Question

The condition of high pumping, limits the operation of a three level laser to pulsed operation. Why is continuous operation is impossible in a three level laser?

Answer 1

In three level laser, to satisfy the condition of high pumping we excite the atoms to higher state from the ground state.but before coming to ground state from excited state ,electrons goes to a intermediate state called metastable state which has a comparatively long lifetime so electrons stays in that state for a long time before making transition to the ground state.

so,there is a time gap between two transitions to the ground state i.e. there is a time gap between emitted photons. that's why it is impossible to get a continuos operation in three level laser.