The Answer Guy

I was cleaning up my inbox the other day and accidentally opened up the box below Admin, which is Answerguy, and one I had forgotten about. Several years ago I was “The Answer Guy” (or at least I had his email address) for our web page, back in the days when I took care of my research group’s web page. The legacy site is still present but in some sense is no longer “official” and has not been maintained in some time (the cesium fountain page has been dormant even longer). The current official page for work is much more sterile and doesn’t even discuss the clock details at all.

I got a number of inquires over the three or four years that the email address was active. A few crackpotty ones, several good questions, and a few from people who couldn’t find contact info on the pages other departments were maintaining, so there were several moon phase and time-of-sunset inquiries I punted. But I like this one, in particular:

To whom it may concern, My name is Christopher I am a sophomore in highschool and conducting a science fair project on time travel. I am not sure if this is the appropriate email address for this, but i am trying to obtain an atomic clock that measures to the nanosecond. If you have any information please please email me as soon as possible.

Sincerely, Christopher

More PITA Than PEW

I align lasers for a living. Or so it seems.

That’s not actually my job description, but if you observed what I do in the lab over a long enough period, it wouldn’t be an unreasonable description of the plurality of my effort. That job doesn’t occupy as much of my time as it once did; because of some engineering advancements we’ve made, once lasers are aligned they stay aligned for much longer, but there’s still a bit of time spent doing this.

And this is the bane of my existence

That’s a multi-axis fiber port. And they are a complete pain in the butt to align.

The reason for using them is the “engineering advancement” I mentioned — if you couple your laser into an optical fiber, it’s very easy to send that light somewhere else. If you sent it through open space, you leave yourself open to all sorts of problems — dust and dirt on your optics, things blocking the beam, and misalignment issues creeping in — if you send the light just 1 meter, each milliradian of error moves your beam a millimeter, and thermal changes can “walk” a mirror mount to steer in a subtly different direction. If you need precision, long-term alignment, fiber is the way to go.

Free-space alignment is relatively easy. This is a mirror pair we call a dog-leg (golfers might observe it’s two dog legs, but this isn’t golf)

I’ve drawn in a laser beam (you wouldn’t see an actual beam unless there was dust or other some particulates to scatter the light, and we try to minimize that) bouncing off of the two mirrors. Each mirror mount has two knobs: one pushes on a corner for horizontal tilting, while the other pushes on the opposite corner for vertical tilting; there is a small ball bearing at the third corner, and a couple of springs to hold it all together. Adjusting a knob will tilt the mirror and deflect the beam, and change the angle of reflection off of the mirror (and also off of the next mirror). You can un-do the angle change with the second mirror, so that the exiting beam is headed in the same direction as you started, but having been translated to one side. The adjustments are orthogonal, so you can “walk” a beam anywhere you want by adjusting pairs of knobs, as long as the beam continues to hit the mirrors.

That’s pretty easy, in principle; in practice it’s a little tougher, because you often do this “blind” — you are looking at the target or a display, telling you how well you are hitting the target, and the lights might be off. But it’s a skill that’s picked up pretty quickly.

The fiber port, though, is tougher. The chuck in the middle is for an optical fiber, and in my case it’s a single-mode optical fiber. The core of such a fiber is a few microns across and is very sensitive to the spatial mode (the shape) of the light it accepts. I’ve previously shown what a poor mode looks like, but assuming you have a nice zeroeth order mode, you need to send the light in with the fiber positioned just right — not only hitting the core of the fiber, but at the proper angle and with the fiber tip just the right distance from the fiber, to match the mode characteristics.

There’s a little divot on the left side of the fiber port that houses the screw for moving the fiber chuck in the x direction, and one at the top for y, and they are both sensitive and subject to a bit of hysteresis (turning through some angle and then getting back to the original position doesn’t reposition the holder exactly), and also have the annoying habit of walking off when you remove the allen key/driver. The small black dots on the face of the holder are the tip/tilt controls, which also suffer a bit from hysteresis. In addition, to translate in the z-direction (moving the fiber closer or further from the lens) you have to change all three tilt controls. In practice this means your coupling efficiency goes down as you change the first two settings, and you have to hope you can recover your signal as you work on the third. If you can’t, it may be because you are moving the wrong way along z, or because you changed one of the tilt settings too much, so you have to try many iterations to find out what’s going on.

Just so you know it’s not all pew, pew, pew when you work with lasers.

