Papers by Alexandro Ruiz de la Cruz
Applied Physics A, 2014
Femtosecond laser pulse temporal shaping techniques have led to important advances in different r... more Femtosecond laser pulse temporal shaping techniques have led to important advances in different research fields like photochemistry, laser physics, non-linear optics, biology, or materials processing. This success is partly related to the use of optimal control algorithms. Due to the high dimensionality of the solution and control spaces, evolutionary algorithms are extensively applied and, among them, genetic ones have reached the status of a standard adaptive strategy. Still, their use is normally accompanied by a reduction of the problem complexity by different modalities of parameterization of the spectral phase. Exploiting Rabitz and co-authors' ideas about the topology of quantum landscapes, in this work we analyze the optimization of two different problems under a deterministic approach, using a multiple one-dimensional search (MODS) algorithm. In the first case we explore the determination of the optimal phase mask required for generating arbitrary temporal pulse shapes and compare the performance of the MODS algorithm to the standard iterative Gerchberg-Saxton algorithm. Based on the good performance achieved, the same method has been applied for optimizing two-photon absorption start-ing from temporally broadened laser pulses, or from laser pulses temporally and spectrally distorted by non-linear absorption in air, obtaining similarly good results which confirm the validity of the deterministic search approach.
Applied Physics A, 2014
Femtosecond laser pulse temporal shaping techniques have led to important advances in different r... more Femtosecond laser pulse temporal shaping techniques have led to important advances in different research fields like photochemistry, laser physics, non-linear optics, biology, or materials processing. This success is partly related to the use of optimal control algorithms. Due to the high dimensionality of the solution and control spaces, evolutionary algorithms are extensively applied and, among them, genetic ones have reached the status of a standard adaptive strategy. Still, their use is normally accompanied by a reduction of the problem complexity by different modalities of parameterization of the spectral phase. Exploiting Rabitz and co-authors' ideas about the topology of quantum landscapes, in this work we analyze the optimization of two different problems under a deterministic approach, using a multiple one-dimensional search (MODS) algorithm. In the first case we explore the determination of the optimal phase mask required for generating arbitrary temporal pulse shapes and compare the performance of the MODS algorithm to the standard iterative Gerchberg-Saxton algorithm. Based on the good performance achieved, the same method has been applied for optimizing two-photon absorption start-ing from temporally broadened laser pulses, or from laser pulses temporally and spectrally distorted by non-linear absorption in air, obtaining similarly good results which confirm the validity of the deterministic search approach.
2013 Conference on Lasers & Electro-Optics Europe & International Quantum Electronics Conference CLEO EUROPE/IQEC, 2013
ABSTRACT form only given. The formation of Laser-Induced Periodic Surface Structures (LIPSS) is a... more ABSTRACT form only given. The formation of Laser-Induced Periodic Surface Structures (LIPSS) is a universal phenomenon [1] that can be observed in a wide variety of materials under certain irradiation conditions. Although the study of sub-wavelength LIPSS in semiconductors and dielectrics irradiated below bandgap has been particularly intense [2], the formation of LIPSS in metals have also been given attention due to its specific features [3,4,5] that have been related to the characteristics of electron-phonon coupling, plasma density and electron diffusion upon fs-laser irradiation at low repetition rates. In this work we report on the unique characteristics of low spatial frequency LIPSS patterns fabricated in Cr upon high repetition rate fs-laser irradiation employing beam scanning. Highly regular patterns with sub-wavelength period can be produced for a wide range of repetition rates (100's kHz range), over large areas (~cm2) and high scan speeds (~m/s).The experiments have been performed with a fs-fibre-laser amplifier (Tangerine, Amplitude Systemes) delivering ≈ 400 fs at 1030 nm and a repetition rate from 50 kHz up to 2 MHz. A galvo-scanner system enables to raster-scan the beam over the sample surface by focusing it with a f=100 mm, f-theta lens to a spot diameter of ≈ 30 μm. Different irradiation conditions (laser repetition rate, beam polarization, pulse energy, scan speed and scan line separation...) have been explored. Figure 1 shows AFM images of LIPSS patters formed by scanning the surface of a sample at v=1500 mm/s, and a repetition rate of 500 kHz for three different pulse energies. In all cases the ripples are formed perpendicular to the polarization axis with period (typically -0.85 μm) smaller than the laser wavelength. The best results in terms of modulation are observed for the scanning direction perpendicular to the polarization axis. High energies (Fig. 1a) produce well-defined and aligned ripples bu- the uniformity is worse than for lower ones (Fig. 1b) featuring a ripple substructure. If the pulse energy is further reduced, (Fig. 1c) the ripple structure becomes discontinuous. Extremely homogenous large-area grating structures with a modulation depth of several hundreds of nanometers have been produced for different repetition rates and scan speeds.In the work we analyse the role of the different irradiation parameters on the characteristics of the generated structures and discuss their origin in the frame of the models presently used for LIPSS formation in metals [3,4,5] with emphasis on the possible influence of thermal accumulation and geometrical factors on the homogeneous propagation of the sub-wavelength structure over macroscopic regions.
