Spatiotemporal characterization of few-cycle laser pulses

Dispersion scan frequency resolved optical gating for consistency check of pulse retrieval

Many methods commonly used to characterize ultrafast laser pulses, such as the frequency-resolved optical gating (FROG) or the dispersion scan (d-scan), face problems when they are used on pulses with a spectrum or phase varying within the laser beam cross section or the acquisition time. The presence of such pulse shape variation leads to discrepancy between the measured FROG trace and its reconstructed counterparts. Nevertheless, it is difficult to reliably discern this shape variation because even the distorted experimental FROG trace can be reasonably reproduced by a realistic pulse shape. In this work, we examine and discern the variation of the pulse shape based on a new method, dispersion-scan FROG (D-FROG), which combines the idea of dispersion scanning with the FROG method. This technique provides a means of careful evaluation of the laser pulse based on a set of FROG traces connected by known dispersion changes. Therefore, this method can disclose seemingly correct pulse retrievals from distorted datasets. The D-FROG method can be used as a simple extension of the FROG technique to provide a consistency check able to identify the shortcomings in the pulse characterization.

Single-scan, dual-functional interferometer for fast spatio-temporal characterization of few-cycle pulses

Accurate and fast characterization of spatio-temporal information of high-intensity, ultrashort pulses is crucial in the field of strong-field laser science and technology. While conventional self-referenced interferometers were widely used to retrieve the spatial profile of the relative spectral phase of pulses, additional measurements of temporal and spectral information at a particular position of the laser beam, however, were necessary to remove the indeterminacy, which increases the system complexity. Here we report an advanced, dual-functional interferometer that is able to reconstruct the complete spatio-temporal information of ultrashort pulses with a single scan of the interferometer arm. The setup integrates an interferometric frequency-resolved optical gating (FROG) with a radial shearing Michelson interferometer. Through scanning one arm of the interferometer, both the cross-correlated FROG trace at the central part of the laser beam and the delay-dependent interferograms of the entire laser profile are simultaneously obtained, allowing a fast three-dimensional reconstruction of few-cycle laser pulses.

Crossed Czerny-Turner Spectrometer with Extended Spectrum Using Movable Planar Mirrors

This study reports a crossed Czerny-Turner spectrometer with multiple mirrors to extend the inspected spectrum. A design example with two movable mirrors and a stationary planar mirror is experimentally demonstrated to offer two additional spectral bands, thereby leading to thrice the spectral range of the original Czerny-Turner spectrometer. The results indicate that the configurations to measure the three bands have almost identical parameters. The moving direction of the planar mirror and the plane of incidence are orthogonal; thus, the influence of mirror movement on the repeatability of the spectrum is minimized. In addition to the merits of cost-effectiveness and rapid inspection, the reported mechanism of mirror movement is applied to general spectrometers to extend the spectral coverage without sacrificing the resolution.

Self-calibrating d-scan: measuring ultrashort laser pulses on-target using an arbitrary pulse compressor

In most applications of ultrashort pulse lasers, temporal compressors are used to achieve a desired pulse duration in a target or sample, and precise temporal characterization is important. The dispersion-scan (d-scan) pulse characterization technique usually involves using glass wedges to impart variable, well-defined amounts of dispersion to the pulses, while measuring the spectrum of a nonlinear signal produced by those pulses. This works very well for broadband few-cycle pulses, but longer, narrower bandwidth pulses are much more difficult to measure this way. Here we demonstrate the concept of self-calibrating d-scan, which extends the applicability of the d-scan technique to pulses of arbitrary duration, enabling their complete measurement without prior knowledge of the introduced dispersion. In particular, we show that the pulse compressors already employed in chirped pulse amplification (CPA) systems can be used to simultaneously compress and measure the temporal profile of the output pulses on-target in a simple way, without the need of additional diagnostics or calibrations, while at the same time calibrating the often-unknown differential dispersion of the compressor itself. We demonstrate the technique through simulations and experiments under known conditions. Finally, we apply it to the measurement and compression of 27.5 fs pulses from a CPA laser.

Spatio-temporal characterization of intense few-cycle 2 μm pulses

We present a variant of spatially encoded spectral shearing interferometry for measuring two-dimensional spatio-temporal slices of few-cycle pulses centered around 2 μm. We demonstrate experimentally that the device accurately retrieves the pulse-front tilt caused by angular dispersion of two-cycle pulses. We then use the technique to characterize 500-650 μJ pulses from a hollow fiber pulse compressor, with durations as short as 7.1 fs (1.3 optical cycles).

Self-referenced characterization of space-time couplings in near-single-cycle laser pulses

We report on the characterization of space-time couplings in high-energy sub-2-cycle 770 nm laser pulses using a self-referencing single-frame method. Using spatially encoded arrangement filter-based spectral phase interferometry for direct electric field reconstruction, we characterize few-cycle pulses with a wavefront rotation of 2.8×10¹¹  rev/s (1.38 mrad per half-cycle) and pulses with pulse front tilts ranging from -0.33  fs/μm to -3.03  fs/μm in the focus.

Miniature anastigmatic spectrometer design with a concave toroidal mirror

An advanced optical design for a low-cost and astigmatism-corrected spectrometer with a high resolution is presented. The theory and method of astigmatism correction are determined with the use of a concave toroidal mirror. The performances of a modified spectrometer and a traditional spectrometer are compared, and the analysis is verified. Experimentally, the limiting resolution of our spectrometer is 0.1 nm full width at half-maximum, as measured for 579.1 nm.

Simultaneous compression, characterization and phase stabilization of GW-level 1.4 cycle VIS-NIR femtosecond pulses using a single dispersion-scan setup

We have temporally characterized, dispersion compensated and carrier-envelope phase stabilized 1.4-cycle pulses (3.2 fs) with 160 µJ of energy at 722 nm using a minimal and convenient dispersion-scan setup. The setup is all inline, does not require interferometric beamsplitting, and uses components available in most laser laboratories. Broadband minimization of third-order dispersion using propagation in water enabled reducing the compressed pulse duration from 3.8 to 3.2 fs with the same set of chirped mirrors. Carrier-envelope phase stabilization of the octave-spanning pulses was also performed by the dispersion-scan setup. This unprecedentedly simple and reliable approach provides reproducible CEP-stabilized pulses in the single-cycle regime for applications such as CEP-sensitive spectroscopy and isolated attosecond pulse generation.

Pulse characterization by THG d-scan in absorbing nonlinear media

We report on few-cycle pulse characterization based on third harmonic generation dispersion scan (THG d-scan) measurements using thin films of different TiO(2)-SiO(2) compositions as nonlinear media. By changing the TiO(2) concentration in the thin film the band gap and therefore the position of the absorption edge were varied. The retrieved pulse durations from different nonlinear media agree within 5%, and the reconstructed pulse shapes prove to be immune against the absorption edges as well. The reason is the robust retrieval algorithm which takes the influence of wavelength dependent nonlinearity into account by a spectral weight function.

Imaging cross-correlation FROG: measuring ultrashort, complex, spatiotemporal fields

We present imaging cross-correlation frequency-resolved optical gating (ImXFROG), a new method for the spatiotemporal phase retrieval of ultrashort pulses. It is demonstrated that ImXFROG can measure phase and intensity of arbitrary, spatiotemporally distorted pulses with femtosecond resolution and up to 10(7) independent variables. ImXFROG is implemented as a plug-in upgrade to an existing correlator and used to demonstrate the reconstruction of highly complex, optical pulses with femtosecond features and massive spatiotemporal distortion.