N.A. Vainos

Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating

A long-period fiber grating (LPFG) humidity sensor is reported utilizing poly(ethylene oxide)/cobalt chloride (PEO/CoCl2) as a hybrid hygrosensitive cladding coating. A thin overlay of the material is deposited on the LPFG and with exposure to different ambient humidity levels, its spectral properties are modified. The material parameters associated with the sensing mechanism may include those of refractive index, absorption, and morphological alterations of the overlaid material. Relative humidity variations in the range from 50% to 95% have been detected with a resolution better than 0.2%. The response time constant of the fiber sensor is of the order of a few hundred milliseconds.

TI:SAPPHIRE PLANAR WAVEGUIDE LASER GROWN BY PULSED LASER DEPOSITION

We document the lasing performance of a waveguiding layer of Ti:sapphire, of ~12-mum thickness, grown by pulsed laser deposition from a 0.12-wt.% Ti(2)O(3) Ti:sapphire single-crystal target onto an undoped z-cut sapphire substrate. Lasing around 800 nm is observed when the waveguide layer is pumped by an argon-ion laser running on all-blue-green lines, with an absorbed power threshold of 0.56 W, with high-reflectivity (R>98%) mirrors. With a 5% pump duty cycle and a T=35% output coupler, a slope efficiency of 26% with respect to absorbed power is obtained, giving quasi-cw output powers in excess of 350 mW.

Modern technology in artwork conservation : a laser-based approach for process control and evaluation

The present work includes a laser-based methodology for the cleaning of artworks, with emphasis on the preservation of their structural integrity and identity. Modern laser-based techniques and instrumentation offer new tools in the field of artwork and antiquities conservation, aiming to alleviate the traditionally applied methods from existing weaknesses. Although in several cases the use of lasers may give rise to superior results, there are still problems to be resolved in relation to the optimization of procedures for safeguarding from potential damage. Furthermore, several operational parameters have to be simultaneously controlled and the long-term effects induced by laser irradiation must be assessed in detail before a full exploitation of the new methods is established. The control of material removal during laser cleaning is achieved by using laser-induced breakdown spectroscopy (LIBS). This control relies on the collection of spectroscopic data by LIBS, which correspond to the in-depth compositional profile of the artifact. This technique may be combined with structural analysis by holographic interferometry. The latter is important for assessing structural changes, which may be induced during laser ablation. Selected examples of this type of applications in a carefully considered methodology are presented.

Microfabrication by UV femtosecond laser ablation of Pt, Cr and indium oxide thin films

We demonstrate the direct deposition of Pt, Cr and In2O3 microstructures on glass using a femtosecond laser assisted technique. A metal (Pt, Cr) or oxide (In2O3) source film is first deposited on an optically transparent quartz carrier and is brought in intimate contact with a receiver glass substrate using an especially designed vacuum cell. An ultrashort excimer laser pulse ablates the source film at the quartz/film interface and results in the forward-transfer deposition of material onto the nearby glass receiver. The morphology of the ablated and transferred features was studied by means of scanning electron and atomic force microscopies. It was found that the good adhesion of the pre-deposited source film on the quartz substrate and the intimate contact between the source and receiver glass are two critical factors for achieving efficient transfer printing. The optimal deposited morphology in terms of spatial resolution and dispersion was produced using 30–40 nm and 50–60 nm thick source films of metals and In2O3 respectively. In addition, the laser fluence had to be just above the threshold for printing (Epr). This was 150±20 mJ/cm2 for Pt, Cr and 60±20 mJ/cm2 for the In2O3. Fluences greater than Epr lead to the development of crater like features with excessive spread on the periphery rim. Similar behaviour was observed for micro-prints obtained using a backward-transfer configuration. Sub-micron Pt dots were obtained from a 30 nm thick Pt source film, irradiated with a 3 μ×3 μm spot at a fluence of 150 mJ/cm2. The production of these sub-micron dots was possible due to limited thermal diffusion and sharp ablation threshold existent in fs laser processing.

