Discussion
In a four level laser system :
Answer: C
In a four level laser the lower lasing level is empty.
In a four level system, atoms are pumped from the ground level to a high pump band, decay quickly to the upper laser level, lase down to the lower laser level, and then decay rapidly back to the ground state. Because the lower laser level lies above the ground state and empties quickly, it stays almost unpopulated, so population inversion between the laser levels is easy to reach, with far less pump power than in a three level laser such as ruby, where the lower level is the ground state. Nd:YAG and He-Ne lasers work this way.
The 'four levels' are energy levels, not blocks of the system, stages of pumping, or a sign of high power.
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