Proc. SPIE. 10010, Advanced Topics in Optoelectronics, Microelectronics, and Nanotechnologies VIII
KEYWORDS: Signal to noise ratio, Signal to noise ratio, Modulation, Sensors, Image processing, Photodiodes, Multiplexing, Image quality, Micromirrors, Reconstruction algorithms, Digital micromirror devices
Single-pixel imaging based on multiplexing is a promising technique, especially in applications where 2D detectors or raster scanning imaging are not readily applicable. With this method, Hadamard masks are projected on a spatial light modulator to encode an incident scene and a signal is recorded at the photodiode detector for each of these masks. Ultimately, the image is reconstructed on the computer by applying the inverse transform matrix. Thus, various algorithms were optimized and several spatial light modulators already characterized for such a task. This work analyses the imaging quality of such a single-pixel arrangement, when various illumination conditions are used. More precisely, the main comparison is made between coherent and incoherent ("white light") illumination and between two multiplexing methods, namely Hadamard and Scanning. The quality of the images is assessed by calculating their SNR, using two relations. The results show better images are obtained with "white light" illumination for the first method and coherent one for the second.
A simple way to make physical encoding of data is to use some common Fourier optics tools, like lenses and some more sophisticated ones like a digital matrix detector (CCD, Charged Coupled Devices), as in Fig. 1. Except now the encoding and the compacting of the data is not made using Hadamard transform but a Fourier transform, which has less compacting power. There is however the big advantage of physically encrypting the data instead of manually or digitally doing the computation. The input data may be anything, a note, a picture, a diagram, anything at all. It may come at a moment notice and does not require any special preparation on the part of the operator. One can accomplish real-time encoding. It may require, however that the message data to be made available in a specific format, independent of the contents of the message. For instance it may have to be inscribed on a transparency and to have certain dimensions.
Although nowadays spectrometers reached a high level of performance, output signals are often weak and traditional slit spectrometers still confronts the problem of poor optical throughput, minimizing their efficiency in low light setup conditions. In order to overcome these issues, Hadamard Spectroscopy (HS) was implemented in a conventional Ebert Fastie type of spectrometer setup, by substituting the exit slit with a digital micro-mirror device (DMD) who acts like a coded aperture. The theory behind HS and the functionality of the DMD are presented. The improvements brought using HS are enlightened by means of a spectrometric experiment and higher SNR spectrum is acquired. Comparative experiments were conducted in order to emphasize the SNR differences between HS and scanning slit method. Results provide a SNR gain of 3.35 favoring HS. One can conclude the HS method effectiveness to be a great asset for low light spectrometric experiments.
A phase-shifting interferometry method to measure the wavefront reflected from a deformable mirror is described.
Usually, an adaptive optics system encompasses a deformable mirror and a Shack-Hartmann wavefront sensor as the
main components. However, an interferometric method is suited for calibration and traceability purposes of such
measurements. In this regard, we developed a system able to characterize the wavefront <i>in situ</i> with both a Shack-
Hartmann sensor and a phase-shifting interferometer. The method encompasses a reflective diffraction grating, where the
plus-first diffraction order reflected off the grating was incident on a deformable mirror. This beam was then reflected
back to the grating where it interfered with the incident beam and the interference pattern was collected on the minusfirst
order. As with the four-step phase-shifting interferometry method, four interference patterns were recorded and
processed to extract the wavefront information. The wavefront sensor recorded the wavefront directly reflected from the
deformable mirror, via a beam splitter.
A grating interferometer that uses the high diffraction orders in conjunction with a Twyman-Green commercial interferometer is used for the measurement of in plane movement of gratings. The high diffraction orders ensures the amplification of the measurement precision with a factor equal to the diffraction order of the measurement in principle, because no imaging of features marking the beginning and the end of the measured length feature is necessary, and therefore the resolution limits associated with microscope imaging are eliminated.
A self-referencing inteferometric method of wavefront sensing based on lateral shearing interferometry, able to measure
the local slope of a wavefront is described. For this, a deformable mirror is used to manipulate the testing wavefront and
a Murty plane-parallel plate interferometer to analyze this wavefront. We demonstrate the interferometer by measuring a
series of different wavefront shapes using coherent light. The principle of operation is presented together with practical
implementation and experimental results.
