Maser Amplification

Hmm, well I don't know all the sticky details, and there are a few 'flavors' of Maser. But in broad general terms, you create a population inversion in the system. For many masers it will be a population difference in the 'magnetic' states of the system. (Magnetic levels could by hyperfine like transitions or Zeeman ones.. or maybe even a combo?) Once you have a population inversion, you can then shine light (E-M radiation) with the right polarization onto the system and cause stimulated emission... you get more radiation out than you put in. That's gain. The details of how you couple E-M into and out of the system, and how you tune the energy level difference between the states to match the frequency you want to amplify... are the sticky details.

George H.

Ah, I see, at least in general terms. Thanks.

Warren.

You can shoot a signal down a waveguide that has a suitable gas, and also inject a higher-frequency pump signal. Molecular down-transitions from the pumped energy states can amplify the microwave photons.

You might like Townes' book "How the laser happened."

So many eminent people declared it to be impossible (including Einstein) that it almost didn't happen. People were convinced that thermodynamics didn't allow the necessary population inversion.

Create the right inverted population and feed in an incredibly weak signal at one end. You get amplification at the other end by stimulated coherent emission. In its day the Telstar receiver on the Goonhilly downstation was the most sensitive low noise receiver on the planet.

Has a short paragraph about the amplifier and dish which was a scaled down version of Jodrell Bank and its drive capability which had to track non geostationary satellites across the sky as they rise and set.

They put the downstation as far south as it is possible to be on the UK mainland to maximise Telstar visibility. The original Telstar was trashed by the US exoatmospheric MT nuclear test Starfish Prime.

Natural OH line masers occur in nature in molecular clouds. UV bright stars pump them into an excited state and stimulated emission give them insane apparent brightness in the maser line at ~1665MHz (and others).

The astrophysical maser Wiki entry gives probably the best explanation of how a maser amplifier can work in a single pass.

Amplified stimulated emission is a more appropriate term with the stimulation being provided by the weak signal from the satellite.

In a classical gas laser the ionised gas provides the amplification and the mirrors at either end act as a Fabry-Perot etalon to increase the effective path length through the medium and force the radiation to match the boundary conditions (integral number of half wavelengths).

There are some single pass gas lasers that don't need mirrors. ISTR a low pressure nitrogen UV laser project in SciAm AmSci back in the 70's.

That amplifier had a noise temperature of only 15 K, an impressive number for the date, but required messing with liquid helium (4 K).

These days HEMT LNAs achieve nearly as good figures at room temperature.

Nice, Feynman does a nice job of describing the ammonia maser.

Good thing chaos theory did

Not so much chaos theory. Townes figured that excited, pumped non-equilibrium inverted states might last long enough before decaying to get some work done.

His original gadget was a cavity oscillator, which was easier to build than an amplifier, but demonstrated the concept.

Thanks John. That looks like an interesting read. I'll certainly get myself a copy.

While a lot of this microwave/quantum stuff is over my head, I still find that the discoveries are fascinating to read about. I especially appreciate when ppl go against popular opinion, only to discover that the impossible was indeed possible.

Einsteins paper on radiation (1917), A and B coefficients etc. Is a gem! I remember reading it in an undergrad laser class, and loving the logic in it. Dan Kleppner wrote a nice piece, and the original paper (in English) can be found here.

George H.

Thanks George. I've read several books about Einstein, though much of the physics is too deep for me. I find it interesting that he never warmed to the probabilistic idea of quantum mechanics.