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[EN]We use self-calibrating dispersion scan to experimentally detect and quantify the presence of pulse train instabilities in ultrashort laser pulse trains. We numerically test our approach against two different types of pulse instability, namely second-order phase fluctuations and random phase instability, where the introduction of an adequate metric enables univocally quantifying the amount of instability. The approach is experimentally demonstrated with a supercontinuum fibre laser, where we observe and identify pulse train instabilities due to nonlinear propagation effects under anomalous dispersion conditions in the photonic crystal fibre used for spectral broadening. By replacing the latter with an all-normal dispersion fibre, we effectively correct the pulse train instability and increase the bandwidth of the generated coherent spectrum. This is further confirmed by temporal compression and measurement of the output pulses down to 15 fs using dispersion scan., The authors acknowledge funding from the Junta de Castilla y León (SA287P18) and FEDER funds; Spanish Ministerio de Economía y Competitividad (MINECO) (FIS2017-87970-R, EQC2018-004117-P, DI-15-07461, PTQ-15-07708); European EUREKA program CDTI-INNO-20171026; CCDR-N via the project Nanotechnology-based functional solutions (NORTE-01-0145-FEDER-000019); Fundação para a Ciência e a Tecnologia (FCT), Portugal (grants ‘UltraGraf’ M-ERA-NET2/0002/2016, M-ERA-NET2/0004/2016, UID/NAN/50024/2013, PTDC/FIS-OTI/32213/2017); Network of Extreme Conditions Laboratories - NECL and CCDR-N (NORTE-07-0124-FEDER-000070, NORTE-01-0145-FEDER-022096); PT2020 (program 05/SI/2017 – SI I&DT Empresarial - DI, grant no. 33573; program 04/SI/2019 Projetos de I&D Industrial à Escala Europeia, grant no. 045932). BA acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 798264.

We implement an in-line and ultrastable spatiotemporal technique based on uniaxial crystals, which solves high-complexity and stability problems associated with other techniques. Moreover, we generate and characterize the temporally resolved wavefronts of time-varying optical vortices.

Mode-locking erbium doped fiber lasers are simple, compact and low-cost sources of ultrashort pulses. However, their characterization could be troublesome when the laser operates in an unstable regime.

The advent of ultrafast optics and its several applications has implied the development of a wide range of pulse time domain diagnostics (autocorrelation, FROG, SPIDER, spectral interferometry, d-scan, etc.). Nowadays, the growing application of ultrafast pulses at out-of-lab environments (e.g., industrial) with noise and instability sources is a challenge, requiring a simple and robust measurement technique.

Special light beams exhibiting a structured field and carrying orbital angular momentum (OAM) appear as interesting tools in different fields such as particle manipulation, telecommunications or in several areas of light-matter interaction, including materials processing and nonlinear optics, among others. A very interesting possibility of these special light beams is the coupling of spatially varying linear polarization fields ( i.e. radial or azimuthal) with spin and orbital angular momenta. Taking advantage of this interaction, we propose an in-line method to obtain collinear optical vortices exhibiting different topological charges [1] . The proposed method can be applied to focused or collimated beams, being continuous wave or pulsed.

Over the past decades, the use of ultrafast laser beams at a highly varied range of applications (e.g., biophotonics, material science, telecommunications, physics, chemistry, etc.) has evolved, demanding more complex light structures not only in time (i.e., ultrafast pulses), but also spatiotemporally and in polarization (e.g., vector beams, time evolving polarization pulses). Therefore, the characterization of such complex structured beams has become a hot topic nowadays.

We demonstrate a novel dispersion-scan (d-scan) scheme for single-shot temporal characterization of ultrashort laser pulses. The novelty of this method relies on the use of a highly dispersive crystal featuring antiparallel nonlinear domains with a random distribution and size. This crystal, capable of generating a transverse second-harmonic signal, acts simultaneously as the dispersive element and the nonlinear medium of the d-scan device. The resulting in-line architecture makes the technique very simple and robust, allowing the acquisition of single-shot d-scan traces in real time. In addition, the technique can be further simplified by avoiding the need of dispersion pre-compensation. The retrieved pulses are in very good agreement with independent FROG measurements. We also apply the new single-shot d-scan to a TW-class laser equipped with a programmable pulse shaper, obtaining an excellent agreement between the applied and the d-scan retrieved dispersions., 7 pages, 5 figures

We present a new concept for temporal pulse measurement, the amplitude swing, based on the amplitude variation of two delayed pulse replicas. The in-line compact set-up is versatile for pulses with different durations and bandwidths.

