Answer to Question #102778 in Optics for YY

Question

What condition(s) must be met to obtain sustained stimulated emission? Compare and contrast three level and four level pumping schemes for a laser.

Answer 1

The word LASER is stands for Light Amplification By Stimulated Emission of Radiation. When the electrons are moving from lower energy level(E1) to the higher energy level (E2), then it absorbs the frequency of $\nu=\dfrac{E_2-E_1}{C}$ , From there it can decay back to the lower energy level, by emitting the photon.

So on the basis of the emission of the photon we have categories it in two types-

In the stimulated emission, an incident photon of frequency stimulates the atom to make a transition from the higher state of the lower energy state and emit another photon of frequency that is in phase with the first photon.

And in the second photon is in phase because the first photon causes the electron charge of the atom to oscillate in phase with it and this oscillation causes the second phont to be in phase with the first, so one photon comes in and two photon leaves.

In the three-level emission state with the pumping from the ground state n1 to the n3 state, but it is meta-stable state. meta state is the state in which the atom can stay for more longer state than the level 4 state.

In the level four emission state, pumping from the ground state n1 to the n4 excited state. From the n4 the atom decay to n3 by spontaneous emission.