Question

Describe why gain saturation in a homogeneously broadened medium occurs

Answer 1

When populations between two energy levels of an atom are inverted, the medium can act as an amplifier. Therefore, as an optical signal passes through it, the signal intensity grows more or less exponentially with distance along the length of the amplifier. However, when the signal intensity increases to a certain value, Is, the population difference and hence the gain coefficient in the laser medium decreases . This behavior is often referred to as gain saturation. Where Is is the saturation intensity at center frequency ν0. Gain saturation is the primary mechanism that determines the power level at which a laser will oscillate. When a laser oscillator begins to oscillate, the oscillation amplitude grows first until the intensity inside the cavity is sufficient to saturate down the laser gain. The steady-state oscillation then occurs when the saturated laser gain becomes just equal to the total cavity losses, so that the net round-trip gain is exactly unity. The saturation behavior is different for homogeneously and inhomogeneously broadened laser media.