Manuel Abreu

EELT-HIRES the high-resolution spectrograph for the E-ELT

The first generation of E-ELT instruments will include an optic-infrared High Resolution Spectrograph, conventionally indicated as EELT-HIRES, which will be capable of providing unique breakthroughs in the fields of exoplanets, star and planet formation, physics and evolution of stars and galaxies, cosmology and fundamental physics. A 2-year long phase A study for EELT-HIRES has just started and will be performed by a consortium composed of institutes and organisations from Brazil, Chile, Denmark, France, Germany, Italy, Poland, Portugal, Spain, Sweden, Switzerland and United Kingdom. In this paper we describe the science goals and the preliminary technical concept for EELT-HIRES which will be developed during the phase A, as well as its planned development and consortium organisation during the study.

3D Finite Element Model for Writing Long-Period Fiber Gratings by CO2 Laser Radiation

In the last years, mid-infrared radiation emitted by CO2 lasers has become increasing popular as a tool in the development of long-period fiber gratings. However, although the development and characterization of the resulting sensing devices have progressed quickly, further research is still necessary to consolidate functional models, especially regarding the interaction between laser radiation and the fibers material. In this paper, a 3D finite element model is presented to simulate the interaction between laser radiation and an optical fiber and to determine the resulting refractive index change. Dependence with temperature of the main parameters of the optical fiber materials (with special focus on the absorption of incident laser radiation) is considered, as well as convection and radiation losses. Thermal and residual stress analyses are made for a standard single mode fiber, and experimental results are presented.

Modeling CO2 laser radiation transmission lap welding of thermoplastic films: energy balance approximation

The characteristics of thermoplastic films, like low conductivity and high transparency, allow simplifying the prediction of the necessary engineering parameters in CO2 laser lap welding by the application of an energy balance approximation. The energy density evaluated by computation is experimentally applied to the welding of transparent high- and low-density polyethylene and polypropylene samples, with thicknesses between 10 and 100 µm. The comparison between prediction and experimental results is also made with an existing thermodynamic model. The latter is developed in accordance with the characteristics of transmission lap welding of the thermoplastics films considered in this work. Results show, for circa 80% of the analyses, a deviation lower than 20% between experimental and predicted values, highlighting the relevance of the energy balance approximation. Also, the similarity between predicted and experimental values confirms the main assumptions made in the development of the thermodynamic model. However, the action of thermal forces in the weld pool prove to be an important factor, restricting the minimum laser beam diameter over the sample. This limits one of the parameters to be evaluated by the energy balance approximation, but from an engineering point of view, its relevance and precision in the process modeling is preserved.

Dual-frequency sweeping interferometry for absolute metrology of long distances

Frequency sweeping interferometry (FSI) is a technique where absolute distance measurements are made without ambiguity, using synthetic wavelengths resulting from frequency sweeps. Accuracy is mainly dependent on the synthetic wavelength measurement using a Fabry-Perot interferometer to count resonances as frequency sweeps. Measurement uncertainty increases with distance due to propagation of uncertainty in the synthetic wavelength measurement. For large distances, the number of fringes dominates performance, leading to a linear decrease of accuracy with range. To overcome this problem, the dual FSI concept was introduced, where the measurement process for large distances is reduced to the close-range case, by limiting the interferometer optical path difference. This is achieved by increasing the reference arm with a long reference fiber, and using an ancillary interferometer to calibrate the fiber length continuously. This dual FSI concept was implemented and fully tested in view of ESA-PROBA3 space mission. The sensor is composed of an external cavity diode laser, a high-finesse Fabry-Perot interferometer, and a dual measurement system. Accuracies better than 32 µm for a measurement range from 51 to 61 m were achieved using a reference fiber with 71 m, while maintaining the reduced complexity inherent to FSI technique, which is mandatory for space applications.

Dual frequency sweeping interferometry with range-invariant accuracy for absolute distance metrology

Coherent absolute distance interferometry is one of the most interesting techniques for length metrology. In frequency sweeping interferometry (FSI), measurements are made without ambiguity, by using a synthetic wavelengths resulting from a frequency sweep. FSI-based sensors are simple devices and fulfill an important role on any demanding space mission metrological chain. Their parameterization flexibility allows either technological or application-related tradeoffs to be performed. Accuracy is mainly dependent on the capability to measure the synthetic wavelength, using a Fabry-Perot interferometer (FP) that counts resonances as the frequency sweeps, and on the number of detected synthetic fringes. For large ranges, the number of fringes dominates performances, leading to a linear decrease of the accuracy with range. By increasing the size of the interferometer reference arm, and by measuring both the distance and the reference arm independently, it is possible to ensure high accuracy for small distance measurements, even for much larger range. In the context of the ESA PROBA3 mission (coronagraph and demonstration of metrology for free-flying formation), we are prototyping a FSI sensor composed of a mode-hop free frequency sweep external cavity diode laser, a high finesse FP (to measure accurately the frequency sweep range) and a dual measurement system to enable the measurements at 150 m with an accuracy at the tens of micrometer level. Its uncertainty budget, interferometers design and preliminary experimental results are detailed in this paper.

