Spatiotemporal characterization of ultrashort laser pulses using spatially resolved Fourier transform spectrometry

Simple few-shot method for spectrally resolving the wavefront of an ultrashort laser pulse

We present a novel, to the best of our knowledge, and straightforward approach for the spatio-spectral characterization of ultrashort pulses. This minimally intrusive method relies on placing a mask with specially arranged pinholes in the beam path before the focusing optic and retrieving the spectrally resolved laser wavefront from the speckle pattern produced at focus. We test the efficacy of this new method by accurately retrieving chromatic aberrations, such as pulse-front tilt (PFT), pulse-front curvature (PFC), and higher-order aberrations introduced by a spherical lens. The simplicity and scalability of this method, combined with its compatibility with single-shot operation, make it a strong complement to existing tools for high-intensity laser facilities.

Spatiotemporal mode-locking and dissipative solitons in multimode fiber lasers

Multimode fiber (MMF) lasers are emerging as a remarkable testbed to study nonlinear spatiotemporal physics with potential applications spanning from high energy pulse generation, precision measurement to nonlinear microscopy. The underlying mechanism for the generation of ultrashort pulses, which can be understood as a spatiotempoal dissipative soliton (STDS), in the nonlinear multimode resonators is the spatiotemporal mode-locking (STML) with simultaneous synchronization of temporal and spatial modes. In this review, we first introduce the general principles of STML, with an emphasize on the STML dynamics with large intermode dispersion. Then, we present the recent progress of STML, including measurement techniques for STML, exotic nonlinear dynamics of STDS, and mode field engineering in MMF lasers. We conclude by outlining some perspectives that may advance STML in the near future.

Measuring spatio-temporal couplings using modal spatio-spectral wavefront retrieval

Knowledge of spatio-temporal couplings such as pulse-front tilt or curvature is important to determine the focused intensity of high-power lasers. Common techniques to diagnose these couplings are either qualitative or require hundreds of measurements. Here we present both a new algorithm for retrieving spatio-temporal couplings, as well as novel experimental implementations. Our method is based on the expression of the spatio-spectral phase in terms of a Zernike-Taylor basis, allowing us to directly quantify the coefficients for common spatio-temporal couplings. We take advantage of this method to perform quantitative measurements using a simple experimental setup, consisting of different bandpass filters in front of a Shack-Hartmann wavefront sensor. This fast acquisition of laser couplings using narrowband filters, abbreviated FALCON, is easy and cheap to implement in existing facilities. To this end, we present a measurement of spatio-temporal couplings at the ATLAS-3000 petawatt laser using our technique.

Simple single-shot complete spatiotemporal intensity and phase measurement of an arbitrary ultrashort pulse using coherent modulation imaging

We propose a simple single-shot spatiotemporal measurement technique called coherent modulation imaging for the spatio-spectrum (CMISS), which reconstructs the full three-dimensional high-resolution characteristics of ultrashort pulses based on frequency-space division and coherent modulation imaging. We demonstrated it experimentally by measuring the spatiotemporal amplitude and phase of a single pulse with a spatial resolution of 44 µm and a phase accuracy of 0.04 rad. CMISS has good potential for high-power ultrashort-pulse laser facilities and can measure even spatiotemporally complicated pulses with important applications.

Multi-gigawatt peak power post-compression in a bulk multi-pass cell at a high repetition rate

The output of a 200kHz, 34W, 300fs ytterbium amplifier is compressed to 31fs with >88% efficiency to reach a peak power of 2.5GW, which to date is a record for a single-stage bulk multi-pass cell. Despite operation 80 times above the critical power for self-focusing in bulk material, the setup demonstrates excellent preservation of the input beam quality. Extensive beam and pulse characterizations are performed to show that the compressed pulses are promising drivers for high harmonic generation and nonlinear optics in gases or solids.

