Femtosecond laser processing of bulk transparent materials can generate localized increase of the refractive
index. Thus, translation of the laser spot give potential access to three dimensionnal photowriting of waveguiding
structures. Increasing the number of machining foci can considerably reduce the processing efforts when complex
photonic structures are envisaged such as waveguide arrays. The present report presents a technique of dynamic
ultrafast laser beam spatial tailoring for parallel writing of photonic devices. The wavefront of the beam is
modulated by a periodical binary (0-π) phase mask of variable pattern to achieve dynamic multispot operation.
The parallel photoinscription of multiple waveguides is demonstrated in fused silica. Using this method, light
dividers in three dimensions relying on evanescent coupling are reported and wavelength-division demultiplexing
(WDD) devices were achieved in single sample scan.
Ultrafast lasers emerged as promising tools to process refractive index changes in band-gap materials, resulting in
waveguiding functions. Positive refractive index changes were often reported in fused silica matrices. However, in
glasses characterized by slow electronic relaxation and high thermal expansion, the refractive index change is usually
negative, detrimental for waveguide writing. This relates to the formation of hot regions, where, due to thermal
expansion, material is quenched in low-density phases. We discuss control mechanisms related to spatio-temporal heat-source
design which may be tuned by temporally shaped laser radiation. Programmable temporal tailoring of pulse
envelopes triggers transitions from thermal expansion to directional inelastic flow. Consequently, material compaction
leads to a positive refractive index change and guiding structures may thus be created. From an application perspective,
the structuring quality degrades with the focusing depth due to wavefront distortions generated at the air-dielectric
interface inducing spatial energy dispersion. Spatial beam tailoring corrects beam propagation distortion, improving the
structuring accuracy. The corrective process is becoming important when laser energy has to be transported without
losses at arbitrary depths, with the purpose of triggering mechanisms of positive index change.
We demonstrate operation of a simple and reliable water-cooled femtosecond laser running at 10 kHz
suitable for industrial micromachining applications. A laser geometry involving only a regenerative
amplifier and delivering 3.5 W average power 60-fs pulses is compared to a more conventional
architecture using an additional multi-pass amplifier. Both laser systems require a moderate pumping
laser of ~30 W average power and deliver high-quality beams (M2<1.2).
PACS : 42.55-f ; 42.60 v; 42.60 Rn
We present programmable focal spot shaping of amplified femtosecond laser pulses by use of an optically
addressed non-pixellated liquid crystal light valve. This extra-cavity phase filtering method is set-up in the frame of
femtosecond micromachining processes. Various focal spot shapes are demonstrated together with drilling and
marking results in metals and dielectrics. Photowriting of waveguides in bulk fused silica is also presented.
Due to a large material removal rate and a minimal collateral damage, subpicosecond laser pulse offers many advantages for material processing. Post-experimental examination of ultrashort laser material ablation shows that the heated surface of the bulk does not exhibit thermal damage. A lot of complicated physical processes follow intense laser irradiation and have often been modeled in over-simplified way to explain experimental results. Because a more complete understanding of the damage mechanisms would be very interesting and would open new applications in the scientific research community and industry. A detailed model of the ultrashort response to reproduce ablation process is presented, describing dynamical electronic properties such as temperature, pressure and energy. To simulate the interaction between the laser and the metallic target, these theoretical models are inserted inside a 1D Lagrangian hydrodynamic code. It is still a very challenging task to disentangle the cumulative/competitive effects of all physical processes if included at once in numerical simulations. Adding the different processes into numerical simulations on a one by one basis allow to assess their contributions to the quantity of ejected matter on a large range of laser intensity. Experimental results related to the influence of multi-pulses or tailored pulses for ablation efficiency with ultrafast pulses(pulse duration up to 5 ps) has been obtained. A significant improvement of the micro structuring quality in metals is demonstrated, and the theoretical approach presented allow to manage the optimal temporal shape of pulses. An efficient process control can be reached and the industrial applications will be evidenced.
A theoretical time-dependent analysis of a high-average power copper HyBrID laser is proposed, pointing out the time-varying properties of beam quality parameters and brightness. Numerical data are compared to experimental measurements performed on a 80 W average power copper HyBrID laser. A significant improvement of the beam quality with time is put in evidence.
We propose a versatile set-up dedicated to programmable beam shaping of femtosecond pulses in a focal plane. A non-pixelated liquid-crystal light valve is used as the phase-front modulator. We demonstrate active and adaptive wavefront correction of a 4-μJ, 100-kHz amplified laser chain, where residual wavefront distorsions are decreased down to λ/15 peak-valley and λ/100 rms. The subsequent improvement for micromachining applications is investigated, and diffraction-limited holes are demonstrated on various materials. Moreover, beam patterning in the focal plane is also presented. Theoretical calculations of the required phase modulation are proposed, and experimental shapes are demonstrated, like square and circular top-hats, as well as triangle or doughnut shapes.
We propose an active and adaptative optics device dedicated to programmable femtosecond beam shaping, based on the use of an optically addressed light valve. A theoretical investigation of the system is presented. The experimental set-up incorporating an active beam shaping device, is depicted. Results are then described and discussed.
Due to the rapid development of ultrashort lasers, quality of the machining is of prime interest for several applications. For instance deep marking of various materials. In this case, the depth can be controlled, knowing the ablation rate for the corresponding material. The evolution of ablation rates of Al, Cu and Ni are given in relation to the energy density. In metals the effect of thermal diffusion has to be taken into account to control collateral effects and especially the heat affected zone.