UV laser beam interaction with painting layers in case of aged mock-ups was investigated and ablation and cleaning
thresholds were estimated as a function of each layer and sub-layer composition. Ablation depth measurements as a
function of incident laser intensities and subsequent irradiation pulse number was measured with white light
interferometry (WLI) and profilometric methods, demonstrating a selectivity of the removal of painting layers from submicrometric
domain to micrometric domain as a function of surface cleaning needs. The laser cleaning station was
designed and developed after careful evaluation of the irradiation conditions proper to the removal of painting layers. A
Q-switched Nd:Yag laser radiation is delivered to the artwork through a mirror system consisting in an articulated arm
and a laser head. A complete control of the incident laser parameters was envisaged with the laser remote control
interface. The system also comprises diagnosis and monitoring tools for the remote control of the cleaning operation.
The prototype is controlled by an integrated interface based on a user-friendly software to perform the available
operations (e.g. laser cleaning, LIBS, colorimetry, live color monitoring, multispectral analysis, database management).
The user interface is also used to start the treatment of a new work, to review or continue a previously started work.
CuIn<sub>1-x</sub>Ga<sub>x</sub>S<sub>2</sub> (CIGS2) thin-films for solar cells were prepared by rf-magnetron sputtering and were deposited on glass substrate. These films were prepared using a stepwise process consisting of succesive deposition of CuInGa (d = 1500 nm) and ZnS (d = 200 nm) layers. Each layer was structurally characterized by X-ray diffraction and atomic force
microscopy. The microstructural and optical properties of CIGS2 component films of the solar cell in comparison with
those of ZnS and CuInGa films separately deposited onto glass substrates under the same conditions, were studied.
Transmission spectra of our thin films are strongly influenced by deposition conditions and nature of the support material
and they were recorded for each component film and CIGS2 solar cell.
Thin films, rf magnetron sputtering, CIGS2 solar cell, Surface roughness, Optical pr
In this paper, a small scale laboratory strainmeter for measuring relative strains is presented. The instrument is a high resolution homodyne interferometer with polarizing optics and special designed electronics for analyzing the output signal of the interferometer. Resolution of the order of λ/8 is obtained in the first instance, with the possibility of improvement by electronic means. Measurement range could vary from microns in the case of earth strains to meters in the case of industrial applications.
A laser strainmeter for in-situ monitoring of an important actively seismic area of Europe, namely Vrancea region in
Romania is proposed. Six groups from four different countries (Romania, Czech Republic, Italy and Greece) with
various areas of expertise (e.g. geophysics, lasers, optics, interferometry, and mechanics) are involved in order to sustain
the complexity of the project. This paper presents some preliminary laboratory experiments related to measuring relative
displacements with a stable interferometer. Displacements of the order of tens to hundreds of nanometers (80 to 285 nm)
were measured with uncertainty of ±1 nm. A computer algorithm was used to process the interferograms.
In micro- and nano- device fabrication technology, localized material removal is one of the basic operations for structure
formation. Classical methods for structure formation on the surface of a silicon wafer are based mainly on chemical
processing, starting with photo etching, chemical etching, and chemical-mechanical linearization. In order to propose
new methods of higher quality from the point of view of both environment protection and processing quality, we have
studied the possibility of thin films controlled removal under the action of laser radiation. We are presenting some
qualitative results of laser induced surface removal of polymer thin films, of interest for microelectronic industry (e.g.
photoresist), under the action of different laser sources. As laser sources we have used the most spread and
commercially available laser systems with different wavelengths and pulse lengths in order to compare their action on
the surfaces and to establish the characteristic parameters for removal of thin layers for surface cleaning.
In this paper a Fourier transform digital holography experimental arrangement is presented. It is actually a hybrid
arrangement, half digital half analog. The Fourier hologram was constructed using the analogous means of the so called
lensless configuration. The hologram was recorded digitally by a camera with a large CCD array in stead of the
recording medium. The recording of the image was analyzed with a computer and the original image was reconstructed
by means of the discrete Fourier transform.
In this paper a simple method for determining the wavelength of an unknown source, (a problem of great theoretical and
practical importance), based on the Moire fringes phenomenon and Fourier analysis is presented and put into practice.
The accuracy and the simplicity of the problem makes it attractive and competitive.
The paper presents measurements of magnetic liquids shaping by use of magnetic fields or gravitation and centrifugal
forces. These measurements are conducted with the goal of evaluating the characteristics of the magnetic fluids in order
to obtain an adaptive system. Values of frequency response domain, pattern imposed configuration response, and free
surface curvature were measured. Three types of optical arrangements were used to determine the parameters: schlieren,
Fizeau type interferometer and triangulation optical scheme.