2013 Conference on Lasers & Electro-Optics Europe & International Quantum Electronics Conference CLEO EUROPE/IQEC, 2013
ABSTRACT In this work we demonstrate the production of a high gain, short length waveguide amplif... more ABSTRACT In this work we demonstrate the production of a high gain, short length waveguide amplifiers and lasers produced by state of the art femtosecond (fs) laser micromachining.
MATEC Web of Conferences, 2013
2013 15th International Conference on Transparent Optical Networks (ICTON), 2013
ABSTRACT The wavelength range between 3 - 4 μm is a highly sensitive window for environmental mon... more ABSTRACT The wavelength range between 3 - 4 μm is a highly sensitive window for environmental monitoring, bio medical application and spectroscopy. Glasses though can offer high bandwidths are often difficult to maneuver in mid IR wavelengths due to its high phonon energies. Tellurite glass have low phonon energies with high transparency up to 6 - 7 μm. We present the successful production of channel waveguides in a rare-earth doped tellurite glass obtaining active operation within the 3 - 4 μm wavelength window.
Scientific Reports, 2015
Ultrafast laser processing applications need fast approaches to assess the nonlinear propagation ... more Ultrafast laser processing applications need fast approaches to assess the nonlinear propagation of the laser beam in order to predict the optimal range of processing parameters in a wide variety of cases. We develop here a method based on the simple monitoring of the nonlinear beam shaping against numerical prediction. The numerical code solves the nonlinear Schrödinger equation with nonlinear absorption under simplified conditions by employing a state-of-the art computationally efficient approach. By comparing with experimental results we can rapidly estimate the nonlinear refractive index and nonlinear absorption coefficients of the material. The validity of this approach has been tested in a variety of experiments where nonlinearities play a key role, like spatial soliton shaping or fs-laser waveguide writing. The approach provides excellent results for propagated power densities for which free carrier generation effects can be neglected. Above such a threshold, the peculiarities of the nonlinear propagation of elliptical beams enable acquiring an instantaneous picture of the deposition of energy inside the material realistic enough to estimate the effective nonlinear refractive index and nonlinear absorption coefficients that can be used for predicting the spatial distribution of energy deposition inside the material and controlling the beam in the writing process.
CLEO/Europe - EQEC 2009 - European Conference on Lasers and Electro-Optics and the European Quantum Electronics Conference, 2009
Abstract In this work we have applied femtosecond pump-probe microscopy to study the regime of st... more Abstract In this work we have applied femtosecond pump-probe microscopy to study the regime of strong excitation, previously not reported on with imaging techniques. The ability of this technique to simultaneously measure reflectivity changes with fs temporal resolution at locations of strong and weak excitation enables a direct comparison of mechanisms occurring in adjacent regions. For the experiments we have used Ti: sapphire amplifier delivering 100 fs pulses, at 800 nm, operating in single pulse mode. In order to enhance ...
