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Laser pulse energy is the radiant energy carried by one specified laser pulse across a measurement plane. For an isolated pulse whose total radiant power is $P(t)\ge 0$ and whose background is zero,Here $P$ is already integrated over the transverse beam area; it is not local irradiance. The unit of $U$ is the joule. Radiant exposure instead divides received energy by a receiving area, so a beam's $U$ alone does not specify the exposure at its centre. The radiant exposure of a Gaussian beam makes that spatial distinction quantitative.

The Gaussian pulse energy and peak power relation gives $P_{\mathrm{pk}}\simeq 0.939437U/\tau$ when $\tau$ is the power full width at half maximum. The pulse energy shape factor records pulse-shape dependence, while the Gaussian pulse field-envelope width is $\sqrt{2}$ times the power width.

For an original example, stipulate $U=0.50\mu \mathrm{J}$ and a Gaussian pulse power full width at half maximum $\tau =10\mathrm{ns}$. Then $P_{\mathrm{pk}}=46.97\mathrm{W}$. Stretching the same model pulse to $40\mathrm{ns}$ while keeping $U$ unchanged reduces its peak power to $11.74\mathrm{W}$. Both curves below integrate to the same energy; their peak heights and widths differ.

A finite detection time gate collects the Gaussian pulse time-gate energy fraction, which depends on its width and alignment. A centred gate as wide as the power full width at half maximum receives only $76.10\%$ of $U$, because Gaussian tails extend beyond it. For a $\tau$-wide gate, a Gaussian gate centring error of $\tau /2$ reduces capture to $49.07\%$.

The average power of a laser pulse train follows by counting complete pulses. If a long observation of duration $T$ contains $N$ pulses of energies $U_1,\dots ,U_N$, with negligible edge contributions and no continuous background,Here $f_{\mathrm{rep}}$ is the pulse repetition frequency, not the inverse pulse width. An average power readout constrains $\overline{U}$ when $f_{\mathrm{rep}}$ is known; it does not resolve pulse-to-pulse energy fluctuation. Identical, well-separated $0.50\mu \mathrm{J}$ pulses at $1.0\mathrm{MHz}$ give $0.50\mathrm{W}$ average power, even though the stipulated $10\mathrm{ns}$ pulses peak near $47\mathrm{W}$.

If a steady optical background is present, state whether the reported energy includes it. For pulse energy above a measured background $P_{\mathrm{bg}}$, the pulse energy baseline correction uses ${\int }_{\mathrm{gate}}[P(t)-P_{\mathrm{bg}}]dt$. Failing to subtract a constant background adds $P_{\mathrm{bg}}{T}_g$ to the inferred energy. This optical-background model is distinct from an offset introduced later by detector electronics.

Historical NIST equipment illustrates what the readouts measure. Its TC-24 reference absorbed the beam in a glass plate, sensed a plate temperature difference with thermocouples, and integrated the thermopile's electric potential difference during beam injection. Before/after readings enabled background subtraction and compensated thermal drift. Counting pulses converted total injected energy to mean pulse energy. The semiconductor pulse photodetector used an integrating amplifier; calibration related the peak output electric potential difference to pulse energy. That electrical peak did not by itself establish optical peak power.^[ref] Pulse energy measurement with a photodiode connects this kind of measurement to collected electric charge; validating the detector response under the actual pulse conditions remains part of that interpretation.

Missing-pulse bias in average-power inference arises when a nominal trigger rate is used as though every trigger emitted a pulse. With background removed and complete pulses counted, let $M$ be the number of triggers and $N$ the number of emissions during $T$. ThenThe quotient is mean energy per trigger, including failed triggers as zero-energy outcomes. It equals the conditional pulse-energy mean given emission only when every trigger emits, or after correction for the measured emission fraction. Historical calorimetric inference uses energy, injection time and pulse rate; those quantities must describe the same collection.^[ref]

For an original five-trigger record, suppose energies are $0,1,0,3,0\mu \mathrm{J}$. Mean energy per trigger is $0.8\mu \mathrm{J}$, while the two emitted pulses average $2\mu \mathrm{J}$. At a nominal $1\mathrm{kHz}$ trigger rate, their average power is $0.8\mathrm{mW}$. The actual emission rate is $400\mathrm{Hz}$, consistent with the same power and emitted-pulse mean.

