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- 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 lenght 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.