Characterization Of Femtosecond Laser Pulses Research Articles

An experimental study of intensity‐phase characterization of femtosecond laser pulses propagated through a polymethyl methacrylate

AbstractThis article presents a comprehensive investigation into the intensity‐phase characteristics of ultrashort laser pulses propagated through a 10 mm thick polymethyl methacrylate (PMMA) using frequency‐resolved optical gating (FROG) and cross‐correlation frequency‐resolved optical gating (XFROG) techniques. The study focuses on discerning characteristic changes that are induced by the PMMA on the laser pulse. The results reveal that the phase variations exhibit a consistent trend and pattern. At the same time, the temporal full width at half maximum (FWHM) of both FROG and XFROG measurements displays only minor changes. The overall analysis indicates that PMMA induces no discernible modifications in the pulse, thus positioning it as a viable optical element. The analysis of reference FROG, FROG, and XFROG with PMMA is compared and discussed, showing that this analysis provides a foundation for optimizing PMMA's utilization in the optical components of ultrafast laser systems. This experimental study strongly affirms that PMMA could be a potential candidate as an optical element for laser pulses.

Generalized central slice theorem perspective on Fourier-transform spectral imaging at a sub-Nyquist sampling rate.

Fourier-transform spectral imaging captures frequency-resolved images with high spectral resolution, broad spectral range, high photon flux, and low stray light. In this technique, spectral information is resolved by taking Fourier transformation of the interference signals of two copies of the incident light at different time delays. The time delay should be scanned at a high sampling rate beyond the Nyquist limit to avoid aliasing, at the price of low measurement efficiency and stringent requirements on motion control for time delay scan. Here we propose, what we believe to be, a new perspective on Fourier-transform spectral imaging based on a generalized central slice theorem analogous to computerized tomography, using an angularly dispersive optics decouples measurements of the spectral envelope and the central frequency. Thus, as the central frequency is directly determined by the angular dispersion, the smooth spectral-spatial intensity envelope is reconstructed from interferograms measured at a sub-Nyquist time delay sampling rate. This perspective enables high-efficiency hyperspectral imaging and even spatiotemporal optical field characterization of femtosecond laser pulses without a loss of spectral and spatial resolutions.

Spectral phase reconstruction of femtosecond laser pulse from interferometric autocorrelation and evolutionary algorithm

We report on the complete temporal characterization of femtosecond laser pulses from second-order interferometric autocorrelation and laser spectrum measurements. The method exploits a newly developed autocorrelator based on a two photon-absorption signal produced directly within a camera sensor so as to provide a single-shot interferometric autocorrelation of great reliability and robustness. Interferometric autocorrelation trace and laser spectrum are exploited for a spectral phase retrieval via an evolutionary algorithm. The quality of the reconstruction for highly modulated spectral phases imprinted by a pulse shaper confirms the reliability of the method. The autocorrelator is compact, robust and easy to use. Possible improvements are discussed.

Temporal characterization of femtosecond laser pulses using tunneling ionization in the UV, visible, and mid-IR ranges

To generalize the applicability of the temporal characterization technique called “tunneling ionization with a perturbation for the time-domain observation of an electric field” (TIPTOE), the technique is examined in the multicycle regime over a broad wavelength range, from the UV to the IR range. The technique is rigorously analyzed first by solving the time-dependent Schrödinger equation. Then, experimental verification is demonstrated over an almost 5-octave wavelength range at 266, 1800, 4000 and 8000 nm by utilizing the same nonlinear medium – air. The experimentally obtained dispersion values of the materials used for the dispersion control show very good agreement with the ones calculated using the material dispersion data and the pulse duration results obtained for 1800 and 4000 nm agree well with the frequency-resolved optical gating measurements. The universality of TIPTOE arises from its phase-matching-free nature and its unprecedented broadband operation range.

Single shot complete characterization of femtosecond laser pulses employing self-phase modulation

We propose a new single shot technique for reconstructing femtosecond pulses utilizing self-phase modulation. The principle of the technique is analyzing multiple spectra belonging to the same pulse but nonlinearly modulated with different intensity levels. The variety of intensities is well defined and obtained from the spatial distribution of the beam. The technique needs no prior information about the pulse and can be implemented by focusing the beam in any thin nonlinear medium and measuring the resulting spectrum by an imaging spectrometer. The fidelity of the technique has been proven with numerical examples including simple and complicated cases and in the presence of noise.