We perform a complete measurement of ultrashort pulsed beams where the polarization evolves spatially and temporally. We apply our technique to characterize ultrafast shaped vector beams which combine temporal polarization gating and radial polarization.

[EN]The use of structured ultrashort pulses with coupled spatiotemporal properties is emerging as a key tool for ultrafast manipulation. Ultrafast vector beams are opening exciting opportunities in different fields such as microscopy, time-resolved imaging, nonlinear optics, particle acceleration or attosecond science. Here, we implement a technique for the full characterization of structured time-dependent polarization light waveforms with spatiotemporal resolution, using a compact twofold spectral interferometer, based on in-line bulk interferometry and fibre-optic coupler assisted interferometry. We measure structured infrared femtosecond vector beams, including radially polarized beams and complex-shaped beams exhibiting both temporal and spatial evolving polarization. Our measurements confirm that light waveforms with polarization evolving at the micrometer and femtosecond scales can be achieved through the use of structured waveplates and polarization gates. This new scale of measurement achieved will open the way to predict, check and optimize applications of structured vector beams at the femtosecond and micrometer scales., We acknowledge funding from Junta de Castilla y León (SA287P18) and FEDER Funds, and from Spanish Ministerio de Economía y Competitividad (FIS2016-75652-P, FIS2017-87970-R, EQC2018-004117-P). C.H.-G. acknowledges support Ministerio de Ciencia, Innovación y Universidades for a Ramón y Cajal contract (RYC-2017-22745), cofunded by the European Social Fund. B.A. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Individual Fellowship grant agreement No. 798264. PGK acknowledges support of ERC project ENIGMA. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 851201).

We apply the self-calibrating d-scan technique to quantify the pulse train instabilities in a fiber laser with supercontinuum generation in a photonic crystal fiber (PCF). Using an all-normal dispersion PCF, stable 15-fs pulses are measured.

We apply the self-calibrating d-scan technique to quantify the pulse train instabilities in a fiber laser with supercontinuum generation in a photonic crystal fiber (PCF). Using an all-normal dispersion PCF, stable 15-fs pulses are measured.

We perform a complete measurement of ultrashort pulsed beams where the polarization evolves spatially and temporally. We apply our technique to characterize ultrafast shaped vector beams combining temporal polarization gating and radial polarization.

We show the versatility of the bulk lateral shearing interferometer characterizing complex spatiotemporal structures in different spectral ranges. Specifically, we have characterized constant and timevarying optical vortices in the visible and near infrared spectral ranges respectively. The high stability of the system combined with its spectral versatility will ease the spatiotemporal characterization of ultrafast phenomena.

[EN]Inscription of optical waveguides by direct femtosecond laser irradiation has become a very versatile tool for the development of integrated photonic devices, such as waveguide lasers, frequency converters or photonic lanterns, among many others. The potential application of such devices for the control and manipulation of ultrashort pulses requires the precise knowledge of the temporal distortions that may be induced in the pulse propagation. Currently, research in this topic is scarce, and to our knowledge there is no previous experimental study on the spatio-temporal characterization at the output of waveguides inscribed inside crystals. Here, we have firstly fabricated depressed-cladding waveguides with different modal behavior in YAG crystal by direct femtosecond laser irradiation. Then, we implemented an experimental method based on the fiber coupler assisted spectral interferometry technique, that allows obtaining: (1) the temporal dispersion of a pulse at the output of an inscribed waveguide and, (2) full spatio-spectral and spatio-temporal characterization of the output of single-mode and multi-mode waveguides. Our results suggest that the main contribution to the pulse dispersion is due to the material dispersion. Moreover, we found that multimodal waveguides may induce an appreciable inhomogeneity in the temporal features of the pulses that needs to be taken into account in the design of complex devices. This work was supported by Junta de Castilla y León (Project SA046U16 and SA287P18) and MINECO (Projects FIS2017-87970-R, EQC2018-004117-P). M.M.-V. acknowledges support from Junta de Castilla y León grant (No. SA046U16) and from Generalitat Valenciana, Spain, under project CDEIGENT/2018/024.

