Light pulses and regular optical pulse trains can be generated e.g. with Q-switched and mode-locked lasers. As important pulse parameters such as pulse duration and energy and also the aspects of interest can be very different, in the following we separately consider pulse characterization for Q-switched and mode-locked lasers.
Pulse Characterization for Q-switched Lasers
The pulse characterization for Q-switched lasers is relatively simple; it typically comprises the following aspects:
- The pulse duration, which is typically in the nanosecond regime, can be directly measured with a fast photodetector.
- The pulse energy may be measured directly (e.g. with a pyroelectric detector). In some cases with repetitive operation, it is calculated from the average power (from a power meter) and repetition rate. One may also use a properly calibrated photodiode signal.
- The peak power may be directly measured with a fast photodiode or calculated from pulse energy, pulse duration and pulse shape.
- The pulse repetition rate is typically determined by the laser driver and thus does not need to be measured. In the case of passive Q switching, one may use a photodiode and an oscilloscope or an electronic spectrum analyzer.
- The timing jitter of a Q-switched laser may be substantial. It may be calculated based on measurements with a sufficiently fast photodiode.
- The optical center frequency and spectral shape can be obtained with an optical spectrum analyzer.
Pulse Characterization for Mode-locked Lasers
- The pulse duration is typically too short for direct measurements even with a very fast photodiode. Therefore, one uses various other methods, e.g. based on an autocorrelator or a streak camera. One may also derive the pulse duration from a complete pulse characterization as explained below. Optical sampling techniques can be used when a shorter reference pulse is available.
- The pulse energy is typically calculated as the average power (from a power meter) divided by the pulse repetition rate.
- The peak power is usually calculated from pulse energy, pulse duration and pulse shape. Note that there may be substantial uncertainties concerning the pulse shape, if no complete pulse characterization is performed.
- The pulse repetition rate (in the megahertz or gigahertz region) is usually measured with a fast photodiode and an electronic spectrum analyzer.
- The optical center frequency and spectral shape can be obtained with an optical spectrum analyzer. In many cases, one does not resolve the lines of the obtained frequency comb, but only obtains the spectral envelope.
Complete Ultrashort Pulse Characterization
The characterization as outlined above is still somewhat incomplete. There are methods of complete pulse characterization , which reveal more details:
- the electric field versus time or the complex spectrum (including spectral shape and spectral phase)
- the precise pulse shape
- the chirp of the pulses
For example, an ordinary intensity autocorrelator always delivers symmetric signal shapes concerning time, even if the pulses are asymmetric (e.g. with steep rise and a slower fall of power). The pulse duration calculated from an autocorrelation trace is often based on the assumption of a certain temporal pulse shape, which cannot be fully validated based on the obtained data. Such an autocorrelator can also not reveal any optical phase properties or a chirp.
The most prominent techniques for complete pulse characterization 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.
Further possibly interesting details are:
- The carrier–envelope offset frequency of the frequency comb is of special interest in optical metrology, and may be measured with an <$f-2f$> interferometer.
- The timing jitter of a pulse train can be measured with various methods, e.g. with a fast photodiode, a fast sampling card and software, or with more sophisticated methods .
- The temporal coherence (e.g. of subsequent pulses) can be characterized e.g. with an interferometer.
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. Therefore, an ultrafast laser system can often be considered as complete only if it comprises comprehensive pulse characterization equipment, which may substantially contribute to the overall cost.
Particularly careful pulse characterization may be required in the laser development, where various effects on the pulse formation need to be investigated.
The RP Photonics Buyer's Guide contains 31 suppliers for pulse characterization instruments. Among them:
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