4D spatio-temporal electric field characterization of ultrashort light pulses undergoing filamentation

We present an experimental method capable of capturing the complete spatio-temporal dynamics of filamenting ultrashort laser pulses. By employing spatially resolved Fourier transform spectrometry in combination with the capability to terminate the filament at any length, we can follow the nonlinear dynamics in four dimensions, i.e. the transverse domain, time and filament length. Our method thus not only enables the full characterization of the filamentation process throughout its evolution, but also allows to identify and select laser pulses with desired parameters.

Survey of spatio-temporal couplings throughout high-power ultrashort lasers

The investigation of spatio-temporal couplings (STCs) of broadband light beams is becoming a key topic for the optimization as well as applications of ultrashort laser systems. This calls for accurate measurements of STCs. Yet, it is only recently that such complete spatio-temporal or spatio-spectral characterization has become possible, and it has so far mostly been implemented at the output of the laser systems, where experiments take place. In this survey, we present for the first time STC measurements at different stages of a collection of high-power ultrashort laser systems, all based on the chirped-pulse amplification (CPA) technique, but with very different output characteristics. This measurement campaign reveals spatio-temporal effects with various sources, and motivates the expanded use of STC characterization throughout CPA laser chains, as well as in a wider range of types of ultrafast laser systems. In this way knowledge will be gained not only about potential defects, but also about the fundamental dynamics and operating regimes of advanced ultrashort laser systems.

Spatiospectral characterization of ultrafast pulse-beams by multiplexed broadband ptychography

Ultrafast pulse-beam characterization is critical for diverse scientific and industrial applications from micromachining to generating the highest intensity laser pulses. The four-dimensional structure of a pulse-beam, E~(x,y,z,ω), can be fully characterized by coupling spatiospectral metrology with spectral phase measurement. When temporal pulse dynamics are not of primary interest, spatiospectral characterization of a pulse-beam provides crucial information even without spectral phase. Here we demonstrate spatiospectral characterization of pulse-beams via multiplexed broadband ptychography. The complex spatial profiles of multiple spectral components, E~(x,y,ω), from modelocked Ti:sapphire and from extreme ultra-violet pulse-beams are reconstructed with minimum intervening optics and no refocusing. Critically, our technique does not require spectral filters, interferometers, or reference pulses.

8 fs laser pulses from a compact gas-filled multi-pass cell

Compression of 42 fs, 0.29 mJ pulses from a Ti:Sapphire amplifier down to 8 fs (approximately 3 optical cycles) is demonstrated by means of spectral broadening in a compact multi-pass cell filled with argon. The efficiency of the nonlinear pulse compression is limited to 45 % mostly by losses in the mirrors of the cell. The experimental results are supported by 3-dimensional numerical simulations of the nonlinear pulse propagation in the cell that allow us to study spatio-spectral properties of the pulses after spectral broadening.

Single-shot spatiotemporal characterization of a multi-PW laser using a multispectral wavefront sensing method

The single-shot spatiotemporal characterization of an ultrahigh intensity laser pulse was performed using a multispectral wavefront sensor. For the measurement of the spatio-spectral electric field, a femtosecond laser pulse was spectrally modulated and separated by a Fabry-Perot etalon coupled with a grating pair, and its spatio-spectral electric field was measured with a wavefront sensor. The spatiotemporal electric field was reconstructed from the measured spatio-spectral electric field of a multi-PW laser pulse. We found that the spatiotemporal distortion could reduce the focused laser intensity by 15%, compared to the case of a diffraction-limited and transform-limited laser pulse.