Mode-locked erbium-doped fibre lasers are ultrashort pulsed sources widely studied due to their versatility and multiple applications in the near infrared range. Here we present the experimental study of the emission of a passive mode-locked erbium-doped fibre laser with an amplification stage outside the cavity by means of Frequency Resolved Optical Gating (FROG) and spectral interferometry. Due to shot-to-shot instabilities, the FROG traces can be understood as the combination of two different traces, corresponding to the coherent artifact and the average pulse characteristics. We have modified a Principal Components Generalized Projections Algorithm, in order to make it able to retrieve efficiently both the coherent artifact and the average pulse. In addition, we study the temporal dependence of the polarization, showing that the pulses present time-dependent polarization with a stable spectral relative phase between the horizontal and vertical projections. Up to our knowledge, this is the first experimental study that shows the FROG measurements of unstable pulse trains associated with the coherent artifact and analyses the time-dependent polarization in ultrafast fibre lasers., This work was supported by the Ministerio de Economía y Competitividad (EQC2018-004117-P, FIS2017-87970-R); the Consejería de Educación, Junta de Castilla y León (SA287P18, SA136P20, VA085G19); and the INTERREG V-A España-Portugal (POCTEP) program (0677_ DISRUPTIVE _ 2_ E)

Many recent applications of vector pulses require the characterization of time-dependent polarization of ultrashort laser pulses [1]. For a precise measurement, the relative phase between the vector components is crucial. We propose a monolithic, in-line, single-channel interferometer setup [2], which allows the full determination of the time-evolving polarization state of the pulse, retrieving with precision the relative phase thanks to its great stability (∼10−3 Λ). This is possible due to the use of a birefringent material (3-mm calcite) to delay the x- and y-components of the pulse (Fig. 1a). A linear polarizer (LP) is used to measure three projections with a spectrometer: 0° and 90° for the spectral amplitude of the x- and y-components, respectively, and 45° for the spectral interferences of the delayed components. By analysing the latter using Fourier-transform filtering [3], it is possible to retrieve the full relative phase (including constant term).

Focusing of ultrashort pulses is an important issue for their applications. Even if free aberration focusing is generally desired to obtain at focus the shorter durations and the higher intensities, the ability to create and control other spatio-temporal radiance patterns can be helpful, or even required, on diverse applications (nonlinear optics, particle acceleration, etc.). For instance, when focused by diffractive elements, ultrashort pulses can show complex spatio-temporal structures [1]. In the present work, we gain a further control over the focusing by shaping the spectral phase of the input pulses. For this purpose, we combine the focusing by a kinoform diffractive lens (KDL) with the control of the spectral phase of the input pulses, to tailor different spatio-temporal patterns of intensity [2]. We have studied spatio-temporal patterns along the propagation at the focal region, for specific values (negative, zero and positive) of second (GDD) and third order dispersion (TOD), introduced by an acousto-optic device to the pulse prior focusing.

The growing use of ultrashort laser pulses exhibiting time-varying polarization (vector pulses) demands simple and robust characterization techniques capable to perform measurements in a broad range of experimental and environmental conditions. Here we present in-line, single-channel setup based on spectral interferometry to characterize ultrashort vector pulses. The use of a bulk interferometer based on birefringence is key for the stability and sensitivity of the technique, thus being simple and highly robust. The technique is used to measure vector pulses corresponding to polarization gates, which are used in many applications. Those results are validated by simulations. The technique here presented has a number of potential applications in nonlinear effects (e.g. transient birefringence and nonlinear phenomena with vector pulses)., Comment: Information: 11 pages, 5 figures. Date of Publication: 27 March 2019. This work was partially supported by the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Action with grant agreement No. 798264, project FastMeasure entitled 'Development and industry transfer of new techniques: full characterization of vector ultrashort pulsed laser beams'

[EN]Special light beams are becoming more and more interesting due to their applications in particle manipulation, micromachining, telecommunications or light matter-interaction. Both spin and orbital angular momenta of light are exploited often in combination with spatially varying linear polarization profiles (e.g. radial or azimuthal distributions). In this work we study the interaction between those polarization profiles and the spin-orbit angular momenta, finding the relation involved in the mode coupling. We find that this manipulation can be used for in-line production of collinear optical vortices with different topological charges, which can be filtered or combined with controlled linear polarization. The results are valid for continuous wave and ultrashort pulses, as well as for collimated and focused beams. We theoretically demonstrate the proposal, which is further confirmed with numerical simulations and experimental measurements with ultrashort laser pulses., This work was partially funded by Junta de Castilla y León (SA287P18) and FEDER Funds; Spanish Ministerio de Economía y Competitividad (MINECO) (FIS2017-87970-R, EQC2018-004117-P); European Research Council (ENIGMA).

