   Sat Apr 11 19:03:58 2015 ZachElectronicsSensorsRL readout PD

Seeing Hartmut's talk at the last LVC meeting about innovative DC photodetector designs (something necessary for future squeezed IFOs) reminded me of some investigation I did into the same while at LLO. One thing I did a fair bit of work on while there was the DC current subtraction idea (c.f. LLO:6449 and 6532), but another thing I spent time modeling was the concept of using an RL network, as Hartmut is exploring now.

The circuit I was considering differs somewhat from Hartmut's idea. In his circuit (at left below), the inductor ("L1") and input resistor ("R1") perform a current branching: at low frequencies, the photocurrent sees low impedance to ground through the inductor, and therefore does not pass through the transimpedance amplifier and get converted into an output voltage; at high frequencies, the inductor looks like an open circuit, and all the current passes through the TIA. Ideally, this leads to an effective frontend whitening that allows for a high Z at audio frequencies. In practice, one would use either the DC resistance (DCR) of the inductor, or perhaps an extra resistor in series, to set the DC Z, which would be $Z(0) = R_2 \frac{R_{DC}}{R_{DC} + R_1}$, where RDC is the DC resistance of the inductor path. One problem with this design is that, since RDC cannot be arbitrarily low due to the DCR of the inductor, one must choose an R1 that is high enough to set the DC Z to a low enough level. Roughly speaking, this means that the value of R1 must be approximately the ratio of the desired AC and DC transimpedances (typically a factor of 100 or so), times RDC. Since RDC will be on the order of 100 Ohms, R1 must be on the order of 10 kOhm. This in turn means that the current noise of the amplifier is fully converted by this high impedance at all frequencies, which ruins the SNR of the detector at low frequencies (you could use a low-current-noise part, but then the voltage noise kills you directly).

The circuit I had in mind is at right below. As you can see, the amplifier in this case is only used as a unity-gain buffer for the passive readout circuit (though one could consider adding a switchable flat gain for low-current operation, as in the ZSWITCH feature of the currently used DCPDs). This design works simply by having a passively different transimpedance at different frequencies: at low frequencies, the inductor shorts the large resistor and the transimpedance is just the inductor DCR plus the additional series resistance to ground (50 Ohms in the schematic); at higher frequencies, the impedance increases until it is limited by the parallel resistance (10 kOhms here, plus the series 50 Ohms). With this topology, the current noise always sees the same impedance to ground as the photocurrent does (i.e., the transimpedance), and there is no extra reduction in SNR. The "DC" section is not necessary in principle, and in fact it always has worse SNR for a finite inductor DCR, but it could be used as a calibration path for the DC response due to potential nonlinearity of the inductor.

As a side project, I've started doing some testing of this design. To start, I bought a ginormous 4-H inductor from DigiKey:

Transfer function

The first thing I did was to verify the transfer function. To do this, I biased one of our 3-mm diodes with the M2 circuit bias supply, then sent the anode into a breadboard version of the RL circuit. The parameters were slightly different: L = 4 H (DCR ~ 60 Ohm), RAC = 10k, RDC = 39 Ohm. I then put one of our SiFi lasers on it and modulated the power using its fiber amplitude modulator. Here is the result:

As you can see, it performs just about as expected. A couple notes:

• The LISO trace above has been adjusted using the inductance and diode capacitance as fit parameters, since they are not known precisely a priori.
• The slow upturn at low frequencies is fairly well explained by the amplitude modulator response at low frequencies (see CRYO:1187).

Noise

Of course, the biggest concern with using such a big inductor is the additional noise it might inject, particularly due to pickup. Below is a summary plot of some measurements I made on this circuit, together with some theory curves and the currently used DCPD for comparison.

