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Summary Lasers
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Unit 2 a level physics notes
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.LASERS 3 state Lasers:
• light amplification by stimulated 3
For light amplification:
emission of radiation 2
I -Ny > NG
structure of a laser • electrons pumped to level 3
→ has an amplifying medium between which is unstable so electrons
2 mirrors : l G drop to level 2
• one of which reflects 99% of the electrons. • level 2 is metastable so the
• the other 1% is transmitted. Pumping level electron will stay in this level
POPULATION INVERSION: = short lived for a long time.
when a photon interacts with an electron Upper state • a photon of laser frequency is
in these energy levels, it can cause absorption = long lived emitted (stimulated emission)
or stimulated emission
For a laser to work there MUST be a Population inversion:
population inversion. • is possible in 3 level system as pumping frequency
and laser frequency are different.
N, > Ny N, < N U • Ny must be more than No
4
mm
U
NB
L
absorption most Stimulated emisson
likely likely
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