Optical prepulse
= Optical prepulse
An \b[optical prepulse] is a secondary <optical pulse> arriving before the reference main pulse. The sign of its delay is part of the definition: choose the main pulse at $t=0$, so a prepulse has negative arrival time. A weak earlier pulse and an equally weak <optical postpulse> can affect a target differently when the material retains a response to the earlier illumination. Temporal order therefore carries information that one total <laser pulse energy> does not.
An original separated rectangular-profile example uses a main pulse of duration $100\,\mathrm{fs}$ and a prepulse of duration $10\,\mathrm{ps}$ centred $50\,\mathrm{ps}$ earlier. Let the prepulse's peak <radiant power> be $10^{-4}$ of the main peak. The <optical pulse temporal contrast> is $10^4$, but the energy ratio is
$$
\frac{U_{\rm pre}}{U_{\rm main}}
=10^{-4}\frac{10\,\mathrm{ps}}{100\,\mathrm{fs}}=0.01.
$$
Thus the prepulse carries $1/101\approx0.990\%$ of the two-pulse energy. If the main pulse carries $1\,\mathrm{mJ}$, the prepulse carries $10\,\mathrm{\mu J}$. These stipulated disjoint power profiles have no field-overlap term, and the main pulse's <full width at half maximum> does not reveal the earlier contribution.
A temporal diagnostic needs both sufficient delay coverage and calibrated dynamic range. The single-shot third-order correlator paper motivates its asymmetric response by contrasting it with the symmetric second-order trace, which loses the distinction between before and after.<A Single Shot 3rd Order Cross-Correlator for Pulse Contrast and Pulse Shape Measurements by J. Collier and others>[printedp194, Introduction and Figure1]{ref} Its delay axis still needs a known physical reference. A <third-order pulse correlation ghost> from a real postpulse can appear on the prepulse side; interpreting every small lobe as earlier light would be incorrect.
Repeating the measurement while varying an independently characterized optical delay helps assign temporal order. Record the <detector baseline subtraction>, coverage and response width along with the trace. A nondetection means a limit within those measurement conditions, rather than proof of zero early optical energy.