Notes:

• Obviously, there is a strong presence of pickup here. These traces are also the result of wrapping the inductor in metal and orienting it to minimize noise. Yes, the pickup is prohibitively bad as it appears here, but I have an idea to get rid of it (see below).
• Below a few Hz and above ~1 kHz, the circuits behave as they should, except that the LT1128 exhibits excess current noise in the high-frequency region. This is no surprise, as we have never acheived the "typical" noise performance advertised by Linear (see ATF:1890---Rana points out that this could potentially be due to our not following proper electrostatic discharge practices, but it seems that they are always out of "typ" spec---not "max"---in the exact same way). In this case, though, the noise is even worse than the "max" level, which is an extra party foul. My proof of this is that the OP27-stuffed circuit does what it says it should at these frequencies. Note also that I had only some modest thermal shielding, and that's why the noise shoots up below a few 100 mHz.
• Given the typical LT1128 performance in the real world, it may be that the OP27 is the ideal buffer amp for this circuit, since it adds a factor of a few at high frequencies (where we are most likely to be sensing noise limited). It's also only a factor of a few worse than the fantasy LT1128 curve at low frequencies.
• For all its nascent imperfections, this is still already a detector with 10x better noise both at 1 Hz and above 1 kHz than what we have in aLIGO, so that's already promising.

Reducing the pickup:

So, we are left with the problem of being highly sensitive to a signal injected into the coil, but not to one induced by external fields. Luckily, the guitar industy has had a solution for almost exactly the same issue for about 80 years now: the humbucker. Of course, it's slightly different, since in the guitar case you want to be sensitive to an induced signal (i.e., the signal from the string, which is deliberately made differential-mode by reversing the polarity of the magnets inside the coils), but I believe the same principle should apply. In our case, we'll put the inductors in series electrically, but adjacent and flipped spatially. In that case, the pickup-induced voltages should cancel while the current-induced voltages should add, as desired. It's hard to find CMRR values for high-end humbuckers, especially since they are usually intentially imbalanced for tone considerations, but I would venture a guess that the ~40-50 dB required in this case is not completely out of the question. I've ordered another identical inductor to see what we can do.

1236   Wed Apr 8 21:12:19 2015 DmassCryostatSiFiRe-evacuated large cryostat

You can make them yourselves if you want with activated charcoal and stycast - you just need to bake and pump to clean it. I got some activated charcoal from Keith a while back (it's either in the glass cabinets in the Cryolab, or the fume hood in the ATF), but I think you'll need to buy some new stycast (which you should do anyways if you're going to be gluing stuff in there - the stuff we had for the Cryolab is past its expiration). The cans of what you want are in the ATF fume hood.

If you have room somewhere by a screw hole, you just make machine a mounting block, cover it in HV compatible epoxy (stycast), and roll it around in activated charcoal. To clean it, just pump and bake (the setup to do this for this should be in the CTN lab, if you cannot figure out what it is / how to use it and are nice to him, Dmass will probably help you). If you do it this way, you can unscrew it and pump/bake it when it loses its vacuumy goodness/cleanliness.

If you don't want to keep vacuum but don't want to ruin your getter, just backfill with N2 gas the valve off. Keeping the cryostat open will unavoidably dirty it up though.

1235   Tue Apr 7 16:24:18 2015 ZachCryostatSiFiGetter quote

## Response

My plan was to use the modulators to pump light into RF sidebands, then use the frequency selectivity of the PMC to measure the SB power and back out the actuation strength (Vpi). I was able to do this, to a degree, but I was thwarted by an unexpected issue: the modulators and the fibers coupling to/from them appear to change the output mode emerging from the collimator. What's worse, the mode seems highly sensitive to any touching of the fiber whatsoever. This was most egregious with S/N 10, with which my new cavity coupling maxed out at 83%(!), even after slight empirical MMT tweaking. S/N 2 wasn't as nasty; I got ~91.5% with it.

Given this, my new plan was to make a quick-and-dirty measurement in the following way:

1. Optimize the mode matching and record the contrast defect (17% and 8.5%, for S/N 10 and S/N 2, respectively, as mentioned above)
2. Drive the modulator at a chosen RF frequency (I chose 30 MHz since this is near where we'll be using them), and determine the amplitude necessary to double the reflected power.
3. The measured amplitude is associated with the modulation depth necessary to pump the same fractional power as the contrast defect out of the carrier (really, you could use any SB power level additively distinguishable from the contrast defect, but doubling it seemed the easiest thing)
4. Use the bessel function to infer that modulation depth, then scale the measured amplitude up to infer Vpi.