Integrated pulse scope for tunable generation and intrinsic characterization of structured femtosecond laser

Numerous techniques have been demonstrated for effective generation of orbital angular momentum-carrying radiation, but intracavity generation of continuously tunable pulses in the femtosecond regime remains challenging. Even if such a creation was realized, the generated pulses-like all pulses in reality-are complex and transitory objects that can only be comprehensively characterized via multidimensional spaces. An integrated lasing system that generates pulses while simultaneously quantifies them can achieve adaptive pulse tailoring. Here, we report a femtosecond pulse scope that unifies vector vortex mode-locked lasing and vectorial quantification. With intracavity-controlled Pancharatnam-Berry phase modulation, continuous and ergodic generation of spirally polarized states along a broadband higher-order Poincaré sphere was realized. By intrinsically coupling a two-dimensional polarization-sensitive time-scanning interferometer to the laser, multidimensional spatiotemporal features of the pulse were further visualized. The proposed methodology paves the way for design optimization of ultrafast optics by integrating complex femtosecond pulse generation and structural customization, facilitating its applications in optical physics research and laser-based manufacturing.

Real-time multispeckle spectral-temporal measurement unveils the complexity of spatiotemporal solitons

The dynamics of three-dimensional (3D) dissipative solitons originated from spatiotemporal interactions share many common characteristics with other multi-dimensional phenomena. Unveiling the dynamics of 3D solitons thus permits new routes for tackling multidisciplinary nonlinear problems and exploiting their instabilities. However, this remains an open challenge, as they are multi-dimensional, stochastic and non-repeatable. Here, we report the real-time speckle-resolved spectral-temporal dynamics of a 3D soliton laser using a single-shot multispeckle spectral-temporal technology that leverages optical time division multiplexing and photonic time stretch. This technology enables the simultaneous observation on multiple speckle grains to provide long-lasting evolutionary dynamics on the planes of cavity time (t) - roundtrip and spectrum (λ) - roundtrip. Various non-repeatable speckly-diverse spectral-temporal dynamics are discovered in both the early and established stages of the 3D soliton formation.

Analysis of angular dispersion induced by wavefront rotation in nanosecond optical parametric chirped pulse amplification

Angular dispersion observed in a nanosecond optical parametric chirped-pulse amplification (ns-OPCPA) amplifier adopted in the frontend of a multi-PW laser was analyzed. The theory on the angular dispersion, extended by including the wavefront rotation and the pulse front tilt of a strongly chirped laser pulse, revealed that the wavefront rotation is a major contributor to the angular dispersion, as compared to the pulse front tilt, in a ns-OPCPA amplifier. It was also shown that the wavefront rotation could be introduced by the phase mismatch and the noncollinear propagation angle in the noncollinear ns-OPCPA amplifier. The theoretical prediction was experimentally verified by measuring the angular dispersion of the ns-OPCPA frontend installed in the 20-fs, 4-PW Ti:Sapphire laser. We emphasize the importance of the proper characterization and control of the angular dispersion in the ns-OPCPA amplifier since the focus intensity of an ultrahigh power laser could be significantly reduced due to the spatiotemporal effect even for small induced angular dispersion.

Directly Measuring the Pulse Front Distortion of High-Peak-Power Femtosecond Lasers

Pulse front distortion occurring in lenses can broaden the temporal profile of the pulse at focus and therefore decrease the focused intensity, especially for large-aperture femtosecond lasers. A previously proposed self-reference cross correlator was improved to directly measure the pulse front distortion of high-peak-power femtosecond lasers. The measured results of a 200 TW/27 fs laser are in good accordance with the calculated value. Moreover, the temporal intensity distribution of the pulse in the focal region was also investigated, in order to better guide and further promote the strong laser-matter interaction investigations. According to the measured PFD, the effective pulse duration at far field of this 200 TW laser was theoretically simulated to be ≈49 fs, which is almost two times the generally regarded 27 fs. As a result, the actually available focused intensity of this laser is only 55% of the case without pulse front distortion.

