REQUIREMENTS   FOR   A RESONATOR

We examined the rate equations which govern the laser and determine the criterion for lasing. One important result, which defines the ratio of the rates of stimulated emission to spontaneous emission:

The equation was simplified further by substituting for the Einstein coefficients, to yield

The important conclusion was that for this ratio to be large (i.e., for the stimulated emission rate to exceed the spontaneous rate), the energy density of incident photons must be high. Unless the gain of the medium is extremely high in order to create a huge flux of photons as they pass down the tube, or the gain medium is very long, cavity mirrors will be required to contain photons within the cavity, keeping ρ high and creating further amplification. Indeed, the vast majority of lasers require cavity mirrors for oscillation. Furthermore, these mirrors are usually very efficient reflectors where typical values for a gas laser are 99.99% reflectivity for the HR and 99 to 98% reflectivity for the OC. The resonator must trap photons completely within the cavity since any photons that escape the resonator represent a loss in the laser, which, as we shall see, can drastically affect output power.

         A brief description of super radiant lasers. These lasers have huge gains, so high that light is amplified to a usable level in a single pass down the tube. These lasers operate without feedback (i.e., without cavity mirrors). Molecular nitrogen, neutral neon, and molecular hydrogen lasers are usually super radiant. Other lasers, such as copper-vapor lasers, have very high gains requiring minimal or no optical feedback to operate