Fresh Wallaby Juice

Been a bit frustrated at work this week – well, okay, for at least the last year now, but at least this is a different frustration. The Task of the Fortnight seems to be to set up an interferometer to measure laser pulse width.

Complication: There are three different lasers in the lab which it could be set up to use. I don’t actually care what the pulse width starts out as – my experiment involves changing the width so a “before and after” measurement is good enough. So choosing a laser table is much more a question of how often I’ll get to use the setup, since some tables are much busier than others. Of course, it wasn’t as simple as choosing just one – first I set it up upstairs and got it nicely aligned on that table. (That was the one table we set up for “sharing” laser time from the start, with beam splitters and such to support about three different experiments at once.) Then it turned out that (A) the microscope on that table is sensitive enough that nobody can work there when it’s being used, and (B) someone is going to need that microscope pretty steadily for the next few months.

Lug it downstairs to the table where I’d originally set it up last fall. But wait, this laser is sick! Pulse width kept climbing and the output power dropped. Go back three spaces…

Now it’s perched happily on a third table, and it seems likely to stay. Draw a “Lab Gremlins” card… aha! The power output from the beam I’m using is too much for the detector (two-photon photodiode). Can’t use a filter, that would affect the different frequencies of light differently, producing what is known as “chirp” – the pulse spreads out and the shape changes.

(Yes, there’s ways to compensate for that. No, I don’t really want to set one of those up, even if we had the parts handy. Which we don’t, because it’d just be silly for a spectroscopy lab to have any extra optics lying around. Some days it’s actually hard to find a piece of plain old glass. Much cheaper to pay three postdocs to spend a day digging around in a closet to find something that will work one-tenth as well, then spending a week setting it up. But I’ve already done that rant.)

So I’m using a single piece of glass as a very lossy mirror, passing about 3 mW of power into the interferometer. Well, that’s supposed to be enough to get signal, but 3 mW at 800 nm is extremely difficult to see. At least our infrared view-o-scope is working this week, usually we’re out of batteries. “Difficult to see” makes the whole thing difficult to align, unfortunately, and so I’ve spent three days on it now and I still don’t see any pulses.

All this is partly with the help of another postdoc, who’s been around forever and has forgotten twice as much about ultrafast lasers than I’ve ever learned, and who’s usually a very good source of info. In this case, not so much – he’s only set this thing up once himself and that was three years ago. “Interferometers are – difficult,” he noted. I agreed. This will be the fourth time I’ve set this thing up, and after I finish I’ll type up a proper set of instructions/notes – the “manual” is both short and poorly translated from Russian. For that matter, so’s the postdoc. (Perhaps I’ll call him “Manuel” – it’d be nice to think of elegant/cute code names for people, the way Mo did.)

His big help today – finally coming to the title of the rant – was to come downstairs and explain exactly how to align the thing. After explaining, again, what I should expect to see on the oscilloscope screen, and how it all works. And demonstrating how to check whether the laser pulses were actually reaching the detector or not. (Hint: Block the beam and see if the output drops.) “It’s very simple once the beam can reach the detector, that’s all you have to do,” he encouraged me. “Yes, that’s what I was trying to say yesterday – that it isn’t reaching the detector. But thanks.”

I’m beginning to wonder if there’s something about working around ultrafast lasers that actually prevents communication. They’re quiet things, and the basement lab is nice and quiet and dark. But sometimes it’s really a stretch to get a point across – and not just with the foreign students, either. On the bright side, all but one of us whose native language is English is extremely easy to communicate with. On the down side, the exception is the boss.

Postdoc (“Manuel”) noted that it took him a week when he did it, so after three days there’s still time to beat his record. Again.

Tags: rants