Why was the AC Stark effect only discovered after the laser was invented? I was reading a paper about the theory of the AC Stark effect, and I was struck by the following assertion:

For instance, the first observation of the AC Stark effect initiated by the laser radiation field was carried out by Bonch-Bruevich and coworkers in 1969 [1].

*

*Bonch-Bruevich A M et al. Zh. Eksp. Teor. Fiz. 56 144 (1969) [Sov. Phys. JETP 29 82 (1969)].


This puts a gap bigger than 50 years between the observations of the DC and AC versions of the Stark effect, a period in which a huge body of atomic and quantum physics was developed.
Is this because the effect was not feasible to observe before the creation of the laser? In other words, would it have been possible to observe this effect with the experimental toolkit available in the mid to late 1950s?
I ask this because it is quite reasonable to imagine that Bonch-Bruevich and coworkers simply stumbled upon this shift by building a laser (as you do with any Hot New Thing) and just shining it on stuff to see what new phenomena they might shake out $-$ but it is also plausible to imagine, at least in principle, that this effect was simply not accessible to any light sources before the advent of the laser. After all, the laser did provide a massive change to the available spatial and temporal coherence as well as focusable intensity.
However, I'm not really sold by that latter argument. Surely it's possible to build a large, bright arc lamp which emits reasonably monochromatic light, and then focus a large fraction of its emission into a gas cell. The resulting radiation will have a much lower temporal coherence than a laser source $-$ but then again temporal coherence isn't particularly important for the AC Stark shift. Would such a light source (either as available in the 1950s or using a modern version) be able to produce a sufficiently high Stark shift to be detectable (using a suitable spectrometer resolution, either from the 50s or from modern capabilities)?
 A: The answer is photon statistics. Before the invention of the laser, i.e. with thermal light sources the mean photon number occupation per mode is
$$
\bar{n} = \frac{1}{e^{\frac{\hbar \omega}{k_B T}} - 1} \text{.}
$$
Using a hot $T = 8000\,\text{K}$ arc lamp there are only $\bar{n} = 0.107$ photons per mode around the D2 transition frequency of $\omega = 2\pi \cdot 389 \,\text{THz}$. After filtering and all the optics it would be reasonable to assume that maybe $0.01$ photons per mode actually make it to the Potassium vapor cell.
For any nonlinear effect like the AC stark shift both, the pump and the probe photon, must interact with the observed atom within a short time interval. Hence, with only $1/100$ photons from each beam, the chances of a nonlinear interaction is $\sim 1/10000$. This might be observable with equipment from the 1960s if you had a lot of signal (high intensity), but again the power is the problem.
In contrast, in the paper they produce pulses (of presumably one or only few modes) with a peak power of $1 \,\text{MW}$, that's $10^{17}$ photons within the pulse of "10–8 – 10–7 sec" duration. So even with a highly multimode laser pulse the strength of nonlinear interactions is many orders of magnitude larger.
What do I mean by mode? And why are they important?
A Fourier-limited pulse is called "single mode", while a pulse of the same duration, but with twice the spectral width would be a two-mode pulse. Analogously a TEM00 Gaussian beam is a spatial single mode, while an incoherent mixture of a TEM00 and a TEM01 beam is two different modes, because the spread in tranversal wavevectors is larger than for a single-mode Gaussian beam of the same waist. 
More generally, the 6-dimensional configuration space (position vector $\times$ momentum vector) can be tiled by Fourier-limited tiles (of any shape) within which particles are indistiguishable. For photons you need to consider a 7th dimension, polarization. Each tile represents a mode, together they form a complete orthogonal basis for the phase space.[1]
I'm formulating this so abstract, because the number of photons per mode can only be decreased by linear optics, like lenses and beamsplitter, but never increased. This is independent of the choice of basis to represent the modes. I think for the frequency, it is pretty clear that there is not linear device taking in photons of multiple different frequencies and outputting the same number of them converted to a single frequency. For the distribution of transversal extent and wavevectors the conservation of photon number per mode is probably better known as the conservation of étendue, which is the phase-space equivalent of Liouville's theorem. I gave 3 different explanations of it in "Is it possible to construct a lens which focuses all the light rays from an extended object in one point?" and it is the reason why the answer to "Is it possible to start fire using moonlight?" is no.
This case
Let's choose a basis for the modes, in which the interaction of an atom with light can be described by a single mode. This is typically the picture people use nowadays, because everybody works with lasers, which emit only a single mode of light.
The mode of interest in this case is the one which matches the radiation pattern of the atom. Spatially, this is the classical dipole emission pattern. Only the part of the light inpinging on the atom in this shape can interact with it. More rigorously one needs to calculate the overlap integral of the ingoing mode with the dipole emission mode.

In the time domain the mode of emission is an expoentially decaying pulse, which is the spectral domain is a Lorentzian. Again, only light which matches this mode interacts with the atom. (Side note: If the mode of light exactly matches the time-reversed spontaneous emission, a single photon can excite the atom with 100% probability.[2]) The spectrum of the D1 and D2 lines of potassium have a linewidth of $\Gamma = 2 \pi \cdot 6 \, \text{MHz}$,[3] which is extremely narrow compared to the blackbody spectrum emitted by an arc lamp (hundreds of $\text{THz}$).
Let's do the above calculation of photons per mode in a more classical way: Optimistically assuming one can build an experiment in which the spatial mode matching is as good as if the atom is placed directly on the surface of a blackbody, i.e. it is illuminated from a solid angle of $2\pi$. Then we only have to care about the spectral overlap. The power per surface area per frequency interval $d \nu$ emitted by a blackbody in $2\pi$ solid angle is
$$
2\pi \frac{2 h \nu^3}{c^2} \frac{1}{e^{\frac{h \nu}{k_B T}} - 1} d \nu \text{,}
$$
that is $5.856 \cdot 10^{-7} \frac{\text{W}}{\text{m}^2 \text{Hz}} d \nu$ for the above used parameters. So in a $d \nu = 6 \, \text{MHz}$ wide frequency interval it is $3.514 \frac{\text{W}}{\text{m}^2}$. Within the scattering cross-section of a single atom $\sigma_0 = \frac{3 \lambda^2}{2\pi}$ this is $9.728 \cdot 10^{-12} \, \text{W}$ or $3.771 \cdot 10^6 \, \text{photons/s}$. The number of photons interacting with the atom per excited-state-lifetime $1/\Gamma$ is therefore $\bar{n} = 0.101$.