[EN]The chirped pulse amplification (CPA) technique applied to Titanium Sapphire lasers has made intense near-infrared (NIR) ultrashort pulses in the 20−100 fs range widely available for scientific, biomedical and industrial applications. Special efforts have been devoted to generate even shorter pulses, in the few- and single-cycle regime, due to a number of interesting applications. In particular, such pulses have paved the way for attosecond physics and metrology, via the extreme ultraviolet (XUV) attosecond pulse trains and isolated attosecond pulses that can be obtained by high-harmonic generation (HHG). The use of few-cycle optical pulses with durations close to or shorter than 10 fs in the near-infrared, visible and near-ultraviolet spectral regions has been extended in recent years to a wide range of spectroscopic techniques such as impulsive vibrational spectroscopy, time-resolved stimulated Raman spectroscopy, and ultrafast pump-probe absorption spectroscopy. Few-cycle optical pulses have also become an interesting tool for transient absorption microscopy, near-field imaging techniques and for generating ultrashort terahertz radiation. While it is possible to obtain sub-10 fs pulses from CPA or from optical parametric amplification systems, the former is not easy to accomplish, and the latter is not commonplace. Therefore, post-compression techniques are usually employed for the generation of intense few- and even single-cycle pulses in the near- and mid-infrared spectral regions. In order to post-compress ultrafast pulses down to the few-cycle regime, two steps are usually needed. First, nonlinear processes broaden the pulse spectrum, thus decreasing the Fourier-limited pulse duration. In a second step, the spectral phase resulting from the previous stage is compensated, typically using chirped mirrors, gratings, prisms, or other dispersive systems, resulting in a temporally compressed pulse. This scheme was first proposed in the context of optical fibers in the 1980s, and enabled achieving 6 fs pulses when compensating simultaneously the outcoming group delay dispersion (GDD) and third-order dispersion (TOD).

Intense few- and single-cycle pulses are powerful tools in different fields of science Today, third- and higher-order terms in the remnant spectral phase of the pulses remain a major obstacle for obtaining high-quality few- and single-cycle pulses from in-line post-compression setups. In this Letter, we show how input pulse shaping can successfully be applied to standard post-compression setups to minimize the occurrence of high-order phase components during nonlinear propagation and to directly obtain pulses with durations down to 3 fs. Furthermore, by combining this pulse shaping of the input pulse with new-generation broadband chirped mirrors and material addition for remnant third-order phase correction, pulses down to 2.2 fs duration have been measured., Ministerio de Economía y Competitividad (MINECO) (FIS2013-44174-P, FIS2015-71933-REDT, FIS2016-75652-P, FIS2017-87970-R); Consejería de Educación, Junta de Castilla y León (SA046U16, SA116U13); Fundação para a Ciência e a Tecnologia (FCT) (M-ERANET2/0002/2016); European Regional Development Fund (ERDF) (NORTE-01-0145-FEDER-022096).

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., 11 pages, 5 figures, Scientific Reports, in press

In this work we study the impact of chromatic focusing of few-cycle laser pulses on high-order-harmonic generation (HHG) through analysis of the emitted extreme ultraviolet (XUV) radiation. Chromatic focusing is usually avoided in the few-cycle regime, as the pulse spatiotemporal structure may be highly distorted by the spatiotemporal aberrations. Here, however, we demonstrate it as an additional control parameter to modify the generated XUV radiation. We present experiments where few-cycle pulses are focused by a singlet lens in a Kr gas jet. The chromatic distribution of focal lengths allows us to tune HHG spectra by changing the relative singlet-target distance. Interestingly, we also show that the degree of chromatic aberration needed for this control does not degrade substantially the harmonic conversion efficiency, still allowing for the generation of supercontinua with the chirped-pulse scheme, demonstrated previously for achromatic focusing. We back up our experiments with theoretical simulations reproducing the experimental HHG results depending on diverse parameters (input pulse spectral phase, pulse duration, and focus position) and proving that, under the considered parameters, the attosecond pulse train remains very similar to the achromatic case, even showing cases of isolated attosecond pulse generation for near-single-cycle driving pulses., We acknowledge support from Consejería de Educación,Junta de Castilla y Le n (Projects No. SA116U13 and No. SA046U16), Spanish MINECO (Grants No. FIS2009-09522, No. FIS2013-44174 P, No. FIS2015-71933-REDT, and No. FIS2016-75652-P). This work was partly supported by Fundacio para a Ciencia e Tecnología, Portugal, co-funded by COMPETE and FEDER, via Grants No. PTDC/FIS/122511/2010 and No. UID/NAN/50024/2013. F.S. acknowledges support from Grant No. SFRH/BD/69913/2010 and B.A. acknowledges support from Post-Doctoral Fellowship Grant No. SFRH/BPD/88424/2012; H.C. acknowledges support from Sabbatical Leave Grant No. SFRH/BSAB/105974/2015. C.H.-G. acknowledges support from a Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development, under REA Grant Agreement No. 328334.

