A. Ferrer

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.

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.

In situ assessment and minimization of nonlinear propagation effects for femtosecond-laser waveguide writing in dielectrics

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.

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.

Independent control of beam astigmatism and ellipticity using a SLM for fs-laser waveguide writing

We have used a low repetition rate (1 kHz), femtosecond laser amplifier in combination with a spatial light modulator (SLM) to write optical waveguides with controllable cross-section inside a phosphate glass sample. The SLM is used to induce a controllable amount of astigmatism in the beam wavefront while the beam ellipticity is controlled through the propagation distance from the SLM to the focusing optics of the writing set-up. The beam astigmatism leads to the formation of two separate disk-shaped foci lying in orthogonal planes. Additionally, the ellipticity has the effect of enabling control over the relative peak irradiances of the two foci, making it possible to bring the peak irradiance of one of them below the material transformation threshold. This allows producing a single waveguide with controllable cross-section. Numerical simulations of the irradiance distribution at the focal region under different beam shaping conditions are compared to in situ obtained experimental plasma emission images and structures produced inside the glass, leading to a very satisfactory agreement. Finally, guiding structures with controllable cross-section are successfully produced in the phosphate glass using this approach.

Femtosecond-Laser Microstructuring of Ribs on Active (Yb,Nb):RTP/RTP Planar Waveguides

We have produced rib waveguides by femtosecond-laser structuring of active (Yb,Nb):RbTiOPO4/RbTiOPO4 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 been obtained by exciting the waveguides at wavelengths of 632 and 972 nm. Micro-Raman spectroscopy study reveals that the damage to the crystalline structure in the rib boundaries, showing no amorphization traces, is around 3 μm in length and depth, which is significantly shorter than the total width of the ribs.

Origin of the refractive index modification of femtosecond laser processed doped phosphate glass

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 ha...

New Characterization Technique for Femtosecond Laser Written Waveguides in Yb/Er-Codoped Glass

We have developed a procedure for the complete characterization of femtosecond-laser written Er/Yb-codoped glass waveguides. The procedure is based on precise measurements of both pump and 1.5 ¿m -band output powers when the waveguide is diode-laser pumped at 980 nm. The dependence of these optical powers on the input pump power is fitted to the results derived from a detailed numerical model of the propagation of the optical powers inside the waveguide. As a result of the fitting procedure the 980-nm absorption cross-section of the Yb3+ ion and the coefficients for both Yb3+ ¿ Er3+ non-radiative energy-transfer and Er3+ ion upconversion are obtained. Moreover, the transmission losses for both pump and signal wavelengths and the coupling losses at each waveguide end are also determined. The model takes into account that the fs-laser writing process may induce transmission losses with a spectral dependence (¿(¿)) departing from the Rayleigh scattering scaling law ( ¿-4) and is valid both for single mode and multimode propagation. It has been tested in fs-laser written waveguides produced under non-optimal conditions in order to test the robustness of the method, providing excellent results.

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.