Nonadiabatic transient response in rapidly swept Brillouin gain spectroscopy

Brillouin gain spectroscopy is widely used in biological microscopy and distributed fiber sensing. Rapid frequency sweeping enables high-speed acquisition but can distort the gain waveform and complicate frequency extraction. Here, we experimentally characterize Brillouin gain under rapid sinusoidal pump-frequency modulation. When detuning changes appreciably within the acoustic lifetime, the gain departs from the quasi-static Lorentzian response showing reduced amplitude, transient ringing, and momentary negative effective gain. A causal acoustic-memory model including multiple Brillouin peaks and the finite interaction length quantitatively reproduces the measured waveforms. The model provides a practical sweep-rate criterion and a framework for interpreting and demodulating dynamically distorted Brillouin signals.