A inteligibilidade da palavra em igrejas católicas, através de análises de carácter objectivo e subjectivo
Lencastre, Margarida Maria Mendes de Freitas de Queiroz e
1988-01-01
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The purpose of this thesis is the fabrication and characterisation of sub-micron period (~500nm) Bragg relief grating reflectors in optical waveguide structures using interferometric excimer laser ablation. Furthermore, the work presented in this thesis describes investigation of the feasibility of the application of excimer laser grating micromachining for the fabrication of waveguide gratings which may be used as wavelength filters in the 1.5µm band for optical communications applications. For this purpose the following studies were carried out: (i) the ablation of sub-micron relief gratings on Tl+ ion-exchanged channel waveguides in Er/Yb-codoped B1664 glass, using 193nm excimer laser radiation and (ii) the ablation of sub-micron relief gratings in InOx and Ta2O5 thin oxide films overlaid on K+ ion-exchanged channel waveguides in BK...
Interferometric ablation and structural modification of materials using pulsed UV lasers is a powerful technique for the straightforward fabrication of relief structures in "hard" optical materials such as glasses and thin polycrystalline films. In this approach, a high intensity periodic UV radiation pattern, generated using two- or multi-beam interference, directly ablates the exposed material or induces structural modification in the irradiated volume. In the case of volume structural modification, a further step of development by selective chemical etching may employed to reveal the periodic relief pattern. The selectivity between ablation and volume structural modification is controlled by adjusting the exposure energy density and the number of pulses according to the specific material. Selected results in direct and chemically-as...
Relief Bragg gratings were imprinted by 248 nm interferometric excimer laser ablation on potassium ion-exchanged channel waveguides in BK-7 glass overlaid with a thin high-index InOx film. Using five pulses of energy density 60 mJ/cm2, a spectral transmittance notch of depth 66% and FWHM<0.1 nm was obtained at 1547 nm in the TE polarization for a waveguide having a nominal width of 8 µm and a 135-nm-thick InOx overlayer. In waveguides coated with 100 nm InOx, with widths increasing from 3 to 8 µm, the reflection wavelength shifted by 0.12 nm/µm and the reflectivity increased monotonically.
Relief gratings of sub-micron periodicity are patterned in InOx thin oxide films using 248nm interferometric excimer laser ablation. The ablation process is studied in terms of grating depth versus exposure conditions, using optical diffraction efficiency measurements. Real time monitoring of grating growth and film resistivity during grating writing are also presented. To study the exact grating morphology, SEM and AFM microscans of the machined structures are performed. A simple ablation model for InOx thin films is proposed in accordance with the experimental data obtained. Relief gratings are patterned in InOx thin films overlaid on ion-exchanged channel waveguides and reflection spectra are also reported.
Relief Bragg grating reflectors inscribed on channel waveguides may be used in optical communications as add-drop wavelength multiplexers, gain-flattening filters, distributed feedback laser mirrors, or in sensing technology as high sensitivity devices for precise monitoring of chemical or biomedical processes. We present strong Bragg grating reflectors in Ta2O5 thin oxide films overlaid on potassium ion-exchanged channel waveguides in BK-7 glass, inscribed using 248nm excimer laser holographic ablation. The grating pattern was created employing two-beam interference using a modified Mach-Zehnder interferometric cavity and the output of a narrow-lined injection cavity 248nm excimer laser. The experimental data presented are divided into two sections: the first section refers to the study of the grating ablation process of thin Ta2O5 fi...
Er-doped phosphate glasses are excellent host materials for the fabrication of high-gain waveguide and fibre lasers and amplifiers emitting in the 1.55μm telecommunication window. The photosensitivity of those glasses under intense UV radiation may be of great significance, since diffraction gratings and waveguide structures may be inscribed in such glasses, leading to the development of functional photonic devices. The photosensitivity of a commercial phosphate glass (Schott IOG-1), which is doped with Er3+ ions and has been ion-exchanged with Ag+ ions, is examined using nanosecond 248nm excimer laser radiation. Grating structures of 510nm period are inscribed in un-treated and ion-exchanged Er-doped IOG-1 glass by employing phase mask interference in contact mode. The dependence of UV-induced refractive index changes upon the Ag+ con...
Thin polycrystalline films of Ta2O5 having high chemical resistance and large dielectric permittivity may be deposited by sputtering and have many applications in microelectronics and optoelectronics. A high-resolution and low-damage method for patterning relief structures in thin Ta2O5 films by chemically assisted UV laser selective etching is presented. The method is based in the initial exposure of the Ta2O5 films to pulsed UV radiation (quadrupled Nd:YAG laser at 266nm) at fluences below the ablation threshold, for the creation of volume damage in the exposed areas. Subsequent immersion of the exposed sample in a KOH solution results in selective etching of the UV-exposed areas, developing relief structures of high quality. Interferometric exposure was used for the patterning of such gratings with periods shorter than 500nm in film...
Control of reflectivity of a relief grating in a high-index film overlaid on a monomode glass waveguide was achieved by adjustment of the superstrate refractive index with liquids. Weak gratings reflecting at 1531 nm were inscribed by UV laser ablation. The grating reflectivity was measured as the ratio of the transmission outside the reflection band, at 1536 nm, to that at the Bragg wavelength. Reflectivity with an air super-strate was <0.3 dB for both polarizations. In the TE polarization the grating strength increased to 20.5 dB after application of a liquid of index 1.45. In the TM polarization the strength increased to 27 dB with a superstrate index of 1.50. Good agreement was found with a theoretical model based on beam propagation.
A high-resolution and low-damage method for patterning relief structures in thin Ta2O5 films by chemically assisted UV laser selective etching is presented. The method is based on the initial exposure of the Ta2O5 films to pulsed UV radiation (quadrupled Nd:YAG laser at 266 nm) at fluences below the ablation threshold, for the creation of volume damage in the exposed areas. Subsequent immersion of the exposed sample in a KOH solution results in selective etching of the UV-exposed areas, developing relief structures of high quality. Interferometric exposure was used for the patterning of such gratings with periods of the order of 500 nm in films with a thickness of 100 and 500 nm. The behaviour of the patterning process is studied using diffraction efficiency measurements and AFM scans. Diffraction efficiency increases by a factor of ~6...
Er-doped phosphate glasses are excellent host materials for the fabrication of high-gain waveguide and fibre lasers and amplifiers emitting in the 1.55μm telecommunication window. The photosensitivity of those glasses under intense UV radiation may be of great significance, since diffraction gratings and waveguide structures may be inscribed in such glasses, leading to the development of functional photonic devices. The photosensitivity of a commercial phosphate glass (Schott IOG-1), which is doped with Er3+ ions and has been ion-exchanged with Ag+ ions, is examined using nanosecond 248nm excimer laser radiation. Grating structures of 510nm period are inscribed in un-treated and ion-exchanged Er-doped IOG-1 glass by employing phase mask interference in contact mode. The dependence of UV-induced refractive index changes upon the Ag+ con...