Several works on laser-matter interaction has shown the differences in sizes for the Heat Affected Zone (HAZ) obtained with nanosecond and femtosecond regimes in laser cutting or drilling. To understand more clearly the basic phenomena that occur in femtosecond regime during the absorption of light by matter, and specially in the case of metals, we have developed both an experimental and a theoretical approach. We use a new method aimed at quantifying the dimensions of the HAZ, using thin-down samples which are micro-drilled and then observed by a transmission electronic microscopy (TEM) technique. The grain size in the samples is analysed near the micro-holes. According to theoretical studies, the thermal diffusion is due to the smaller value of the electron specific heat compared to the lattice one. The thermal diffusion length is found to be a few hundred of nanometers in the case of metals. We use a thermal model to describe the heat diffusion in the sample in order to obtain a theoretical estimation of the HAZ. Holes are drilled in Aluminum using nanosecond and femtosecond laser pulses and characterized by Transmission Electronic Microscopy (TEM). The method for quantifying the dimensions of the heat affected zone (HAZ) surrounding micro-holes is based on the analyze of the grain size evolution. The experiments are using the same Ti-Sapphire laser source (1 kHz, 800 nm). The regeneratively amplified ultra-short pulses (150 fs) are utilized at a low fluence regime (typically 0.01-0.5 mJ/pulse), while the longer pulses (ns) are obtained from the regenerative amplifier without oscillator seeding (0.5 mJ,τ approximately 7-8 ns). The main conclusion is that a 40 micrometers wide HAZ is induced by nanosecond pulses, whereas the femtosecond regime does not produce any TEM observable HAZ. It has to be noticed that the width of the femtosecond HAZ is roughly less than 2 micrometers , which is our observation limit. These results are in agreement with theoretical predictions.
Holes drilled in Aluminum using nanosecond and femtosecond laser pluses are characterized by Transmission Electronic Microscopy (TEM). Hence we present a method for quantifying the dimensions of the heat affected zone (HAZ) surrounding micro-holes by analyzing the grain size evolution. Drilled samples investigations are performed after electrolytic thinning down to 100 nm. The experiments require a real time imaging system to shot close to the located thinner zone with an accuracy in the micrometers range. Thin Al samples are drilled both in nanosecond and femtosecond regimes using het same pulses number and the same Ti-Sapphire laser source. The regeneratively amplified ultra-short pulses are utilized at a low fluence regime, while the longer pulses are obtained from the regenerative amplifier without oscillator seeding. The main conclusion is that a 40 micrometers wide HAZ is induced by nanosecond pulses, whereas the femtosecond regime does not produce any TEM observable HAZ. It has to be noticed that the width of the femtosecond HAZ is roughly less than 2 micrometers , which is our observation limit.
It is now well known that rhodium doped barium titanate (BaTiO3:Rh) exhibits a significant photorefractive response at near infrared wavelengths .We studied and characterized this crystal at 1 .06 jtm by two-wave mixing experiments. In a 45°-cut crystal with a low absorption (0.1 cm1), we measured a photorefractive gain F of 23 cm1 with cw illumination and 16.6 cm1 with nanosecond illumination. Using spectroscopic determinations of the photorefractive sites 2(Rh3+, Rh4+, RhS+), we demonstrated that the photorefractive properties of BaTiO3:Rh are well described by a three charge state model '. Internal parameters of the material were derived using these experimental characterizations which allowed to accurately predict its performances at 1 .06 tm. Comparative characterizations of several BaTiO3 :Rh samples proved that this material is now well reproducible. Reproducibility, high photorefractive gain, low absorption and accurate theoretical description make BaTiO3:Rh a good candidate for realization and optimization of non linear functions like optical phase conjugation. The application we are interested in, is the dynamic wavefront correction of nanosecond Nd:YAG master-oscillator power amplifier (MOPA) laser sources. We implemented a ring self-pumped phase conjugate mirror using a BaTiO3:Rh crystal . This geometry brings several advantages. The threshold in terms of "gain X interaction length" product, is low (F1=2) 6 This phase conjugate mirror does not require a source of long coherence length 7and the gratings involved in the four wave mixing process can be well controled. Moreover the phase conjugate beam can be efficiently selected among the backscattered light by inserting optical elements in the ring 8,9• Such a self-pumped mirror is self-starting and the four-wave mixing process is initiated by the beam-fanning. To avoid spurious internal oscillations in total reflection on the crystal faces that initially developed in several of our experiments and prevented phase conjugation we optimized the geometry of the crystal. Roof-cut, 45° orientation of the c axis and antireflection coatings of the crystal suppressed these internal oscillations and efficient phase conjugation was demonstrated.
Photorefractive rhodium doped barium titanate (BaTiO3:Rh) is now well known for its significant response at near infrared wavelengths .We studied and characterized this crystal at 1.06 j.tm. By twowave mixing experiments in a 45°-cut crystal, we measured a maximum photorefractive gain F of 23 cm1 with cw illumination and 16.6 cm1 with nanosecond illumination, together with a low absorption (0. 1 cm1). Using spectroscopic determinations of the photorefractive sites 2 (Rh3, Rh4, Rh5), we showed that the photorefractive properties of BaTiO3 Rh could be well described by a three charge state model .This enabled to determine the internal parameters of the material using experimental characterizations and to accurately predict its performances at 1 .06 tm. Comparative characterizations of several BaTiO3:Rh samples proved that this material is now well reproducible, which is of prime importance for applications. Reproducibility, high photorefractive gain, low absorption and accurate theoretical description make BaTiO3:Rh a good candidate for realization and optimization of non linear functions like optical phase conjugation. The application we are interested in, is the dynamic wavefront correction of nanosecond Nd:YAG master-oscillator power-amplifier (MOPA) laser sources.
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