## Nd:YAG Laser

Hey guys,

A simplified diagram for the energy levels involved in Nd:YAG laser can be found here:

Two (short) questions:

1. Selection Rules
The lasing transition happens between $^{4}F_{3/2}$ and $^{4}I_{11/2}$. In the process J (of the electrons) changes from 3/2 to 11/2. Since a photon only carries J=1, how does the process not violate conservation of angular momentum? (Is it something to do with the emission of phonons? If phonons are also required to be emitted in the lasing process, would this affects down the lasing effect?)

2. Transition Intensity
Besides $^{4}I_{11/2}$, there are also many other close levels such as $^{4}I_{13/2}, ^{4}I_{15/2}$ etc. Why is the coupling between $^{4}F_{3/2}$ and $^{4}I_{11/2}$ the strongest? Is there a rule of thumb for determining which transition is the strongest?

Recognitions:
 Quote by paultsui 1. Selection Rules The lasing transition happens between $^{4}F_{3/2}$ and $^{4}I_{11/2}$. In the process J (of the electrons) changes from 3/2 to 11/2. Since a photon only carries J=1, how does the process not violate conservation of angular momentum? (Is it something to do with the emission of phonons? If phonons are also required to be emitted in the lasing process, would this affects down the lasing effect?)