We discuss an experiment for detecting small deformations by speckle interferometry. Vibration modes of an
aluminium plate are observed by digital speckle pattern interferometry (DSPI). A Mach-Zender interferometer
arrangement is used and the speckle interferograms are recorded with a CCD camera and processed on a computer.
These fringes depend on the path differences due to the vibration of the aluminium plate from its original state.
Vibration amplitudes between 0.3-0.6 &mgr; were measured for seven vibration modes.
The refractive index of a material medium is an important optical parameter since it exhibits the optical properties of the
material. The adulteration problem is increasing day by day and hence simple, automatic and accurate measurement of
the refractive index of materials is of great importance these days. For solid thin films materials Abeles method
was reconsidered. Quick, measurements of refractive index using simple techniques and refractometers can help
controlling adulteration of liquids of common use to a greater extent. Very simple interferometric set-up using Fizeau
fringe patterns compares the fringe pitch as obtained in a cell with two levels: one down level with the unknown
refractive index liquids and the upper level with gas air. A CCD matrix and a PC can handle the data and produce the
results up to for digits.
Proper calibration of any instrument is vital to an investigator's ability to compare laboratory experiments, as well as to
draw quantitative relations between experimental results and the real objects. Traceability is a term used to certify an
instrument's accuracy relative to a known standard. Because traceability to meter is a very expensive and complicated
process, accurate and traceable calibration of lateral and vertical standards (e.g. 1D and 2D gratings) is a basic
metrological task for nano- and micro- technology. On the other hand laser interferometry is the <i>de facto</i> method to
transfer the meter standard to practical measurement. In this lecture, we describe interferometric vertical and lateral
calibration of a grating used to quantify the parameters necessary for proper translation of AFM data into physically
A new approach to metrology for the range below 100 nm is based on large fiducial grids optical encoders produced by
interference / lithography. Since the encoder can only be as accurate as the grating scale, advance in this area depends
on the availability of encoder plates of nanometer accuracy. Various commercially available or home made holographic
gratings were checked using interferometric methods and compared with the AFM device results. The budget of errors
was analyzed and the necessary improvements of measuring technology are presented.
In this paper we investigate the vibrations of a square aluminium plate by speckle interferometry means. Modes of
vibration of this plate are shown as speckle interferograms. As usually is the case with such interferograms, enhancement
and filtering of these images is needed after recording. The speckle index and the signal-to-noise ratio (SNR) of the preprocessed
interferograms before and after filtering are calculated. An improvement of the SNR between 1.37 and 1.81 is
obtained for the vibration modes presented here.
This paper is concerned with the determination of in-plane displacements and deformations, by using digital speckle
correlation. A special algorithm for determining the position of the maximum of the correlation function is presented. An
example on how to apply this method is presented.
We present a practical numerical method for processing the fringes obtained when two waves, with a quadratic phase difference function, interfere. This kind of fringe includes straight equispaced fringes and Newton's rings as particular cases. The numerical method we present is based on the discrete Fresnel (Fourier) transform of the data and has the same precision as least square fitting (LSF). Compared to the LSF method, this new method is better, as it is more efficient and does not require initial approximations for the fringe parameters to be determined.
This paper presents a numerical method for processing the fringes obtained when two waves, with a quadratic phase
difference function, interfere. As a particular case of this kind of fringes are the Newton's rings. The numerical method
we present is based on the discrete Fresnel (Fourier) transform of the data and it has the same precision as the least
square fitting (LSF).
Laser interferometer displacement measuring transducers have a well-defined traceability route to the definition of the
meter. The laser interferometer is <i>de-facto</i> length scale for applications in micro and nano technologies. However their
physical unit -half lambda is too large for nanometric resolution.
Fringe interpolation-usual technique to improve the resolution-lack of reproducibility could be avoided using the
principles of absolute distance measurement. Absolute distance refers to the use of interferometric techniques for
determining the position of an object without the necessity of measuring continuous displacements between points.
The interference pattern as produced by the interference of two point-like coherent sources is fitted to a geometric model
so as to determine the longitudinal location of the target by minimizing least square errors. The longitudinal coordinate
of the target was measured with accuracy better than 1 nm, for a target position range of 0.4μm.
Presented in this paper are numerical algorithms necessary to determine the surface error by means of optical
interferometry. These algorithms are based on digital processing of phase-modulated fringe patterns, and are using the
discrete Fourier transform method.