So the other day I got to build a fibre laser (hence the cool colours in the picture above). Technically it’s just an amplifier, it needs some input light, and it makes more of it.
So you feed in 0.006 Watts of seed light at one end, and out the other end of the optical fibre you get 5 Watts. This is all invisible light in the infrared.
Where does the extra energy come from, you say? Well. You also put in a whole lot of light of a different colour. That’s the red looking light above. You can’t see it with your eyes (nor would you want to, it could blind you), but your camera can probably see it. We put in 10 Watts of this light.
The mixture of chemicals which have been put into the glass which makes up the optical fibre, allow the pump light (red) to be absorbed, and the seed light (invisible) to be amplified.
But they do other things too. They can absorb two photons of pump light (red), and emit a photon with twice the energy (green in the photo). This is known as frequency doubling. Sometimes you want this, but not here. Here it is just a loss mechanism. But a very pretty loss mechanism none the less.