Do You Believe in Rock 'n Roll?

Yes, we label many of the drawers in the lab. Things get lost for too long otherwise. It’s not like school, where the grad students are always there and can tell you what happened to the special wrench for tightening the marzul vanes or the tremmy pipe in the turboencabulator. (I’ve also taken to labeling many of our unique belongings, so one can put it back in a consistent location when finished with it. Assuming one actually puts it away. I’m looking at you, everyone. And me.)

This was a colleague and no, he did not screw up the spelling. The drawer contains audio cassettes for the boom box. (Kids: ask your parents, or go to Google)

Seven Ferrite Rings For the Laser-Lords in Their Dark Labs of Cinderblock

OK, more than seven. Ferrite rings (aka cores) are used as inductive loads in circuits, among other things. Any alternating current in a wire creates a magnetic field. By looping the wire through the ring a few times, you will create a changing magnetic field inside, which will produce eddy currents. These currents create fields which oppose the induced changes (this is Lenz’s law). From the circuit’s point of view (I hope it will excuse me anthropomorphizing it; I know circuits dislike that) this is an inductive load, so the alternating current sees a higher impedance than the DC, and this knocks down any high-frequency noise you might have and reduces what you write onto the components being powered.

Igor, Bring Me a Flat, Horizontal Surface

Carts always seem to be at a premium in the lab. They are supposed to be used to transport equipment — the whole reason there are wheels involved — but they often turn into squatters. They take up residence next to an experiment for an extended period of time, often hosting some equipment that’s also supposed to be temporary, such as an oscilloscope, and become the equivalent of a mobile home in a trailer park: Mobile only in name.

Which is why we got a couple of new carts. I assembled them and took the opportunity to give them the equivalent of vanity license plates:

The two numbers are the hyperfine frequency splittings of Cesium (on the left) and Rubidium. To four digits, at least.

The bulkiness of our electric drill/screwdriver meant that the casters blocked access to the screw-holes. I’m not sure who originally said If necessity is the mother of invention, then laziness must be its father, but I didn’t see any reason to tighten 32 big screws by hand, so I kludged this together with help from a few pieces from our socket-wrench set

Now we can race the carts around the hallway, until someone loads them up. Oops, too late.

Clang II: The Wrath of Oersted

When last we left our intrepid physicists, they (meaning we) had, after many trials and tribulations, found a magnetic washer that was messin’ with our clock. We removed it, killed it and had it stuffed and mounted on the wall. A couple of you posted congratulatory notes on the success of the mission. I had delayed posting the story until we had confirmed that the vacuum was intact, because Murphy has a way of penalizing premature celebration. And the vacuum was fine the next day. Nope, no problems with the vacuum.

But I still should have waited. The problem is that there was still an icky nasty foreign magnetic field. At first we thought that maybe we (meaning I) had screwed up the shields by getting a magnet too close. And by too close, I mean I accidentally waved my hand too close to one part and heard a resounding THUNK! as the magnetic attraction (however the funk that works) overwhelmed my grip. We routinely degauss the shields, but there’s always the spectre of “burning” in a strong enough localized field that the degauss-o-tron can’t handle. That made for a bit of stomach acid on my part until we confirmed that the shields were fine. The problem was localized to the location of another bolt.


But it wasn’t just a washer this time. There was another one, even though we had asked the magic eight-ball if the problem was fixed and we got a “Signs point to YES” answer. At this point we were incredibly paranoid and with good reason — the pathology was far more sinister. We had noticed that some of the titanium bolts we used would have titanium chips inside the socket head, from the broaching process used to create the hexagonal shape. But they’re titanium chips, right? It’s only a problem if they keep you from tightening the bolt. Or so we had thought, and while we did clean the bolts up, we weren’t as anal meticulous as we needed to be: the problem (most likely one, at least) is that the tool itself is steel, and tools will occasionally chip. And that chip can get caught in with the broaching chips, lodged inside the cap-screw, and that, my friends, is pure evil.

So we scraped and tested — I did my best Nick Stokes impersonation and gathered the specks on some tape so I could wave it in front of the detector, and pretty soon we found one that buried the needle (metaphorically, at least; it was a digital meter)

That’s the one who slimed me. The ugly little spud above the 5″ line. (Yes, we have English-system rulers in the lab. They are a nanosecond long.)

Everything seems to be fine now. This time I waited until I could check that things were running well, and confirm we’re moving on to tackling the next gremlin in the lab.