Contents
Source: Wikipedia
The Duration of Light Pulses
Light pulses, also known as pulse width or pulse length, can vary significantly in duration depending on the method of generation:
– Modulating a continuous-wave light source can produce pulses ranging from tens of picoseconds to high values.
– Gain switching of laser diodes can lead to pulses with durations from a few nanoseconds to hundreds of picoseconds.
– Q-switched lasers typically generate pulses with durations between 100 ps and hundreds of nanoseconds.
– Mode-locked lasers can produce pulses with durations from approximately 5 fs to hundreds of picoseconds.
– High harmonic generation enables the formation of single attosecond pulses with durations of a few hundred attoseconds.
Common Prefixes for Pulse Duration
– 1 millisecond (ms) = 10^-3 s
– 1 microsecond (μs) = 10^-6 s
– 1 nanosecond (ns) = 10^-9 s
– 1 picosecond (ps) = 10^-12 s
– 1 femtosecond (fs) = 10^-15 s
– 1 attosecond (as) = 10^-18 s
Definition of Pulse Duration
There are various definitions of pulse duration:
– Full width at half-maximum (FWHM) is commonly used, based on the optical power versus time profile.
– For soliton pulses, a duration parameter is often used, calculated as FWHM duration divided by 1.7627.
– The second moment of the temporal intensity profile is suitable for complicated pulse profiles.
– In laser-induced damage contexts, an effective pulse duration is defined as the pulse energy divided by the peak power.
Time–Bandwidth Product and Minimum Possible Pulse Duration
The time–bandwidth product, the product of pulse duration and spectral bandwidth, sets limits on pulse duration and bandwidth relationships. For instance, a 10-fs pulse must have a bandwidth of approximately 30 THz.
Measurement of Pulse Durations
Pulse durations can be measured using photodiodes, streak cameras, or optical sampling techniques like autocorrelators. Techniques such as FROG or SPIDER provide detailed pulse characterization beyond just duration and energy.
Spatial Width of a Pulse
The spatial width of a pulse in the propagation direction is determined by the group velocity and temporal pulse width. Ultrashort pulses can have very short spatial lengths, with few-cycle pulses having spatial lengths of just a few wavelengths.
Effects on Pulse Duration
Chromatic dispersion, optical nonlinearities, and optical filters can affect pulse duration. Mode-locked lasers may experience changes in pulse duration during operation, but these effects are usually balanced out.
Spatio-Temporal Effects
Coupling of spatial and temporal pulse properties can complicate pulse duration definitions and measurements, such as in cases of pulse front tilt.
Conclusion
Understanding the duration of light pulses is crucial in various applications, from laser technology to pulse characterization. By considering the generation methods, measurement techniques, and effects influencing pulse duration, researchers and engineers can optimize their systems for desired pulse characteristics.
Source: Sill Optics GmbH
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