# Photon pumping in Laser

Let's consider a ring laser where the laser must pass through the gain material before it is sent toward a partially reflective surface $\ R=1-T$. The other mirrors are perfect reflectors with $\ R_1=R_2=1$. Furthermore the output irradiance of the setup is give as $\ I_{out} = I_{sat}(\gamma - \gamma_{th})L$ where $I_{sat}$ is the saturation irradiance, $\gamma_{th}$ is the threshold gain coefficient, and $\gamma$ is the small-signal gain coefficient. Now I am trying to understand what needs to happen in order to have the scenario when every pump event results in an output photon. That is to say a theoretical one to one correspondence, every photon I put in I will get right back. (Neglect spontaneous emission). Does this happen when $\gamma > \gamma_{th}$ ? or when they are equal?

• So you mean, 100% efficiency? No cavity losses? Then, I guess your gain threshold would be zero, right? Sep 22, 2016 at 12:28

## 1 Answer

Well this is a sort of question that goes back and forth. Since stimulated emission really is a quantum optics concept, there is never a practical case with 100% stimulation. The stimulated emission cross section is proportional to the Einstein A-coefficient which is basically a probability of absorption of a photon to achieve population inversion which in turn is required for stimulated emission in the manner of lasers. As @flippiefanus mentioned, this would be a theoretical case where the threshold would be 0 and the pump infinite.