Active Physics Lab - Laser

Purpose:
To analyze what's going on inside a laser.

Materials:
Decent computer with java installed
http://wps.aw.com/aw_young_physics_11/13/3510/898597.cw/index.html

Question 1: Absorption
At any given time, the number of photons inputted into the cavity must be equal to the number that have passed through the cavity without exciting an atom plus the number still in the cavity plus the number of excited atoms. Verify this conservation law by stopping the simulation and counting photons.

At any given time, N_in = N_out + n_2 (# atoms in excited state) + #photons in cavity.
In this case, N_out (2) + n_2 (6) + 0 = N_in = 8.


Question 2: Direction of Spontaneous Emission
During spontaneous emission, does there appear to be a preferred direction in which the photons are emitted?

No, the direction of emitted photons appeared to be completely random and having no preferred direction.


Question 3: Lifetime of Excited State
Does there appear to be a constant amount of time in which an atom remains in its excited state?

No. The time an atom stays in excited state is random, because the traveling photons appear to be traveling randomly; whichever atom happens to be in the way of the photon will transition. 

Question 4: Stimulated Emission
Carefully describe what happens when a photon interacts with an excited atom. Pay careful attention to the phase and direction of the subsequent photons. (Can you see why this is called stimulated emission?)

When a photon interacts with an excited atom, the excited atom will transition and emits a photon, emitting a net photon of 2 (incident photon + emitted photon). When those photon group hits another excited atom, that atom will also emits one more photon, emitting 3 net photons. 
Every time a photon(group) interacts with an atom, it will come out having one more photon.


Question 5: Pumping
Approximately what pumping level is required to achieve a population inversion? Remember, a population inversion is when the number of atoms in the excited state is at least as great as the number of atoms in the ground state.

A pumping level of at least 75 is required to have an average of at least as many excited atoms as ground state. 

Question 6: Photon Emission
Although most photons are emitted toward the right in the simulation, occasionally one is emitted in another direction. Are the photons emitted at odd directions the result of stimulated or spontaneous emission?

Spontaneous emission. It cannot be stimulated by stimulated emission because stimulated emission's photons must go the same direction as the incident photons, which is to the right. 

Since spontaneous emission occurs naturally, without external interference, photons emitted from spontaneous emission could go to any direction. 

Consequently however, a spontaneous- stimulated emission could also cause the emitted photon to travel to any directions, although the probability of this happening is relatively small.