Your mission should you chose to accept it is to point this laser straight down the centre of the cavity. This sounds simple doesn't it? Lasers point in a straight line, and the cavity is also straight.
Oh, did I forget to mention that to get it into the cavity you will have to use 2 mirrors to reflect the beam, and then the cavity has a mirror on each end which the beam must be exactly perpendicular to and hit centrally.
This is what we were faced with a 9am today:
We had had been briefed yesterday by our supervising professor, he spent over an hour talking us through it but had warned that there was a good chance we wouldn't manage to get our laser aligned perfectly in the first lab session. Despite the fact we knew just a fraction of a degree of misalignment on any of the 4 mirrors would lead to a huge walking of the beam (due to the nature of technique) we were still confident - how hard can it be?
We spent 3 hours this morning (the total of our lab session) turning various screws to adjust the mirrors, moving the laser beam by fractions of a degree, to be visualised in various ways. Several times we were sure it was perfect (the first of which came after just 45 minutes!), but each time we fired up the photomultiplier tube, hoping to see the characteristic ringing down on the oscilloscope we were greeted by a simple laser pulsing. We had at least managed to get the laser through the tube, but no matter how hard we tried we couldn't get it aligned enough to collect the data we needed.
We had a chat with the professor midway through the session, and afterwards too and decided that although part of the problem may just be our ineptness a could be down to the fact that the laser had been mounted too high (and due to the safety procedures in place this was something we couldn't fix ourselves), and this was making it difficult to get a perfectly horizontal beam. They're going to lower it for us in time for the next session on Monday.
Really hoping we don't spend all day Monday having to do the same thing, it would be really nice to actually collect some data. It's also making my lab notes really boring, given that we've just been following basically the same procedure each time we've tried to align. I should ave written it out once and then just said repeated lots of times!
Despite our epic failure to achieve anything it was a remarkably enjoyable sessions (high praise coming from me 'cos I really don't like spending time in labs!), there was no pressure on us because it was expected that we wouldn't manage it today (and if we did it would have been a pleasant surprise), and we were just left to it so it was nice to be given the opportunity to work out how to do it all ourselves and work independently (and without anyone breathing over our shoulders!). We could happily do this knowing that with the possible exception of the PMT we couldn't break anything, and there were various mircoswitches and safety features in place so we couldn't do any harm by getting it wrong.
So yes, that was labs today. Tomorrow is 3 hours of biology/organic chemistry lectures (mainly somewhere in the middle) followed by seeing new specialist doctor guy. Already getting a bit nervous...