Gas Lasers This is what is inside. But how do they work ?

L.A.S.E.R. Light Amplification by the Stimulated Emission of Radiation

Lasers lead double Lives The Laser part is a RANDOM PROCESS.

Lasers lead double Lives The beam is generated by OSCILLATION.

Stimulated Emission First an electron is excited and caught in an energy TRAP. Then an incident photon will release the electron from the trap. This allows the electron to emit a second “stimulated” photon

What’s a Trap ? When electrons are excited, they decay to their ground states again very quickly. This process is called FLUORESCENCE. A TRAP is an excited energy level which holds the electron and stops it from decaying without some outside help.

But Wait, There’s More ! To maintain laser action, we must have a lot of traps AND a way for the decayed electrons to get out of the ground state. This is so that decaying electrons have somewhere to go. So we need more than one ground state.

Inversion If we can arrange for a lot of traps and a reasonably empty ground state, then there can be more excited electrons than decayed ones. This is population inversion and is essential to maintain laser action.

Nearly There..... If laser action can be maintained and we arrange for the light to bounce back and forth along the laser tube, without too much loss, then the system will oscillate. This oscillation is what produces the laser BEAM.

Mechanics The oscillation is created by mirrors placed at each end of the laser tube. One mirror, the HIGH REFLECTOR has 100% reflection. The other is called the COUPLER and has about 99% reflection.

Review the Laser Parts

More Mechanics The electrons are excited by a plasma beam which is caused by an electrical discharge through the laser gas. Free electrons for the gas discharge are generated by a heated cathode. These electrons are accelerated towards an anode. They strike and ionise gas molecules in the laser tube, so creating excited electrons.

Why doesn’t everything Ionise ? Actually, everything does, but only for an instant. To create the gas discharge, a short pulse of very high voltage is applied between anode and cathode. This starts the formation of the plasma. Once established, the discharge needs only moderate voltages to continue.

Why doesn’t everything Ionise ? The plasma is restricted to a central tube by electric, and sometimes magnetic fields. The tube is made of Beryllium Oxide. BeO is a good electrical insulator, a great heat conductor. .......and a deadly poison.

Krypton-Argon has a Problem The krypton ion is HUGE. It eventually erodes the BeO tube walls and causes refraction losses. This causes the tube to fail. Also, the Kr is depleted. This is why we need a reservoir of gas connected to the laser tube.

To get the Light Out Gas lasers usually have external mirrors. To couple the light from the inside to the outside of the tube, BREWSTER WINDOWS are used.

Brewster Window Brewster windows have the property that they cause NO light loss for one particular polarisation of the light passing through them. This means that light from the laser will be LINEARLY POLARISED.