Einstein Coefficients: A (Spontaneous Decay) B (Absorption/Emission)
E2
E1
N2
N1
AN2
BN2
BN1
dN2
dt
______ = BN1 - BN2 - AN2
dng
dt
_____ = Wng(N2 - N1) - ng/tloss
exponential growth
cavity losses
LASING REQUIRES N2>N1 (population inversion)
N2-N1 > 8šf2/c3tloss
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