Complex spatiotemporal coupling distortion pre-compensation with double-compressors for an ultra-intense femtosecond laser

In an ultra-intense femtosecond chirped-pulse amplification laser, the imperfect diffraction wave-fronts of the second and the third gratings of the compressor, where spatio-spectral coupling exists, could introduce a complex spatiotemporal coupling distortion (STCD) and degrade the pulsed beam in both near- and far-fields. Here, we propose a method of double-compressors for pre-compensation. By inserting a scaled down compressor (small compressor) with a deformable retro-reflection mirror into the beam-line, the frequency-dependent wave-front distortion, i.e., the complex STCD, could be removed. We simulate the results in two different ultra-intense femtosecond lasers with 80 and 400 nm bandwidths for comparison, and near ideal focused peak intensities could be obtained in both cases. Meanwhile, the influences of several miss-matching effects, which might appear in engineering, are also analyzed and discussed for applications.

Arbitrarily distorted 2-dimensional pulse-front measurement and reliability analysis

A method of 2-dimensional (2-D) space-scanned (in the x-y plane) spatiotemporal double-slit interference is used to reconstruct the 2-D pulse-front (in the x-y-t domain) of a femtosecond pulsed beam. While comparing with recent other methods, the method possesses two advantages: no reference pulse/beam is required anymore, and an arbitrarily distorted pulse-front, not just pulse-front tilt and pulse-front curvature, could be detected. Meanwhile, the influence of different factors of unknown pulsed beams and optical elements on the measurement reliability is also analyzed for engineering applications.

Low-energy/pulse response and high-resolution-CMOS camera for spatiotemporal femtosecond laser pulses characterization @ 1.55 μm

In this work, we present a commercial CMOS (Complementary Metal Oxide Semiconductor) Raspberry Pi camera implemented as a Near-Infrared detector for both spatial and temporal characterization of femtosecond pulses delivered from a femtosecond Erbium Doped Fiber laser (fs-EDFL) @ 1.55 µm, based on the Two Photon Absorption (TPA) process. The capacity of the device was assessed by measuring the spatial beam profile of the fs-EDFL and comparing the experimental results with the theoretical Fresnel diffraction pattern. We also demonstrate the potential of the CMOS Raspberry Pi camera as a wavefront sensor through its a nonlinear response in a Shack-Hartmann array and for the temporal characterization of the femtosecond pulses delivered from the fs-EDFL through TPA Intensity autocorrelation measurements. The direct pulse detection and measurement, through the nonlinear response with a CMOS, is proposed as a novel and affordable high-resolution and high-sensitivity alternative to costly detectors such as CCDs, wavefront sensors and beam profilers @ 1.55 µm. The measured fluence threshold, down to 17.5 µJ/cm², and pJ/pulse energy response represents the lowest reported values applied as a beam profiler and a TPA Shack-Hartmann wavefront sensor, to our knowledge.

Spatiotemporal coupling of attosecond pulses

In most optics textbooks, one writes the electric field describing an optical wave as a product of temporal and spatial amplitudes. This approximation often breaks down for short optical pulses. An example of such spatiotemporal coupling is chromatic aberrations, where the focal properties of the radiation vary with frequency over the pulse bandwidth. In this work, we point out significant chromatic aberrations of attosecond pulses, which depend on the geometry of the generation process. These aberrations are intrinsic to the generation process and need to be eliminated in applications requiring attosecond pulses to be focused over a small region.

The shortest light pulses produced to date are of the order of a few tens of attoseconds, with central frequencies in the extreme UV range and bandwidths exceeding tens of electronvolts. They are often produced as a train of pulses separated by half the driving laser period, leading in the frequency domain to a spectrum of high, odd-order harmonics. As light pulses become shorter and more spectrally wide, the widely used approximation consisting of writing the optical waveform as a product of temporal and spatial amplitudes does not apply anymore. Here, we investigate the interplay of temporal and spatial properties of attosecond pulses. We show that the divergence and focus position of the generated harmonics often strongly depend on their frequency, leading to strong chromatic aberrations of the broadband attosecond pulses. Our argument uses a simple analytical model based on Gaussian optics, numerical propagation calculations, and experimental harmonic divergence measurements. This effect needs to be considered for future applications requiring high-quality focusing while retaining the broadband/ultrashort characteristics of the radiation.

