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Gas Lasers

Gas Lasers. This is what is inside. But how do they work ?. L.A.S.E.R. L ight A mplification by the S timulated E mission of R adiation. Lasers lead double Lives. The Laser part is a RANDOM PROCESS. Lasers lead double Lives. The beam is generated by OSCILLATION.

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Gas Lasers

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

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

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

  4. Lasers lead double Lives • The beam is generated by OSCILLATION.

  5. 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

  6. 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.

  7. 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.

  8. 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.

  9. 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.

  10. 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 theCOUPLER and has about 99% reflection.

  11. Review the Laser Parts

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. Brewster Window • Brewster windows have the property that they causeNO light loss for one particular polarisation of the light passing through them. • This means that light from the laser will be LINEARLY POLARISED.

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