selfmodulated

Selfmodulated, often written as self-modulated, describes processes in which a system’s own conditions cause the modulation of its signals or outputs, rather than an external modulating input. This self-action arises from nonlinearities, feedback, or dynamic coupling within the system. The term is used across physics, engineering, and signal processing to distinguish internally driven modulation from externally imposed modulation.

Common mechanisms include nonlinear index changes, such as the Kerr effect, where an intensity-dependent refractive index converts part of a signal’s power into phase changes, producing amplitude and spectral modulation. Saturable absorption or gain dynamics in lasers can cause the medium’s transmission or amplification to depend on the evolving signal, effectively modulating the output. Feedback loops—where a portion of the output is fed back into the input—also enable self-modulation. In wave and plasma contexts, modulational or self-modulation instabilities can cause a carrier to break into a modulated envelope during propagation.

Examples span several domains. In nonlinear and fiber optics, self-modulation accompanies phenomena like self-phase modulation and modulational instability, contributing to spectral broadening and the formation of pulse trains. In plasma physics, the self-modulated laser wakefield acceleration regime involves a long laser pulse whose envelope becomes modulated by its interaction with plasma, leading to microbunching of the driven structure. In electronic oscillators, intrinsic nonlinearities and saturation can induce amplitude or phase modulation without external driving signals.

See also modulational instability, self-phase modulation, and nonlinear optics.