 beat PD is an 1811 with BW ~ 125 MHz, while the cavities have an FSR of >2 GHz), scan the second laser until the beat is found.
4. Once beating, scan the second (still unlocked) laser in both directions and find the closest resonance, then lock.
5. In the worst case, the two cavities' closest modes are ~1 GHz apart (i.e., FSR/2), so it should be possible to see a beat if the procedure is done properly. For the real experiment, we'll have some slow length actuation (e.g., via temperature) so that we can bring the two cavity modes as close together as we want.

Returning to the actuation situation, Dmass actually prefers that I hijack his diode drivers temporarily, rather than trade for his ITC510 unit (even though he is not using the diode driver side of that unit, he is worried about having to retune the temperature loops with a new unit). This is actually way better for me, as well, since it will really let me tune up my servos the way I'll want them when I grow up and get my own Rich drivers. Now that the optical layout is more or less complete (at least for the short term), I can focus almost exclusively on the loops. There are some spare long BNC->DSub cables for exactly this purpose that I can use to drive my lasers without moving the drivers from the CryoCav rack, so I'll get on that shortly.

1249   Thu Apr 30 17:53:20 2015 ZachCryostatPurchasesGetters gotten

Being the go-getters we are, we got our spare getters gotten. They'll be stored in the cabinet for when we need them.

1250   Thu Apr 30 18:00:47 2015 ZachUpdateSiFiDiced wafers received

We re-received the 6" wafers we had sent to American Precision Dicing to be chopped up last week. One wafer was cut into 10 cm x 1 cm rectangles, and the other into 5 cm x 1 cm ones. They came back in some super cool packaging.

1251   Tue May 5 17:10:14 2015 ranaUpdateSiFiwafers received

NOVA wafers are here and on the VIP desk in Koji's office.

ZK: Wafers unpacked and stored in 050.

1252   Wed May 6 18:01:02 2015 nicolasLab InfrastructureDrawingsCryostat base plate

To lift the cap screws of the cryostat off the table, I've designed this base plate.

It has 8 holes that are coincident with the screw holes on the bottom of the cryostat. 4 of them let the full cap pass through, the other 4 will fasten the cryostat to the base plate.

There are slotted holes for mounting to the table, but dogs could also be used.

The four corners are slightly thicker than the rest of the plate (0.5mm) to make feet.

Edrawings and solidworks files are in the SVN @ 40mSVN/SiCryoSus/Designs/SiFiDewar/BasePlate

Attachment 1: baseplate.png
Attachment 2: baseplatebottom.png
1253   Thu May 7 03:35:49 2015 ZachElectronicsSensorsRL readout PD with humbucking

I got the second inductor in recently, so I tested out the humbucking circuit. Here are the results of a quick trial:

Notes:

• As you can see, having the humbucking coil greatly reduces the influence of noise pickup---particularly the flurry below a few hundred Hz.
• As measured by the mains lines, the rejection is not that great at the moment, at something like a factor of 10. This measurement was made simply by putting the reversed coils next to one another, but it's likely that I'll have to do some more tuning to get them matched to the ~1% level I'd like.
• Aside from residual pickup, the noise is exactly what is predicted by LISO
• A caveat is the noise peak feature at ~40 kHz, which is probably something to do with the op amp loop. Investigating...
• Considering that this is in a relatively high-EM environment, things are already looking pretty good. The DC Z of this circuit is ~100 ohms, so it can measure RINs of ~10-11/rtHz above 100 Hz

Hartmut says that they see excess noise in their circuit when using it for DC readout, which he believes is likely Barkhausen noise. So, the next thing to do is inject a reasonable, mildly fluctuating DC current and investigate the noise performance in that case. The only method I know for creating a current with low enough audio noise to see this level is using the buffered active-lowpass-reference scheme (e.g., what we used to drive the LEDs in our OSEM testing days and what is used as the bias on the M2 board). Not thinking too much in advance, I connected the M2 bias (5 V) to the circuit, which therefore drove a ~50 mA current through the coil, and the output noise was not changed significantly. In hindsight, this is not surprising: the mentioned current generation scheme is actually just a voltage reference, and so the voltage atop the inductor was actively fixed at 5 V, so that what I saw at the output of the entire circuit was just the noise of the 5 V reference circuit.

So, I need a true current source, and I'm not sure a good enough one exists to inject tens of mA without swamping the dark noise of this detector completely. I think the best I can do is to use whatever I can and then just verify that no excess noise is observed above the input current's own noise. Perhaps it's easiest to just do it optically: using balanced PDs, stabilize the light to around the shot noise level with a normal M2 transimpedance channel, then read out the OOL PD with the RL humbucker.

 Quote: 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.... I've ordered another identical inductor to see what we can do.

1254   Wed May 13 06:37:30 2015 ZachPhotosSiFiA taste of things to come...

I've been working on an early-stage mockup of the in-vacuum setup for the main experiment for my GWADW poster. Here is a cutaway shot of what I have so far and also a detail of the suspension. The breadboard at the bottom is what I'd like to get made custom by ThorLabs (plus a couple mods we'll have to do ourselves, like the holes for the OMC-style captive ferrule joints). The blades are OMC bottom-stage blades, but HSTS lower stage will also work. We need to see what spares are available.

More later.

1255   Sat May 16 15:34:47 2015 ranaPhotosSiFiA taste of things to come...

Might be able to get blades made instead of just using spares. Crackle might have some spares and they also have a place they get them made. Gabriele ought to be able to give us advice on whether to use as is or modify by adding constrained layer damping ala the ISI.

1256   Sat May 16 22:06:55 2015 ZachPhotosSiFiA taste of things to come...

Talking with him, I got the impression that going with spares, if they are available, is the best course. We can probably do better by designing new ones, but I think this can be a time-expensive process. I'm going to see what options we have "off the shelf" and then we can consider a custom job.

 Quote: Might be able to get blades made instead of just using spares. Crackle might have some spares and they also have a place they get them made. Gabriele ought to be able to give us advice on whether to use as is or modify by adding constrained layer damping ala the ISI.

1257   Sun May 17 18:39:48 2015 Rana(?)PhotosSiFiA taste of things to come...

Sounds wise. But if you have a good idea of what's needed in terms of mass for the platform (with appropriate extra trim mass added for margin), then you can get some going into the machine shop queu now and not have to wait for 3 months later on.

1258   Mon May 18 12:31:25 2015 ZachPhotosSiFiA taste of things to come...

EDIT (ZK): The rough estimate for the payload as it is currently envisioned is about 3 kg, considerably lower than the OMC's 7 kg. This can be brought up closer to 5 kg by making the breadboard out of SS instead of Al. That makes it a little closer to a reasonable pre-trim weight, but it's also the case that the clamp-and-cantilever side and the macromirror side are balanced to below 10%, as it turns out, so this would just be a 2-kg dead CM addition. Back to the materials question, the bending mode issue is a wash, since the frequencies are roughly the same in both cases here.

So, I'm going to talk to