Module 7 : Laser- I
Lecture   : Line Broadening in Lasers
 

If we define absorption coefficient by

$\displaystyle \alpha(\nu_s)=(\frac {g_2}{g_1}n_1-n_2)\sigma_{21}$

where the stimulated emission cross section $ \sigma_{21}$is defined as

$\displaystyle \sigma_{21}=\frac {h\nu_s g(\nu_s,\nu_0)} {c} B_{12} $

we have

$\displaystyle u(\nu_s)=u_0(\nu_s)e^{-\alpha(\nu_s)x}$

which gives the penetration depth to be $ \frac{1}{\alpha(\nu_s)}$. The signal get exponentially attenuated. $ \alpha(\nu_s)$is maximum when all atoms are in the ground state.

Analogous to $ \sigma_{21}$, we may define an absorption cross section

$\displaystyle \sigma_{12}=\frac{h\nu_sg(\nu_s,\nu_0)}{c}B_{21}$
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