The core principle behind optical resonators is the phenomenon of resonance, where certain frequencies of light are amplified due to constructive interference. When light is confined within the resonator, it can only exist at specific wavelengths that satisfy the condition for constructive interference. These wavelengths are known as the resonant modes of the cavity. The quality of an optical resonator is often described by its quality factor (Q-factor), which measures how well the resonator confines the light.
