Cross-correlation Frequency-resolved Optical Gating Research Articles

Generation of shaped ultraviolet pulses at the third harmonic of titanium-sapphire femtosecond laser radiation

We experimentally demonstrate a method to generate shaped femtosecond laser pulses in the ultraviolet at a central wavelength of 267 nm, the third harmonic of conventional titanium-sapphire femtosecond laser systems. Employing a 128-pixel liquid-crystal spatial light modulator, we impose variable spectral phase modulations upon the near-infrared laser pulses. By this, complex laser pulses can be shaped whose overall spectrum is still conserved. Our experiments show that it is possible to easily transfer these pulses into the ultraviolet at 267 nm via sum-frequency mixing in nonlinear crystals and to predictably generate multistructured ultraviolet femtosecond laser pulses. We analyze the temporal and spectral composition of these pulses after frequency conversion into the ultraviolet using difference-frequency cross-correlation and XFROG (cross-correlation frequency-resolved optical gating) techniques with an unmodulated fundamental laser pulse. The method can be employed to facilitate adaptive quantum control experiments in the ultraviolet wavelength regime, where the major absorption bands of many organic molecular systems are located.

Coherent synthesis using carrier-envelope phase-controlled pulses from a dual-color femtosecond optical parametric oscillator

By using a dual-color femtosecond optical parametric oscillator (OPO), a coherent waveform was synthesized from two coresonant near-infrared signal pulses whose center wavelengths had a separation of 100 nm. Immediately after the OPO cavity the pulses had independent carrier-envelope phase-slip frequencies, and synthesis was achieved by shifting these frequencies using an acousto-optic modulator driven by an internally generated difference frequency. Soliton self-frequency shifted pulses from a photonic crystal fiber and a cross-correlation frequency-resolved optical gating (XFROG) measurement were used to analyze the result of the synthesis experiment and revealed that the synthesized waveform was a train of high-contrast 30 fs pulses.

Complete characterization of molecular vibration using frequency resolved gating

The authors propose a new approach to vibration spectroscopy based on the coherent anti-Stokes Raman scattering of broadband ultrashort laser pulses. The proposed method reveals both the amplitude and the phase of molecular vibrations by utilizing the cross-correlation frequency resolved optical gating (XFROG) technique. The spectrum of the anti-Stokes pulse is measured as a function of the time delay between the laser-induced molecular vibrations and a well characterized broadband femtosecond probe pulse. The iterative XFROG algorithm provides a simultaneous complete characterization of molecular vibrations both in frequency and time domains with high resolution. They demonstrate experimentally the feasibility of the proposed method and show one of its potential applications in disentangling the time behavior of a mixture of vibrationally excited molecules. The technique of femtosecond pulse shaping is used for further improvement of accuracy and stability against noise.

Pulse optimization for Raman spectroscopy with cross-correlation frequency resolved optical gating

We propose to employ the technique of femtosecond pulse shaping for improving the performance of the recently suggested method of complete characterization of molecular vibrations, in which both the amplitude and phase of the laser induced vibrational coherence are detected with high resolution. The amplitude-phase information is retrieved from the cross-correlation frequency resolved optical gating of Raman modes. By creating rich interference pattern in the measured two-dimensional spectrogram of coherent anti-Stokes Raman scattering we enhance the accuracy of the retrieved spectral and temporal response and increase the robustness of the method against noise.

High-fidelity line-by-line optical waveform generation and complete characterization using FROG

A stable optical frequency comb with 20-GHz spacing is shaped by a compact integrated silica arrayed waveguide grating (AWG) pair to produce optical waveforms with unprecedented fidelity. Complete characterization of both the intensity and phase of the crafted optical fields is accomplished with cross-correlation frequency resolved optical gating (XFROG) which has been optimized for periodic waveforms with resolvable modes. A new method is proposed to quantify, in a single number, the quality of the match in both the amplitude and phase between the measured optical waveform and the target waveform.

Phase-sensitive scattering of a continuous wave on a soliton

Using cross-correlation frequency-resolved optical gating, we observe the phase-sensitive resonance in the interaction of a soliton with a continuous wave in a photonic crystal fiber. This interaction strongly depends on the difference in the phase velocities of the orthogonally polarized fiber modes and leads to generation of a new spectral peak. The spectral and temporal structure of this signal is revealed in our measurements, which are supported by analytical theory and numerical simulations.

