A. Ruiz de la Cruz

Ultrafast imaging of transient electronic plasmas produced in conditions of femtosecond waveguide writing in dielectrics

Femtosecond laser-induced plasmas in bulk dielectrics are imaged under waveguide writing conditions, for different polarizations, pulse durations, and processing depths, and their temporal evolution is measured using ultrafast pump-probe microscopy. The irradiation beam profile is elliptically shaped yielding a disklike focal volume. We demonstrate for doped phosphate glass that increasing the pulse duration improves the spatial distribution of deposited energy by minimizing beam filamentation and prefocal depletion effects. As a consequence, energy deposition in the desired volume is greatly enhanced. Our results identify key parameters for optimizing femtosecond laser processing of dielectrics and different strategies to minimize energy loss channels.

High speed inscription of uniform, large-area laser-induced periodic surface structures in Cr films using a high repetition rate fs laser

We report on the fabrication of laser-induced periodic surface structures in Cr films upon high repetition rate fs laser irradiation (up to 1 MHz, 500 fs, 1030 nm), employing beam scanning. Highly regular large-area (9  cm2) gratings with a relative diffraction efficiency of 42% can be produced within less than 6 min. The ripple period at moderate and high fluences is 0.9 μm, with a small period of 0.5 μm appearing at lower energies. The role of the irradiation parameters on the characteristics of the 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 LIPSS, which, for perpendicular orientation, allows the coherent extension of the sub-wavelength structure over macroscopic distances. The processing strategy is robust in terms of broad parameter windows and applicable to other materials featuring LIPSS.

Deep subsurface waveguides with circular cross section produced by femtosecond laser writing

A combination of low numerical aperture focusing optics, in order to minimize spherical aberration, and beam shaping with a slit has been used to produce waveguides in fused silica by femtosecond laser writing. Waveguides with circular cross section and low losses are produced over a large depth window (>7mm) without changing any experimental parameter. Diffraction induces beam divergence along the axis perpendicular to the slit, leading to a shift of the focal plane. The focal intensity distribution can be predicted by a hybrid model combining Gaussian beam propagation with imaging of the slit into the material.

Photoinscription domains for ultrafast laser writing of refractive index changes in BK7 borosilicate crown optical glass

Ultrashort laser pulses can induce structural modifications in BK7 borosilicate optical glass leading to refractive index variations. These changes and the related optical functions are determined by specific electronic and thermo-mechanical factors, namely network modifiers and high expansion coefficient. We investigate here laser-induced soft positive index changes underpinning photo-physical transformations involving electronic and polarizabillity changes in a narrow processing window (type I), as well as contrasted positive or negative index changes based on thermomechanical mechanisms, non-uniform material expansion, and density redistribution in dense and rarefied regions (type II). These index changes resemble apriori those of pure silica networks despite different thermo-mechanical behavior of BK7 glass upon heating and cooling. The associated structural changes are discussed on the basis of phase contrast microscopy, photoluminescence of defects, and Raman spectroscopy. Material influences are emphasized, pointing out the increasing role of defects as compared to structural reorganization in type I refractive index changes, and mechanical rarefaction and compaction in type II, departing thus from a pure silicate scenario. The resulting optical properties are discussed based on their ability to guide light.

Rapid assessment of nonlinear optical propagation effects in dielectrics

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.

Structural origin of the nonlinear optical properties of lead niobium germanate film glasses

The structural origin of the nonlinear optical susceptibility (/χ(3)/) of lead-niobium-germanate film glasses with large Nb2O5 contents has been investigated. /χ(3)/ shows a strong enhancement with the Nb content in the films with /χ(3)/ values close to 2 × 10−11 esu at 800 nm for a Nb content as high as 0.71. Boling-Glass-Owyoung and Lines’ semiempirical models predict accurately the values of /χ(3)/ for transparent bulk glasses but not for film glasses. This discrepancy is related to the remarkable structural differences between them. Raman spectroscopy suggests the formation of a three-dimensional (3D) structure of [NbO6] octahedra in the case of film glasses having large Nb contents, while X-ray photoelectron spectroscopy shows that a significant fraction of these units contain Nb4+ ions. The combination of a 3D structure of [NbO6] with the presence of Nb4+ polarons and their migration through electron intervalence transfer is proposed as the origin of the observed enhancement of /χ(3)/ in the film gl...

Performance of ultrafast laser written active waveguides by rigorous modeling of optical gain measurements

An Er:Yb co-doped P2O5-La2O5 based glass has been synthesized and used for producing 1.6 cm-long active optical waveguides using a low repetition (1 kHz) rate Ti:Al2O3 fs-laser amplifier. Before processing, the laser energy deposition profile for an elliptically shaped beam was simulated, and the best processing conditions for optimizing the focal volume shape, minimizing non-linear propagation effects, were determined. Under these conditions, a multi-scan writing approach was used to maximize the refractive index change induced and to minimize the transmission losses. After processing, the optical powers propagating inside the waveguide (pump absorption, co- and counter-propagating ASE, low signal gain, …) were measured for uni- and bi-directional pumping schemes, and the measurements were simulated and fitted using an ad hoc developed model to describe the behavior of laser written waveguides. The measurements provide internal gain figures comparable to the best ones reported in phosphate glasses for low repetition rate writing even with larger insertion losses. The simulations provide access to key parameters of the waveguide characteristics (coupling losses and propagation losses, Yb3+ ⇔ Er3+ energy transfer rates, Er3+ upconversion coefficient), which have been used to model the expected performance of these structures in terms of length and doping level. A moderate increase of the Er3+ and Yb3+ doping level would potentially lead to net gain values up to 9.4 dB for a waveguide length of 25 mm.

Rapid calculation of the energy deposition profiles for processing of dielectrics with femtosecond lasers

The energy deposition profile of a focused beam inside a dielectric material is conditioned by linear and nonlinear propagation effects. This can substantially distort the desired shape of the transformed region in laser processing applications [1]. By solving the nonlinear Schrödinger equation (NLSE) using the split-step propagation technique or similar calculation procedures it is possible to perform accurate estimations of the energy deposition profile [2,3]. However, this is usually not practical in terms of a balance between computation time and accuracy. Thus, an experimental study considering various parameters (pulse energy, duration, processing depth, beam ellipticity, etc.) is the typical approach to determine a set of optimized “laser writing” parameters that yield the best result.

Fabrication of ridge waveguides by femtosecond-laser structuring of (Yb, Nb):RTP/RTP using beam multiplexing with a Spatial Light Modulator

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.

Study of the refractive index modification mechanisms of femtosecond laser processed waveguides in doped phosphate glass through its micro photoluminescence properties

Doped phosphate glasses are very attractive materials for the production of active photonic devices (i.e. waveguide amplifiers or lasers) using femtosecond-laser micro-machining. Light confinement in these devices is usually caused by the local the refractive index increase that is induced under certain irradiation conditions. The mechanisms that lead to such a refractive index modification have been shown to be strongly dependent in both: the irradiation parameters and the particular composition of the glass [1,2].