• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 37
  • 8
  • Tagged with
  • 45
  • 45
  • 45
  • 45
  • 45
  • 43
  • 2
  • 1
  • 1
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Fabrication and characterization of optical waveguide components in epitaxial (Pb,La)(Zr,Ti)O3 thin films.

Kjellman, Jon Øyvind January 2009 (has links)
<p>This report presents a method for fabrication of ridge-type waveguide electro-optic modulators in 500 nm thick, c-oriented, epitaxial, (Pb0.92,La0.08)(Zr0.4,Ti0.6)O3 thin films on a MgO substrate. The method is based on ordinary photolitography and wet etching techniques with lift-off metallisation of gold electrodes. To achieve good input coupling of light the device end faces were polished using diamond lapping film swith grain sizes as small as 0.5 um. Several devices with both Mach-Zehnder electro-optic modulators and phase modulators with co-planar gold electrodes and an interaction length of L = 3 mm were fabricated using this method. These modulators were used to examine the electro-optic efect in the thin films at the wavelength 1.55 um. Coherent, linearly polarised light was coupled into the waveguide end face using tapered fibres while a modulation voltage was applied to the electrodes. The phase modulators were used as intensity modulators by exciting them with light polarised 45 degrees on the optical axis and placing a linear polariser at the output. Practical modulation was achieved using voltages as low as 46 V with the phase modulators and 80 V with the Mach-Zehnder modulators. Using a numerical curve fitting technique, the measured data was fitted to a proposed theoretical model for the modulators. The model was based on Jones matrix formalism and the assumption that the electro-optic effect in the thin films corresponds with the Pockels effect. Good fits were achieved indicating that the effect is indeed a Pockels effect and estimates of the Pockels coecient r51 ranged from 4.4 pm/V to 11 pm/V depending on the device. These values were obtained under the uncertain assumption that the birefringence of (Pb,La)(Zr,Ti)O3 is 0.005. It is found that if the actual value of the birefringence is lower, then r51 will also be lower than estimated. Based on previous works the Pockels coeficient was expected to be on the order or ~102 pm/V. The low measured values of r51 and the variation between films is believed to be due to the thin films being multi domain with a domain structure created by a stochastic process.</p>
22

Fabrication of a Photonic Crystal using self-assembly : A potentially new rear reflector for thin solar cells

Granlund, Håvard January 2009 (has links)
<p>The major issue with solar cells today, is the price per watt. To tackle this challenge several approaches are being made. One possible approach is to reduce the amount of silicon raw material used in todays wafer-based solar cells by making the wafers thinner. This approach does however bring with it another problem; much of the incident light will escape the cell before being absorbed. One possible way to solve this problem is to use a photonic crystal (PC) as a rear reflector. PCs is a periodic dielectric structure that can reflect light in a non-specular manner, which can substantially increase the optical pathlength for the light within the cell, and hence increase the probability of absorption- One such PC is the inverted opal. To make an inverted opal, one first fabricates a synthetic opal, consisting of microspheres of some kind, that can be used as a template. The gaps between the spheres of such an opal can then be filled with a suitable dielectric, e.g. silicon, and the spheres can be removed by a selective process, leaving the inverted opal structure. In this master project, opals were attempted made with Ugelstad spheres and polysterene (PS) spheres. With the Ugelstad spheres, wafer sized monolayers were made, but opals were not achieved. The PS spheres however, proved very suitable, and opals with a grain size of 150x300$mu m$ were successfully made. These opals were then partially inverted, by first filling in the gaps between the spheres with silicon, using a low temperature low pressure PECVD process, and then heating the samples to 600 degrees Celsius for 2.5 hours.</p>
23

