Fringe-resolved Autocorrelation Research Articles

High-gain harmonic generation with temporally overlapping seed pulses and application to ultrafast spectroscopy.

Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG) process in a free-electron laser (FEL) is a promising approach to facilitate various coherent nonlinear spectroscopy schemes in the extreme ultraviolet (XUV) spectral range. However, in collinear arrangements using a single nonlinear medium, temporally overlapping seed pulses may introduce nonlinear mixing signals that compromise the experiment at short time delays. Here, we investigate these effects in detail by extending the analysis described in a recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order fringe-resolved autocorrelation and wave packet interferometry experiments at photon energies > 23 eV are performed, accompanied by numerical simulations. It turns out that both the autocorrelation and the wave-packet interferometry data are very sensitive to saturation effects and can thus be used to characterize saturation in the HGHG process. Our results further imply that time-resolved spectroscopy experiments are feasible even for time delays smaller than the seed pulse duration.

Efficient generation of below-threshold harmonics for high-fidelity multi-photon physics in the VUV spectral range.

We demonstrate the generation of microjoule level, sub-20-fs, Ti:Sa fifth-harmonic pulses utilizing a loose-focusing geometry in a long Ar gas cell. The VUV pulses centered at 161.8 nm reach pulse energies of 1.1 μJ per pulse, while the corresponding pulse duration is measured with a second-order, fringe-resolved autocorrelation scheme to be 18±1 fs. Nonresonant, two-photon ionization of Kr and three-photon ionization of Ne verify the fifth-harmonic pulse high-intensity content and indicate the feasibility of multi-photon VUV pump-VUV probe studies of ultrafast atomic and molecular dynamics.

Pulse duration and wavelength stability measurements of a midinfrared free-electron laser

We report the pulse duration and wavelength stability measurements of a midinfrared free-electron laser (FEL) where the wavelength fluctuation may not be negligible. The technique we employ is a fringe-resolved autocorrelation (FRAC) method that has good sensitivity on not only the pulse duration and the chirp but also the wavelength stability. By the simple manipulation of experimental FRAC signals, we can obtain the pulse duration even if the amounts of the chirp and the wavelength stability are not known in advance, which is further used to estimate the wavelength stability. Through this procedure we find that the pulse duration of the Kyoto University FEL at 12 μm is about 0.58 ps without any notable chirp, and the wavelength stability is about 1.3%. We also carry out separate experiments for intensity autocorrelation and sum-frequency mixing. The difference we find for pulse duration and wavelength stability by the different measurements is attributed to the different operation conditions of FEL.

Use of Fringe-Resolved Autocorrelation for the Diagnosis of the Wavelength Stability of a Free Electron Laser

Use of Fringe-Resolved Autocorrelation for the Diagnosis of the Wavelength Stability of a Free Electron Laser

Use of Fringe-Resolved Autocorrelation for the Diagnosis of the Wavelength Stability of a Free Electron Laser

Although monitoring the wavelength stability of a free-electron laser provides users valuable useful information, a single-shot measurement of the laser spectrum is not always an easy task in the mid/far-infrared wavelength region due to the lack or very high cost of an array-type photodetector, etc. We propose to use fringe-resolved autocorrelation to monitor the wavelength stability of an oscillator-type free-electron laser. The idea is verified by the numerical experiments for intensity and fringe-resolved autocorrelations which include shot-to-shot fluctuations of intensity, duration, and central wavelength of micropulses as well as the chirps of both micro/macro pulses.

Highly efficient 1 GHz repetition-frequency femtosecond Yb^3+:KY(WO_4)_2 laser

We present a highly efficient, diode-pumped, femtosecond Yb3+:KY(WO4)2 (Yb:KYW) laser operating at a 1.024 GHz repetition rate. The output was centered at a wavelength of 1042 nm and had a bandwidth of 3.8 nm, leading to transform-limited pulses with durations of 278 fs determined by fringe-resolved autocorrelation measurements. The optical-to-optical conversion efficiency and slope efficiency were 61% and 69%, respectively, and the relative intensity noise was <0.1%.

Non-iterative characterization of few-cycle laser pulses using flat-top gates

We demonstrate a method for broadband laser pulse characterization based on a spectrally resolved cross-correlation with a narrowband flat-top gate pulse. Excellent phase-matching by collinear excitation in a microscope focus is exploited by degenerate four-wave mixing in a microscope slide. Direct group delay extraction of an octave spanning spectrum which is generated in a highly nonlinear fiber allows for spectral phase retrieval. The validity of the technique is supported by the comparison with an independent second-harmonic fringe-resolved autocorrelation measurement for an 11 fs laser pulse.

In situ third-order interferometric autocorrelation of a femtosecond deep-ultraviolet pulse

The interferometric measurement of an ultrashort optical pulse is demonstrated using three-photon ionization of Xe in a time-of-flight mass spectrometer (MS). This approach allowed measurement of the third-order fringe-resolved autocorrelation trace of an ultrashort optical pulse in the deep-ultraviolet (DUV) region. The pulse can be measured in situ using the same MS that is employed in ultrafast spectroscopy. Sensitivity (>3×107 W) was sufficient to measure a DUV pulse used in such applications. This setup has the potential to measure the temporal duration of a broadband (204–306 nm) ultrashort optical pulse with no additional distortion in the temporal characteristics.

Ultrashort pulse characterization by ultra-thin ZnO, GaN, and AlN crystals

Ultra-thin semiconductor crystals were investigated as nonlinear materials for second-harmonic generation. Nonlinear susceptibilities of sub-micrometer- thick ZnO, GaN, and AlN crystals were measured, and these crystals were used for sub-10-fs pulse measurement by a fringe-resolved autocorrelation method. We found that a one-cycle pulse could be characterized by using these ultra-thin-film crystals.