Thermal modeling CO2 laser radiation transmission welding of superposed thermoplastic films

The low absorption presented by thermoplastic films to 10.6-μm CO 2 laser radiation makes the engineering use of welding parameters, predicted by models developed for thicker thermoplastics, very difficult. A new theoretical model is developed describing the temperature distribution in thin thermoplastic material during the laser welding process. The heat conduction equation is solved analytically by the Green function method and heating and cooling thermal stresses are taken into consideration. Engineering parameters predicted by the model are applied to lap welding of high- and low-density polyethylene and polypropylene samples, both transparent and white, with thicknesses between 10 and 100 μm, and experimentally validated. This validation is also accomplished by comparison with the measured temperature through the use of two diagnostic methods: schlieren interferometry and photothermal deflection spectroscopy. The first of these methods, combined with direct observation of Mie scattering, also puts in evidence the absorption of about 30% of the incident energy due to plasma formation in the air above the interaction interface. This plasma ignites after the initial release of chunks of material during the first moments of interaction. Proper modeling, and the introduction of a reflective substrate under the samples, allows an increase in process efficiency and the achievement of lap welding speeds up to 14 m s –1 with this new transmission welding technique.

Adoption of new software and hardware solutions at the VLT: the ESPRESSO control architecture case

ESPRESSO is a fiber-fed cross-dispersed echelle spectrograph which can be operated with one or up to 4 UT (Unit Telescope) of ESOs Very Large Telescope (VLT). It will be located in the Combined-Coudé Laboratory (CCL) of the VLT and it will be the first permanent instrument using a 16-m equivalent telescope. The ESPRESSO control software and electronics are in charge of the control of all instrument subsystems: the four Coudé Trains (one for each UT), the front-end and the fiber-fed spectrograph itself contained within a vacuum vessel. The spectrograph is installed inside a series of thermal enclosures following an onion-shell principle with increasing temperature stability from outside to inside. The proposed electronics architecture will use the OPC Unified Architecture (OPC UA) as a standard layer to communicate with PLCs (Programmable Logical Controller), replacing the old Instrument Local Control Units (LCUs) for ESO instruments based on VME technology. The instrument control software will be based on the VLT Control Software package and will use the IC0 Field Bus extension for the control of the instrument hardware. In this paper we present the ESPRESSO software architectural design proposed at the Preliminary Design Review as well as the control electronics architecture.

Automation methodology for the development of LPFG using CO2 laser radiation

The mid-infrared radiation produced by CO2 lasers is being widely used to produce long period fiber gratings (LPFG) with several advantages over other methods. Several techniques can be used to irradiate the fiber in order to produce the necessary effect. Using a cylindrical lens to create a line of light or scanning of a spot over the fiber are the most common approaches. Usually, the period is produced either by a translation stage moving perpendicular to the incident beam or by using a two mirrors scanner that inscribes the entire period directly on the fiber. In both cases, the synchronization between the laser and the moving elements is critical. Also, when using a two mirrors scanner, the dimension of the LPFG is limited by the focusing lens diameter and its focal length. All this become critical when one needs to increase the LPFG’s length or reduce its period. The later usually implies shorter laser emission times, which is limited by the laser emission physics (at least for cheap low power CW lasers). In order to overcome the disadvantages of each method, a combined approach is presented and analyzed. A mirror scans vertically the beam over a cylindrical lens and a translation stage moves the fiber to create the different periods. The laser keeps emitting during the complete process increasing laser power stability and thus, improving grating homogeneity. To guarantee the synchronization between the translation table and the one mirror scanner, special hardware and software was created.

Application of schlieren interferometry to temperature measurements during laser welding of high-density polyethylene films

Schlieren interferometry is found to be an alternative tool for temperature measurement during thermoplastic laser welding with regard to methods based on thermocouples or optical pyrometers. In fact, these techniques are not easily applied when materials to be processed have reduced thickness, negligible heat conduction, and low emissivity, as is the case of welding high-density polyethylene films with 10.6-microm CO2 laser radiation, even if the method reaches its applicability limit after approximately 1 s of the interaction process. The schlieren method provides the means and the results to probe the thermal variations of the laser-thermoplastic interaction on both the surface and the interface between the sample material and the air.

Influence of laser spot shape on welding of thin thermoplastics

Lap welding of thin thermoplastics at very high speeds has been studied and the influence of laser beam spot shape, dimensions, and position relative to sample displacement were analysed.A 2700 W CO2 laser was used and a rotating drum allows plastic samples, with thickness between 0.1 and 0.01 mm, to reach constant linear velocities up to 20 m s−1.The focusing system, using spherical or cylindrical lenses, and adjusted towards or away of the sample, produced on the thermoplastic, laser spots of circular or elliptical contour with several dimensions. This allowed changing spot area, laser energy density and the interaction time of the beam on the sample. Influence of each of them on welding parameters was analysed. The influence of an angular deviation between welding direction and elliptical spot position on the sample was also studied.Experimental results show that weld strength increases for larger beam spot diameters, and that elliptical beam spots increase weld efficiency, allowing higher processing speeds or decreasing required laser power. Consequence of angular deviation of elliptical beam spot relative to sample movement direction is also an increase of weld strength, but maximum welding speed decreases.Lap welding of thin thermoplastics at very high speeds has been studied and the influence of laser beam spot shape, dimensions, and position relative to sample displacement were analysed.A 2700 W CO2 laser was used and a rotating drum allows plastic samples, with thickness between 0.1 and 0.01 mm, to reach constant linear velocities up to 20 m s−1.The focusing system, using spherical or cylindrical lenses, and adjusted towards or away of the sample, produced on the thermoplastic, laser spots of circular or elliptical contour with several dimensions. This allowed changing spot area, laser energy density and the interaction time of the beam on the sample. Influence of each of them on welding parameters was analysed. The influence of an angular deviation between welding direction and elliptical spot position on the sample was also studied.Experimental results show that weld strength increases for larger beam spot diameters, and that elliptical beam spots increase weld efficiency, allowing higher processing s...