Measurements of Optical Penetration Depth in Smoked Salmon

Johansen, Remi Andre Ursin January 2008 (has links)
Optical spectroscopy is a common method used in the determination of quality parameters in groceries. An optical characterization of smoked Atlantic salmon was carried out in this thesis. The optical penetration depth in salmon was found at 531 nm and 632 nm from measurements with lasers as light sources, and from 550 nm to 880 nm with a tungsten halogen lamp as light source. The spectrum of the halogen lamp combined with the absorption spectrum of the salmon made it difficult to obtain results for wavelengths below 550 nm with the halogen lamp. Two variations in the measurements on smoked salmon were performed; measuring on needle insertion versus needle extraction and measuring across several layers of muscle tissue versus measuring along one layer of muscle tissue in salmon. The absorption coefficient and the reduced scattering coefficient of smoked Atlantic salmon was calculated. Significant differences were found dependent on needle insertion or needle extraction, and whether the measurements were made along one layer or across several layers. The penetration depths were found to be 6.79±0.33 mm across several layers and 10.76±1.03 mm along one layer in the measurements with the He-Ne laser. The diffusion approximation was found to be a good approximation for wavelengths from 600 nm to 700 nm. With further development, it may be possible to determine the astaxanthin content of salmon with the method used in this thesis.
24

Reduction of speckle contrast in HDTV laser projection display.

Apeland, Knut Øyvind January 2008 (has links)
Abstract In this thesis the focus has been on laser speckle. It is done in collaboration with poLight. They are developing a projector, where laser light is the source of illumination. In such projectors, laser speckle degrades the image quality. The aim of this project is to construct a speckle reduction device to be used in the laser projector. The theory covers a description of laser speckle, how to reduce the speckle contrast, and five methods to so. We explain why speckle arises and which parameters we can manipulate to reduce the speckle contrast. The five speckle reduction methods included in this thesis are; vibrating diffuser, slowly moving diffuser, Hadamard matrices, scattering tube, and vibrating mirror. Large vibrational motions are unwanted, considering the size of the device, generation of noise, and problems with alignment of the optical components in the projector that this would lead to. The quality of the laser beam is prominent in order to produce a sharp image, thus the use of diffusers with large scattering angles is not a good solution. The scattering tubes, designed by poLight, are tubes filled with micro pearls in a polymer gel. The size of the pearls decides the nature of the scattering. Larger pearls will give less back scattering and more light transmitted in the forward direction. If the tubes are rotated in a well balanced device we can avoid generating vibrations. The Hadamard matrices is the only one of the five methods which is not based on a motion. The challenge is to find a SLM to implement the matrices. It requires a low response time in order to present enough matrices during the exposure time of the eye. The laboratory setup we use to measure the speckle contrast is an improved version of the setup constructed in the specialisation project. A screen was removed from the old setup, and the speckle is now imaged directly from the speckle reduction device. The measured speckle reduction is thus due to the device alone, and not affected by the screen. The results were reproducible and in agreement with what we expected. We implemented a vibrating diffuser, both the single and the slowly moving. A piece cut from a plastic bag and some Scotch Magic tape were used as diffusers. The tape is the strongest diffuser and gives the lowest speckle contrast, however, it also has the largest scattering angle. The single tape diffuser reduced the speckle contrast to $C = 0.112$. With two tape difusers in series the intensity in the images becomes too low to exploit the dynamic range of the CCD sensor. The result is a higher calcualted speckle contrast with two diffusers, $C=0.131$, even though it ought to be smaller. We tested five prototypes of the scattering tube with different concentrations. The tube with the highest concentration has the highest speckle reduction abilities. It also has the strongest scattering effect. The scattering is less than with the tape diffuser, and so is the speckle reduction. The speckle contrast is reduced to $C=0.320$ when the tube is rotated, and to $C=0.389$ when it is vibrated. The tubes was also tested in series with a ground glass. The ground glass acted as a second diffuser. In this setting, vibration and rotation of the tubes reduced the speckle contrast equally, $C approx 0.283$ From the measured speckle contrast of the diffusers and tubes in stationary conditions, a polarization analysis should show a depolarization of the laser beam. This were the case only for the plastic diffuser. It is assumed that the error lays with the polarization analysis. There should be a depolarization in the tape and a partial depolarization in the tubes. A calculation of the speckle size was performed as well. Based on the theory we expected the size of the speckle grains to be $sigma_s = 37.77~mu m$. From the Fourier analysis of a speckle image from the setup we calculated the speckle size to be $sigma_s = 5.35$~mm, which is approximately 140 times bigger. The expected speckle size is too small, because we did not take into account a small magnification in the setup. The Fourier analysis of discrete and limited sets of data points is probably the main explanation of the difference, but a more thorough study is needed.
25