Intensity and phase evolutions of transmitted and reflected femto-second optical pulses in GaAs

We report on intensity and phase evolutions of transmitted and reflected femto-second optical pulses in bulk (300 μm) and thin film (300 nm) crystals of GaAs by cross-correlation frequency-resolved optical gating method. A beating phenomenon is observed at the transmitted optical pulse through the GaAs thin film. The temporal phase shows a phase jump of almost π, which is equivalent to a change of sign of the field. The oscillatory behavior arises from the interference between upper and lower branches of the polariton excited coherently. The reflection pulse from GaAs thin film shows more complicated behavior in the temporal and spectral domain, because the film is as thick as the wavelength of the optical pulse in the crystal and the polariton effect cannot be ignored. On the other hand, the reflection pulse from bulk GaAs shows an instantaneous pulse response. These phenomena are in good agreement with numerical calculations based on the Lorentz model.

Numerical simulations of optical parametric amplification cross-correlation frequency-resolved optical gating

We perform numerical simulations of cross-correlation frequency-resolved optical gating with the nonlinearities, optical parametric amplification, and difference-frequency generation for measuring broadband pulses. We show that use of a noncollinear beam geometry that matches the group velocities of the pump, signal, and idler pulses permits use of relatively thick crystals for high gain without significant distortion in the measured trace, yielding bandwidths of ~100 nm.

Recovery of time evolving fluorescence spectra via sum-frequency cross-correlation frequency resolved optical gating

Sum-frequency conversion of sample emission with short light pulses has been used for time-resolved fluorescence of chromophores in solution. In contrast to traditional fluorescence upconversion techniques where the time resolved fluorescence signal is approximated from 10 to 15 different sum-frequency gated decays distributed across the fluorescence spectrum, sum-frequency cross-correlation frequency resolved optical gating offers the opportunity to determine the electric field of the sample fluorescence as a function of time. Dynamic quantities associated with changes in the sample emission, such as solvent relaxation, can then be obtained directly from the electric field without assumption of a particular spectral line shape.

Reconstruction of the phase of matter-wave fields using a momentum-resolved cross-correlation technique

We investigate the potential of the so-called cross-correlation frequency-resolved optical gating (XFROG) technique originally developed for ultrashort laser pulses for the recovery of the amplitude and phase of the condensate wave function of a Bose-Einstein condensate. Key features of the XFROG method are its high resolution, versatility, and stability against noise and some sources of systematic errors. After showing how an analog of XFROG can be realized for Bose-Einstein condensates, we illustrate its effectiveness in determining the amplitude and phase of the wave function of a vortex state. The impact of a reduction of the number of measurements and of typical sources of noise on the field reconstruction are also analyzed.

Cross-correlation frequency-resolved optical gating for studying ultrashort-pulse nonlinear dynamics in arbitrary fibers

We describe a cross-correlation frequency-resolved optical-gating system specifically designed for studying nonlinear pulse-propagation dynamics in fibers of arbitrary length at telecommunication wavelengths. The formation of optical solitons and the appearance of temporal phase slips are observed in 100 m of fiber. The wide phase-matching bandwidth and high sensitivity of this system allow us to visualize femtosecond-pulse evolution in a range of linear and nonlinear propagation regimes.

Transition between different coherent light–matter interaction regimes analyzed by phase-resolved pulse propagation

We present phase-resolved pulse propagation measurements that allow us to fully describe the transition between several light-matter interaction regimes. The complete range from linear excitation to the breakdown of the photonic bandgap on to self-induced transmission and self-phase modulation is studied on a high-quality multiple-quantum-well Bragg structure. An improved fast-scanning cross-correlation frequency-resolved optical gating setup is applied to retrieve the pulse phase with an excellent signal-to-noise ratio. Calculations using the semiconductor Maxwell-Bloch equations show qualitative agreement with the experimental findings.

Frequency-resolved optical gating in a quasi-phase-matched LiNbO/sub 3/ waveguide

We demonstrate the complete characterization of short pulses around 1.55 /spl mu/m by cross-correlation frequency-resolved optical gating based on sum-frequency generation in an integrated LiNbO/sub 3/ device. Excellent retrieval is achieved for pulsewidths ranging from 30 to 4 ps.