### S/N 10:

Measured amplitude to double REFL power: 0.78 Vpp --> 0.39 Vpk.

2*J1^2 = 17% --> gamma = 0.611

Vpi = 0.39 * (pi / 0.611) ~ 2.00 V

### S/N 2:

Measured amplitude to double REFL power: 0.52 Vpp --> 0.26 Vpk.

2*J1^2 = 8.5% --> gamma = 0.422

Vpi = 0.26 * (pi / 0.422) ~ 1.93 V

The datasheet claims 3.5 V typical, so this seems pretty good (though the spec is only officially at 50 kHz drive). Holding the amplitudes constant, I also swept the frequency down from 30 MHz to 10 MHz, and the reflected power was stable to around 5%.

Again, this is only really a quick-and-dirty measurement. Unfortunately, the only real way to get a good measurement is to reprofile the beam again with each modulator in place. Then, the contrast defect can presumably be brought down closer to 2% or better again, and the measurement can be made more cleanly. I'm hesitant to waste time doing so, though, given the observed mode dependence on the fiber resting position.

1195   Fri Feb 6 04:46:26 2015 ZachLaserSiFiBeam profile remeasured, test PMC aligned well and locked

I was having some issues with the beam(s) I had previously mode matched into the PMC. Apart from not having gotten great coupling to begin with, the alignment seemed to have drifted over a few days (I noticed this last week). I attributed this to 2 things: 1) the MMT I had was a pretty sensitive one, owing partly to the fact that I had to work with the beam far outside the Rayleigh zone due to the beam beat recombination being upstream, and 2) having the recombining BS in the way, I was susceptible to clipping in the output path I was using for the PMC. I don't really need the beat setup at the moment, and I can do the modulator characterization using a single laser, so I decided to rebuild the PMC test setup using a single laser.

As a first step, I simply remeasured the output beam profile of the West laser using the razor blade technique. The beam seems very circular and not astigmatic, so I only profiled in the horizontal direction. The result:

Using this, I recalculated a better MMT:

------------------------------------------------------------
Other solution:

mismatch: 0.00011786
w0x = 303.7849 um
w0y = 303.7849 um

lens 1: f = 103.2118 mm
lens 2: f = 206.4236 mm
Distances:
d1 = 6.161 cm
d2 = 14.3007 cm
d3 = 29.5383 cm
(Total distance = 50 cm)

I then installed this, aligned the PMC and was able to get ~96% coupling with little trouble. By locally optimizing the second lens, I pushed this to about 97.5%. While a bullseye was faintly evident on the card in the first case, it was very hard to tell what was reflected after the reoptimization.

I borrowed the RF electronics from the steel gyro PMC temporarily (splitter, mixer, bias tee and filters). For some reason, the 1-MHz dither I used with that PMC did not work with this one, but I was able to derive a nice error signal using a 300-kHz dither at 3 Vpp. I wanted to use the uPDH box I used to use before I had the digital servo for the gyro PMC, but I forgot that Eric Q had borrowed it for the 40m. Instead, I was actually able to lock robustly and stably with just an SR560 and a single pole at 10 Hz. The control signal stays within its output range over ~10 min+ time scales. (I didn't bother measuring the loop---all I needed for my phase modulator characterization is essentially a DC lock, and the bandwith was easily 10s-100s of Hz).

The transmission dither lock leaves the REFL port open so that I can measure the rejected sideband light pumped by the modulator as planned.

1194   Fri Feb 6 04:23:20 2015 ZachDailyProgressSiFi - ringdownNo big Q increase at low temperature

I monitored the reservoir level periodically over the day and night. As of the evening, there appeared to be ~1 cm of LN2 still there. As of around 4am, it appears empty, so it should be OK to open tomorrow. I've sealed the vacuum and shut off the pump in preparation.

 Quote: I'm going to wait for things to warm up and then vent the chamber so that we can: Improve the clamp Fix our wiring issues

1193   Thu Feb 5 02:04:39 2015 ZachDailyProgressSiFi - ringdownNo big Q increase at low temperature

Dmass helped me solve the Great Funnel Problem of 2015 by fashioning a foil extender to put in the tip of his metal funnel, since my glass funnel has a spout that is too narrow to get enough nitrogren through it. We spent some time yesterday afternoon filling the reservoir, after which I waited and then came back to see if it was still holding liquid. It was, so I added some more and left it overnight, and there still seemed to be some liquid by late this afternoon.

