Characterization Of Laser Pulses Research Articles

Characterization of a broadband interferometric autocorrelator for visible light with ultrashort blue laser pulses

We present a compact interferometric autocorrelator that allows the characterization of ultrashort laser pulses in the visible light domain (370–740nm). The presented device uses a GaN photodiode with corresponding two-photon absorption. Different GaN and AlGaN photodiodes were characterized for this purpose. Despite AlGaN diodes have a better matched bandgap for this application, we have found that only the GaN diodes show sufficient nonlinear behavior. Using the autocorrelator we were able to characterize ultrashort frequency doubled Ti:Sapphire laser pulses with a pulse duration down to 18fs in the second harmonic having just a few hundred nanojoules of pulse energy. The broadband behavior and extension towards the UV along with the need for only low energetic pulses are the novelties of this device.

Frequency-resolved optical gating capable of carrier-envelope phase determination

Recent progress of the coherent light synthesis technology has brought the generation of single-cycle pulses within our reach. To exploit the full potential of such a single-cycle pulse in any applications, it is highly important to obtain the full information of its electric field. Here we propose a novel pulse characterization scheme, which enables us to determine not only the intensity and phase profiles of ultrashort pulses but also their absolute carrier-envelope phase values. The method is based on a combination of frequency-resolved optical gating and electro-optic sampling, which can be extended to a self-referencing scheme to determine the electric field evolution of few-cycle ultrashort pulses. We have experimentally demonstrated the technique to characterize sub-single-cycle infrared pulses, and numerically studied the capability of the scheme to incorporate a self-referencing technique and to extend the wavelength range to visible region. The time-resolved characterization of laser pulses is important in particular for the development of optical characterization techniques at ultrashort timescales. Here the authors develop a scheme that is able to characterize the electric-field evolution of femtosecond laser pulses.

Self-referenced characterization of femtosecond laser pulses by chirp scan

We investigate a variant of the d-scan technique, an intuitive pulse characterization method for retrieving the spectral phase of ultrashort laser pulses. In this variant a ramp of quadratic spectral phases is applied to the input pulses and the second harmonic spectra of the resulting pulses are measured for each chirp value. We demonstrate that a given field envelope produces a unique and unequivocal chirp-scan map and that, under some asymptotic assumptions, both the spectral amplitude and phase of the measured pulse can be retrieved analytically from only two measurements. An iterative algorithm can exploit the redundancy of the information contained in the chirp-scan map to discard experimental noise, artifacts, calibration errors and improve the reconstruction of both the spectral intensity and phase. This technique is compared to two reference characterization techniques (FROG and SRSI). Finally, we perform d-scan measurements with a simple grating-pair compressor.

Molecular applications of attosecond laser pulses

We review the present state of the application of attosecond lasers in molecular physics. Following the first synthesis and characterization of attosecond laser pulses a little more than a decade ago, the first applications in molecular physics have been published only in the last few years. These have yielded new insight into the coupling of multiple electronic degrees of freedom and that of electronic and nuclear degrees of freedom on the attosecond to few-femtosecond timescale. We review these first experiments as well as theoretical work that was carried out during the same period, and sketch some future molecular applications of attosecond pump–probe spectroscopy.

Regularization of an autoconvolution problem in ultrashort laser pulse characterization

An ill-posed inverse problem of autoconvolution type is investigated. This inverse problem occurs in non-linear optics in the context of ultrashort laser pulse characterization. The novelty of the mathematical model consists in a physically required extension of the deautoconvolution problem beyond the classical case usually discussed in literature: (i) For measurements of ultrashort laser pulses with the self-diffraction SPIDER method, a stable approximate solution of an autocovolution equation with a complex-valued kernel function is needed. (ii) The considered scenario requires complex functions both, in the solution as well as in the right-hand side of the integral equation. Since, however, noisy data are available not only for amplitude and phase functions of the right-hand side, but also for the amplitude of the solution, the stable approximate reconstruction of the associated smooth phase function represents the main goal of the paper. An iterative regularization approach will be described that is specifically adapted to the physical situation in pulse characterization, using a non-standard stopping rule for the iteration process of computing regularized solutions. The opportunities and limitations of regularized solutions obtained by our approach are illustrated by means of several case studies for synthetic noisy data and physically realistic complex-valued kernel functions. Based on an example with focus on amplitude perturbations, we show that the autoconvolution equation is locally ill-posed everywhere. To date, the analytical treatment of the impact of noisy data on phase perturbations remains an open question. However, we show its influence with the help of numerical experiments. Moreover, we formulate assertions on the non-uniqueness of the complex-valued autoconvolution problem, at least for the simplified case of a constant kernel. The presented results and figures associated with case studies illustrate the ill-posedness phenomena also for the case of non-trivial complex kernel functions.