We present observations of the emission of XUV continua in the 20-37 eV region by high harmonic generation (HHG) with $4$-$7\ \mathrm{fs}$ pulses focused onto a Kr gas jet. The underlying mechanism relies on coherent control of the relative delays and phases between individually generated attosecond pulse, achievable by adjusting the chirp of the driving pulses and the interaction geometry. Under adequate negative chirp and phase matching conditions, the resulting interpulse interference yields a continuum XUV spectrum, which is due to both microscopic and macroscopic (propagation) contributions. This technique opens the route for modifying the phase of individual attosecond pulses and for the coherent synthesis of XUV continua from multicycle driving laser pulses without the need of an isolated attosecond burst., 14 pages, 5 figures. Submitted to Physical Review A

We have designed and developed a pulse compressor with volume transmission holographic gratings to be implemented in post-compression experiments based on filamentation in gases. Pulse compression down to 13 fs has been demonstrated. The gratings have been recorded in commercial PFG-04 dichromated gelatin emulsions with a recording wavelength of 532 nm, attaining sufficient index modulation to achieve high efficiency when they are illuminated by an 800-nm laser., This research has been supported by the Diputación General de Aragón (Grupo Consolidado TOL), the Universidad de Zaragoza (under grant UZ2010-CIE-02), the Spanish Ministerio de Ciencia y Tecnología (FEDER funds, grant FIS2006-04151) and grant Consolider Programme SAUUL (CSD2007-00013)), COST (grant CM0702-CUSPFEL), and the Junta de Castilla y León (grants GR27 and SA002B08). We also acknowledge support from the Centro de Laseres Pulsados, CLPU, Salamanca, Spain. B. Alonso and I.J. Sola acknowledge support from the Spanish Ministerio de Ciencia e Innovación through the ‘Formación de Profesorado Universitario’ and ‘Ramón y Cajal’ grant programs, respectively. Valuable assistance by J. Rodríguez Vázquez de Aldana and R. Torres is also acknowledged.

The influence of the input pulse chirp and energy on the filamentation process is studied. Output beam profiles, spectra and energies were systematically measured by changing the pulse chirp for different input energies. A map of the different energy-chirp regions was compiled. It shows that high-energy stable single filament can be obtained by using chirped input pulses, allowing the scaling-up of the energy throughput in the filament. Moreover, under high-energy regimes nonlinear effects induced by chirped pulses could produce pulse post-compression.

We present a complete experimental characterization and simulation of the spatiotemporal and spatio-spectral effects taking place when a femtosecond pulse is shaped by a diffractive optical element.

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.

We present, for the first time, the complete pulse characterization of the infrared pulse after generating harmonics. A systematic study of the high harmonic generation process, and the generating infrared pulse characterization, has been done by changing the focus-gas-jet relative position. We have concluded, supported by nonlinear propagation simulations, that there is a correlation between the spectral and temporal nonlinear evolution of the infrared generating field and the structures shown in the harmonic signal. We have identified two different pressure regimes: the low pressure regime, characterized by the effects produced by the plasma generated by the infrared pulse, and the high pressure regime where the plasma and the Kerr effect generated by the infrared field are both present. These observations highlight the important role played by the nonlinear propagation of the generating field in the high harmonic generation context., Spanish Ministerio de Ciencia e Innovación (MICINN) through the Consolider Program SAUUL ( CSD2007-00013) and Research Project FIS2009-09522, from the Junta de Castilla y León (Project No. SA116U13) and from Centro de Láseres Pulsados. B. A. acknowledges Fundaçao para a Ciencia e a Tecnologia (FCT) through grant No. SFRH/BPD/88424/2012. W. H. and I. J. S. also acknowledge support from the Spanish Ministerio de Ciencia e Innovación through the Formación de Personal Investigador and Ramón y Cajal grant programs respectively.