Assuming the cold volume had had enough time to reach low temperature, I made a quick ringdown measurement, only to find that the Q had only increased from ~4000 to ~8000 between room temperature and now. I think this means that the clamp integrity afforded by the sapphire washer sitting on just the lip of the steel clamp is not good.

I'm going to wait for things to warm up and then vent the chamber so that we can:

1. Improve the clamp
2. Fix our wiring issues
1192   Wed Jan 21 15:21:19 2015 ZachLab InfrastructureCryoNew LN2 dewar delivered

I ordered a new LN2 dewar and it has just arrived. Appropriately, for me, it is #305.

1191   Thu Jan 15 18:27:02 2015 ZachDailyProgressSiFi - ringdownSapphire washers added, ringdown setup rebuilt, higher Q measured

[Nic, Zach]

Yesterday, we opened up the small cryostat and installed the sapphire washers (SwissJewel SP-175). This is hypothesized to increase the resonator Q by reducing the strain energy leaking into the lower-Q steel clamp.

We found that the inner diameter of the washers is slightly too small to accomodate the inner lip of the lower part of the clamp. We were able to make do just by having the lower sapphire washer sitting on this lip---rather than on the full wider area of the lower clamp section---but it is not ideal.

Nevertheless, we clamped it, resealed and pumped the chamber down. As it pumped, I rebuilt the HeNe optical lever readout. When I finished, I was quickly able to tap the cryostat and see a mode ringing at almost exactly 250 Hz, which is known to be the frequency of this cantilever at room temperature. At a respectable pressure of several x 10-5 Torr, I made a quick-and-dirty ringdown measurement using a scope and a stopwatch. I estimated $\tau$ at roughly 2.5 seconds, giving Q ~ 2000. This was already a few times higher than Marie was able to measure at room temperature (see below).

I went down today and did an actual measurment, using the Zurich box sampling at 7 kHz as DAQ. Fitting the envelope by eye, I found a time constant closer to $\tau$ = 5.55 s, giving Q ~ 4300 (I don't think my stopwatch method was all that wrong yesterday, but I do think the residual gas might have been contributing at the time---the pressure is now at 10-7 Torr). This is not only much better than the previous result, but also within a factor of less than 3 of the expected result for Si, according to Marie's data. Given how cavalier we were with the clamping, I'm fairly confident that the sapphire washer idea (and therefore also the monolithic thicker-clamp idea) works as intended.

1190   Wed Jan 14 02:38:43 2015 ZachLaserSiFiPMC set up as test cavity

To continue with the laser/modulator testing, I have added Dmass's old PMC to the temporary characterization setup. I have used the other output of the 50/50 BS that combines the two laser diode outputs, so that we can keep the beat setup intact while also being able to send either of the two beams into the PMC.

To do this, I:

• Made a cursory razor beam scan of the beam emerging from the BS
• Calculated a MMT solution to the PMC mode using some of our new lenses
• Installed the telescope and directed the beam towards the PMC
• Macropositioned the PMC by hand to rougly center it on the transmission of the single-pass beam, as measured using a power meter
• Scanned the PZT using a 0-10 V triangle from an SRS function generator, then used the diode temperature as a coarse adjustment to look for modes
• Maximized the first found mode (a horizontal HOM)
• Looked for nearby lower-order modes, then maximized them and iterated to get to TEM00
• Installed HWP upstream and then maximized visibility by rotating polarization

The coupling isn't stellar yet, at roughly ~66%, but the MMT is fairly tight and I'm sure I can improve easily. The laser and cavity are stable to well within a linewidth at high frequencies, and only drift apart over many seconds.

Some things I plan to do with this setup:

1. Dither lock the PMC to the laser(s)
2. Characterize the phase modulators
3. Set up reflection PDH lock and feed back to lasers
4. More stuff
1189   Mon Jan 12 13:35:31 2015 DmassNotesCavityCavity Parameters from Dec 2014 Labwork

The boring way:

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