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The first "Laser", the Maser, amplified stimulated microwave emissions, rather than visible light that is the most common thing today. Why?

If we take a simple example, and consider the ratio of the rate of spondaneous emission and stimulated emission

$$ Ratio = \frac{R_{spont.}}{R_{stim.}} = e^{\frac{h\nu}{kT}} -1 $$

Comparing two different cases: 1) a microwave source operating at a frequency of $10^{10}$Hz and at 300K, and 2) a tungsten lamp with operating at a frequency of $5\times10^{14}$ at 2000 K. The ratios become $R_{Tungsten} = 163605$ and $R_{Microwave} = 1.60198\times 10^{-9}$. So, clearly, the probability of stimulated emission is much higher for longer wavelengths.

Using these results, why did the Maser come before the Laser? I can understand that perhaps the Maser resulted in a higher intensity beam than a Laser would at that time, but I suppose they could still make a working Laser.

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The problem with making a laser is getting a population inversion. Optical lasers tend to us a three level system i.e. they excite atoms/molecules from the ground state to an excited state, and this excited state then decays rapidly to a relatively long lived third level. Lasing then involves transitions between this third state and the ground state.

But finding systems that can manage this trick in not a trivial business. It's hard to predict in advance what systems will be suitable so building the first optical laser required considerable trial and error.

By contrast creating a population inversion in a maser is relatively easy. The energy spacing is smaller than kT so the ground and excited states start with nearly the same population and no pumping is needed. Furthernore the lifetimes of the excited states are long enough that a population inversion can be obtained by simple physical methods.

For example in the ammonia maser (the first maser built) the ground and excited states have different electric dipoles so they can be separated using an electric field. You start with a molecular beam containing ammonia molecules in thermal equilibrium and use a nonlinear electric field to split the beam into separate ground state and excited state beams. The ammonia molecules in the excited state beam are then in a population inversion and can lase. This is how the first maser was built in 1953. This approach wouldn't work for an optical laser because the lifetime of the excited state is too short for you separate it from the ground state.

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  • $\begingroup$ Thank you. Could you try and relate your answer to the calculations in my question? Can this difficulty in population inversion for visible light Lasers be understood in terms of the ratio of the rates? $\endgroup$ – Yoda Nov 15 '16 at 17:11
  • $\begingroup$ @Yoda: it's not the ratio that matters, it's that spontaneous emission at microwave frequencies is slow enough for a population inversion to be created by physical methods. $\endgroup$ – John Rennie Nov 15 '16 at 17:14
  • $\begingroup$ Okay, I see. The reason I'm asking is that the above question is basically an exam question. Using the results from calculating the two ratios, we were to motivate why the Maser preceded the Laser. $\endgroup$ – Yoda Nov 15 '16 at 18:29

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