= Optical postpulse
An \b[optical postpulse] is a secondary <optical pulse> arriving after the chosen main pulse. Specify the delay, relative peak <radiant power>, duration and collected <laser pulse energy>: a relative peak alone does not determine the energy of a differently shaped replica. It is distinct from an extended <optical pulse pedestal> and from electronic ringing from a <photodetector>.
A transparent plate provides a concrete mechanism. At normal incidence, a transmitted replica that makes one additional internal round trip has approximate delay
$$
d=\frac{2n_g\ell}{c},
$$
where $\ell$ is plate thickness and $n_g$ is its <group refractive index>. Neglect absorption, dispersion and overlap, and let each internal reflection retain power fraction $R$. The first delayed replica then has approximately $R^2$ times the directly transmitted energy. This <etalon round-trip delay> can create a physical optical echo.
For an original chosen $\ell=1.2\,\mathrm{mm}$, $n_g=1.5$ and $R=0.04$, the delay is about $12\,\mathrm{ps}$ and the energy ratio is $0.0016$. A $2.0\,\mathrm{mJ}$ main pulse would be followed by a $3.2\,\mathrm{\mu J}$ replica under that model. The historical single-shot correlator experiment deliberately inserted a BK7 <etalon> to generate and detect such a delayed pulse.<A Single Shot 3rd Order Cross-Correlator for Pulse Contrast and Pulse Shape Measurements by J. Collier and others>[printedp195, Results and Figure4, double-reflection postpulse near1.5×10^-3]{ref}
Changing the plate thickness tests the delay prediction; changing optical alignment tests whether a stray reflection is involved. A <third-order intensity autocorrelation> can distinguish temporal sides under its model, while a symmetric second-order <intensity autocorrelation> cannot alone assign a feature to a prepulse or postpulse. An opposite-delay correlation feature may instead be a <third-order pulse correlation ghost>; it need not identify another physical pulse.
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