Helium Neon Laser Calibration 2008-04-23, NIKON D70s, 46mm at 6 seconds ƒ/25.0 College Science Laser Lab
This is a quick snapshot of what I’m working on now. The really bright line is the lasing medium. It’s a glass tube filled filled with helium and neon. We run electricity through it to excite the helium atoms, which then run into the neon atoms and excite those too (they share an energy level.) That excited state of the neon atoms is metastable, which means that because of the quantum selection rules about how atoms can change states, they’ll stay at that energy level for a long time. When enough of the neon atoms are excited, you get what’s called population inversion, and stimulated emission (where one photon hits a neon atom and two identical photons are emitted) becomes a chain reaction.
If you look closely, you’ll see a thin beam in the right side of the image: that’s the laser light itself. It’s going through an iris, which we use to force the laser to operate in lower-level transverse modes. Right behind that is a 99% reflective concave mirror–the output coupler, which reflects photons back into the medium and lets the laser reach criticality. The beam is just barely visible in air.
2 exposures, composited so you can see clearly.