 Detection time gate

A detection time gate is a selected interval during which a photodetector's output contributes to a measurement. Its start delay and duration $T_g$ are measured relative to a trigger or another time reference. The gate may control an integrating amplifier, an electronic counter or a software selection of recorded events; it need not physically switch off the light.

A gated integrator supplies a concrete apparatus. Stanford Research Systems' SR250 uses a triggerable gate generator, a fast integrator and a readout normalized by gate width. Its gate monitor can be displayed alongside the signal on an oscilloscope to check their relative timing.^[ref] An integrated area and a gate-averaged amplitude are different output quantities: dividing an area by $T_g$ changes its units.

An original photon counting example shows the gate-width tradeoff. Suppose an ideal decaying signal has expected count rate $s(t)=S_{\infty }{e}^{-t/\tau }/\tau$ for $t\ge 0$, and a constant background has rate $b$. A gate from0 to $T_g$ collects expected countsIf signal and background counts are independent Poisson distributions and the background mean is known exactly, the background-corrected count has variance $S+B$. Its signal-to-noise ratio is $S/\sqrt{S+B}$. With chosen values $S_{\infty }=100$ and $b\tau =100$, a gate of width $\tau$ gives about63.2signal counts,100background counts and ratio4.95. A gate of width $3\tau$ collects95.0signal counts but300background counts, reducing that ratio to4.78. More collected signal alone does not guarantee a better measurement.

This example assumes ideal event registration; detector dead time, non-Poisson backgrounds or an estimated rather than known background alter the calculation. Gate timing jitter can also vary the admitted part of a rapidly changing signal. Gate placement and duration should therefore be checked against the actual waveform, background and measurement objective, rather than chosen solely from the nominal pulse width.