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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., Spanish Ministerio de Ciencia e Innovación (MICINN) through the Consolider Program SAUUL (CSD2007-00013), Research projects FIS2009-09522, and grant programs Formación de Profesorado Universitario (No. AP2007-00236 for B. Alonso) and Ramón y Cajal (for I. J. Sola); and from the Junta de Castilla y León through the Program for Groups of Excellence (GR27). H. Crespo acknowledges Fundos FEDER, through Programa Operacional Factores de Competitividade – COMPETE, and Fundação para a Ciência e a Tecnologia (FCT) under grants No. PTDC/FIS/115102/2009 and PTDC/FIS/122511/2010. M. Miranda acknowledges FCT and FEDER grant No. SFRH/BD/37100/2007. We also acknowledge support from Centro de Láseres Pulsados (CLPU), Salamanca, Spain.

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. (C)2012 Optical Society of America

[EN] We demonstrated a simple diffractive method to map the self-similar structure shown in squared radial coordinate of any set of circularly symmetric fractal plates into self-similar light pulses in the corresponding temporal domain. The space-to-time mapping of the plates was carried out by means of a kinoform diffractive lens under femtosecond illumination. The spatio-temporal characteristics of the fractal pulses obtained in this way were measured by means of a spectral interferometry technique assisted by a fiber optics coupler (STARFISH). Our proposal allows synthesizing suited sequences of focused fractal femtosecond pulses potentially useful for several current applications, such as femtosecond material processing, atomic, and molecular control of chemical processes or generation of nonlinear effects. © 2012 Optical Society of America., This research was funded by the Spanish Ministerio de Ciencia e Innovación (MICINN) and FEDER, through the projects FIS2010-15746, DPI2008-02953, and SAUUL (CSD2007-00013) and the Fundació Caixa Castelló (P1-1B2010-26).

We make a review of the high harmonic generation (HHG) process, its characteristics and properties, presenting our main results in the field. The HHG is a very interesting source on the deep ultraviolet region with high potential in applications such as time resolved measurements and high spatial resolution imaging

The spatio-temporal characterization of the electric field of an ultrashort laser pulse after passing through a diffractive optical element has been done. The expressions derived for the total diffraction field in the time and spectral domain are validated using spatially resolved spectral interferometry assisted by an optical fiber coupler (STARFISH).

In the last decade the use of laser beam filamentation has become of great interest, not only as a complex physical process, but also applied to the compression of femtosecond pulses. In particular, it has been reported that at some particular experimental conditions the pulse can experience a self-compression effect [1]. This phenomenon has been recently obtained by controlling the chirp of the input [2]. For this reason we have studied the impact of the input chirp over the temporal structure of the pulses generated after the filamentation process. During those scans, very rich pulse structures and pulse splitting dynamics has been observed. Particularly, for each input energy studied, we found two different input chirp values (oppositely signed) that produce a single filament self-compressed pulse. Fig. 1 shows the data measured in the 5 mJ input energy case, obtaining 22 and 25 fs when using input pulses with positive or negative input chirp, respectively. Similar results were obtained from a systematic study realized for input energies between 3 and 5 mJ [2]. The energy transmission in the inner core remains constant at 20 % in all the cases.

Due to the spatiotemporal and spatiospectral coupling of the light field, the characterization of ultrashort pulses cannot be usually accomplished in separated spatial or temporal domains. For this reason, considerable efforts have been done during last years to measure the full spatiotemporal amplitude and phase of ultrashort pulses by using several techniques [1–3].

Self-compressed (SC) pulses have been achieved through the filamentation process in air without any additional dispersion compensation, using the input pulse chirp as the control parameter. For any studied input pulse energy (3-5 mJ), we have found two opposite sign input group-delay dispersion values for which SC pulses can be achieved systematically. In addition, we have observed that the energy coupled into the inner core of the filament is always of the order of 20% of the total input pulse energy, which opens the way to a scalable technique to obtain intense short pulses directly from the filamentation process.

