externalcavity

External cavity refers to an optical resonator placed outside the primary light-emitting element, used to provide optical feedback and to select or stabilize the emission wavelength. In practice, the most common example is the external cavity diode laser (ECDL), where a laser diode is fed back by an external element such as a diffraction grating or an etalon. This feedback narrows the emission to a single longitudinal mode and makes the laser tunable over a limited range by adjusting the external cavity.

The external cavity typically consists of the laser diode, a collimating optic, and one or more mirrors or refractive elements that define the feedback path. Diffraction gratings are frequently used as wavelength-selective feedback elements: the first-order diffracted beam returns to the diode, selecting the lasing wavelength, while the zeroth order may be used as the output. Common configurations include the Littrow arrangement, where the grating serves as both the dispersive element and the feedback mirror, and the Littman–Metcalf arrangement, which uses a movable mirror to tune the cavity with high mode stability. The cavity length and the feedback spectrum determine the mode structure, stability, and tunability of the laser.

External cavities can be built in free space or integrated with optical fibers. They typically require careful mechanical and thermal stabilization because environmental changes affect the cavity length and thus the emission wavelength. Practical advantages include narrow linewidths, tunable single-mode operation, and flexible wavelength selection, while limitations include sensitivity to vibration, temperature drift, and alignment.

Applications include high-resolution spectroscopy, laser cooling and trapping, precision metrology, and communications where narrow, tunable laser sources are advantageous.