Experiment 15: LASER

Introduction
                   The purpose of this experiment is to understand how a laser works by investigating the difference between the spontaneous and stimulated emission. This experiment was conducted by using animations of light absorption and emission from a laser on Modern Physics website.
Clink here to go to "The Laser" page
Images

Figure 1: Absorption

 Figure 2: Spontaneous Emission

Figure 3: Stimulated Emission

Discussion
                  According to figure 1, it reveals that the number of photons (N_in) is equal to the addition of the number of photons out (N_out) and the number of electrons in the excited state (n₂). Specifically, 15=1+14. In addition, as shown in figure 2, the emission of photons from these excited states does not have a preferred direction, and the lifetime of each excited state is unpredictable. However, when a photon interacts with an excited electron, the emitted photons are in same direction, phase, and polarization. This is because the incident photon has the same frequency that the excited electron is needed to emit a photon. Therefore, the input of one photon stimulates the emission of another photon along with the incident photon as shown in figure 3. Therefore, the light is being amplified. In order to achieve a population inversion, where the number of excited atoms is more than that of the ground state atoms, the rate of pumping level has to be large enough so that the stimulated emission dominates. By changing the pumping level from 0 to 100, we found that the pumping level at least 70 is required to achieve this population inversion. If one of the photons are emitted in another direction, not aligned with the incident photon, it is possible that the photon is emitted spontaneously.