Dechirping

Dechirping is the process of removing chirp from a signal or optical pulse. A chirp is a signal whose instantaneous frequency changes with time. Dechirping, therefore, aims to counteract that frequency sweep so the resulting waveform is shorter in time or has a simpler frequency structure.

In ultrafast optics, dechirping is used to compensate dispersion that broadens laser pulses. When a pulse travels through dispersive media or is amplified in a system, its spectral components acquire different delays, producing a frequency sweep. Dechirping introduces opposite dispersion to cancel this effect, ideally returning the pulse toward its transform-limited duration. Common implementations include grating pairs, prism pairs, chirped mirrors, and fiber-based compressors. Managing higher-order dispersion is often necessary to achieve sub-picosecond pulse durations.

In radar, sonar, and communications, dechirping (or despreading) refers to mixing a received chirped signal with a reference chirp of opposite sweep. This operation converts a frequency-modulated signal into a near-DC or narrowband tone, enabling easier detection, correlation, and parameter estimation. In radar, dechirp supports range and velocity calculations; in chirp-spread spectrum systems, it simplifies demodulation and improves signal-to-noise ratio.

Practical considerations include residual higher-order dispersion, noise amplification, and sensitivity to the accuracy of the reference chirp. Dechirping effectiveness depends on precise dispersion compensation, stable reference signals, and appropriate filtering. Overall, dechirping is a foundational technique across fields that seek to reverse frequency-time distortions in pulses or modulated signals.