Ultrafast laser spatial beam shaping based on Zernike polynomials for surface processing.

In femtosecond laser machining, spatial beam shaping can be achieved with wavefront modulators. The wavefront modulator displays a pre-calculated phase mask that modulates the laser wavefront to generate a target intensity distribution in the processing plane. Due to the non-perfect optical response of wavefront modulators, the experimental distribution may significantly differ from the target, especially for continuous shapes. We propose an alternative phase mask calculation method that can be adapted to the phase modulator optical performance. From an adjustable number of Zernike polynomials according to this performance, a least square fitting algorithm numerically determines their coefficients to obtain the desired wavefront modulation. We illustrate the technique with an optically addressed liquid-crystal light valve to produce continuous intensity distributions matching a desired ablation profile, without the need of a wavefront sensor. The projection of the experimental laser distribution shows a 5% RMS error compared to the calculated one. Ablation of steel is achieved following user-defined micro-dimples and micro-grooves targets on mold surfaces. The profiles of the microgrooves and the injected polycarbonate closely match the target (RMS below 4%).

Large-mode-area infrared guiding in ultrafast laser written waveguides in sulfur-based chalcogenide glasses.

Current demands in astrophotonics impose advancing optical functions in infrared domains within embedded refractive index designs. We demonstrate concepts for large-mode-area guiding in ultrafast laser photowritten waveguides in bulk Sulfur-based chalcogenide glasses. If positive index contrasts are weak in As2S3, Ge doping increases the matrix rigidity and allows for high contrast (10(-3)) positive refractive index changes. Guiding with variable mode diameter and large-mode-area light transport is demonstrated up to 10 µm spectral domain using transverse slit-shaped and evanescently-coupled multicore traces.

Control of ultrafast laser-induced bulk nanogratings in fused silica via pulse time envelopes.

Employing a method of in-situ control we propose an approach for the optimization of self-arranged nanogratings in bulk fused silica under the action of ultrashort laser pulses with programmable time envelopes. A parametric study of the influence of the pulse duration and temporal form asymmetries is given. Using the diffraction properties of the laser-triggered subwavelength patterns we monitor and regulate the period and the quality of the periodic nanoscale arrangement via the effective nonlinear excitation dose. Periodicity tuning on tens of nanometers can be achieved by pulse temporal variations, with a minimum around 0.7 ps at the chosen powers. Equally, strong sensitivity to pulse asymmetries is observed. The driving factor is related to increasing carrier densities due to nonlinear confinement and the development of extended nanoroughness domains upon multiple exposure, creating a pulse-dependent effective accumulation dose via a morpho-dimensional effect. The result may impact the associated optical functions.

Single-pulse ultrafast laser imprinting of axial dot arrays in bulk glasses.

Ultrafast laser processing of bulk transparent materials can significantly gain flexibility when the number of machining spots is increased. We present a photoinscription regime in which an array of regular dots is generated before the region of main laser focus under single-pulse exposure in fused silica and borosilicate crown glass without any external spatial phase modulation. The specific position of the dots does not rely on nonlinear propagation effects but is mainly determined by beam truncation and is explained by a Fresnel propagation formalism taking into account beam apodization and linear wavefront distortions at the air/glass interface. The photoinscription regime is employed to generate a two-dimensional array of dots in fused silica. We show that an additional phase modulation renders flexible the pattern geometry.

Nanosize structural modifications with polarization functions in ultrafast laser irradiated bulk fused silica.

Laser-induced self-organization of regular nanoscale layered patterns in fused silica is investigated using spectroscopy and microscopy methods, revealing a high presence of stable broken oxygen bonds. Longitudinal traces are then generated by replicating static irradiation structures where the nanoscale modulation can cover partially or completely the photoinscribed traces. The resulting birefringence, the observed anisotropic light scattering properties, and the capacity to write and erase modulated patterns can be used in designing bulk polarization sensitive devices. Various laser-induced structures with optical properties combining guiding, scattering, and polarization sensitivity are reported. The attached polarization functions were evaluated as a function of the fill factor of the nanostructured domains. The polarization sensitivity allows particular light propagation and confinement properties in three dimensional structures.

Ultrafast laser photoinscription of polarization sensitive devices in bulk silica glass.

Ultrashort pulsed laser irradiation of bulk fused silica may result under specific energetic conditions in the self-organization of subwavelength material redistribution regions within the laser trace. The modulated structures have birefringent properties and show unusual anisotropic light scattering and reflection characteristics. We report here on the formation of waveguiding structures with remarkable polarization effects for infrared light. The photoinscription process using 800 nm femtosecond laser pulses is accompanied by third harmonic generation and polarization dependent anisotropic scattering of UV photons. The photowritten structures can be arranged in three-dimensional patterns generating complex propagation and polarization effects due to the anisotropic optical properties.

Dynamic ultrafast laser spatial tailoring for parallel micromachining of photonic devices in transparent materials.

Femtosecond laser processing of bulk transparent materials can generate localized positive changes of the refractive index. Thus, by translation of the laser spot, light-guiding structures are achievable in three dimensions. Increasing the number of laser processing spots can consequently reduce the machining effort. In this paper, we report on a procedure of dynamic ultrafast laser beam spatial tailoring for parallel photoinscription of photonic functions. Multispot operation is achieved by spatially modulating the wavefront of the beam with a time-evolutive periodical binary phase mask. The parallel longitudinal writing of multiple waveguides is demonstrated in fused silica. Using this technique, light dividers in three dimensions and wavelength-division demultiplexing (WDD) devices relying on evanescent wave coupling are demonstrated.

Ultrafast laser writing of homogeneous longitudinal waveguides in glasses using dynamic wavefront correction.

Laser writing of longitudinal waveguides in bulk transparent materials degrades with the focusing depth due to wavefront distortions generated at the air-dielectric interface. Using adaptive spatial tailoring of ultrashort laser pulses, we show that spherical aberrations can be dynamically compensated in optical glasses, in synchronization with the writing procedure. Aberration-free structures can thus be induced at different depths, showing higher flexibility for 3D processing. This enables optimal writing of homogeneous longitudinal waveguides over more significant lengths. The corrective process becomes increasingly important when laser energy has to be transported without losses at arbitrary depths, with the purpose of triggering mechanisms of positive refractive index change.

Analysis of the effects of spherical aberration on ultrafast laser-induced refractive index variation in glass.

We propose a comprehensive analysis of the effects that spherical aberration may have on the process of ultrafast laser photowriting in bulk transparent materials and discuss the consequences for the generated refractive index changes. Practical aspects for a longitudinal photowriting configuration are emphasized. Laser-induced index variation in BK7 optical glass and fused silica (a-SiO(2)) affected by spherical aberration are characterized experimentally using phase-contrast optical microscopy. Experimental data are matched by analytical equations describing light propagation through dielectric interfaces. Corrective solutions are proposed with a particular focus on the spatial resolution achievable and on the conditions to obtain homogeneously photo-induced waveguides in a longitudinal writing configuration.