 Optical pulse pedestal

An optical pulse pedestal is lower-level optical power extending around a pulse's dominant short peak. It can precede the peak, follow it or extend on both sides. A broad pedestal can carry substantial laser pulse energy while barely changing the peak's full width at half maximum. Optical pulse temporal contrast describes the relative power levels; integrated energy also depends on how long each contribution persists.

For an original power-profile model, choose a central rectangular contribution of height $P_0$ and duration $\tau$, together with a pedestal of height $\alpha P_0$ and duration $T>\tau$, centred at the same time. Let $0<\alpha <1$. The peak power is $(1+\alpha )P_0$, while the pedestal lies below half of that value. The power FWHM is therefore still $\tau$, butThese are stipulated additive power contributions, not a rule for adding coherent electric fields.

Set $\alpha =10^{-3}$ and $T=1000\tau$. The pedestal then contains half the total energy even though its power is about a thousand times below the main peak. With $P_0=1\mathrm{MW}$ and $\tau =1\mathrm{ps}$, each contribution carries $1\mu \mathrm{J}$. The peak-to-pedestal power ratio is exactly $(1+\alpha )/\alpha =1001$ in this example. An inverse convention would quote a small pedestal-to-peak ratio instead; state which contrast definition is being used.

A measurement needs temporal range as well as dynamic range. A narrow scan can miss broad wings, while a readout that resolves only the peak can bury a pedestal in its own background. A commercial example is the Sequoia third-order cross-correlation instrument, whose manufacturer identifies temporal-contrast measurement as its purpose and separately specifies scan range and temporal resolution.^[ref] The cited instrument description establishes the measurement approach; the numerical profile above is an independent model.

A real optical pedestal must also be distinguished from detector baseline subtraction errors. Blocked-beam and reference measurements characterize the detector background, while a suitable optical measurement tests whether low-level light accompanies the pulse. A detection time gate that excludes a pedestal changes the admitted energy relative to a detector integrating the full waveform.