Questions on two, three, and four-level lasers

1. What is population inversion and why is it important for lasing?
2. Why is it not easy to use a two-level atom in a laser?
3. Are there any two-level atom lasers? What are they?
4. Draw a three-level laser energy level diagram. What is an example of such a laser?
5. Why is the ruby laser a pulsed laser and not a CW laser?
6. What is ruby?
7. Who made the first ruby laser? What is its output wavelength?
8. Why is the ruby laser not very efficient?
9. Draw the energy level diagram for a four-level laser.  On it specify the requirement for each energy level.
10. Why is a four-level laser preferred to a three-level laser?
11. Give an example of a four-level laser.