That's how a dye laser works, more or less.
Depending on the pump wavelength (325nm, 514nm, 532nm) you can pump an organic dye solution of with whatever ratio of dye to solvent (ethanol for instance) is necessary, and get a different WL output.
I plan to make my own dye laser when I'm in a location where a loud N2 laser won't disturb everybody and drive them mad.
They have tunable lasers that use gratings and/or a prism to "tune" to wavelengths.
The way it works in a HeNe is quite simple. Take your above average single brewster tube. This is a tube with only one mirror, the other end is a slanted portion of the bore. You can actually see whatever internal gain is in the tube in the area between this brewster window, and your external window. Anyway. If on one end you have your broadband OC, and on the brewster end, you put your prism, boom tunable laser (in a perfect world.)
(see reference picture)
How does this work? To explain that requires the explanation of a basic HeNe setup (let me reference a previously sent PM).
A HeNe tube is a large tube filled with a ratio of around 6:1-10:1 He:Ne. In the center of the tube is a thinner tube, known as the bore. This is where the action happens. The anode and cathode gets power, and sends HV through this bore. That is where the ionization process takes place. The outer tube of gas does nothing but provide fuel for this process.
On a regular tube, on each end of the tube is a mirror. Generally on the cathode end, you will find the OC, or output coupler, on the other end you will find the HR, or high reflector. The way a HeNe works is quite simple. The ionization causes the atoms to get really excited, causing the electrons to elevate in orbit, and fall back down. This process releases photons which travel and some hit the bore wall, some collide and do nothing, others get carried through the bore and hit a mirror. Both of these mirrors are... mirrored. The average OC for a 632.8nm tube is mirrored to reflect around 99% of the 632.8nm and pass just about everything else (more on that later). the HR relfects damn near 100% (absolutely no absolutes ). Those traveling photons then bounce back and fourth, and collect, and build, and eventually lase through. The internal gain can be in the hundreds of milliwatts into WATTS of power. The purpose of the OC is to allow just enough through so that there's enough left to build, and gain, and excite other photons, as well as actually allow light through. If the OC had too small a reflectance, then you'd get nothing in output. <97% and you're not going to really see anything (in a standard 632.8nm laser). Green lasers have OC's around 99.95%, far IR (3.39um) have some as low as 50%.
These mirrors fit a general "rule". The HR is generally going to be planar (flat). Whereas the OC is spherical. There are various trade secrets involving the angle at which it is spherical, to get that extra 100uW out, but for the most part that's what you will see. There are planar-planar setups, but you won't see them in a HeNe due to it's GREAT instability, and nearly impossible alignment requirements.
About that coating. What causes multiline lasers can be a combination of many things. Thicker bores, close planar lenses, broadband OCs, etc. To keep it simple, my REO 612nm has a broadband OC. This means the coating isn't just meant for 632.8nm, whether by design or fault, the OC actually reflects a certain amount of the 604nm and 594nm back into the cavity to build and lase.
That last part about multiline lasers pertains to this. That broadband OC on the other end will allow for multiple lines to become available (again, in a perfect setup). The Littrow prism on the other end is a regular prism, but with an HR embedded within the prism. This prism absorbs the line exiting the brewster window, splits it up, and reflects it back. Now, if you've seen how a prism works with a multiline laser, then this will be simpler to visualize.
When a prism is turned, the spectrum emitting from it also moves. Thus allowing separate lines to be reflected perpendicularly into the OC, travel through the tube, gain, and emit. Whereas the other lines are going to smash right into the bore and do nothing. Turn the prism, and another line is not perpendicular.
I think that was a long, but effective, way to explain how a tunable laser system works. In the world of gas, ion, and dye.
Enjoy your night/morning! Let me know if you need anything else clarified
Reference picture:
On the bottom left is the slanted brewster window. Top left is the additional external mirror