2013 Conference on Lasers & Electro-Optics Europe & International Quantum Electronics Conference CLEO EUROPE/IQEC, 2013
ABSTRACT In this work we show the ultrafast laser fabrication of ridge waveguides in a (Yb, Nb):R... more ABSTRACT In this work we show the ultrafast laser fabrication of ridge waveguides in a (Yb, Nb):RbTiOPO4 (RTP) epilayer grown on a RTP substrate by liquid phase epitaxy [1]. The ribs are produced by machining trenches on the surface of the (Yb, Nb):RTP epilayer. In order to improve the verticality and roughness of the walls of the trenches, we have used the approximation scanning technique [2]. This approach has been enhanced in terms of speed and flexibility by using a Spatial Light Modulator (SLM) to multiplex the irradiation beam.
Optics Letters, 2014
Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc.... more Received Month X, XXXX; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX We report on the fabrication of low-spatial-frequency patterns in Cr films upon high repetition rate fs laser irradiation (up to 1 MHz, 400 fs, 1030 nm), employing large-area beam scanning. Highly-regular large-area (9 cm 2 ) gratings with a relative diffraction efficiency of 42% can be produced in less than 6 minutes. The ripple period at moderate and high fluences is 0.9 µm, with a small period of 0.5 µm appearing at low energies. The role of the irradiation parameters on the characteristics of the generated laser-induced periodic surface structures (LIPSS) is studied and discussed in the frame of the models presently used. We have identified the polarization vector orientation with respect to the scan direction as a key parameter for the fabrication of high-quality, large-area LIPPS, which, for perpendicular orientation, allows the coherent extension of the subwavelength structure over macroscopic distances. The processing strategy is robust in terms of broad parameter windows and applicable to any other material featuring LIPSS.
Laser Physics Letters, 2013
A 16.5 mm long, heavily doped erbium-ytterbium phosphate glass-waveguide amplifier was fabricated... more A 16.5 mm long, heavily doped erbium-ytterbium phosphate glass-waveguide amplifier was fabricated by the femtosecond laser (fs-laser) inscription technique. The femtosecond laser inscription of waveguides was carried out at 500 kHz repetition rate using a 0.68 NA aspheric lens. The energy deposition profile in the dielectric material was initially simulated using a generalized adaptive fast-Fourier evolver (GAFFE) algorithm. The size and shape of the guiding structures were carefully controlled by the slit shaping technique to reduce the coupling losses, with achievable values down to less than 0.1 dB. Rigorous simulations of the response of the active waveguides were carried out to optimize their performance as optical amplifiers. A maximum of 8.6 dB internal gain at 1534 nm was obtained upon bidirectional laser pumping at 976 nm, leading to a gain per unit length of 5.2 dB cm −1 . Laser action was also achieved for both ring and linear cavity configurations.
Journal of Lightwave Technology, 2000
We have produced rib waveguides by femtosecond-laser structuring of active (Yb, Nb): RbTiOPO_4/Rb... more We have produced rib waveguides by femtosecond-laser structuring of active (Yb, Nb): RbTiOPO_4/RbTiOPO_4 epitaxial layers. The ribs were produced by the approximation scanning technique combined with beam multiplexing. The so-obtained waveguides are trapezoidal in shape and show propagation losses with an upper bound of~ 4 dB/cm. A simulation of the rib waveguides with real geometry parameters reveals high levels of light confinement at 632 and 972 nm. The near-field patterns of the fundamental modes have ...
Applied Physics A, 2007
Optical waveguides have been produced by femtosecond-laser writing in a high linear and nonlinear... more Optical waveguides have been produced by femtosecond-laser writing in a high linear and nonlinear refractive index glass (SF57). Light guiding occurs nearby a central damaged zone due to the collateral densification caused by shockwaves generated in the focal volume. High pulse energies are required to induce a modified region capable of supporting a guided mode. An alternative processing method, based on using multiple structures, has been successfully used for the production of waveguides with controllable core size at low energies.