We carry out a complete spatio-temporal characterization of the electric field of an ultrashort laser pulse after passing through a diffractive optical element composed of several binary amplitude concentric rings. Analytical expressions for the total diffraction field in the time and spectral domain are provided, using the Rayleigh-Sommerfeld formulation of the diffraction. These expressions are experimentally validated. The spatio-temporal amplitude and phase structure of the pulse are measured at different planes beyond the diffractive optical element using spatially-resolved spectral interferometry assisted by an optical fiber coupler (STARFISH). Our results allow corroborating theoretical predictions on the presence of multiple pulses or complex spectral distributions due to the diffraction-induced effects by the hard-edge ring apertures.

We have investigated femtosecond multi-filamentation process in a mixture of gases controlling the concentration of atoms versus molecules in the gas cell. The experimental results show that this control could provide a new freedom degree to deterministic spatial distribution control of the multiple filaments. Our simulation indicates surprisingly that only difference of the gases nonlinearity (referred to as “synthesised nonlinearity”) is sufficient to be responsible for this control. This study opens the way to provide few-cycle pulses spatial distributed source for spatially encoded measurements and experiments.

We make a review of our main results in the field of femtosecond pulses diffracted by diffractive optical elements (DOEs). These results show how amplitude and/or phase DOEs can be used to change the spectral and temporal characteristics of the diffracted pulses. We experimentally demonstrate that shaped pulses diffracted by DOEs are useful in nonlinear optics or chemical applications.

In this work we design and construct a pulse compressor with volume transmission holographic gratings, to compensate the second order dispersion in femtosecond laser pulses emitting at 794 nm with a spectral broadband of 10 nm. The gratings (730 lines/mm) are recorded in PFG-04 dichromated gelatine emulsion with a wavelength of 532 nm, reaching enough index modulation to use the gratings illuminated with 800 nm light source with high efficiency (around 80% of efficiency in each grating). This efficiency is expected to be increased with an antireflection coating. We measure the factor of compression as a function of the grating distance using an autocorrelator, finding a good agreement with theoretical curve. A dispersed pulse (580 fs) is reduced to the bandwidth limited value of 106 fs with the grating pair separated by 27 mm.

We propose a novel method to reconstruct the spatio-temporal amplitude and phase of the electric field of ultrashort laser pulses using spatially-resolved spectral interferometry. This method is based on a fiber-optic coupler interferometer that has certain advantages in comparison with standard interferometer systems, such as it being alignment-free and selection of the reference beam at a single point. Our technique, which we refer to as STARFISH, offers compactness and simplicity. We report its application to the experimental characterization of chirped pulses and to spatio-temporal reconstructions of a convergent beam as well as plane-plane and spherical-plane waves interferences, which we check with our simulations., Comment: 9 pages, 7 figures

We present evidence for self-stabilization of the relative spectral phase of high-order harmonic emission against intensity variations of the driving field. Our results demonstrate that, near the laser focus, phase matching of the harmonic field from a macroscopic target can compensate for the intensity dependence of the intrinsic phase of the harmonics emitted by a single radiator. As a consequence, we show experimentally and theoretically the insensitivity of the harmonic spectra produced at the laser focus against variations of the carrier-envelope phase (CEP) of a sub-two-cycle driving field. In addition, the associated attosecond pulse trains exhibit phase locking against CEP changes of the few-cycle driver., The authors acknowledge support from MINECO ( FIS2013-44174-P), from Junta de Castilla y León (Project No. SA116U13) and from Centro de Láseres Pulsados. C.H.-G. acknowledges support from the Marie Curie International Outgoing Fellowship within the EU Seventh Framework Programme for Research and Technological Development (2007–2013), under REA grant Agreement No. 328334. W.H. acknowledges ESF support through SILMI Research Networking Programme. B. A. acknowledges Fundação para a Ciência e a Tecnologia (FCT) through grant No. SFRH/BPD/88424/2012. W. H. and I. J. S. also acknowledge support from the Spanish Ministerio de Ciencia e Innovación through the Formación de Personal Investigador and Ramón y Cajal grant programs respectively. This work was partly supported by grant PTDC/FIS/122511/2010 from Fundação para a Ciência e Tecnologia, Portugal, co-funded by COMPETE and FEDER.