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Manfra27-2011 - Lasers Fast decay Stimulated Emission Gain...

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Stimulated Emission Gain and Inversion The Laser Four-level System Threshold Some lasers Laser Transition Pump Transition Fast decay Fast decay * L ight A mplification by S timulated E mission of R adiation Lasers
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Achieving inversion: Pumping the laser medium Now let I be the intensity of (flash lamp) light used to pump energy into the laser medium: R = 100% R < 100% I 0 I 1 I 2 I 3 Laser medium I Will this intensity be sufficient to achieve inversion, N 2 > N 1 ? It’ll depend on the laser medium’s energy level system.
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Rate equations for a two-level system Rate equations for the densities of the two states: 2 1 2 2 ( ) dN BI N N AN dt = - - 1 2 1 2 ( ) dN BI N N AN dt = - + 2 2 2 d N BI N AN dt = - + Absorption Stimulated emission Spontaneous emission 1 2 N N N - 1 2 N N N + If the total number of molecules is N : 2 1 2 1 2 2 ( ) ( ) N N N N N N N = + - - = - ∆ 2 d N BI N AN A N dt = - + - 2 1 N 2 N 1 Laser Pump Pump intensity
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Why inversion is impossible in a two-level system 0 2 BI N AN A N = - + - 1 / sat N N I I = + / 2 sat I A B = In steady-state: ( 2 ) A BI N AN + = where: N is always positive, no matter how high I is! It’s impossible to achieve an inversion in a two-level system!
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