What is a LASER and how does it work?

We love lasers here at Laser Classroom – who doesn’t? Lasers are easily one of the most important scientific breakthroughs of the century – impacting technological advances in fields as diverse as medicine, communications and defense, just to name a few!

But what IS a laser, how does it work and why does it matter? A laser produces light with three properties that are unique to lasers: monochromatic, coherent and collimated. You can take a closer look at the properties of laser light with these hands-on activities.

A laser is produced via a process called Light Amplification by Stimulated Emission of Radiation – which is how the laser got is acronym of a name. How lasers are created is complex and fascinating. Let’s unpack LASER and see if we can understand a little better.

Light Amplification

Let’s start with light. To understand how light is amplified, we’ll need to consider properties of light as wave and as a particle (photon). A light wave is distinct from other waves in that light is actually comprised of two waves that are actually synchronized oscillations of electric and magnetic fields propagating through space.  To amplify a light wave, you need to increas the amplitude without changing its frequency or phase. We can accomplish this becuase light simultaneously has the properties of particles (photons).  It just so happens that if you add one photon, with a certain wavelength, direction and polarization, to another photon with the same wavelength, directio and polarization (a copy, if you will), the amplitude of the electromagnetic wave doubles, while its temporal course (frequency and phase) remains unchanged.

So, one thing that is done to create a laser beam is to amplify the light by copying photons, We ultimately do this by passing an electromagnetic current through what’s called  a gain medium, to force photons to copy themselves, and therefore amplify.

Spontaneous Emission of Photons

Almost all of the light we observe originates via a process known as spontaneous emission. There are lots of ways that atoms and molecules can be stimulated to an energy state that is higher than their ground state. Such excitation causes outer electrons to leave their regular orbits for “higher ground”, or more energetic states.  But sooner or later (usually much sooner), excited electrons spontaneously return home to their original state, and when they do, they emit the excess energy in the form of a photon.a spontaneously emitted photon will be of a specific wavelength, but will be of random direction and phase. This is not the kind of photon emission we need for a laser – what we need is for all the photons to be of the same direction and phase as well as of the same wavelength.

Stimulated Emission of Photons

In order to force the emission of photons that are of the same wavelength, phase and direction, we use a method called Stimulated Emission of Photons. To stimulate emission of such photons, an electromagnetic (EM) field (photons) of a very specific frequency is passed though a gain medium. The electrons of the atoms/molecules in the gain medium are stimulated by specific frequency f so that the emission of light is not spontaneous, but predictable and regular. The specific frequency of EM introduced into the gain medium is the same frequency of light that would be emitted if it were allowed to emit spontaneously. This frequency induces a quantum interaction which causes the photons emitted to be not only of the same frequency as the original EM field, but also the same direction and phase – THAT is what is needed to produce laser light.

stimulated-emission

Population Inversion

In order to actually amplify light, we need a higher concentration of atoms in an excited (high energy) state than in the ground (low energy) state.  This excess of excited atoms is called a population inversion and it’s an essential requirement for optical amplification. But… the excess of excited atoms is very unstable and nearly impossible to maintain. In order to overcome this, lasers make use of more than two excitation levels for the atoms being excited.

Optical Oscillation and creating the beam

Once amplified, we have lots of photons that have the essential properties of laser light: same wavelength and phase – but, those photons leave the amplifier at the speed of light (literally), and do so in all directions. So the next step in creating an actual laser beam is to wrangle the photons into a narrow beam. this is accomplished with a combination of mirrors and lenses that cause the photons to bounce back and forth inside the tube of gain medium and then exit in a controlled way resulting in a narrow beam that diverges very little as it travels.

Those are the basics; of course there are many more highly technical details that were not covered here. If you are interested in going a little deeper without a whole lot of math,  the video below is a great overview of how lasers work and are created.