One of the main technical difficulties in the fabrication of optical antennas working as light detectors is the proper
design and manufacture of auxiliary elements as load lines and signal extraction structures. These elements need
to be quite small to reach the location of the antennas and should have a minimal effect on the response of
the device. Unfortunately this is not an easy task and signal extraction lines resonate along with the antenna
producing a complex signal that usually masks the one given by the antenna. In order to decouple the resonance
from the transduction we present in this contribution a parametric analysis of the response of a bolometric stripe
that is surrounded by resonant dipoles with different geometries and orientations. We have checked that these
elements should provide a signal proportional to the polarization state of the incoming light.
Fractal antennas have been proposed to improve the bandwidth of resonant structures and optical antennas. Their multiband characteristics are of interest in radiofrequency and microwave technologies. In this contribution we link the geometry of the current paths built-in the fractal antenna with the spectral response. We have seen that the actual currents owing through the structure are not limited to the portion of the fractal that should be geometrically linked with the signal. This fact strongly depends on the design of the fractal and how the different scales are arranged within the antenna. Some ideas involving materials that could actively respond to the incoming radiation could be of help to spectrally select the response of the multiband design.
The practical application of optical antennas in detection devices strongly depends on its ability to produce
an acceptable signal-to-noise ratio for the given task. It is known that, due to the intrinsic problems arising
from its sub-wavelength dimensions, optical antennas produce very small signals. The quality of these signals
depends on the involved transduction mechanism. The contribution of different types of noise should be adapted
to the transducer and to the signal extraction regime. Once noise is evaluated and measured, the specific
detectivity, D*, becomes the parameter of interest when comparing the performance of antenna coupled devices
with other detectors. However, this parameter involves some magnitudes that can be defined in several ways
for optical antennas. In this contribution we are interested in the evaluation and comparison of D_ values for
several bolometric optical antennas working in the infrared and involving two materials. At the same time,
some material and geometrical parameters involved in the definition of noise and detectivity will be discussed to
analyze the suitability of D_ to properly account for the performance of optical antennas.
We present an imaging technique for the 3D-form metrology of optical surfaces. It is based on the optical absorption in fluids situated between the surface and a reference. An improved setup with a bi-chromatic light source is fundamental to obtain reliable topographic maps. It is able to measure any surface finish (rough or polished), form and slope and independently of scale. We present results focused on flat and spherical optical surfaces, arrays of lenses and with different surface finish (rough-polished). We achieve form accuracies from several nanometers to sub-lambda for sag departures from tens to hundred of microns. Therefore, it seems suitable for the quality control in the production of precision aspheric, freeform lenses and other complex shapes on transparent substrates, independently of the surface finish.
The use of optical excitation in MEMS elements triggers several mechanisms that can be properly
used for enhancing its mechanical response. This is particularly interesting when MEMS components are
used as the transducer element on energy harvesting applications involving opto-mechanical conversion
schemes. One of the pathways is based on the thermal response of the vibrating structures. In this
contribution we have analyzed how a MEMS structure consisting on a clamped-clamped beam responds
mechanically to the heating of the element. This heating is produced by the partial absorption of an
incident radiation at the IR band. A thin metal layer evaporated on top of the suspended beam acts as the
infrared light absorber. The gap forms an optical interferometer which couples the light absorption to the
mechanical deflection of the CC-beam. This effect can be enhanced by a proper design of the whole
mechanical geometry. Both, the optical absorbance and the energy conversion to the thermal domain of
the MOEMS device are analyzed. Additionally, the transduction to the mechanical domain in the form of
beam vibrations of the optical energy absorbed by the structure and transformed into heat is also modeled.
This paper focus on the analytical model that is necessary to understand the involved physical
mechanisms and the results obtained from the simulation of the device.
Non-dispersive infrared (NDIR) is a well known technique for gas concentration monitoring. Lead salt photoconductors
and thermopile detectors are typically used. Together with gas filter correlation (GFC) they are the basis for a reference
standard in environmental gas monitoring like carbon monoxide determination and other gas species. To increase gas sensitivity, a multi-pass optical cavity is often used. In this contribution we propose a new optical design that allows for auto-reference multiple gas detection. It basically consists of an array of White's cell multi-pass camera that allows multiple channels with independent lengths inside the same volume. We explore its performance for carbon monoxide detection and based on recent commercial developments in infrared detector and emitter technologies.
