Jesus del Hoyo

Femtosecond laser written 16.5 mm long glass-waveguide amplifier and laser with 5.2 dB cm-1 internal gain at 1534 nm

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.

Controlling plasma distributions as driving forces for ion migration during fs laser writing

The properties of structures written inside dielectrics with high repetition rate femtosecond lasers are known to depend strongly on the complex interplay of a large number of writing parameters. Recently, ion migration within the laser-excited volume has been identified as a powerful mechanism for changing the local element distribution and producing efficient optical waveguides. In this work it is shown that the transient plasma distribution induced during laser irradiation is a reliable monitor for predicting the final refractive index distribution of the waveguide caused by ion migration. By performing in-situ plasma emission microscopy during the writing process inside a La-phosphate glass it is found that the long axis of the plasma distribution determines the axis of ion migration, being responsible for the local refractive index increase. This observation is also valid when strong positive or negative spherical aberration is induced, greatly deforming the focal volume and inverting the index profile. Even subtle changes in the writing conditions, such as an inversion of the writing direction (quill writing effect), show up in the form of a modified plasma distribution, which manifests as a modified index distribution. Finally, it is shown that the superior control over the waveguide properties employing the slit shaping technique is caused by the more confined plasma distribution produced. The underlying reasons for this unexpected result are discussed in terms of non-linear propagation and heat accumulation.

Control of waveguide properties by tuning femtosecond laser induced compositional changes

Local compositional changes induced by high repetition rate fs-laser irradiation can be used to produce high performance optical waveguides in phosphate-based glasses. The waveguide refractive index contrast is determined by the local concentration of La, which can be changed by the action of the writing laser pulses. In this work, we have investigated the degree of control that can be exerted using this waveguide writing mechanism over the cross-section of the guiding region, and the local refractive index and compositional changes induced. These variables can be smoothly controlled via processing parameters using the slit shaping technique with moderate Numerical Aperture (NA 0.68) writing optics. The combined use of X-ray microanalysis and near field refractive index profilometry evidences a neat linear correlation between local La content and refractive index increase over a broad Δn interval (>3 x 10^2). This result further confirms the feasibility of generating efficient, integrated optics elements via spatially selective modification of the glass composition.

Mirrorless Yb 3+ -Doped Monoclinic Double Tungstate Waveguide Laser Combining Liquid Phase Epitaxy and Multiplexed Beam fs Laser Writing

In this paper, a mirrorless Yb³⁺-doped KY_1-x-yGd_xLu_y(WO₄)₂ waveguide laser with cw operation at 981.5 and 1001 nm, fabricated by liquid phase epitaxy and femtosecond laser microstructuring, is reported. A planar waveguide was fabricated by growing an Yb³⁺-doped KY_1-x-yGd_xLu_y(WO₄)₂ epitaxial layer by liquid phase epitaxy over a KY(WO₄)₂ substrate. This planar waveguide was then microstructured by means of a multiplexed beam femtosecond laser writing technique in order to define ridge waveguides. Mirrorless laser action is demonstrated in ridge waveguides with different fabrication parameters, obtaining maximum slope efficiency of 78% versus absorbed power, which confirms the feasibility of this technique for the development of integrated laser devices.

Strong ion migration in high refractive index contrast waveguides formed by femtosecond laser pulses in phosphate glass

Strong ion migration in shown to enable the production of high refractive index contrast waveguides by fs-laser writing in a commercial (Er,Yb)-doped phosphate based glass. Waveguide writing was performed using a high repetition rate fslaser fibre amplifier operated at 500 kHz and the slit shaping technique. Based on measurements of the NA of waveguides, the positive refractive index change (Δn) of the guiding region has been estimated to be ∼1-2 x10-2. The compositional maps of the waveguides cross-sections performed by X-ray microanalysis evidenced a large increase of the La local concentration in the guiding region up to ~25% (relative to the non-irradiated material). This large enrichment in La was accompanied by the cross migration of K to a neighbouring low refractive index zone. The refractive index of the La-phosphate glass increases linearly with the La2O3 content (Δn per mole fraction increase of La2O3 ≈ 5x10-3) mainly because of the relative mass of the La3+ ions. The density increase without substantial modification of the glass network was confirmed by space-resolved micro-Raman spectroscopy measurements showing minor variations in the (PO2)sym vibration Raman band. These results provide evidence for the feasibility of adapting the glass composition for enabling laser-writing of high refractive index contrast structures via spatially selective modification of the glass composition.