We report on the statistical assessment of the properties of directional couplers based on silicon waveguides, growth by IMEC fab in Belgium in the framework of the Europractice partnership. We characterized 25 chips from a multi-project wafer, each one containing several passive add-drop ring resonators with different coupling strengths. The analysis was repeated for chip temperatures ranging from 25°C to 55°C. The measurements we performed confirmed the reliability of the iSiPP50G platform used to growth the considered components.
Using Synopsys® Software Suite, we developed a Process Design Kit (PDK) targeting the commercial software OptSim Circuit® for the design of integrated photonics devices on transparent materials. We combined the physical properties and optical parameters of the material from laser direct writing (LDW) experimental data in the literature with theoretical model to carry out numerical Finite Differences Time Domain and Beam Propagation Method simulations. The final PDK includes the main building blocks library, namely straight and bent waveguides, directional coupler, splitter, and mode-converter, which give the opportunity to easily design and simulate more complete layouts, simplifying the creation of complex photonic integrated circuits on transparent materials.
In this work, we report on the fabrication of laser induced periodic surface structures (LIPSS) on stainless steel, using bursts of 200 fs sub-pulses at a wavelength of 1030 nm. A cascade of birefringent crystals was used to generate the bursts with tunable number of sub-pulses and intra-burst delays varying between 1.5 ps and 24 ps. Being such a delay shorter than the typical electron-lattice relaxation time in metals, the sub-pulses impinge on the sample surface when the material is still in a transient state after excitation from the first sub-pulse, thus allowing peculiar structures to be generated depending on the burst features. We obtained 1-D and 2-D periodic surface structures and investigated the influence of number of sub-pulses and polarization on their morphology. In particular, when bursts composed by all-aligned linearly polarized sub-pulses were used, 1-D LIPSS were obtained with different periodicity and depths depending on the number of sub-pulses. Bursts with crossed linear polarization or circular polarization sub-pulses produced 2-D LIPSS with morphology varying from triangular structures arranged in hexagonal lattice to pillar-like ordered or disordered structures depending on the bursts features. In most cases these structures exhibit a superhydrophobic behavior, as assessed by static contact angle measurements, which is achieved after a time of exposition to laboratory air. By XPS analysis we investigated the chemical variations occurring on the surfaces over this time.
We report on an experimental study of the incubation effect during irradiation of stainless steel targets with bursts of femtosecond laser pulses at 1030 nm wavelength and 100 kHz repetition rate. The bursts were generated by splitting the pristine 650-fs laser pulses using an array of birefringent crystals which provided time separations between sub-pulses in the range from 1.5 ps to 24 ps. We measured the threshold fluence in Burst Mode, finding that it strongly depends on the bursts features. The comparison with Normal Pulse Mode revealed that the existing models introduced to explain the incubation effect during irradiation with trains of undivided pulses has to be adapted to describe incubation during Burst Mode processing. In fact, those models assume that the threshold fluence has a unique value for each number of impinging pulses in NPM, while in case of BM we observed different values of threshold fluence for fixed amount of sub-pulses but different pulse splitting. Therefore, the incubation factor coefficient depends on the burst features. It was found that incubation effect is higher in BM than NPM and that it increases with the number of sub-pulses and for shorter time delays within the burst. Two-Temperature-Model simulations in case of single pulses and bursts of up to 4 sub-pulses were performed to understand the experimental results.
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