A pulse monitor with a threshold above the weaker pulse could detect only the $3\mu \mathrm{J}$ event. Its detected-event average would then be $3\mu \mathrm{J}$, also different from the true emitted-pulse mean. Failure to detect is not automatically failure to emit. A calibrated pulse-resolving receiver and a trigger counter distinguish emitted, detected and commanded events. The average power of a laser pulse train article additionally treats continuous backgrounds and observation-window accounting.

Pulse-to-pulse energy fluctuation is variation in the laser pulse energy of successive emitted pulses. For a stationary population with finite variance, its standard deviation is ${\sigma }_U=\sqrt{\mathbb{E}[(U-\mathbb{E}U{)}^2]}$. A finite record supplies an estimate, whose convention should be stated rather than being confused with the uncertainty of the record's mean.

For an original four-pulse record, take measured energies $9,11,11,9\mu \mathrm{J}$. Their mean is $10\mu \mathrm{J}$. The sample standard deviation isIts coefficient of variation is 11.55 percent. The root-mean-square deviation of this particular list, using divisor four rather than three, is instead $1\mu \mathrm{J}$ or 10 percent. The definition matters when comparing reported stability figures. The familiar $s_U/\sqrt{N}$ estimate of uncertainty in a mean additionally assumes independent, identically distributed observations; an ordered record alone does not establish that assumption.

A historical NIST stability assessment compared an internal excimer-laser pulse monitor with a pyroelectric laser-energy meter. Its calorimeter integrated a pulse train, so averaging and individual-pulse variation entered different parts of the analysis.^[ref]

Observed variability also includes readout noise. Separating detector noise from pulse-energy variance requires an appropriate measurement model and independent information about that noise. Detector saturation, drift and correlated pulse errors are not removed merely by collecting a longer record. An average power of a laser pulse train measurement by itself does not resolve this sequence.

A pulse train is a sequence of pulses. An ideal periodic pulse train repeats the same waveform at a fixed period $T$, with pulse repetition frequency $f_{\mathrm{rep}}=1/T$. A finite burst, varying pulse heights or pulse timing jitter can break exact periodicity without removing the useful description as a pulse train. The repetition interval describes spacing; it is distinct from one pulse's duration.

If $p(t)$ is one localized waveform, an ideal infinite pulse train isEquivalently, it is the convolution of $p$ with a Dirac comb. Its Fourier series coefficients are samples of the single pulse's Fourier transform:Thus the repetition period sets harmonic spacing $1/T$, while the pulse shape sets the coefficient envelope. This statement concerns the repeated waveform itself; an optical electric field additionally has carrier phase information that a measured optical-power trace does not contain.

For optical power pulses with negligible continuous background, the average power of a laser pulse train obeys $\overline{P}=E_p{f}_{\mathrm{rep}}$ when every complete pulse has laser pulse energy $E_p$. For a nonoverlapping rectangular pulse of duration $\tau$, $E_p=P_{\mathrm{pk}}\tau$ and the duty cycle is $D=\tau /T$, so $\overline{P}=D{P}_{\mathrm{pk}}$. For other shapes, a width such as the full width at half maximum needs a pulse energy shape factor before it determines peak power.^[ref]

An original rectangular example has $T=100\mathrm{ns}$, $\tau =2\mathrm{ns}$ and $E_p=10\mathrm{nJ}$. The pulse repetition frequency is $10\mathrm{MHz}$, the duty cycle is $0.02$, the peak power is $5\mathrm{W}$ and the average power is $0.10\mathrm{W}$. A slow power meter can report that average while failing to resolve either the $2\mathrm{ns}$ width or individual pulse-to-pulse energy fluctuation.

A fast photodetector and oscilloscope instead display successive electrical responses. Their bandwidth and impulse response limit the optical timing information available. Counting the spacing tests the pulse repetition frequency; integrating calibrated individual responses tests laser pulse energy. Pulse energy measurement with a photodiode explains the charge-integration step. Separating those measurements avoids inferring the pulse width from an average-power reading alone.