Cross-Correlation Frequency-Resolved Optical Gating for Test-Pulse Characterization Using a Self-Diffraction Signal of a Reference Pulse

A diagnostic system using three frequency-resolved optical gating (FROG) techniques—cross-correlation, second harmonic generation, and self-diffraction—is reported for the reliable characterization of femtosecond laser pulses. The latter two FROG techniques are employed to evaluate suitability in measurements of the reference pulse. A train of optical pulses generated by the superposition of two femtosecond pulses emitting at 800 nm and 1180 nm has been characterized by the cross-correlation FROG to evaluate the reliability of the present diagnostic system.

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.

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.

Compact spectral shearing interferometer for ultrashort pulse characterization

A simple, compact, and robust implementation of spectral shearing interferometry using a single nonlinear crystal for both ancilla generation and upconversion is demonstrated. The device is capable of accurate characterization of femtosecond laser pulses over the 740-900 nm range with a KDP crystal.

Optical cross-correlator based on supercontinuum generation

A novel cross-correlator that can be used for temporal characterization of femtosecond laser pulses has been developed. The correlation trace is obtained by "sampling" the structure of the laser pulse with a single, high-contrast pulse produced through femtosecond white-light generation in a line focus. This correlator has, therefore, fewer "ghosts" than a conventional third-order cross-correlator and it can be used with laser pulses that span across a wide wavelength range. Both scanning and single-shot experimental arrangements are described.

Second-order autocorrelation measurements of attosecond XUV pulse trains

The unequivocal demonstration of attosecond XUV pulse train generation based on the synthesis of a comb of harmonics of an infrared femtosecond laser pulse is inevitably linked to a reliable method of pulse characterization in this temporal regime. The most commonly used method to date relies on a cross-correlation technique between the fundamental laser frequency and the XUV light from which a measurement of the relative phases of the consecutive harmonics is obtained and the attosecond temporal structure in a train of XUV pulses is inferred. Here we report on an alternative method that renders possible the direct observation of a train of attosecond pulses in form of a second-order autocorrelation trace. The method is an extension in the XUV spectral range of the well-established technique commonly used for the characterization of femtosecond laser pulses. Besides being the first direct visualization of a periodic attosecond structure, the approach supplies quantitative information on the overall pulse duration including the influence of the macroscopic conditions of generation.

Generation and characterization of polarization-shaped femtosecond laser pulses

We demonstrate the generation and complete characterization of femtosecond laser pulses which change their intensity, frequency, and light polarization almost arbitrarily within a single pulse employing the new technique of femtosecond polarization pulse shaping. Specifically, the degree of polarization ellipticity as well as the orientation of the elliptical principal axes can be varied as a function of time in a completely controllable manner, using a 128-pixel, two-layer, liquid-crystal display (LCD) inside a zero-dispersion compressor. A mathematical formalism is presented with which polarization-shaped pulse parameters can be calculated and used to generate intuitive quasi-three-dimensional electric field representations. However, laboratory realization requires accurate and complete experimental pulse characterization methods. For this purpose, the technique of dual-channel spectral interferometry is employed. Furthermore, Jones calculus for polarization-shaped pulses with experimentally determined Jones matrices is developed. It can be used to predict and account for all pulse-shape modifications occurring in various optical elements of the pulse shaper and the experimental setup.

Temporal characterization of femtosecond laser pulses induced plasma for spectrochemical analysis of aluminum alloys

This paper reports studies on time-resolved space-integrated laser induced breakdown spectroscopy (LIBS) of plasmas produced by ultrashort laser pulses at atmospheric pressure, on aluminum alloy targets. The temporal behavior of specific ion and neutral emission lines of Al, Mg and Fe has been characterized. The results show a faster decay of continuum and spectral lines, and a shorter plasma lifetime than in the case of longer laser pulses. Spectroscopic diagnostics were used to determine the time-resolved electron density, as well as the excitation and ionization temperatures. In comparison with plasmas produced by ns laser pulses, the plasma generated by ultrashort pulses exhibits a faster thermalization. Analytical performances of fs-LIBS were also evaluated. Linear calibration curves for minor elements (Mg, Fe, Si, Mn, Cu) presented in aluminum alloys were obtained. The limits of detection are in the parts per million (ppm) range and are element-dependent.

Characterization of femtosecond laser pulses with GaN thin films

The application of n-type GaN thin films (thickness of 4.4 μm) for measurement of the correlation spectrum of femtosecond laser pulses is demonstrated for the entire spectral region of the fundamental (720–820 nm) and second harmonic beams of a Ti:sapphire laser. Phase matching conditions are not required. The GaN thin film is stable and robust. Its wide band gap (3.4 eV) allows the characterization of femtosecond laser pulses over a wide wavelength range from 370 nm to 1.1 μm.