Prepulses

Prepulses are small stimuli or pulses that occur shortly before a main event or signal. The term is used in fields such as ultrafast laser physics and neuroscience to describe preceding inputs that influence the response to the subsequent main stimulus.

In ultrafast lasers and laser–matter experiments, prepulses refer to low-intensity light that precedes the main high‑intensity pulse. They can arise from amplified spontaneous emission, residual light in the cavity, or reflections from optical components. Prepulses can pre-ionize or pre-heat the target, modify surface conditions, or seed instabilities, thereby degrading the contrast and altering experimental outcomes. The quality of a pulse is often described by the contrast ratio, the main-pulse energy divided by the prepulse energy. Suppression of prepulses is a central concern, and techniques such as plasma mirrors, cross-polarized wave generation, spectral and spatial filtering, and precise pulse shaping are routinely employed to improve contrast.

Diagnostics used to characterize prepulses include fast photodiodes, cross-correlation techniques, and frequency-resolved optical gating or SPIDER measurements, which help quantify the temporal structure and spectral content of pre-pulses and ASE.

In neuroscience, prepulses denote brief, non-startling stimuli that precede a startling stimulus. Prepulse inhibition (PPI) describes the reduction in the subsequent startle response, reflecting the integrity of sensory gating and information processing. PPI is commonly assessed by presenting a low-intensity prepulse before a loud acoustic startle and measuring the inhibited response. The strength and timing of PPI depend on interval and intensity and are reduced in several psychiatric and neurodevelopmental disorders, including schizophrenia and fragile X syndrome.

Overall, prepulses are a cross-disciplinary concept involving preceding signals that modulate responses to subsequent events, with practical implications for experimental design, measurement, and interpretation.