Presentation on theme: "Gas Lasers l This is what is inside. l But how do they work ?"— Presentation transcript:
Gas Lasers l This is what is inside. l But how do they work ?
L.A.S.E.R. l Light l Amplification by the l Stimulated l Emission of l Radiation
Lasers lead double Lives l The Laser part is a RANDOM PROCESS.
Lasers lead double Lives l The beam is generated by OSCILLATION.
Stimulated Emission l First an electron is excited and caught in an energy TRAP. l 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 ? l When electrons are excited, they decay to their ground states again very quickly. This process is called FLUORESCENCE. l 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 ! l 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. l This is so that decaying electrons have somewhere to go. l So we need more than one ground state.
Inversion l 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. l This is population inversion and is essential to maintain laser action.
Nearly There..... l 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. l This oscillation is what produces the laser BEAM.
Mechanics l The oscillation is created by mirrors placed at each end of the laser tube. l One mirror, the HIGH REFLECTOR has 100% reflection. l The other is called the COUPLER and has about 99% reflection.
Review the Laser Parts
More Mechanics l The electrons are excited by a plasma beam which is caused by an electrical discharge through the laser gas. l Free electrons for the gas discharge are generated by a heated cathode. These electrons are accelerated towards an anode. l They strike and ionise gas molecules in the laser tube, so creating excited electrons.
Why doesn’t everything Ionise ? l Actually, everything does, but only for an instant. l To create the gas discharge, a short pulse of very high voltage is applied between anode and cathode. l This starts the formation of the plasma. l Once established, the discharge needs only moderate voltages to continue.
Why doesn’t everything Ionise ? l The plasma is restricted to a central tube by electric, and sometimes magnetic fields. l The tube is made of Beryllium Oxide. l BeO is a good electrical insulator, a great heat conductor. l and a deadly poison.
Krypton-Argon has a Problem l The krypton ion is HUGE. l It eventually erodes the BeO tube walls and causes refraction losses. l This causes the tube to fail. l Also, the Kr is depleted. This is why we need a reservoir of gas connected to the laser tube.
To get the Light Out l Gas lasers usually have external mirrors. l To couple the light from the inside to the outside of the tube, BREWSTER WINDOWS are used.
Brewster Window l Brewster windows have the property that they cause NO light loss for one particular polarisation of the light passing through them. l This means that light from the laser will be LINEARLY POLARISED.