Laserresonator

A laser resonator, or optical cavity, is the arrangement of mirrors and sometimes other optics that provides optical feedback to a laser’s gain medium. It sustains light by allowing multiple passes through the medium, selects discrete resonant frequencies, and defines the spatial mode and beam quality of the emitted light.

Most common resonators are linear two-mirror cavities, with the gain medium placed between the mirrors. Mirrors can be plane or curved, and their radii together with the cavity length determine stability. For a two-mirror cavity, stability requires the product g1g2 to lie between 0 and 1, where gi = 1 − L/Ri. Ring cavities and other multi-mirror configurations are also used to achieve different output properties or directional light.

Light makes a round trip in the cavity and must satisfy a resonance condition: the round-trip phase must be an integer multiple of 2π. Longitudinal modes form discrete frequencies separated by about c/(2nL) in a simple cavity. Transverse confinement leads to higher-order Gaussian modes (TEMmn). Mode selection is achieved with intracavity elements such as etalons, apertures, or polarization optics to favor the fundamental mode and improve beam quality.

Practical design also contends with issues like thermal lensing, astigmatism, and alignment sensitivity, all of which affect threshold, efficiency, and stability. The resonator is tailored to the gain medium and application, influencing the laser’s performance, coherence, and brightness. Common resonator types include configurations used in Nd:YAG, CO2, and diode-pumped solid-state lasers, as well as various ring and confocal variants.