HOW LASERS ACTUALLY WORK
Lasers are optical amplifiers, optical oscillators, and in a way, the most sophisticated light source ever invented. Not only are lasers extremely useful, but they are also champions of magnitude: While different laser types cover the electromagnetic spectrum from radiation (<10 nm) over the visible spectrum to far infrared light (699 μm), their individual output band can be as narrow as a few µHz. Their high temporal and spatial coherence lets them cover hundreds of meters in a tight beam of lowest divergence as a perfectly sinusoidal, electromagnetic wave. Some lasers reach peak power outputs of several exawatts, while their beams can be focused down to the smallest spot sizes in the hundreds and even tens of nanometers. Laser is the acronym for Light Amplification by StimulatedEmission Of Radiation, which suggests that it makes use of a phenomenon called stimulated emission, but well, how exactly do they do that? It’s time to look the laser in the eye (Disclaimer: don’t!).
THE OPTICAL AMPLIFIER
When we talk about the amplification of electrical signals, we are typically not too concerned about whether or not the amplified signal is actually the original signal – just amplified. The minuscule flow of electrons in a bus line may transport a signal, but this signal is not bound to that exact representation. We can send it through a transformer, optocoupler or piezo-transducer and, if we do it right, it just won’t matter. We are really ok if an amplified signal is a good, enlarged copy, as long as the copy retains the subset of properties of the original signal we are interested in.
Light is however indeed bound to its inherent representation as an electromagnetic wave. To understand how light is amplified – or copied – we need to look at the different properties of light both as a wave and as a particle. As a wave, it’s really not much more than synchronized oscillations of electric and magnetic fields propagating through space. To amplify it, we would want to somehow increase the amplitude of this oscillation while leaving its temporal course, such as its phase and frequency, intact. As a particle, however, a photon of a certain wavelength, direction, and polarization – all we can do is add more. So, if take this photon and we add another photon with this very same properties – a copy so to speak – we will see that the amplitude of our electromagnetic wave also doubles. It is amplified.