Spectral-temporal dynamics of ultrashort Raman solitons and their role in third-harmonic generation in photonic crystal fibers

We use cross-correlation frequency-resolved optical gating to obtain spectral-temporal portraits of ultrashort Raman solitons in photonic crystal fibers at telecommunication wavelengths. Power-dependent Raman frequency shifts of 200 nm in 63 mm of fiber are observed accompanied by spectral broadening and 2.5-times soliton compression. Complete time-frequency dynamics at the fundamental wavelength thus visualized enables us to explain the details of the intermodally phase-matched third harmonic generation by the propagating solitons.

Phase Evolution of Solitonlike Optical Pulses during Excitonic Rabi Flopping in a Semiconductor

We demonstrate that the temporal pulse phase remains essentially unaltered before separate phase characteristics are developed when propagating high-intensity pulses coherently on the exciton resonance of an optically thick semiconductor. This behavior is a clear manifestation of self-induced transmission and pulse breakup into soliton-like pulses due to Rabi flopping of the carrier density. Experiments using a novel fast-scan cross-correlation frequency-resolved optical gating (XFROG) method are in good agreement with numerical calculations based on the semiconductor Bloch equations.

Phase and amplitude comparison between experiment and calculation of ultrabroad-band pulses generated in a taper fiber

Using a cross-correlation frequency-resolved optical gating apparatus, the phases and amplitudes of ultrabroad-band optical pulses from a taper fiber have been measured for various input pulse peak powers. The measured waveforms have been compared with calculated waveforms using a finite-difference in the frequency domain method, in which no envelope approximations were used and the variation of the taper shape was taken into account. Excellent agreement between the measured and the calculated waveforms was obtained when the additional dispersion of a normal fiber and an objective was considered.

Time-spectrally-resolved ultrafast nonlinear dynamics in small-core photonic crystal fibers: Experiment and modelling

Nonlinear dynamics of ultrashort optical pulses in the vicinity of the second zero-dispersion point of a small-core photonic crystal fiber is visualized and studied using cross-correlation frequency-resolved optical gating. New spectral features observed in the experiments match well with recent theoretical predictions of the generation of new frequencies via mixing of solitons and dispersive waves. Power- as well as length-dependent dynamics is obtained showing strong interaction between solitons and dispersive waves, soliton-soliton interaction, soliton stabilization against Raman self-frequency shift and Cherenkov continuum generation.

Cross-correlation frequency-resolved optical gating coherent anti-Stokes Raman scattering with frequency-converting photonic-crystal fibers

Photonic-crystal fibers provide a high efficiency of frequency upconversion of regeneratively amplified femtosecond pulses of a Cr: forsterite laser, permitting the generation of subpicosecond anti-Stokes pulses with a smooth temporal envelope and a linear positive chirp, ideally suited for femtosecond coherent nonlinear spectroscopy. These pulses from a photonic-crystal fiber were cross correlated in our experiments with the femtosecond second-harmonic output of the Cr: forsterite laser in toluene solution, used as a test object, in boxcars geometry to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene molecules (XFROG CARS).

Spectrally and temporally isolated Raman soliton features in microstructure fibers visualized by cross-correlation frequency-resolved optical gating

We study soliton phenomena accompanying the propagation of femtosecond Cr: forsterite-laser pulses through a microstructure fiber in the regime of efficient anti-Stokes frequency conversion. The dispersion of the fiber is designed in such a way as to minimize the group delay of the 1.25-μm pump and the Stokes pulse within the length of soliton pulse compression in the regime of anomalous dispersion. Spectrally and temporally isolated solitonic features, resulting from soliton self-frequency shift, are detected at the output of such a microstructure fiber by means of cross-correlation frequency-resolved optical gating.

Generation of femtosecond anti-stokes pulses through phase-matched parametric four-wave mixing in a photonic crystal fiber.

Phase-matched parametric four-wave mixing in higher-order guided modes of a photonic crystal fiber is shown to result in an efficient decay of 40-fs 800-nm Ti:sapphire laser pump pulses into an anti-Stokes signal with a central wavelength around 590-600 nm and a Stokes signal centered at 1.25 microm. The photonic crystal fiber is designed in such a way as to minimize the group-velocity dispersion at the pump wavelength, phase match the parametric four-wave-mixing process, and reduce the group delay between the pump and the anti-Stokes pulses. The duration of the anti-Stokes pulse under these conditions, as shown by cross-correlation frequency-resolved optical gating measurements, is less than 200 fs.