Calculating Gamma, L, R1,2 for Excited Ions

[Jozefina Gramatikova]

Homework Statement

A Nd:YAG laser with a crystal Nd:Y3Al5O15 rod. Determine the number of Nd ions in the excited state at threshold in this laser.

Relevant Equations

$\gamma+\frac{1}{2L} ln(\frac{1}{R_{1}R_{2})=(N_{1}-N_{2})\sigma} $

For part b)

Gamma is the cavity loss coefficient, L is the length of the rod, R1,2 are the reflectivities. I am not sure how to find the number of ions in the excited state since I don't know the number of ions in the ground state.

 

[Gordianus]

The Nd:YAG laser is a four-level system. Thus, it is sensible to assume the lower level population $N_1$ is negligible.