ActivPhysics The Laser

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.

Answer: Consevation verifyed.Nin=Nout+20 +3（in the field）

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

Answer: In spontaneous emission, the photons are emitted in all directions with equal probability . 

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?

Answer: In spontaneous emission,  the amount of time a n atom will stay in excited state is unpredictable.

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?)

Answer: in this simulation, once all the atoms have returned to the ground state no more lasing can take place. An important mechanism, which is missing from this simulation, is a method to re-excite the atoms into the higher energy state in order for the lasing action to continue. This re-excitation is referred to as pumping. The actual method of pumping the laser varies from laser to laser. The goal of pumping is to always keep a larger number of atoms in the excited state than in the ground state. 

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.

Answer: The interaction of photons and atoms is a very dynamic process; atoms are continually absorbing and emitting photons. Nonetheless, a pumping level of around 75 is sufficient to achieve a population inversion.

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?

  

Since in stimulated emission the photon is emitted following the interaction of an incident photon with an excited atom, and since the incident photons are traveling toward the right, the stimulated emission photons must travel toward the right. Therefore, the photons emitted at odd angles must be the result of spontaneous emission.