Optical pulses and regular optical pulse trains can be characterized in various respects:
- The pulse repetition rate is usually measured with a fast photodiode and an electronic spectrum analyzer.
- The pulse duration can be measured with various methods, e.g. with an autocorrelator or a streak camera. Optical sampling techniques can be used when a shorter reference pulse is available.
- The pulse energy may be measured directly or (for pulse trains) calculated from the average power and repetition rate.
- The peak power may be directly measured with a photodiode or calculated from pulse energy, pulse duration and pulse shape.
- The optical center frequency and spectral shape can be obtained with an optical spectrum analyzer.
- The carrier–envelope offset frequency is of special interest in optical metrology, and may be measured with an f−2f interferometer.
- The chirp can be measured e.g. with frequency-resolved optical gating.
- The timing jitter of a pulse train can be measured with various methods.
- The coherence (e.g. of subsequent pulses) can be characterized e.g. with an interferometer.
There are methods of complete pulse characterization , which reveal the electric field versus time or the complex spectrum (including spectral shape and spectral phase) of ultrashort pulses. The most prominent techniques for this purpose are FROG (frequency-resolved optical gating ) and SPIDER (spectral phase interferometry for direct electric-field reconstruction , → spectral phase interferometry). The results can be visualized in various ways, e.g. with graphs of time- or frequency-dependent functions, or with spectrograms.
Note that apart from the temporal aspect, there is also the spatial aspect . Both aspects are often approximately separated in the sense that the whole spatio-temporal profile of the electric field of a pulse can be specified as the product of two functions, one depending only on time and the other only on the spatial position. However, a significant coupling of temporal and spatial properties can occur in various situations. For example, pulses from Kerr lens mode-locked lasers often exhibit a time-dependent beam radius, which makes the complete characterization (and modeling) very challenging. Another spatio-temporal aspect is pulse front tilt, which is related to angular dispersion and can, e.g., result from a misaligned pulse compressor.
Accurate and reliable pulse characterization is essential for many applications. For example, if an ultrafast laser system does not work properly, e.g., due to misalignment of components, this can greatly affect the operation of a larger system. The problem can be located and fixed only if the pulse properties can be monitored.
Particularly careful pulse characterization may be required in the laser development, where various effects on the pulse formation need to be investigated.
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The RP Photonics Buyer's Guide contains 29 suppliers for pulse characterization instruments. Among them:
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See also: pulses, spectral phase, carrier–envelope offset, autocorrelators, frequency-resolved optical gating, spectral phase interferometry, streak cameras
and other articles in the categories light detection and characterization, optical metrology, light pulses