Table of Contents
WHAT MAKES A LASER…A LASER?
What makes laser light different from say, a light bulb, you may ask? One of the most important distinctions is the coherence of laser light both temporal and spatially. Temporal coherence basically means that if you have the phase front at a given point in time, then a temporally coherent beam will keep that phase front for many future points in time (the amount of time that it will remain coherent is directly related to its bandwidth, which is discussed both in the definition of temporal coherence and in the “Coherence” section of “Classical Optics”). A spatially coherent beam means that if you have the phase front at one point in space, then a spatially coherent beam will have that phase front over the entire beam. We will go through the origins of both temporal and spatial coherence classically and through the lens of quantum mechanics in the coming sections. Or, if these explanations seem like entirely too much jargon to you, it’s not essential to understand coherence to understand the more basic functioning of a laser, so you can continue on in peace!
Another important distinction of a laser from a light bulb is the (quasi-)monochromatic nature of laser light. Light from a light bulb is white, which means that it spans the full visible spectrum of light, from the “R” in “Roy” to the “V” in “Biv”. That’s about 300 nm of bandwidth. Conversely, a typical continuous-wave laser might have a bandwidth of about 1-2 nm, and sometimes less (depending on a little thing called the Shawlow-Townes limit, discussed in a proceeding section). That’s a 300x difference! Having a small bandwidth allows for a long coherence time, which is why we can shoot lasers very long distances (up to 15 miles!) and the range of light bulbs is a few yards at maximum.
Finally, the directionality of laser light compared to a light bulb is our last distinction between these two light sources. As you have probably noticed, we don’t use laser lights as our main means of lighting up a room. We use light bulbs instead, and rejoice in the glorious non-directionality of the light from the light bulb. This difference in directionality comes from the difference in how the photons are created between these two sources. Not to sound like a broken record, but we will cover this later as well!
4 MAIN COMPONENTS OF A LASER
Before we dive into how a laser works, we should go through the ingredients of how to make a laser. First up, a laser gain medium! As the name may imply, a gain medium is what supplies gain to an initial beam of spontaneous emission which was triggered by a pumping source. There are many different types of gain media: solid-state, diode, gas, quantum well, free electron, and so on. New items are added to this list as scientists become more and more creative, so do a quick meander around your favorite search engine if you are interested in all the types of gain media out there! The most important property that a potential gain medium must have is unequal energy orbital spacing, but we will leave that there for now.
Next, we already kind of gave it away, but we need a pumping source. The pumping source is essential to provide an initial excitation to electrons or atoms (depending on the gain medium) hanging out in the ground state of the gain medium. The excitation will excite them into a higher energy state, which means that they will decay down and we can start the process of population inversion, which will be explained in a moment.
This brings us to our third component of lasers, and that is a feedback mechanism. The basic task of a feedback mechanism is just to send the spontaneously emitted beam back through the gain medium to provide more gain, and to begin to produce a coherent beam of light.
Finally, all of these components are necessary to produce population inversion, which is how we get a laser! This is part of the reason why it took so long to create a laser; we needed to have a better understanding of the constituents of atoms before we even knew that lasers were possible.
SIMPLE EXPLANATION OF A LASER
Considering how long it took to make first a maser (operating at microwave frequencies), and then a laser, they are obviously very complex setups…but for our purposes here, and not to step on any dead guys toes, we will simplify the explanation a bit.