Fresnel lenses and other faceted or micro-optic devices are increasingly used in multiple applications like solar light
concentrators and illumination devices, just to name some representative. However, it seems to be a certain lack of
adequate techniques for the assessment of the performance of final fabricated devices. As applications are more exigent
this characterization is a must. We provide a technique to characterize the performance of Fresnel lenses, as light
collection devices. The basis for the method is a configuration where a camera images the Fresnel lens aperture. The
entrance pupil of the camera is situated at the focal spot or the conjugate of a simulated solar source. In this manner,
detailed maps of the performance of different Fresnel lenses are obtained for different acceptance angles.
Fresnel lenses and other faceted or micro-optic devices are increasingly used in multiple applications like solar light
concentrators and illumination devices. As applications are more exigent this characterization is of increasing
importance. We present a technique to characterize the surface topography of optical surfaces. It is especially well
adapted to Fresnel lenses where abrupt surface slopes are usually difficult to handle in conventional techniques. The
method is based on a new photometric strategy able to codify the height information in terms of optical absorption in a
liquid. A detailed topographic map is simple to acquire by capturing images of the surface. Some experimental results
are presented. A single pixel height resolution of ~0.1 μm is achieved for a height range of ~50 μm. A surface slope
analysis is also made achieving a resolution of ~±0.15°.
KEYWORDS: Optical fibers, Light sources and illumination, Sensors, Light emitting diodes, LED lighting, Prototyping, Cultural heritage, Signal attenuation, Reflection, Waveguides
Recent advances in LED technology have relegated the use of optical fibre for general lighting, but there are
several applications where it can be used as scanners lighting systems, daylight, cultural heritage lighting,
sensors, explosion risky spaces, etc. Nowadays the use of high intensity LED to inject light in optical fibre
increases the possibility of conjugate fibre + LED for lighting applications. New optical fibres of plastic
materials, high core diameter up to 12.6 mm transmit light with little attenuation in the visible spectrum but there
is no an efficient and controlled way to extract the light during the fibre path. Side extracting fibres extracts all
the light on 2π angle so is not well suited for controlled lighting. In this paper we present an extraction system for
mono-filament optical fibre which provides efficient and controlled light distribution. These lighting parameters
can be controlled with an algorithm that set the position, depth and shape of the optical extraction system. The extraction system works by total internal reflection in the core of the fibre with high efficiency and low cost. A 10 m length prototype is made with 45° sectional cuts in the fibre core as extraction system. The system is tested with a 1W white LED illuminator in one side.
We present a variable spectral and angular light source generator. The design and presented results are focused on solar
radiation simulation, reproducing the spectral and angular distribution observed from the sun. This system is particularly
interesting in the area of solar concentration. It permits to measure and test multi-junction photovoltaic cells alone or
together with concentrating optics. We present some system setups and its performance in reproducing solar radiation
around the visible band.
KEYWORDS: Diffraction, Polarization, Picosecond phenomena, 3D modeling, Refractive index, Phase shifts, Control systems, Process control, 3D metrology, Interferometry
The most common technique for diameter measurement of thin metallic cylinders is optical diffractometry. It consists in
illuminating the cylinder with a collimated monochromatic light beam, determining the diameter from the location of
the minima of the far field diffraction pattern. Babinet principle is normally assumed, being the diffraction pattern of the
cylinder equivalent to that of a strip whose width is equal to the cylinder diameter. Due to the three dimensional nature
of the cylinder, this model is not valid for accurate measurements. It has been experimentally shown that, when
compared to interferometry, Fraunhoffer model presents a systematic overestimation in the cylinder diameter. Rigorous
models which assume that the wire presents an infinite conductance have been developed. However, the refraction index
of the material has also appeared important for an accurate estimation since it produces a phase shift of the reflected
wave by the wire surface, modifying the state of polarization of the incident light beam and, as a consequence, the
location of the diffraction minima. In this work we propose a model based on the Geometrical Theory of Diffraction that
assumes both the three-dimensional nature and a finite conductance of the wire. Results for several materials are
presented, showing that the overestimation of the wire diameter depends on the state of polarization and wavelength of
the incident light beam, as well as the diameter and refraction index of the metallic wire.
