Thursday, July 5, 2007

Some Knurdly Background about Lasers II

So the trick of getting “amplification through stimulated emission” is the “inverted population,” having enough excited atoms or molecules in your mix that they predominate, rather than having the “ground state” atoms or molecules absorb your photons and shut it all down.

The first gizmo that worked got its inverted population by separating out the excited molecules. Then this guy, Townes, obviously obsessive about the matter, got even more clever. He had the idea of using a “metastable state.”

I’ve been talking as if there were only two states, a ground state and a single excited state. But atoms and molecules actually have a lot more quantum states than that. And some of them are what’s called “metastable,” which just means that they take a long time to decay, relatively speaking.

If you hit an atom with enough energy to put it into a state higher than the metastable state, say state 2, or 3, or whatever, with the metastable state being state 1 and ground state being 0, then it will quickly decay from the higher states, but will linger in the metastable state. Presto, an inverted population.

Then Townes’ brother-in-law (okay, it was the respected Bell Labs’ physicist Arthur Schawlow, but he’d married Townes’ sister) had the idea of putting mirrors at the two ends of a lasing cylinder. This increased the path length of the photons going through the laser medium, but only in the direction at right angles to the mirrors. Moreover, if properly adjusted, the mirrors could be used to “tune” the laser to a single frequency, just like a microwave cavity. Finally, because Bell Labs was very big on solid state physics in those days, Schawlow suggested using solid state materials for the lasing medium.

By this time, Townes’ work had attracted attention and a lot of other bright guys had been added to the mix, guys like Gordon Gould, Nikolay Basov, Aleksandr Prokhorov and Theodore H. Maiman. Some of them had overlapping ideas, some had novel ideas, and pretty much all of them got involved in priority, patent, and other kind of squabbles over the next years and decades. They also won a lot of prizes, made a lot of money, got famous, the usual, and there were plenty of it all to go around.

The result was the first laser, made by Ted Maiman at Hughs Research Labs, which used a synthetic ruby as the lasing medium and was “optically pumped” via a flash tube wrapped around it. Within months, the Iranian physicist Ali Javan, working with William Bennet and Donald Herriot, made the first gas laser using helium and neon. Laser diodes were developed within two years, though we had to wait into 1970 for some that worked at room temperature.

When the laser was first announced, practically every news story referred to it as “a solution looking for a problem.” That was about as ignorant a statement as has ever been made. A strong, monochromatic, coherent light source? Man, there were scientists and engineers who had been making do with crappy things like sodium light through a pinhole for decades. Holography had been invented in 1947, but it needed lasers to make it work. There’s a patent that was filed in 1961 using a technique called a “two-dimensional fourier transform” (ask me how I knew to do a search on that phrase sometime) to interpolate between images in an animated film, that calls for a “small, coherent, monochromatic light source.” God only knows how long the inventors had been sitting on that one, waiting for the laser to be invented.

But that wasn’t the really big deal. No, the first thing we all heard about lasers doing was punching holes in a diamond. And fan boys everywhere went, “Hurrah! We have a Disintegrator! Or a Death Ray!” Frankly, we didn’t much care which. ‘Cause the ghosts of Hugo Gernsback and Amazing Stories were yelling, “Hot damn!”

And yes, Gernsback didn't technically die until 1967, and Amazing Stories has died and been resurrected so many times I've lost count. But their ghosts still walked the land in 1961, because you don't have to die to leave a ghost.

1 comment:

Blogger said...

Quantum Binary Signals

Professional trading signals delivered to your mobile phone every day.

Follow our signals NOW and make up to 270% daily.