Femtosecond laser pulse systems allows to modify in a precise and permanent way the optical properties of a transparent materials. This process enables the direct writing of guiding structures in materials, commonly known as waveguides, which are the base for optical circuit fabrication. It is our interest to study the main characteristics of the waveguides manufactured by the laser micromachining technique. Here, an analysis of the resulting refractive index profile has been carried out. This characteristic is essential for the design and simulation of integrated optical circuits. In particular we have developed our research on the study of light coupling in a pair of type II waveguides made in Lithium Niobate (LiNbO<sub>3</sub>). These experimental backgrounds provide us with elements to adjust and test the retrieved profile. Taking into account different distance between tracks and writing energies, it is well known that the coupling length changes and the coupling ratio too. Then this study allows us to reconstruct the refractive index profile according to its manufacturing conditions. Modeling of the refractive index distribution profile is a key parameter to perform beam propagation mode simulations (BPM) to achieve more realistic results. So, by means of this method it is possible to obtain a general procedure to describe the characteristics of these kinds of waveguides. As a model test, integrated waveguides were built to corroborate their light coupling. In a first stage it is designed through BPM simulations then it is manufactured in an X-cut LiNbO<sub>3</sub> crystal in order to check its operation according to the simulations carried out.
An optical fiber ring resonator (OFRR), a wavelength sensor for testing a single-mode laser system in a wide range of temperature, is presented. We will show that it is possible to calibrate, in relative form, a scale of wavelength, to determine, accurately enough, a thermally induced laser detuning, using the free spectral range of an OFRR. The optical circuit was constructed using 2 × 2 (50/50) optical fiber coupler obtaining an OFRR of 10-cm ring radius. A single-mode diode laser system has been launched into the OFRR, and different experiments have been performed. We tested the OFRR performance, considering fluctuations in the laser wavelength caused by small temperature instabilities, measuring the output intensity from the ring resonator. Theoretical simulations and experimental results were in agreement with the expected behavior. Furthermore, OFRR systems can be used as an excellent control tool to test the wavelength stability for a narrowband laser diode and act as a part of a control system.
In this work we have studied the fragmentation of gold nanoparticles (NPs) after generation by femtosecond laser
ablation of a solid target in deionized water. The fragmentation process was carried out using two different types of
radiation: direct ultra-fast pulses and super-continuum radiation focused in the colloidal solution. In the former case, IR
pulses were applied both in low and high fluences regime, while in the latter, super-continuum was generated by an
external sapphire crystal. In this last case, to assess the effects of the different spectral bands present in the super-continuum
for fragmentation, we have determined different efficiency regions. From the analysis of optical extinction
spectra and Transmission Electron Microscopy (TEM) histograms we can conclude that the main mechanism is linear
absorption in the visible region. Likewise, the super-continuum generated in water during fragmentation resulted more
efficient than that obtained externally by the sapphire crystal. This fact can be attributed to the broadening of the water
continuum band originated due to large intensity used for generation. TEM and Small Angle X-ray Scattering (SAXS)
measurements support the results found from optical extinction spectroscopy.
This work deals on the fabrication process of diffraction gratings made on Lithium Niobate substrates by means of focusing femtosecond laser pulses. The main optical features of these photonic structures are presented in this paper. As it was expected the relief gratings showed high diffraction efficiency in accordance to the index modulated profile of the crystal-air border resulting in this kind of diffractive structure. On the other hand, for the inside grating a higher diffraction efficiency for the first orders were found, however the overall diffraction efficiency was found to be similar to that obtained for the ablation structures made in this work; this result suggests that the index increment in the inside grooves should be very important. The thermal stability of these structures is also studied and discussed in this paper. It is found that for the relief gratings the diffraction efficiency is temperature independent up to 400°C degrees, while for the inside gratings a slight decrease on diffraction efficiency was observed after making a thermal annealing at 400°C during two hours. The grating fabrication method presented in this work can be a powerful tool for development of several photonic devices made inside/on Lithium Niobate by using femtosecond laser writing by means of the <i>one step </i>process.
Fabrication characterization and operation stability of Zn-indiffused integrated optical devices in lithium niobate are described. Two examples of the operation of active waveguides fabricated by this technique are presented: laser operation of Nd<sup>3+</sup> doped channel waveguides, and blue light generation by Quasi Phase Matching (QPM) using periodically structured substrates. In both cases n<sub>e</sub>-polarized high power denstiy CW-optical beams are involved and stable room temperature operation is sustained.