The accuracy and precision of the thin wires still requiring special attention. Both theoretical and experimental studies together may give closest approximation to the "real" value. Concerning the optical technique of measurement, perhaps one may analyze more in detail the interaction between light and matter (wire) which can lead to a simple mathematical approach. Besides this, a calibrating system and robust technique of measurement is required both in the industrial sector and laboratories. Measuring the wires depends especially on how much accuracy and precision we want to achieve, we have static or dynamic measurement, which kind of wire we need to measure...etc. This report shows some work about the diffraction models and some measurement of the thin wire (30-500 μm). Statistical technique of measurement is provided as well.
We provide a simple solution for the particular problem of the in-service inspection of aeronautic surfaces in order to detect bumps that otherwise it is made visually and usually less reliably. Bumps and other surface defects can be a symptom of a more serious internal damage. We have extended the use of the well know shadow Moire technique to be implemented in convex curved surfaces. The use of flexible Ronchi grating allows its adaptation to a convex general surface. The effect is the contrast enhancement of only the small defects sweeping-off other misleading height profile fringes. The result obtained are quite satisfactory compared to the use of flat reference gratings. The experimental set- up consists of a plastic foil with a printed Ronchi grating stretched between three points which adapts to nay cylindrical or conical convex surface independently of the relative grating-surface orientation. Static quantification of defects profiles is also possible by means of with an attached CCD camera. Visual detectability of local surface structures lye in the range of approximately 30 micrometers in depth.
We have developed a prototype for in-line detection of surface defects in metallic wires, specially for scratches. A simple geometrical relationship between surface topography and conical reflection, permits to correlate the defects with intensity patterns in a simple way. The presented apparatus consists basically in a grating-divided laser beam incident on angular equidistant points. A CCD and an associated optics capture the information of the whole wire perimeter at once. Analytic rudiments are provided in agreement with the experimental results.
The purpose of this paper is to improve the metallic wire diameter measurement. The size of some thin wires and slits has been achieved using the spectrogoniometer which represents good alternative system of detection. The fact that, the technique of measurement may be regarded as bringing more calibrated diameter. Since it provides wide range of detection, possibility to control the output signal readjusting a minimum of points per fringe and few image error to deal with comparing to other measurement techniques. The target-arm of detection rotated and drifted by a stepping motor of high resolution. As we know the diffraction of the thin wires using Fraunhofer technique is very suitable for an automatic control in the process of fabrication. Whereas, the more precise diameter we want the more deeper study is required (both theory and experiment). Concerning the physical approach of the phenomena, we fit our data to a polynomial of the third order. The error of each coefficient is given. The odd term presented in the polynomial function may be due to the deviations from the ideal properties of the components. Once the measurements are made, a special care of the experimental data is required in order to deduce the right diameter confining us only to the minima of the diffraction pattern. The wire diameter accuracy depends not only how good is our signal but as well how good our analysis of data.
The laser diffraction is a robust and precise technique to monitor wire diameters in-line. However, classical Fraunhofer diffraction formulas are not appropriate for 3-dimensional object size determination. The Babinet's principle allow to use such formulas only for angles of diffraction that tend to zero. A real diffraction measurement necessarily takes a finite angular range (approximately 10 degrees) and therefore, an error will be introduced if using classical formulas. The exact electromagnetic formulation is not appropriate to deal with 3-D objects, basically because it does not provide explicit formulas to determine the wire diameter. We have worked a pseudo-empirical approach out to reach simple accurate and reliable diffraction formulas that use exclusively the fringe pattern. To validate the diffraction formulas we need a calibration of the wire diameter. To accomplish this, we have introduced a hybrid set-up which allows interferometric and diffraction measurements over the same area of the wire. Using a He-Ne laser and a plus or minus 10 degree measurement range we observe, typically, a diameter overestimation of approximately 0.5 microns for different metallic wires (approximately 30 - 300 micrometer). From this work, we can also extract a practical physical insight to diffraction phenomena in terms of the Geometrical Theory of Diffraction (GTD). Many optical techniques for metrology of high resolution must take into account diffracted light.
The laser diffraction is a robust and precise technique to monitor wire diameters in-line. However, classical Fraunhofer diffraction formulas are not appropriate for 3D object size determination. The Babinet's principle allow to use such formulas only for angles of diffraction that tend to zero. A real diffraction measurement necessarily takes a finite angular range (approximately 10 degree(s)) and therefore, an error will be introduced if using classical formulas. The exact electromagnetic formulation is not appropriate to deal with 3D objects in a simple way. On the other hand we observed a systematic overestimation of the diameter, even very small angle of observation, in other words, a small misfit with the Babinet's principle.
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