The pulse energy Ep is simply the total optical energy content of a pulse, i.e., the integral of its optical power over time. For single pulses, e.g. from a Q-switched laser, the pulse energy may be measured e.g. with a pyroelectric device.
For regular pulse trains, the pulse energy is often calculated by dividing the average power (measured e.g. with a powermeter) by the pulse repetition rate. However, this is a valid procedure only if the energy emitted between the pulses is negligible. There are cases where, e.g., a mode-locked laser emits a pulse train together with a low-level background emission. Even if the background power level is far below the peak power, the background can significantly contribute to the average power. If, e.g., a photodetector has an insufficient dynamic range for testing this, it can be useful to test the conversion efficiency of a frequency doubler in a carefully controlled situation in the low-conversion regime.
Typical pulse energies from Q-switched lasers range from microjoules to millijoules, and for large systems to multiple joules or even kilojoules. Mode-locked lasers achieve much lower pulse energies (picojoules, nanojoules or sometimes several microjoules) due to their high pulse repetition rates and sometimes due to limiting nonlinear effects in the laser resonator. Much higher energies of ultrashort pulses can be achieved by amplifying pulses at a lower repetition rate, as obtained e.g. with a pulse picker or a regenerative amplifier.
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