Journal of the Optical Society of America B, 2013
ABSTRACT Fused silica surface structuring has been performed using temporally shaped femtosecond ... more ABSTRACT Fused silica surface structuring has been performed using temporally shaped femtosecond laser pulses. For this purpose we have designed pulse bursts with a triangular intensity envelope and different slope sign and interpulse separation that were experimentally generated using a home-made temporal pulse shaper. We have found that pulse bursts with decreasing intensity envelopes are remarkably more efficient in terms of surface ablation than bursts with increasing intensity envelopes. The results reveal that laser energy coupling in the material is enhanced as the interpulse spacing decreases. A study of the ablation depth using stretched single pulses was carried out and compared to results obtained for pulse bursts with different interpulse spacing. We find that the deepest crater was achieved with bursts of 0.5 ps interpulse separation and decreasing envelope. This pulse form also induced the largest change of the surface reflectivity after irradiation. The results are discussed in terms of how the laser energy coupling efficiency is linked to the temporal pulse shape.
Journal of Applied Physics, 2011
The origin of the local refractive index modification in femtosecond laser inscribed structures h... more The origin of the local refractive index modification in femtosecond laser inscribed structures has been investigated with confocal microfluorescence imaging. We have identified the origin of both, positive and negative refractive index changes in a commercial Er-Yb codoped phosphate glass upon irradiation in the low repetition rate regime (1 kHz). Consistent relations among the photoluminescence behavior of the dopants (erbium and ytterbium ions), the local sign of the density change and the local modification of the refractive index by means of characteristic emission features such as the intensity and the spectral peak position have been established. Blue or redshift in the photoluminescence emission have been observed and related to a local perturbation in the crystal field caused by a modification of the mean distance among the dopant ions, and thus of the local matrix density. These conclusions are additionally supported by the spatial distribution of photoluminescence emission intensities, which have been interpreted in terms of energy transfer mechanisms underlying the overall erbium-ytterbium emission process.
Journal of the Optical Society of America B, 2010
The effect of nonlinear propagation on the shape of the focal volume has been assessed by in situ... more The effect of nonlinear propagation on the shape of the focal volume has been assessed by in situ plasma emission imaging during the subsurface processing of a commercial phosphate glass. The sample was processed with an elliptically shaped femtosecond-laser beam at 1 kHz repetition rate and scanned transversely with respect to the writing beam axis. As a consequence, optimal conditions for minimizing undesirable nonlinear propagation effects during the production of optical waveguides by direct laser writing have been determined. Under these conditions, it is possible to induce structural transformations and still preserve the focal volume shape associated with the linear propagation regime. While at low pulse energy a single scan laser-written structure does not support a guided mode, the use of multiple scans with minimized nonlinear propagation effects enables the production of optical waveguides. The latter show a significantly improved performance in terms of the refractive index change and propagation losses when compared to single scan waveguides.
Applied Surface Science, 2007
In the present work, we have analyzed the use of elliptical beam shaping along with low numerical... more In the present work, we have analyzed the use of elliptical beam shaping along with low numerical aperture focusing optics in order to produce circular cross-section waveguides in different materials at large processing depths by direct femtosecond laser writing (100 fs, 800 nm, 1 kHz). A variable slit located before the focusing optics allows to generate a nearly elliptical beam shape and also to reduce the effective numerical aperture of the beam along the shat axis of the ellipse. The focusing optics allows to focus the beam deep inside the sample, which is translated at a constant speed transversely to the writing beam direction. The influence of several experimental parameters (energy per pulse, slit width, processing depth) on the properties of the produced waveguides has been analyzed. The influence of the intrinsic properties of the material (refractive index, composition) has been analyzed by comparing results obtained in fused silica and Er:Yb co-doped phosphate glass. The results obtained show that this approach leads to the successful production of deep subsurface (up to 7 mm) waveguides with circular cross-sections. Preliminary results using chirped pulses in the phosphate glass suggest that temporal pulse shaping can be used as an additional parameter to optimize the guided mode symmetry. #
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Papers by Alexandro Ruiz de la Cruz