LaserFest

Road Show

Experiment 1: How a laser works

This experiment demonstrates the basic principles of laser operation and the critical difference between incoherent light (like a gas discharge lamp) and coherent laser radiation.

With a high enough electric potential, one can produce a glowing discharge in a neon gas – we have neon signs everywhere! In such a discharge, atoms are excited into high energy states and then decay, emitting photons. It is not hard to check that such emission consists of  many distinct colors, called an emission spectrum. The exact combination of emission lines is individual for every atom, and, for example, can be used to identify gas composition.

The photo above shows the discharge of a Ne tube. There are several important things illustrated there. First, even though the discharge color is soft pink to the eye, it in fact has many colors that you can separate using a diffraction grating. Second, the emitted light goes in all different directions. It is good for illuminating a room, but would make a very lousy laser pointer.

One more critical component is required to turn the incoherent light from a discharge to a coherent laser beam – a feedback to amplify only a specific fraction of the emitted light. In most lasers this is done by placing two mirrors (called the resonator) at the end of a light-emitting element (called the optical gain medium). The mirrors are aligned to let, for example, only red light propagate between the two mirrors, bouncing back and forth many times. As a result only this particular light gets amplified. In this case a bright collimated beam appears.   

By fine-tuning the position of one of the mirrors, we can change the spatial distribution of light inside the cavity, producing a variety of shapes in the output beam.

Our huge thanks to Sam Goldwasser for lending us his He-Ne laser system for the demonstrations. Do you want to build a laser? Check out Sam’s page for He-Ne laser kits.

Experiment 2: Liquid fiber optics

 Did you know that most of the data you see on-line or on television, and sometimes even phone calls, are transmitted using light pulses traveling in optical fiber? This experiment is designed to explain how optical fiber works.

Optical fibers are made of a very thin glass wire where light pulses can propagate for very long distances. This is much more efficient than sending an electrical signal through metal wires, since people have learned to make fibers with extremely low losses. But what makes light stay inside a fiber core?

Everyone knows that if a light beam hits an interface between two materials, some of the light is transmitted, and some of the light is reflected. How much of the original beam goes which way depends on the properties of the media, in particular on their refractive indices.

But it is not always possible to transmit the light through the boundary even if two media are transparent! When a light beam gees from a medium with ahigher refractive index to a medium with a lower refractive index (for example from water or glass to the air) at a large angle, all the light is reflected back, and nothing is transmitted. This effect is called total internal reflection.

Optical fibers take full advantage of this effect. Commercial fibers are made such that their core has a slightly higher refractive index than the surrounding part of the wire (called cladding), so that the light is always completely reflected off the interface.

But you don’t need a fancy optical fiber to see this light guiding effect. In our experiment we can “trap” light inside a water stream, using the same idea: the laser beam continuously reflects off the water-air interface and thus follows the bending water.
 

Experiment 3: Mechanical action of a laser

You can probably rather easily distinguish a laser from a lamp, because light emitted by a lamp   shines in every direction, while the laser beam is collimated (i.e., forms a single streight beam). Another important difference is that the spectrum of the lamp consists of many colors, and a laser operates on a very specific wavelength. In fact, there is a deeper difference: light emitted by the lamp is incoherent, which means that any portion of the light us independent from others. On the other hand, laser radiation is coherent, and all the emitted light oscillates in unison. As a result, its action on the object is much more pronounced.

We can see that difference in our “Pop the balloon” experiment. First, a regular 20W light bulb is focused on a balloon and nothing happens. Then a 200x weaker (100mW) laser beam is directed to the balloon and pops it in a few seconds.

We can put several balloons of different colors in a row to see how easily our laser pops all of them with two exceptions. A baby pink balloon is very light - it scatters more light than it absorbs. A green balloon is also very hard to pop - even though you cannot see it in the clip, the laser was shining on it for several minutes without any effect. Can you guess why the green balloon is tough for the green laser to pop? The balloon appears green because this is the color that penetrates it the best, and thus it is more transparent for the green laser compare to all others.

 

We can even use this effect to pop a balloon inside another balloon!

While popping balloons is just fun, it illustrates much more serious applications of lasers in industry, medicine, and other areas. A focused laser beam can cut out metal parts or shape a cornea in a human eye with great precision.

At the same time think of these popping balloons next time you are playing with a laser pointer. Human eyes are very delicate objects, and can be damaged with as little as a few milliWatt of coherent laser radiation. Never aim a pointer or any other laser at your eyes, or at any people around you.