Coherence
Quantum and Nuclear | Light, Sound and Waves

Episode 504: How lasers work

Lesson for 16-19 IOP TAP

This episode considers uses of lasers, and the underlying theory of how they work.

Lesson Summary

  • Demonstration: Seeing a laser beam (10 minutes)
  • Discussion: Uses of lasers (15 minutes)
  • Discussion: Safety with lasers (10 minutes)
  • Discussion: How lasers work (20 minutes)
  • Worked examples: Power density (10 minutes)
  • Student calculations (10 minutes)

Safety

Ensure that you are familiar with safety regulations and advice before embarking on any demonstrations (see

episode 504-2 (Word, 26 KB)


Demonstration: Seeing a laser beam

A laser beam can be made visible by blowing smoke or making dust in its path. Its path through a tank of water can be shown by adding a little milk.

Show laser light passing through a smoke filled box or across the lab and compare this with a projector beam or a focussed beam of light from a tungsten filament light bulb.

Show the principle of optical fibre communication by directing a laser beam down a flexible plastic tube containing water to which a little milk has been added.

Show a comparison between the interference pattern produced by a tungsten filament lamp (with a monochromatic filter) and that produced by a laser.

Discussion: Uses of lasers

Talk about where lasers are used – ask for suggestions from the class. As far as possible this should be an illustrated discussion with a CD player, a laser pointer, a set of bar codes, a bar code reader and the school’s laser with a hologram available for demonstration.

Episode 504-1: Uses of lasers (Word, 25 KB)


Show the list of uses. Invite students to consider the uses shown in the list. Can they say why lasers are good for these? The reasons might be:

  • a laser beam can be intense
  • a laser beam is almost monochromatic
  • a laser beam diverges very little
  • laser light is coherent

Discussion: Safety with lasers

Lasers must be used with care. Use the text as the basis of a discussion of the precautions which must be taken.

Episode 504-2: Lasers and safety (Word, 26 KB)


Discussion: How lasers work

If students are familiar with energy level diagrams for atoms, and of the mechanisms of absorption and emission of photons, you can present the science behind laser action. Point out the difference between:

(a) excitation – an input of energy raises an electron to a higher energy level

(b) emission – the electron falls back to a lower energy level emitting radiation and

(c) stimulated emission – the electron is stimulated to fall back to a lower energy level by the interaction of a photon of the same energy.

Define population inversion: Usually the lower energy levels contain more electrons than the higher ones (a).

In order for lasing action to take place there must be a population inversion. This means that more electrons exist in higher energy levels than is normal (b).

For the lasing action to work the electrons must stay in the excited (metastable) state for a reasonable length of time. If they fell to lower levels too soon there would not be time for the stimulating photon to cause stimulated emission to take place.

Laser stands for Light Amplification by Stimulated Emission of Radiation. The diagrams in

episode 503


up to the higher energy level using photons. hey then drop down and accumulate in a relatively stable energy level, where they are stimulated to all drop back together to the ground state by a photon whose energy is exactly the energy difference to the ground state.

Discuss coherent and non-coherent light. Coherent light is light in which the photons are all in step – in other words the change of phase within the beam occurs for all the photons at the same time. There are no abrupt phase changes within the beam. Light produced by lasers is both coherent and monochromatic (of one colour).

Incoherent sources emit light with frequent and random changes of phase between the photons. (Tungsten filament lamps and ordinary fluorescent tubes emit incoherent light).

Worked examples: Power density

The laser beam also shows very little divergence and so the power density (power per unit area) diminishes only slowly with distance. It can be very high.

For example consider a light bulb capable of emitting a 100 W of actual luminous radiation.

At a distance of 2 m the power density is

100 W4 π 22 = 2 W m-2.

The beam from a helium-neon gas laser diverges very little. The beam is about 2 mm in diameter close to the laser spreading out to a diameter of about 1.6 km when shone from the Earth onto the Moon!

At a distance of 2 m from a 1 mW laser the power density in the beam would be

0.001 W4 π 0.0012 = 320 W m-2.

This is why you must never look directly at a laser beam or its specular reflection.

Student calculations

Ask the class to calculate the power densities for a 100 W lamp and a 1 mW laser at the Moon.

(Distance to Moon is 400 000 km; diameter of laser beam at Moon is 1.6 km)

Coherence
is a special case of Interference
is used in analyses relating to Laser
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