What results in amplification within a laser system?

In a laser system, amplification occurs primarily through the process of stimulated emission. During stimulated emission, an incoming photon interacts with an excited electron in the gain medium, prompting the electron to transition to a lower energy state. This transition results in the emission of a photon that is coherent with the incoming photon, meaning it has the same phase, frequency, and direction.

This coherent light contributes to the overall light intensity within the laser cavity, leading to amplification of the light. The gain medium must have a sufficient number of excited electrons to sustain this process, and when the conditions are met, the emitted photons form a cascading effect that amplifies the light.

Other options such as thermal energy loss, spontaneous emission rates, and diffused energy levels do not contribute to the amplification in the same way. While spontaneous emission can initiate the laser action, it does not produce the coherent light necessary for amplification; rather, it's stimulated emission that leads to the desired reinforcement of the optical signal characteristic of lasers.