Pulse Duration

The duration of an optical pulse (also known as pulse width or pulse length) can vary over a wide range:

• By modulating a continuous-wave light source, for example, using an electro-optical modulator, pulses of duration from tens of picoseconds to arbitrarily high values can be generated.
• For example, the gain switching of laser diodes results in pulses lasting as low as a few nanoseconds or even a few hundred picoseconds.
• The pulse duration of Q-switched lasers typically ranges from 100 ps to hundreds of nanoseconds.
• Mode-locked lasers can generate pulses lasting between ≈5fs and hundreds of picoseconds.
• Higher harmonic generation can form a single attosecond pulse or a sequence of attosecond pulses with pulse durations ranging from a few hundred attoseconds to less than 100 attos.

Conversion of common units

• 1ms=10^3s
• 1μs=10^-6s
• 1ns=10^-9s
• 1ps=10^-12s
• 1fs=10^-15s
• 1as=10^-18s

Definition of pulse duration

There are actually different definitions of pulse duration:

• The most commonly used definition is Full Width at Half Height (FWHM) based on optical power versus time.
• For calculations on soliton pulses, a duration parameter τ is usually used, which is approximately the FWHM duration divided by 1.76.
• Especially in the case of laser-induced damage, the effective pulse duration, which is defined as the pulse energy divided by the peak power, is sometimes used.
• Especially in the case of a pronounced pulse base, different methods can lead to completely different pulse duration values.

Time-bandwidth product

The product of the pulse width and the spectral bandwidth is called the time-bandwidth product and is typically calculated using the FWHM values of the duration and bandwidth. It cannot be less than ≈0.3 and depends on the pulse shape and the precise definition of the pulse duration and bandwidth. This means, for example, that 10-fs pulses must have a bandwidth of at least 30THz, and if attosecond pulses have such a bandwidth, their center frequency must be much higher than that of any visible light.

Measurement of pulse duration

• Using the fastest photodiodes and fast-sampling oscilloscopes, pulse durations as low as about 10ps can be measured. For measurements with short pulse durations, a fringe camera can be used.
• Another approach is optical sampling (or cross-correlation), using another source that produces a shorter reference pulse. However, in most cases, people use optical autocorrelators and do not require any reference pulses.
• Note that there are also techniques such as FROG or SPIDER (spectral phase interferometry) that can be used to get more information about the pulse, not just the pulse duration and energy.