Ultrashort laser pulse characterization techniques

There are several competing techniques for the characterization of ultrashort laser pulses. Here we report the studies that we have done in the laboratory. These techniques use the autocorrelation or cross-correlation of the signals. The spectral properties of almost-Gaussian functions were applied for the second-order approximation of the coefficients of Hermite-Gaussian functions that described almost perfect pulses. Other techniques are discussed such as frequency-resolved optical-gating (FROG), which is a time-dependent intensity and phase of the laser pulse, the sonogram of an ultrashort pulse using a two-photon detector which is sensitive and robust, SPIDER which measures the interference between two pulses separated in time which are identical except for their central frequencies. Finally, the ambiguity-Wigner function properties were applied for the characterization of ultrashort pulses.

All-fiber time-delay spectrometer for simultaneous spectral and temporal laser pulse characterization in the nanosecond range

We have realized a modified time-delay spectrometer based on a step-chirped fiber Bragg grating array. This method allows simultaneous spectral and temporal characterization of pulsed light sources in the nanosecond regime, which can also be applied to the investigation of single pulses. With a spectral resolution in the 100 pm range, pulse spectrograms are measured and exemplarily used to explore the emission behavior of a wavelength-stabilized laser diode directly modulated in the nanosecond range.

Spatio-temporal characterization of mid-infrared laser pulses with spatially encoded spectral shearing interferometry

We report on the spatially resolved full amplitude and phase characterization of mid-infrared high intensity laser pulses generated in a three stage OPA. We use a spatially-encoded arrangement (SEA-)SPIDER with spectral filters for ancilla generation for spatially resolved characterization. Using five interchangeable filter sets we are able to characterize pulses from 1 to 2 μm with one single device with minimal adjustments.

Temporal, Spectral and Spatial Characterization of High-Energy Laser Pulse with Small Bandwidth Propagating through Long Path

Temporal, spectral and spatial characters of 0.3-nm-bandwidth high-energy laser pulse propagating through a long path are studied in detail in one newly constructed beamline of our laser facility. The evolution of propagation, pulse energy and near-field deterioration are analyzed theoretically and experimentally. Substituting argon for air is demonstrated effectively to suppress stimulated rotational Raman scattering and the experimental result provides operating criterion, and engineering parameters for the under-constructed beamlines.

Characterization of broadband few-cycle laser pulses with the d-scan technique

We present an analysis and demonstration of few-cycle ultrashort laser pulse characterization using second-harmonic dispersion scans and numerical phase retrieval algorithms. The sensitivity and robustness of this technique with respect to noise, measurement bandwidth and complexity of the measured pulses is discussed through numerical examples and experimental results. Using this technique, we successfully demonstrate the characterization of few-cycle pulses with complex and structured spectra generated from a broadband ultrafast laser oscillator and a high-energy hollow fiber compressor.

Spatiotemporal characterization of few-cycle laser pulses

In this paper we apply a broadband fiber optic coupler interferometer to the measurement of few-cycle laser pulses. Sub-8-fs pulses delivered by an ultrafast oscillator were characterized spatiotemporally using STARFISH, which is based on spatially resolved spectral interferometry. The reference pulse was measured with the d-scan technique. The pulses were focused by an off-axis parabolic mirror and were characterized at different transverse planes along the focusing region. The evolution of the retrieved pulses is analyzed, exhibiting small variations in the temporal (and spectral) amplitude and phase during propagation. Finally, the peak irradiance evolution is estimated from the integration of the spatiotemporal intensity.

