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Frequency-resolved Optical Gating

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Acronym: FROG

Definition: a technique for the complete characterization of ultrashort optical pulses

Categories: methods, optical metrology, light pulses

How to cite the article; suggest additional literature

Frequency-resolved optical gating [1, 2] is a technique for the “complete” characterization of ultrashort pulses, i.e. for measuring not only pulse parameters such as the pulse energy or pulse duration, but also the full time-dependent electric field or (equivalently) the optical spectrum including the frequency-dependent spectral phase. (The carrier–envelope offset and the arrival time of pulses can not be measured.) This technique has been pioneered by Rick Trebino's research group at the Georgia Institute of Technology.

setup for frequency-resolved optical gating (FROG)

Figure 1: Setup for frequency-resolved optical gating in the form of SHG FROG. The spectrum of the nonlinear mixing product of the two beams is measured as a function of the relative time delay.

A typical setup for a FROG measurement (Figure 1) is similar to that of an intensity autocorrelator, except that the photodetector is replaced with a spectrometer. A FROG measurement involves recording some tens or hundreds of spectra for different settings of the arrival time difference of the two pulses. These data can be illustrated in the form of a so-called FROG trace (see Figure 2), which is a kind of spectrogram and displays with a color scale the intensity as a function of time delay and optical frequency (or wavelength).

A sophisticated iterative phase retrieval algorithm, implemented as a computer program, can then be used for reconstructing the pulse shape from the FROG trace. As the recorded data are substantially redundant, the FROG retrieval algorithm can not only deliver the pulse shape, but also carry out a consistency check. It may thus be noticed when errors occur, e.g. due to wrong calibration of the spectrometer. On a typical PC, the procedure may take a few minutes with a non-optimized algorithm, and optimized algorithms have been developed which make it possible to do the calculations in less than 0.1 s, at least for simply shaped pulses.

FROG trace

Figure 2: SHG FROG trace for a higher-order soliton pulse after some propagation length in the optical fiber. The color scale (near the right axis) is applied to the logarithm of the FROG intensity. The simulation was done with the software RP ProPulse.

The term “frequency-resolved optical gating” originates from the idea that a short gate pulse can be used to obtain a sample from a longer pulse by nonlinear mixing (gating) in a nonlinear crystal material. As an additional gate pulse, shorter than the pulse to be investigated, is usually not available, FROG actually uses the pulse itself for gating. This makes the method much simpler to apply, but at the same time conceptually and computationally more sophisticated.

Variants of Frequency-resolved Optical Gating

There are different versions of FROG, which rely on different nonlinear gating mechanisms, generate different kinds of FROG traces (thus requiring different phase retrieval algorithms), and have different strengths and weaknesses:

Beyond these traditional FROG measurement methods, refined versions of FROG have been developed, which can be applied even to very short pulses (with angle dithering of the crystal to remove strong effects of group velocity mismatch in the nonlinear crystal) or to fairly long pulses (where a high spectrometer resolution is required). A particularly compact setup is achieved with the GRENOUILLE geometry [8], which has no moving parts and even allows the measurement of additional features such as spatial chirps. A waveguide as the nonlinear component allows detection at ultralow power levels, and polarization scrambling makes possible polarization-independent measurements, which facilitate e.g. the delivery of pulses via fibers [12].

A possible alternative to frequency-resolved optical gating is spectral phase interferometry for direct electric-field reconstruction (SPIDER), as explained in the article on spectral interferometry.


[1]D. J. Kane and R. Trebino, “Characterization of arbitrary femtosecond pulses using frequency-resolved optical gating”, IEEE J. Quantum Electron. 29 (2), 571 (1993)
[2]FROG tutorial of Rick Trebino's group at the Georgia Institute of Technology, http://www.frog.gatech.edu/tutorial.html
[3]K. W. DeLong et al., “Frequency-resolved optical gating using second-harmonic generation”, J. Opt. Soc. Am. B 11 (11), 2206 (1994)
[4]R. Trebino et al., “Measuring ultrashort laser pulses in the time–frequency domain using frequency-resolved optical gating”, Rev. Sci. Instrum. 68, 3277 (1997)
[5]A. Baltuška et al., “Amplitude and phase characterization of 4.5-fs pulses by frequency-resolved optical gating”, Opt. Lett. 23 (18), 1474 (1998)
[6]S. Linden et al., “Amplitude and phase characterization of weak blue ultrashort pulses by downconversion”, Opt. Lett. 24 (8), 569 (1999)
[7]L. Gallmann et al., “Collinear type II second-harmonic-generation frequency-resolved optical gating for the characterization of sub-10-fs optical pulses”, Opt. Lett. 25 (4), 269 (2000)
[8]P. O'Shea et al., “Highly simplified device for ultrashort-pulse measurement”, Opt. Lett. 26 (12), 932 (2001)
[9]J. Zhang et al., “Measurement of the intensity and phase of attojoule femtosecond light pulses using Optical-Parametric-Amplification Cross-Correlation Frequency-Resolved Optical Gating”, Opt. Express 11 (6), 601 (2003)
[10]S. Akturk et al., “Extremely simple device for measuring 20-fs pulses”, Opt. Lett. 29 (9), 1025 (2004)
[11]G. Stibenz and G. Steinmeyer, “Interferometric frequency-resolved optical gating”, Opt. Express 13 (7), 2617 (2005)
[12]H. Miao et al., “Polarization-insensitive ultralow-power second-harmonic generation frequency-resolved optical gating”, Opt. Lett. 32 (7), 874 (2007)
[13]X. Liu et al., “Numerical simulations of ultrasimple ultrashort laser-pulse measurement”, Opt. Express 15 (8), 4585 (2007)
[14]D. Lee et al., “Experimentally simple, extremely broadband transient-grating frequency-resolved-optical gating arrangement”, Opt. Express 15 (2), 760 (2007)
[15]J. Gagnon et al., “The accurate FROG characterization of attosecond pulses from streaking measurements”, Appl. Phys. B 92, 25 (2008)
[16]R. Trebino, Frequency-Resolved Optical Gating: the Measurement of Ultrashort Laser Pulses, Kluwer, Boston (2002)

(Suggest additional literature!)

See also: pulses, pulse characterization, autocorrelators, spectral interferometry
and other articles in the categories methods, optical metrology, light pulses

In the RP Photonics Buyer's Guide, 10 suppliers for equipment for FROG measurements are listed.

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