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Pulse Characterization

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: the measurement of various properties of an optical pulse

More specific terms: pulse duration measurement, carrier–envelope frequency measurement

Categories: article belongs to category light detection and characterization light detection and characterization, article belongs to category optical metrology optical metrology, light pulses

DOI: 10.61835/wly Cite the article: BibTex plain text HTML Link to this page LinkedIn

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

Basics

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 [6], 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 [2]) and
SPIDER (spectral phase interferometry for direct electric-field reconstruction [9], → spectral phase interferometry).

The results can be visualized in various ways, e.g. with graphs of time- or frequency-dependent functions, or with spectrograms.

Generally, some optical nonlinearities are exploited for characterizing ultrashort pulses. Purely linear optical effects, e.g. interference effects, could give information on the optical spectrum but not on the spectral phase, which is crucial for the temporal pulse shape.

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 [17].
The temporal coherence (e.g. of subsequent pulses) can be characterized e.g. with an interferometer.

Spatial Aspects

Note that apart from the temporal aspect, there is also the spatial aspect [16]. 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.

Applications

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.

More to Learn

Encyclopedia articles:

Bibliography

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(Suggest additional literature!)

Suppliers

The RP Photonics Buyer's Guide contains 31 suppliers for pulse characterization instruments. Among them:

Femto Easy

Femto Easy offers different kinds of devices for the characterization of ultrashort light pulses:

Single-shot and scanning autocorrelators are easy tools for measuring pulse durations.
FROG devices allow for full pulse characterization. They are also available in single-shot and scanning versions.

All devices are optimized for easy installation and handling.

Thorlabs

The FSAC benchtop interferometric autocorrelator manufactured by Thorlabs is designed to characterize ultrafast pulse durations from 15 – 1,000 fs in the 650 – 1100 nm range. This autocorrelator for use with femtosecond lasers compliments our ultrafast family of lasers, amplifiers, and specialized optics, including nonlinear crystals, chirped mirrors, low GDD mirrors/beamsplitters, and dispersion compensating fiber.

Edmund Optics

Our compact ultrafast autocorrelator is used to characterize ultrafast laser pulses originating from Ti:sapphire and Yb:doped lasers. Featuring a built-in two-photon absorption (TPA) detector, this autocorrelator is ideal for measuring ultrafast femtosecond and picosecond laser pulses at wavelengths from 700 to 1100 nm. The highly sensitive TPA detector allows for measurements of ultrafast laser pulses with high sensitivity by eliminating the need for angle tuning of the SHG nonlinear crystal.

APE

APE offers a range of products for pulse characterization in the picosecond and femtosecond domain:

ALPHALAS

Ultrafast photodetectors from ALPHALAS in combination with high-speed oscilloscopes are the best alternative for measurement of optical waveforms with spectral coverage from 170 to 2600 nm (VUV to IR). For example, photodetectors with rise time 10 ps and bandwidth 30 GHz in combination with 50 GHz sampling oscilloscope can be successfully used to measure optical pulse widths down to 10 ps using deconvolution. Configurations of the photodetectors include free-space, fiber receptacle or SM-fiber-pigtailed options and have compact metal housings for noise immunity. The UV-extended versions of the Si photodiodes are the only commercial products that cover the spectral range from 170 to 1100 nm with a rise time < 50 ps. For maximum flexibility, most models are not internally terminated. A 50 Ohm external termination supports the specified highest speed operation.

Quantifi Photonics

Quantifi Photonics' IQFROG Frequency-Resolved Optical Gating Pulse Analyzer is a spectrally resolved Second Harmonic Generation (SHG) autocorrelator suitable for intensity and phase measurement for pulses 300 fs to 50 ps long.

Fluence

The Blueback advanced ultrashort laser pulse characterization device is a real-time ultrashort laser pulse characterization device specifically engineered to provide a high-resolution measurement for ultrafast oscillators and amplifiers. It is an essential piece of equipment for everyone who depends on accurate information about properties of their ultrashort pulses. With Fluence Blueback you get more than just a single result. You can watch your pulse evolving in real time.

RPMC Lasers

Serving North America, RPMC Lasers offers a compact, state-of-the-art, real-time, ultrashort laser pulse characterization device, designed to provide high-resolution measurements for ultrafast oscillators and amplifiers. With input specifications including 80 fs – 4 ps pulses, 1 kHz – 200 MHz rep. rate, and 1010 nm – 1060 nm wavelength range, it offers unparalleled precision in ultra-short pulse measurements, providing flexibility for a wide range of laser characterization. A high-resolution spectrometer provides detailed spectral information about their pulses, while real-time data acquisition & display help users monitor and adjust processes on the fly.

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