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Pulse Repetition Rate

Acronym: PRF

Definition: the number of pulses emitted per second e.g. by a mode-locked or Q-switched laser

Alternative term: pulse repetition frequency

German: Pulsrepetitionsrate

Categories: optical metrology, light pulses

Units: Hz

Formula symbol: <$f_\textrm{rep}$>


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The pulse repetition rate (or pulse repetition frequency) <$f_\rm{rep}$> of a regular train of pulses is defined as the number of emitted pulses per second, or more precisely the inverse temporal pulse spacing.

Depending on the technique of pulse generation (concerning light pulses), typical pulse repetition rates can be in different parameter regions:

  • Typical mode-locked solid-state lasers emit with pulse repetition rates between 50 MHz and a few gigahertz, but in extreme cases < 10 MHz or > 100 GHz are possible. In most cases, there is a single pulse circulating in such a laser, so that the pulse repetition rate is the inverse round-trip time in the laser resonator.
  • Q switching of solid-state lasers typically allows repetition rates from below 1 Hz to the order of 100 kHz. For active Q switching, the pulse repetition rate is determined by an external drive signal, while for passive Q switching its depends on the magnitude of loss modulation, the pump power and various other parameters.
  • Gain switching of semiconductor lasers can provide repetition rates from below 1 Hz to many megahertz.
  • Attosecond pulse trains of finite length can be generated via high harmonic generation with repetition rates of hundreds of terahertz. Here, the pulse repetition rate is twice the optical frequency of the pump light.

The article on pulse trains for methods of multiplying or dividing the pulse repetition rate.

For burst mode lasers, there are two different repetition rates: the pulse repetition rate within each burst and the much lower repetition rate of the bursts.

If a pulse train is regular and the pulses are mutually coherent, the optical spectrum of the pulse train is a frequency comb, where the spacing of the lines is determined by the pulse repetition rate.

The deviation from perfect periodicity of the optical power is called timing jitter.

See also: pulse generation, light pulses, pulse trains, frequency combs, timing jitter

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