Pulse Compression Using Direct Feedback of the Spectral Phase From Photonic Crystal Fiber Output Without the Need for the Taylor Expansion Method

Characterization and compensation of the complex spectral phase and the temporal profile of output pulses from a photonic crystal fiber (620-945-nm spectral broadening) were performed using a computer-controlled feedback system that combines a modified spectral-phase interferometry for direct electric-field reconstruction apparatus and only a 4-f chirp compensator having a spatial light modulator. These pulses were adaptively compressed from 12-fs input pulses to 6.8-fs. In addition, the compressed pulse profile showed excellent agreement with results measured independently with fringe-resolved autocorrelation.

Experimental and theoretical demonstration of validity and limitations in fringe-resolved autocorrelation measurements for pulses of few optical cycles

Using 3.6- and 5.3-fs pulses, we demonstrated theoretically and experimentally that fringe-resolved autocorrelation (FRAC) traces are distorted by bandwidth limitations of the second-harmonic generation (SHG) in 10-microm-thick, type I ss-BaB2O4 for pulses shorter than sub-5 fs. In addition, detailed numerical analysis of the SHG showed that the optimum crystal angle where the FRAC trace distortion becomes minimum is in disagreement not only with the phase-matching angle but also with the angle where the FRAC signal intensity becomes maximum. Furthermore, the apparent pulse duration measured at a nonoptimum angle was confirmed to become shorter than that of its transform-limited pulse, in excellent agreement with the calculated result.

Simple real-time fringe-resolved autocorrelator for measuring picosecond optical pulses

We propose a simple configuration of a real-time fringe-resolved autocorrelator for measuring a duration and a frequency chirp of ultrafast optical pulse, which is composed of two glass plates, a convex lens and a two-photon detector. The autocorrelator enables us to estimate relatively longer pulsewidths easily and stably by spinning one of the glass plates. The feature of the autocorrelator is examined numerically using the diffraction formula. We demonstrate a real-time fringe-resolved autocorrelation of picosecond optical pulse using the proposed autocorrelator and reconstruct the pulse intensity and phase by applying the technique of the iterative reconstruct interference signals (IRIS).

Measurement of high-intensity laser pulse widths by use of tunneling ionization of atoms and ions

We measured the pulse duration of amplified Ti:sapphire laser pulses by using the tunneling ionization of atoms and ions. Fringe-resolved autocorrelation signals with various effective nonlinear orders are measured by means of counting the number of ions at different charge states by the time-of-flight (TOF) method. The measured pulse widths agree well with the independently measured pulse width if we consider the pulse distortion by a focusing lens. Measurement of multiply charged ions by a TOF device is useful for extending the measurable intensity range and wavelength ranges.

High-average-power, 1-MW peak-power self-mode-locked Ti:sapphire oscillator

We describe a novel self-mode-locked Ti:sapphire ring laser that produces 13-fs pulses with peak powers exceeding 1 MW, pulse energies of 13 nJ, and average mode-locked output powers of 1.5 W at a cavity repetition frequency of 110 MHz. A complete resonator analysis describing the optimum mode-locking configuration is presented, together with fringe-resolved autocorrelation and sonogram measurements of the output pulses.

All-solid-state cavity-dumped sub-5-fs laser

We discuss in detail a compact all-solid-state laser delivering sub-5-fs, 2-MW pulses at repetition rates up to 1 MHz. The shortest pulse generated thus far measures only 4.6 fs. The laser system employed is based on a cavity-dumped Ti:sapphire oscillator whose output is chirped in a single-mode fiber. The resulting white-light continuum is compressed in a novel high-throughput prism chirped-mirror Gires–Tournois interferometer pulse compressor. The temporal and spectral phase of the sub-5-fs pulses are deduced from the collinear fringe-resolved autocorrelation and optical spectrum. The derived pulse shape agrees well with the one retrieved from the measured group delay of the continuum and calculated characteristics of the pulse compressor.

Measurement of interferometric autocorrelations: comment

Conventional Michelson interferometers that are widely used for autocorrelation of ultrashort optical pulses are incapable of producing two identical replicas of an optical pulse, hence yielding a correct autocorrelation signal. The problem is acute when one records the interferometric autocorrelation of pulses consisting of just a few optical cycles. With the use of sub-10-fs pulses we demonstrate asymmetric autocorrelation traces recorded with a conventional Michelson interferometer, refer to previously reported results that exhibit the same deficiency, and elucidate the origin of the problem. A simple modification of the conventional interferometer configuration yields the correct fringe-resolved autocorrelation trace.

Femtosecond Pulse Phase Measurements of Mode-locked Ti3+:Al2O3 Lasers.

A simple iterative-calculation-based method for reconstructing time histories of intensity and phase for a commercially available Ti3+: Al2O3 mode-locked femtosecond laser pulse is demonstrated. Time histories of intensity and phase, which are reconstructed under an assumption of symmetric property for the intensity profile, are cross-checked with the measurements of fringe-resolved autocorrelation and spectrum-resolved autocorrelation. Once the intensity and phase are determined with a high accuracy for an oscillator output pulse, phase distortion caused on the pulse in post processes can be reconstructed by the same iterative calculation using a crosscorrelation signal with the standard oscillator pulse.This phase measurement scheme is experimentally demonstrated for a pulse transmitted through a single-mode fiber.

Characterisation of a train of subpicosecond laser pulses by fringe resolved autocorrelation measurements

Fringe resolved autocorrelation functions generated by a train of subpicosecond laser pulses from a synchronously pumped ring dye laser were recorded. By comparison with calculated correlation functions one can not only derive the pulse duration of the center pulse, but also draw conclusions about the quasicontinuous background or satellite pulses of lower intensity.