Dentist Time

[Title from my grandfather's favorite joke: When is the best time to go to the dentist? 2:30! (Tooth-hurty)]

Earlier I said I would describe what I'm doing here in Florida, and this week I'm going to talk about my work on LIGO. I'm part of the engineering department here, so rather than the data analysis that I usually do, I'm more involved in the mechanics of the detectors. I described in my first post on LIGO how gravitational waves stretch and squeeze space. That stretching is a proportional factor, so the longer the distance, the greater the change. The scaling factor is so small though that to have any hope of picking it up, we need an enormous distance. The detectors are 2.5 miles long, but on top of that, we use resonant cavities that bounce the laser beam back and forth many times:

The laser goes through two partially-reflective mirrors that concentrate the light in a small area. The mirrors have to be precisely aligned to make the light add, instead of cancel out:

The laser we use in LIGO has a wavelength of 1064 nm, which means the difference between making the peaks add or cancel is only 0.000000266 meters!

This past week I was attending some (virtual) workshops on a tool we use to simulate cavities called Finesse. Adapting some of the code my colleague Luis Ortega wrote, I made a plot of the laser power that builds up in the cavity if we send a 1 Watt laser in, and have 85% reflective mirrors on either end:

One quality of a resonant cavity is how quickly the power falls off when the length is changed. Here you can see that if we have things just right, we can increase the power by 6.5x, but any error, and the output quickly drops to near zero.

The part that I'm working on is making simulations of the suspensions that help prevent the mirrors from moving out of alignment. Maybe in a future post I can discuss that, but I wanted to start with the basics, since I haven't been thinking about this stuff for as long as my coworkers.