Flashcards
Lasers
Cards in this deck (13)
- 1What does LASER stand for?Light Amplification by Stimulated Emission of Radiation.
- 2Why is stimulated emission uncommon in nature?it is unlikely that the right photons will pass exited atoms in the brief time before they undergo spontaneous emission
- 3What characteristic must the amplifying medium have?An energy level above the ground state that is almost stable (a metastable state) that allow a population inversion to be set up
- 4What is a population inversion?a situation where there are more atoms in an excited state than in the ground state
- 5What is the purpose of the laser cavity?--increases amplification
--makes the beam collimated
--the ends of the cavity select certain optical standing waves, which improves the spectral purity of the beam - 6Why is it not possible to achieve a population inversion with a two-level system?The probability than electrons will fall back to the ground state increases as more enter the excited state, and with a two-level system this approaches an equilibrium, regardless of how had the system is pumped
- 7Why are 4 level systems preferable to 3 level systems?In a 4 level system, the laser transition does not end in the ground state. If the bottom level of the laser transition is depopulated faster than the top level, a population inversion is guaranteed, regardless of strength of pumping. [In a three level laser system, over half of the electrons in E1 must be pumped to E2 (via E3. In a 4-level system, this is not so.]
- 8Explain some applications of lasers.reading bar codes
optical fibres
microsurgery
laser printers
reading CDs and DVDs - 9Describe the structure of a laser.The amplifying medium (in which the population inversion exists) sits between two mirrors: one fully reflecting mirror, and one which lets through about 1% of the light incident upon it. Because only 1% of the light passes through the mirror each time the beam reflects off it, the beam travels back and forth through the amplifying medium an average of 100 times before it exists. This compounds the effects of the exponential increase in intensity due to stimulated emission considerably.
- 10Why is it acceptable to ignore spontaneous emission in a laser?Inside the amplifying medium, the light intensity is so high that excited electrons do not have time to drop down an energy level spontaneously—they are continually stimulated to do so.
- 11Why are lasers so inefficient?1. The pumping energy is significantly greater than the output photon energy.
2. Amplifying medium gets very hot due to combined effects of high intensity pumping light and high intensity laser beam. Heat losses are made worse as heat is intentionally transferred away from the laser in order to cool it (to protect it). - 12What are the advantages of semiconductor lasers?Small
Cheap
More efficient
Easy to mass produce - 13What are the uses of semiconductor lasers?Inside DVD and CD players
Inside barcode readers
Telecommunications (via optical fibres)
Image scanning
Laser surgery
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- A level Physics (A level) (Paper 3) Flashcards on Lasers, created by muon neutrino on 30/05/2017.
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- muon neutrino
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