Laser Basics

There aren’t many simple explanations available for learning about lasers, optics, electronics, or their combinations. In this blog I will tell you the primary things you need to know about lasers.

Before we really start a discussion on lasers, you need to know a couple things.

  1. Light is made of photons.
  2. Photons are particles AND waves (at the same time! This means we can count individual photons, and multiple photons can interfere. Interference can only happen with waves).
  3. Each wave has its own wavelength, which is the distance between peaks.
  4. The energy of a single photon depends on its wavelength.

Here is a quick graphic to show what happens when waves interfere:

If I lost you already, that’s ok! Check out this YouTube video by Physics Girl, she will help you understand waves: https://www.youtube.com/watch?v=Y1LbQBgvISs

The word ‘laser’ started as the acronym for Light Amplification by Stimulated Emission of Radiation.

A laser is essentially a set of mirrors facing each other (we call this an optical cavity) with a gain medium in between. Of course, there are a few more details. The mirrors need to reflect the light back and forth between them and not lose light faster than the gain medium can contribute light. One of the mirrors must transmit a little bit of light so we can get the laser light out of the optical cavity. If the light stays between the mirrors, we cannot use it!

A gain medium is any material that can produce photons (light) at the right wavelength/energy (if you want more details on this, email me at [email protected]).  Many lasers use doped semiconductors. When a bunch of photons pass the gain medium, it stimulates the gain medium to emit more light. This is the “stimulated emission of radiation” (the radiation here is light, meaning wavelengths that we can see with our eyes).  Visible light is only a tiny portion of the electromagnetic spectrum:

You already know that somehow a laser is special. No other light sources form strong beams that can be seen about the same size as a spot landing far away. This happens because the photons bouncing between the mirrors travel as a group. When more photons are emitted by the gain medium, they join the same group. What this really means is that all the photons’ waves are lined up in time and space. When the waves are lined up they can generate a strong beam and be focused to a small spot.