Simultaneous measurement of two ultrashort near-ultraviolet pulses produced by a multiplate continuum using dual self-diffraction dispersion-scan

We present a new method based on self-diffraction dispersion-scan (SD d-scan) that enables the simultaneous measurement of two distinct ultrashort laser pulses in a region where they spatially and temporally overlap. This situation can arise when sampling and focusing two different spatial portions of a single inhomogeneous beam onto a medium. We demonstrate this new dual SD d-scan method by simultaneously characterizing two intense broadband ultraviolet pulses at 400 nm, with durations in the 10 fs range, originating from two different spatial portions of a beam produced by a multiplate continuum (MPC).

Spatio-spectral metrology at focus of ultrashort lasers: a phase-retrieval approach

The complete characterization of an ultrashort laser beam ultimately requires the determination of its spatio-temporal electric field E(x, y, t), or its spatio-spectral counterpart Ẽ(x, y, ω). We describe a new measurement technique called INSIGHT, which determines Ẽ(x, y, ω), up to an unknown spatially-homogeneous spectral phase. Combining this information with a temporal measurement at a single point of the beam then enables the determination of the spatio-temporal field E(x, y, t). This technique is based on the combination of spatially-resolved Fourier-transform spectroscopy with an alternate-projection phase-retrieval algorithm. It can be applied to any reproducible laser source with a repetition rate higher than about 0.1 Hz, relies on a very simple device, does not require any reference beam, and circumvents the difficulty associated with the manipulation of large beam diameters by working in the vicinity of the beam focus. We demonstrate INSIGHT on a 100 TW-25 fs laser, and use the measurement results to introduce new representations for the analysis of spatio-temporal/spectral couplings of ultrashort lasers.

Single-shot real-time detection technique for pulse-front tilt and curvature of femtosecond pulsed beams with multiple-slit spatiotemporal interferometry

Spatiotemporal coupling (STC) of femtosecond pulsed beams could significantly reduce the focal-spot intensity of ultra-intense lasers. We theoretically present a very simple method for single-shot real-time detecting pulse-front tilt, curvature, or tilt and curvature (PFT, PFC or PFT&PFC) by using multiple-slit spatiotemporal interferometry (MSTI). An unknown input pulsed beam is spatially cut by a high-density multiple-slit and changed into a series of spatially separated sub-pulses. By only measuring the spatial distribution of the interference pattern in the far-field, PFT, PFC, or PFT&PFC can be detected. Comparing with recent methods, no reference pulses or beams, no temporal or spatial scanning, and no temporal or spectral measurement is required. The single-shot and spatial-only measurement will greatly simplify the real-time detection of PFT and PFC.

Simulating ultra-intense femtosecond lasers in the 3-dimensional space-time domain

Femtosecond petawatt (fs-PW) lasers, with femtosecond pulses and sub-meter-sized beams, could be easily distorted by spatiotemporal coupling (STC). In 2016, a femtosecond terawatt pulsed beam was experimentally reconstructed in the 3-dimensional (3D) space-time domain for the first time, and showing STC induced distortions. Referring to recently developed laser techniques, traditional first-order STCs can be controlled and then removed. However, the complex STC induced by wavefront errors in a meter-sized grating compressor, where the spatial and spectral coordinates of beams and pulses are coupled, would introduce a non-negligible and complicated distortion. Herein, we theoretically simulated this complex STC in the 3D space-time/spectrum domain and presented its evolution with various factors, which opens a new perspective to analyze CPA lasers in the 3D domain.

