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Optical autocorrelators are used for various purposes, in particular for the measurement of the duration of ultrashort pulses with picosecond or femtosecond durations, where an electronic apparatus (based on, e.g., a photodiode) would be too slow.
The basic principle of operation of an autocorrelator for a pulse duration measurement is explained in the following. A beam splitter creates two copies of the incoming pulses. These copies are superimposed in a nonlinear medium, where they interact on the basis of some nonlinearity, provided that they overlap temporarily.
In an intensity autocorrelator as shown in Figure 1, a beam splitter splits an incoming pulse into two pulses, which are then focused and sent into a crystal with a χ(2) nonlinearity. The arm length difference and thus the relative timing of the pulses can be mechanically adjusted via the variable delay line. (Different kinds of delay lines are used, e.g. using rotating glass blocks or mirrors mounted on loudspeakers.) If the arm length difference is made small, so that the pulses meet in the nonlinear crystal, the process of sum frequency generation occurs, leading to an output with a shorter wavelength. If the relative time delay is increased, so that the overlap of the two pulses in the crystal is reduced, the mixing product becomes weaker.
For measuring the pulse duration, the power of the mixing product is recorded as a function of the arm length difference (this can be done under computer control, using a motorized translation stage to move the delay line, or simply a loudspeaker moving a corner cube). The dependence of the autocorrelation signal on the temporal delay is given by
Figure 2 shows the autocorrelation signal for sech2-shaped pulses. The horizontal axis shows the time delay as calculated from the arm length difference.
For sech2-shaped pulses, the pulse duration is ≈ 0.65 times the width of the autocorrelation signal, but this conversion factor depends on the pulse shape. A rough evaluation is often based on some assumption concerning the pulse shape. Fitting the recorded autocorrelation with a theoretical curve for that assumed pulse shape provides a kind of sanity check, but not a strict one. Note that the autocorrelation is e.g. always symmetric with respect to its center, even for asymmetric pulse shapes. (However, asymmetric autocorrelation shapes can be obtained from misaligned autocorrelators.)
Note that very short pulse durations can be measured without requiring a fast photodetector: the detector only has to measure an average power (assuming that a regular pulse train is sent into the autocorrelator), rather than resolving the power variations of the pulses. For very short pulse durations (e.g. < 20 fs), various difficulties arise: the beam splitter may have a limited bandwidth, and its substrate and the other optical elements introduce chromatic dispersion. Furthermore, a geometric smearing effect leads to a too long measured pulse duration, if the beam angles are too steep, and group velocity mismatch can restrict the phase-matching bandwidth. Very thin nonlinear crystals are used in that domain, sometimes with a thickness less than 10 μm.
A collinear setup for intensity autocorrelation measurements can exploit type II phase matching in the crystal. The two pulse copies then have different polarization directions, rather than different propagation directions. In that way, the mentioned geometric smearing effect is avoided.
The intensity autocorrelation is called background-free, since the signal vanishes for large time delays. This is different for interferometric autocorrelators, as discussed below.
In most cases, an autocorrelation trace is measured using different pulses for different time delays. However, it is also possible to measure a whole autocorrelation trace with a single pulse, if this has a sufficiently high pulse energy. In a single-shot autocorrelator, the focusing into the nonlinear crystal is done with a cylindrical (rather than spherical) lens, and the converted signal is recorded with a camera. Different spatial positions in the crystal then correspond to different time delays.
The setup of an interferometric autocorrelator (Figure 3) contains a Michelson interferometer with a variable arm length difference. The superimposed copies of the pulse are collinearly propagating into the nonlinear crystal (after focusing with a lens or curved laser mirror) and have the same polarization.
An interferometric autocorrelation is obtained by recording the average power of the frequency-doubled signal:
This kind of autocorrelation trace exhibits a fast oscillation with a period of half the optical wavelength. The maximum signal (Figure 4) is obtained when the two pulses after the beam splitter undergo perfect constructive interference, leading to twice the amplitude compared with a single pulse, and thus four times the intensity, and after frequency doubling 16 times the intensity. For a large arm length difference, the pulses do not overlap in the nonlinear crystal, and the intensity is only twice that generated by a single pulse. Hence the peak signal is always eight times higher than the background, provided that the interferometer is properly aligned.
The interferometric autocorrelation is sensitive to chirps and thus in principle makes it possible to extract more information on the pulses. However, the pulse duration of a chirped pulse is underestimated if one simply uses the width of the autocorrelation signal (see Figure 5). An improved method called modified-spectrum autointerferometric correlation (MOSAIC)  is based on an interferometric autocorrelation, which is numerically post-processed such that the resulting trace makes it much easier to diagnose a chirp.
For relatively long pulses, the many oscillations of the interferometric autocorrelation trace can be averaged out. In that case, the peak signal is three times the background (not four times, due to the non-sinusoidal oscillation).
Owing to its simpler setup, interferometric autocorrelators are more suitable than intensity autocorrelators for measuring very small (few-femtosecond) pulse durations. In particular, the above-mentioned geometric effect is avoided. However, other methods of pulse characterization (e.g. FROG or SPIDER) are more precise in the regime below 10 fs.
Choice of Nonlinear Crystal
The choice of a suitable nonlinear crystal and the crystal thickness involves various considerations, in the femtosecond regime particularly concerning group velocity mismatch. Thin KDP crystals are a good choice for pulse durations down to a few femtoseconds. Lithium iodate (LiIO3) is also often used due to its particularly wide phase-matching bandwidth.
Autocorrelators Based on Two-photon Absorption
Particularly compact and simple autocorrelators can be constructed using photodiodes with large bandgaps, where only two-photon absorption in the photodiode contributes to the photocurrent, whereas there is no linear response due to the too small photon energy. Such two-photon detectors sometimes allow for a very wide optical measurement bandwidth.
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See also: pulses, ultrashort pulses, pulse duration, pulse characterization
and other articles in the categories optical metrology, photonic devices, light pulses
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