Stimulated Scattering

What happens when your laser turns the air into a laser? You get stimulated scattering, and it can be a big problem for some kinds of beams.

There is a physical phenomenon called Raman scattering where a photon can bounce off an atom or molecule and create two lower energy photons that add up to the same energy. Usually one of them will be deep in the infrared, the other just has all the energy that wasn’t used up making that infrared photon (which is usually most of it). This is usually a very weak process, and for practical purposes we wouldn’t usually need to worry about it. However ...

If you have a whole bunch of photons of the same energy (like a laser beam) going past atoms (like if the beam is in the air), and a photon of just the right energy happens to come past corresponding to the Raman scattered photons (like if one of the other photons in the beam had undergone Raman scattering), it can make a photon that normally would have just minded its own business undergo Raman scattering to give a photon that matches the incoming scattered photon. Then this scattered photon can make even more of the original photons also scatter. This is just the same stimulated emission process that makes lasers work, but now it is removing photons from your beam!

In order for this to become an issue, you need high powers and long distances over which the scattering amplification can occur. Unfortunately, the two things a laser weapon wants to have are high powers and long ranges, Oops.

So will it affect your laser? Let’s calculate it! The distance $z_{0}$ where we expect stimulated scattering to ruin your beam is about

$z_{0}={\frac {25\,\pi \,S\,D}{4\,g\,P}}$

where $S$ is the spot diameter at the target, $D$ is the diameter of the focusing aperture, $P$ is the beam power, and $g$ is the gain. In Earth-like air, the gain for any particular wavelength $\lambda$ and pulse duration $t$ can be estimated with

$g=g_{0}\,{\frac {\lambda }{\lambda _{0}}}\,{\frac {t}{t+\tau }}$

where $g_{0}=2.6\times 10^{-14}{\mbox{m}}/{\mbox{W}}$, $\lambda _{0}=1.057\times 10^{-6}{\mbox{m}}$, and $\tau =5\times 10^{-9}{\mbox{s}}$.

It may be possible to preempt unwanted stimulated scattering by emitting small amounts of light in the Raman-shifted wavelengths in laser modes that focus along with the main pulse. Now these additional modes will cause all the stimulated scattering first, before random Raman scattering from the air can build up. So even though you still get runaway Raman scattering, it will still be adequately focused on your target. If this mode will in turn succumb to stimulated scattering before it reaches its target, inject a second mode to preempt its stimulated scattering, and so on.

A less complicated method is to do what the laser fusion people do, and just use laser pulses with a large spread in frequencies. Any one frequency of your laser now has a much lower intensity to stimulate the Raman scattering.

Credit

Author: Luke Campbell