1. Saturation effects Pump Ip np sp Signal I n0 s 2 level system

2. Ip= 0 ph/s/cm2

3. Ip= ph/s/cm2

4. Ip= ph/s/cm2

5. Ip= ph/s/cm2

6. Ip= 1, ph/s/cm2

7. Ip= ph/s/cm2

8. Ip= ph/s/cm2

9. Ip= ph/s/cm2

10. Ip= ph/s/cm2

11. Comments
Differences between 2, 3 and 4 level systems
The 3 level system shows clearly its properties : absorbant medium at the beginning and then a gain medium when the pumping intensity is high enough
The 4 level is always a gain medium whereas the 2 level system is always an absorbing medium
Saturation on the laser transition
When the laser intensity increases, all the curves are closer to the x axis : it is the saturation of the laser transition : absorption and stimulated emission rates become more important than the other rates.
Saturation intensity versus pumping intensity
The curves show that it is more difficult to saturate a medium if the pump intensity is important