Chromatic diversity: a new approach for characterizing spatiotemporal coupling of ultrashort pulses

Two-dimensional chromatic aberrations are characterized by a single-shot scheme based on a simultaneous measurement of chromatically diversified focal spots. The chromatic diversity is introduced by a 2-D grating with holographic defocus terms. The chromatic aberrations in the beam are either subtracted or added by the additional known chromatic aberrations in the grating, depending on the diffraction order. By analyzing the asymmetry in the size of diffracted focal spots, input beam chromatic aberrations can be deduced. Theoretical discussions and experimental results are presented.

Characterization of two ultrashort laser pulses using interferometric imaging of self-diffraction

Noncollinear pulse characterization methods can be applied to over-octave spanning waveforms, but geometrical effects in the nonlinear medium such as beam smearing and critical sensitivity to beam alignment hinder their accurate application. Here, a method is introduced for the temporal and spatial characterization of two pulses by interferometric, spectrally resolved imaging of self-diffraction. Geometrical effects are resolved by the method and, therefore, do not limit the accuracy. Two methods for quantitative pulse retrieval are presented. One method is analytical and very fast; the other method is iterative and more robust if applied to noisy data.

Spatiotemporal evolutions of ultrashort vortex pulses generated by spiral multi-pinhole plate

We use the spiral multi-pinhole plate to generate ultrashort vortex pulses and study their spatiotemporal evolutions involving intensity, phase, orbital angular momentum, and energy current. In the experiment, a Mach-Zehnder-type interferometer is employed to perform the investigation of ultrashort vortices. Combining the experimental results and the theoretical analyses, we discuss the spatiotemporal evolutions of ultrashort vortex pulses in femtosecond regime. The results show that the distribution of orbital angular momentum in the cross-section of the vortex pulse is maintained almost invariable in the pulse duration, while both the intensity and the energy current obey a Gaussian-like distribution. With time evolution, the phase contour lines of such vortex pulses rotate around the propagation axis.

Close to transform-limited, few-cycle 12 µJ pulses at 400 kHz for applications in ultrafast spectroscopy

Non-collinear optical parametric amplification has become the leading technology for amplifying few-cycle carrier-envelope phase (CEP) stable pulses to high energy at extreme repetition rates. In this work, a parametric amplifier system devoted to ultrafast photoionization experiments with coincidence detection is reported. The amplifier delivers CEP-stable few-cycle pulses with an average power of 5 W, and operates at repetition rates between 400 and 800 kHz. Close to transform-limited compression of the few-cycle pulses is achieved with minimized spatio-temporal distortions. Potential limitations introduced by spatio-temporal couplings to applications in attosecond science are analyzed. In particular, it is shown that pulse front tilt resulting from non-collinear amplification can considerably reduce the asymmetry in stereo above threshold ionization (stereo-ATI) experiments.

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.

Numerical simulations of holographic spatiospectral traces of spatiotemporally distorted ultrashort laser pulses

We simulate traces for a catalog of spatiotemporally complex pulses measured using a single-shot complete spatiotemporal pulse-measurement technique we recently developed, called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH). The only such technique ever developed to our knowledge, STRIPED FISH measures the complete spatiotemporal intensity I(x,y,t) and phase ϕ(x,y,t) of an arbitrary laser pulse using an experimentally recorded trace consisting of multiple digital holograms, one for each frequency present in the pulse. To understand the effects of various spatiotemporal distortions on the STRIPED FISH trace, we numerically investigate STRIPED FISH trace features for a catalog of pulses, including the spatially and temporally transform-limited pulse, temporal and spatial double pulses, spherically focusing and diverging pulses, self-phase modulated and self-focusing pulses, spatiotemporally coupled pulses, and pulses with complex structures. As a practical example, we also analyze an experimentally recorded trace of a focusing pulse with spatial chirp. Overall, we find that, from STRIPED FISH's informative trace, significant spatiotemporal characteristics of the unknown pulse can be immediately recognized from the camera frame. This, coupled with its simple pulse-retrieval algorithm, makes STRIPED FISH an excellent technique for measuring and monitoring ultrafast laser sources.