Direct phase and amplitude characterization of femtosecond laser pulses undergoing filamentation in air

Measurement of the temporal (spectral) phase and amplitude of a 50 fs laser pulse approaching and exceeding the critical power for self-focusing (P(crit)) in air reveals the formation of an isolated 17 fs pulse at 3P(crit). The dynamics of self-shortening are measured directly in the filament using transient-grating cross-correlation frequency-resolved optical gating with a noble gas serving as the nonlinear medium. Our results support recent filamentary propagation simulations, suggesting that a Kerr-dominated temporal reshaping process toward the end of the filament is largely responsible for the generation of short pulses.

Sub-4-fs laser pulse characterization by spatially resolved spectral shearing interferometry and attosecond streaking

We demonstrate the generation of high-energy sub-2-cycle laser pulses generated through hollow core fibre pulse compression. We demonstrate their full characterization with two independent methods. For all-optical characterization in amplitude and spectral phase, we employ spatially encoded arrangement spectral phase interferometry for direct electric-field reconstruction using spectrally filtered ancilla beams to characterize the sub-4-fs pulses with spatial resolution. For field-sensitive pulse characterization, we generate isolated attosecond pulses around 93 eV. The attosecond pulse as well as the infrared few-cycle pulse is characterized in amplitude and phase using the frequency resolved optical gating for complete reconstruction of the attosecond bursts technique. We find good agreement between the two methods.

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.

Simultaneous compression and characterization of ultrashort laser pulses using chirped mirrors and glass wedges

We present a simple and robust technique to retrieve the phase of ultrashort laser pulses, based on a chirped mirror and glass wedges compressor. It uses the compression system itself as a diagnostic tool, thereby making unnecessary the use of complementary diagnostic tools. We used this technique to compress and characterize 7.1 fs laser pulses from an ultrafast laser oscillator.

Characterization of high-intensity sub-4-fs laser pulses using spatially encoded spectral shearing interferometry

We report on the full amplitude and phase characterization of high-intensity few-cycle laser pulses generated in a single-stage hollow core fiber system with subsequent compression by ultrabroadband chirped mirrors. We use a spatially-encoded arrangement (SEA) spectral phase interferometry for direct electric field reconstruction (SPIDER) with spectral filters for ancilla generation to characterize the sub-4 fs pulses with spatial resolution.

Complementary ultrashort laser pulse characterization using MOSAIC and SHG FROG

A new (to our knowledge) method for generating the modified spectrum autointerferometric correlation (MOSAIC) trace from the second-harmonic generation frequency-resolved optical gating (SHG FROG) dataset is shown. Examples are presented illustrating enhanced visual sensitivity, applicability, and complementary qualitative pulse characterization using SHG FROG.

Complete real-time temporal waveform characterization of single-shot few-cycle laser pulses

A method for complete characterization of the waveform of individual few-cycle laser pulses is presented. By analyzing the ``left'' and ``right'' asymmetries of high-energy photoelectrons along the polarization axis using the recently developed quantitative rescattering theory, we show that the carrier-envelope phase (CEP), pulse duration, and peak intensity of each single-shot pulse can be readily retrieved. By CEP tagging each laser shot, the method permits the study of waveform-dependent processes be extended to relativistic beams and to wavelengths, where CEP stabilization is not yet possible.

Frequency-resolved optical gating system with a tellurium crystal for characterizing free-electron lasers in the wavelength range of 10–30 μm

A second-harmonic generation frequency-resolved optical gating (SHG-FROG) system has been developed for the complete characterization of laser pulses in the wavelength range of 10-30 microm. A tellurium crystal is used so that spectrally resolved autocorrelation signals with a good signal-to-noise ratio are obtained. Pulses (wavelength approximately 22 microm) generated from a free-electron laser are measured by the SHG-FROG system. The SHG intensity profile and the spectrum obtained by FROG measurements are well consistent with those of independent measurements of the pulse length and spectrum. The pulse duration and spectral width determined from the FROG trace are 0.6 ps and 5.2 THz at full width half maximum, respectively.

Pulse measurements by randomly quasi phase matched second harmonic generation in the regime of total internal reflection

We demonstrate experimentally the frequency doubling in disordered quadratic nonlinear media in the regime of total internal reflection of the fundamental beam. We show how the transversely emitted second harmonic can be employed for an efficient characterization of short laser pulses.