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

so that

$\displaystyle \frac{\sigma_{12}}{\sigma_{21}}=\frac{B_{12}}{B_{21}} =\frac{g_{2}}{g_{1}}$

If $ g_1=g_2$$ \sigma_{12}=\sigma_{21}$

If $ \alpha<0$,

$\displaystyle u(\nu_s)=u_0(\nu_s)e^{+\mid\alpha\mid x}\equiv u_0(\nu_s)e^{gx}$

where $ g$is known as the gain , which amplies that the laser radiation is amplified.

Exercise:

Using Boltzmann distribution for the ratio $ \frac {N_2}{N_1}$show that for a 2 level system, $ \alpha$ is always positive, i.e. in a two level system only absorption can take place.

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