Thursday, July 29, 2004


So, now we're sending laser light into my little rubidium vapor cell, which looks like an empty glass tube. Doing this makes the rubidium gas glow brightly -- in the infrared range, so it still looks like an empty glass tube.

Today was about trying to focus a laser on the center of the cell, and then trying to focus the invisible glow on my little APD ("Avalanche Photo-Diode"), a light-detector which is connected to an amplifier, which is connected to an oscilloscope. If we set the laser to scan through a range of frequencies, the glow blinks on and off, as the laser hits the "resonant" frequency and then scans past it. On the 'scope, the little dancing dot draws two nice big peaks when the glow is "brightest" (but still invisible), and smaller peaks for less intense spectral lines. It's still a kick for me to see any theory actually work, especially in an aparatus that I built. Part of me never really believed in quantum mechanics, and spectral lines, even after seeing them in undergraduate labs. Subconsciously I must have assumed there was some trick to it. But this, I set up -- I know for a fact that it's not a special effect.

But I must admit it's not really the oscilloscope view which I find most convincing, even though it produces these text-book peaks. It's watching the cell wink which actually excites me. I say that this glow is infrared, invisible... But we have an infrared viewer. And if you look through it, there it is! A bright line of glowing gas, inside my empty cell. I can see the beam narrow when I put in a lens to focus the laser. I can see it flash as the laser scans frequencies. I can see it reflect off the post that's holding another lens, and I can see it cast shadows that aren't there in the room light. It's beautiful and uncanny. Of course I'm willing to believe in quantum mechanics after seeing something like that... Flickering "spectral" lines that can be seen as a green glow only if one looks through this special crystal viewing device... I'm half willing to believe in ghosts!

But even so I can't believe that anything I've built actually works.


Anonymous said...

Uh... congratulations?

-Simon W, recalling a 4th-year project (not mine) which basically went "Build a laser. From scratch".

Mary said...

We've got plenty of homemade lasers in the lab, but they're not exactly "from scratch" in the sense that you have to pump them with laser light of a different frequency. Eg, put green light into the crystal, get infrared out.

Most dye lasers work the same way, so I think to get laser light that doesn't depend on already having laser light, you have to start from a nice box of low pressure gas with mirrors at each end, which may take more manufacturing than a student can do by herself...

But if you're given the mirrors and the vapor cell, in principle building that kind of laser from scratch is easy too. Just align everything *perfectly* and swear when it doesn't work. [g]

Mary said...

Incidentally, this is a fun page: