Optical resonator

An optical resonator confines light through repeated reflection or feedback, reinforcing selected optical modes. It is a defining component of most lasers and many photonic devices.

Fabry–Pérot interferometer: An optical cavity formed by two parallel reflecting surfaces that produces interference through repeated reflections. Its two-mirror geometry is the simplest model of resonant optical feedback.
Dielectric mirror: A mirror made from layered dielectric materials that reflects selected wavelengths through interference. Its reflectivity sets how much light remains in a mirror-based resonator.
Laser: A device that generates coherent light by stimulated emission, typically using optical feedback and gain. The resonator provides feedback that lets a laser sustain selected optical modes.
Distributed feedback laser: A semiconductor laser whose periodic grating provides wavelength-selective feedback throughout the gain region. Its distributed reflection replaces the separate mirrors of a conventional cavity.
Optical mode: A spatial and frequency pattern of electromagnetic fields that satisfies a system's boundary conditions. Only modes compatible with the resonator’s boundaries build up strongly.
Distributed Bragg reflector: A periodic stack or grating that reflects light near wavelengths satisfying the Bragg condition. It provides wavelength-selective reflection in compact resonator designs.
Ring laser: A laser whose light circulates around a closed optical path rather than reflecting between two end mirrors. Its traveling-wave cavity avoids the simple standing-wave pattern of a linear resonator.
External-cavity laser: A laser in which an external optical path provides feedback and often narrow wavelength selection. Its resonator extends beyond the gain chip, unlike a wholly integrated cavity.
Longitudinal mode: A resonant field pattern whose standing-wave structure varies along an optical cavity's axis. Its frequency spacing follows the round-trip phase condition in a linear cavity.
Optical gain: Amplification of light through stimulated emission or another process that increases optical intensity. Gain compensates cavity losses so selected modes can grow.

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