Microdeposition of metals by femtosecond excimer laser

Abstract Microablation and transfer of thin metal films using ultrashort, ultraviolet laser radiation has been studied. A KrF excimer laser (λ=248 nm) having 500-fs pulse duration is coupled to a high-power image projection micromachining workstation. The laser irradiation is focused onto thin Cr films through the supporting transparent quartz substrates. Single pulses are used to completely remove the metal film. The ablated material is transferred onto a receiving target glass substrate placed parallel to the source film. Experiments were conducted in a miniature vacuum cell under a pressure of 10−1 Torr. The distance between the source and target surfaces is variable from near-contact to several hundreds of microns. Serial writing of well-defined metal lines and isolated dots, is accomplished using the x–y sample micropositioning system. Optical microscopy and surface profilometry showed deposition of highly reproducible and well-adhering features of a few microns in width for a source–target distance in the neighborhood of 10 μm. The short pulse length limits thermal diffusion, thereby enabling superior definition of the deposited features. Metal patterns were also directly deposited via a parallel-mode mask projection scheme. In a first demonstration of this method, deposited diffractive structures were shown to produce high-quality computer-generated holograms.

Performance of a low loss pulsed laser deposited Nd:Gd3Ga5O12 waveguide laser at 1.06µm and 0.94µm

We report the laser performance of a low-propagation-loss neodymium-doped Gd(3)Ga(5)O(12) (Nd:GGG) waveguide fabricated by pulsed-laser deposition. An 8- mum -thick crystalline Nd:GGG film grown upon an undoped Y(3)Al(5)O(12) substrate lases at 1.060 and 1.062 microm when pumped by a Ti:sapphire laser operating at 740 or 808nm.Using a 2.2% output coupler, we observed a 1060-nm laser threshold of 4mW and a slope efficiency of 20%. Laser action was also achieved, for what we believe is the first time in Nd:GGG, on the quasi-three-level 937-nm transition. With a 2% output coupler at this wavelength a laser threshold of 17mW and a 20% slope efficiency were obtained. This demonstration of low propagation loss combined with the fact that these waveguides have a very high numerical aperture (0.75) makes pulsed-laser-deposited thin films attractive for high-power diode-pumped devices.

Etching and printing of diffractive optical microstructures by a femtosecond excimer laser

Diffractive optics fabrication is performed by two complementary processing methods that rely on the photoablation of materials by ultrashort UV laser pulses. The spatially selective ablation of materials permits the direct microetching of high-quality surface-relief patterns. In addition, the direct, spatially selective transfer of the ablated material onto planar and nonplanar receiving substrates provides a complementary microprinting operation. The radiation from the ultrashort pulsed excimer laser results in superior quality at relatively low-energy density levels, owing to the short absorption length and minimal thermal-diffusion effects. Computer-generated holographic structures are produced by both modes of operation. Submicrometer features, including Bragg-type structures, are microprinted onto planar and high-curvature optical-fiber surfaces, demonstrating the unique ability of the schemes for complex microstructure and potentially nanostructure development.

Growth of Nd:potassium gadolinium tungstate thin-film waveguides by pulsed laser deposition

Thin Nd-doped potassium gadolinium tungstate [KGW or KGd(WO4)2] films are grown by pulsed laser deposition by ablation of a stoichiometric monocrystal target. Rutherford backscattering, x-ray diffraction, atomic force microscopy, and waveguide propagation analyses are performed. The as-grown films are optically active, as evidenced by the photoluminescence spectra centered at 1.068 μm. In some of the films, fine photoluminescence spectra between Stark levels are observed.

Characterisation of GaLaS chalcogenide glass thin-film optical waveguides, fabricated by pulsed laser deposition

Abstract The fabrication of stoichiometric thin-film optical waveguides of GaLaS via a pulsed laser deposition technique is reported. Stoichiometric films are grown by ablating GaLaS bulk glass with a KrF excimer laser (λ = 248 nm) at an incident laser flux ⩾ 3.5 J/cm 2 . The composition of the films is determined by energy-dispersive X-ray analysis and the refractive index is measured by a dark-mode prism coupling technique. Photoinduced structural rearrangement of the as-deposited films leads to a blueshift in the visible absorption edge and a permanent refractive index change, Δ n , of −1%. On the basis of these results, grating structures have been written with both blue light, and e-beam addressing, and their suitability for integrated optical structures assessed.

Photosensitivity of lead germanate glass waveguides grown by pulsed laser deposition.

We report very large photoinduced refractive-index changes Dn, of the order of ~10(2), in lead germanate glass waveguides grown by pulsed-laser deposition. The magnitude of Dn was derived from measurements of diffraction efficiency for gratings written by exposure to 244-nm light through a phase mask, whereas the sign of Dn was determined from ellipsometric data. Results are shown for films grown under oxygen pressures ranging from 1 chi 10(-2) to 6 chi 10(-2)mbars (1.33mbars=1 Torr).