Frequency sweeping interferometry (FSI) is a technique where absolute distance measurements are made without
ambiguity, by using synthetic wavelengths resulting from a frequency sweep. In FSI, the measurement uncertainty
increases with the distance, as consequence of the propagation of the uncertainty in the synthetic wavelength
measurement. For long ranges, this component of the uncertainty budget is one of the major drawbacks of the technique.
To overcome this problem, we introduced the concept of the dual FSI mode, where the measurement process for longer
ranges is reduced to the close range case, by limiting the Optical Path Diference in the interferometer. This was achieved
by increasing the reference arm with a long reference fiber, and using a second ancillary interferometer to calibrate
continuously the fiber length and compensate temperature variations.
In the context of the ESA PROBA3 space mission (coronagraph and demonstration of metrology for free-flying
formation), we implemented a FSI sensor composed of a mode-hop free frequency sweep external cavity diode laser, a
high finesse Fabry-Perot interferometer (to measure accurately the frequency sweep range) and a dual measurement
This dual FSI concept, presented in San Diego in 2008, was now implemented and fully tested in view of the PROBA3
mission. Accuracies smaller than 32 μm for a measurement range from 51 m to 61 m were achieved using a reference
fiber with 71 m, maintaining the reduced complexity inherent to FSI technique, a mandatory condition for space
applications. Implementation issues and performance results are also discussed in this paper.
The present paper summarizes the results of a study of the morphological, structural and compositional changes caused
on dentin by processing with KrF excimer laser (λ= 248 nm). Different surface textures are achieved depending on the
structure of the samples and on the processing parameters. Independently of the radiation fluence used, a significant
reduction of the organic material content is observed in a surface layer a few nanometers thick, but no significant
changes in the mineral phase occur.
Optical interferometry for the absolute calibration of standard accelerometers is based on displacement amplitude
measurements considering a uniaxial sinusoidal excitation movement at a given frequency.
In reality, the movement generated by a shaker also contains components perpendicular to the oscillation axis,
introducing a rocking motion effect.
In the primary calibration of vibrations by laser interferometry, the rocking motion is a critical issue to be considered for
high accuracy measurements. The knowledge of the impact of this effect in the performances of acceleration amplitude
measurement is fundamental for the definition of a robust calibration approach. Generally, this effect increases with the
excitation frequency and, beyond a certain threshold, its influence in the final result may become quite relevant.
In this work, we study the influence of the rocking motion in the calibration of one accelerometer with two shaker
models. The study comprises a nominal acceleration of 100 m.s-2 for frequencies between 1 kHz and 9 kHz, considering
a sinusoidal excitement. An interferometric system based on heterodyne detection was used for the high frequency
Measurements were performed for 12 incidence points equally spaced along the border of the surface of a dummy mass
attached to the standard accelerometer, and the corresponding average was estimated, allowing the characterisation of
the rocking motion effect and the estimation of the corresponding component in the expanded uncertainty budget.
Laser treatment is a promising technique for dental applications such as caries removal, dental hypersensitivity
reduction and improvement of the bond strength between dentin and restoration materials. In this study the
topographic and morphological changes induced in enamel and dentin surfaces by treating with KrF excimer
laser radiation were studied as a function of the number of laser pulses and radiation fluence by scanning electron
microscopy and optical profilometry. For enamel, independently of the fluence used, material removal occurs
preferentially at the prisms sheaths, leading to the formation of surface pits of a few micrometers. For dentin,
a cone-like topography develops when the tubules are approximately parallel to the laser beam direction and
the radiation fluence is within the range 0.5 to 1.5 J/cm2. For higher fluences, the treated surfaces are flat and
covered with a layer of re-solidified materials.
In this work is presented a relative based interferometry technique with homodyne and heterodyne detection for accelerometers absolute calibration in the range between 10 Hz and 10 kHz with accelerations of 0.5 to 400 ms-2. The experimental systems implemented and the acquisition and the developed data processing modules are described. The results obtained as the uncertainty budget for all the measuring range is evaluated, describing the mathematical models and identifying the corresponding uncertainty sources.
Coherent interferometric optical interferometry is one of the most interesting techniques for the primary calibration of standard accelerometers. By measuring the displacement caused by a sinusoidal excitation applied to the accelerometer, it is possible to determine the value of the acceleration. An experimental system was developed and implemented, based in relative interferometry with heterodyne interferometry, for the calibration of standard accelerometers in high frequency regime, up to 10 kHz. Heterodyne detection is needed to overcome a limitation of homodyne based systems in terms of measuring range, both in acceleration and frequency. In terms of performances, the system was capable of calibrations for frequencies from 100 Hz to 10 kHz and accelerations from 0.5 ms-2 to 400 ms-2. In this paper, we present a detailed description of the selected method, the implemented experimental systems and data processing algorithms, and the test results obtained in the calibration of a standard accelerometer, evaluating systems performances in terms of working range.
This paper aims to contribute to the understanding of column formation mechanisms in Al2O3-TiC ceramic composites due to processing with excimer laser radiation. The mechanisms proposed in the literature to explain the formation of such columns can be grouped in four categories: shadowing mechanisms, hydrodynamic mechanisms, vapour phase deposition mechanisms, and spatial modulation of absorbed energy mechanisms. In the case of Al2O3-TiC ceramics, the hydrodynamic and vapour phase deposition mechanisms can be excluded because experimental results show that the column core is composed of material in a pristine condition. A theoretical simulation of the spatial modulation of absorbed energy due to radiation reflected from preexisting topographic artefacts reveals that this mechanism can explain the growth of columns from those artefacts, but does not explain column growth in Al2O3-TiC, because it predicts that the height of the columns will increase indefinitely with increasing number of pulses, whereas it has been experimentally observed that columns only grow during the first 100-200 laser pulses. This model does not explain the observed variation of the columns height with laser fluence either. By contrast, predictions of the shadowing mechanism with TiC globules formed during the first laser pulses shielding the substrate and favouring column growth are in semiquantative agreement with experimental observations. The evolution of surface topography in Al2O3-TiC ceramics composite during processing with KrF excimer laser radiation is controlled by the ablation behaviour of individual phases and by the chemical changes of the material surface during laser processing.