What’s in the box? Part 1 – lasers

There’s less than one month until we launch our optical clock into the stratosphere and next week we’ll be assembling all the components of our experiment into the compact box that will strapped to the balloon gondola. Want to know what’s in the box? We’ll go through it, part by part!

The basis for our clock, the very thing that ticks, is a laser. What exactly is a laser and what’s so special about it? The word laser actually started off as an acronym, standing for “Light amplification by stimulated emission of radiation”. Let’s break that down.

Stimulated emission is the process in which a photon, a particle of light, bumps into an atom that happens to be in an excited state. (Remember excited atoms? We talked about them here if you need a reminder!) When this happens, the photon knocks the atom back to the ground state and in the process, the photon stimulates the atom to emit a photon that is identical to the one that collided with it! When I say identical, I mean that it has the same color, the same polarization, and it even travels in the same direction. Now there are two photons. These may very well bump into two new atoms, stimulating the emission of two more photons. Now there are four photons! This process can occur many many times leading to the amplification of the light that was there originally.

A laser consists of a “gain medium”, which just means a tube or chamber filled with excited atoms, that is usually placed between two mirrors. The mirrors allow the photons to bounce back and forth many times through the gain medium so that a very large number of photons can be produced. A small hole in one of the mirrors allows a stream of these photons to exit in a narrow beam, what we see as the laser beam.

Laser light is special because it differs from “regular” light, such as sunlight or light from an incandescent light bulb, in two ways:

  1. Laser light is monochromatic. All of the photons in a laser beam have the same colour which is related to the wavelength of the light waves. Sunlight consists of photons of all the colours of the rainbow, which our eye interprets as white.
  2. Laser light is all “in phase”. This means that the crests of the waves all line up, and the troughs of the waves all line up. This is the property that gives laser beams their “punch”, enabling technology like laser cutters and laser eye surgery.

Here is a picture of one of our lasers. The laser itself is the little gold block in the centre. The light exits from the small round glass window in the side of the laser package. Surrounding it are a lot of electronics that enable us to control the frequency of this laser so precisely and use it as a clock!


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