In-class group activity 13:

Building a Laser

Use an on-line simulation from the University of Colorado PhET group to build a laser. 
Link to the simulation http://phet.colorado.edu/en/simulation/lasers.  Click "Run Now!".

Open a Microsoft Word document to keep a log of your procedures, results and discussions.

• Explore the interface.
  • Non obvious control:  You can grab the lines of the excited states in the energy level diagram and move them up and down.
• Open the One Atom Panel in the Laser Simulation and start exploring the two-level atom. 
  • You are optically pumping an atom.  Use the preset wavelength and move the lamp control to a medium level.  Monitor the photons emitted by the atom?  Do you observe spontaneous emissions, stimulated emissions or both?   How can you tell?
  • Move the lamp control to a low level.  What do you notice about the emitted photons.
  • Move the lamp control to a high level.  What do you notice about the emitted photons?
  • Decrease the lifetime of the atom.  What do you notice about emissions?
  • Change the wavelength?  What happens?

   

• Explore the three-level atom.
  • Turn off the red lamp an try pumping the atom with the blue lamp.  Characterize the emitted photons.
  • Click on “display photons emitted from upper state.”  Characterize those photons.
  • Change the lifetime of the upper state or lower state.  What happens?

   

• Switch to the Multi-Atom Panel and build a laser.
  • You want to produce many identical photons. Describe what you have to do to achieve this goal.
  • First try to establish a population inversion.
    • How many energy levels do you need?
    • Which energy level do you need to pump?
    • Do any of the energy levels have to be metastable?
    • Does adjusting the energy level and the wavelength of the pump photons and the emitted photons change the ease with which you can achieve a population inversion?
  • Now build a laser.
    • Do you need mirrors?  Do they need to be highly reflective?
    • Can you blow up your laser?