Antiferromagnetic Domain Contrast in LaFeO3 Thin Films : Examined with X-ray Magnetic Linear Dichroism and Photo Electron Emission Microscopy

Kristiansen, Tom January 2006 (has links)
This study will focus on strain induced by thermal history and thickness on antiferromagnetic epitaxial LaFeO3(110) thin films grown on Nb:SrTiO3(001) substrates. Lattice mismatching between film and substrate induce strain in the film lattice during growth of the film. This strain can be relaxed by thermal treatment after growth and may show tendencies in the domain size, shape and distribution related to strain. Thin films are grown in different thicknesses by rf magnetron sputtering and selected films are relaxed by thermal anneal treatment. The samples are investigated in room temperature and in heating experiments to obtain images of antiferromagnetic domain contrast along the L2 and L3 absorbtion egde of Fe by XMLD spectromicroscopy. Size of domains areas and contrast level are measured and related to the strain in the thin films. Averaged spectra of L2 absorbtion edge antiferromagnetic contrast showed a clearly observable domain contrast with consistently shaped energy spectrums. No difference in antiferromagnetic contrast due to lattice strain caused by lattice mismatching was observed. All as-grown samples showed comparable size, distribution and shape of antiferromagnetic domains on both polished and etched substrates. Relaxation of prepared samples in 1000±C for 12 hours in a 1 atm atmosphere of oxygen gave a distinct increase in size for the domains. Area calculation show a increase from typically 0.2 ¹m2 for as-grown films to 2 ¹m2 for relaxed films. Heating experiments estimate a Néel temperature of 625oK for as-grown films and 740oK for relaxed samples. The thermal anneal thus has a significant effect on LaFeO3 thin films as the domains increase in size and the Néel temperature in which the film is no longer antiferromagnetic increases to the Neel temperature of bulk LaFeO3. This preparation approach may be useful for further investigations of the exchange-bias effect.
26

Optical Detection of Surface Acoustic Waves

Rostad, Torbjørn January 2006 (has links)
This project was worked on during the autumn 2005 at the Norwegian University of Science and Technology, Department of Electronics and Telecommunications. The assignment was to write a new LabVIEW programme that is to run the measurement procedure of a laser probe setup. The setup is used in characterization of surface acoustic waves(SAW). A programme was written that contained the necessary functionality and proved to operate satisfactorily. Several measurements were made on a SAW transducer, accurately picturing the wave. Fourier analysis were performed on the collected data in order to separate the propagation directions. An absolute amplitude measurement was made on a heterodyne interferometer, and the result was compared to a similar scan made using the laser probe. The work shows that the setup is ready for calibration against the heterodyne interferometer, in order to enable the laser probe to measure absolute amplitude by itself.
27

Polarization Dependent Loss (PDL) in Polarization Multiplexed and Hybrid Optical Networks

Chapa Gordero, Felisa January 2007 (has links)
Study of the Polarization Dependent Loss caused by the combined effect of orthogonality degradation and dynamic power fluctuations in a hybrid network, which combines circuit switching with packet switching to transmit applications with very different quality requirements on the same wavelength channel.
28

Reduction of speckle contrast in laser based HDTV projection displays

Lifjeld, Anders January 2007 (has links)
In this assignment the theoretical background for the nature of speckle is presented and practical work was done to reduce the speckle effect in a display system based on a laser source. This was done without any picture modulators, or any kind of line scan or flying spot scanning. Work was done to find the right setup to be able to as easy as possible characterize the statistics of the speckle in an image. A still image of an expanded laser spot worked as an image. A series of test sets were carried out to address the different factors which could make a difference on the speckle contrast and their role in such systems.
29

Polarization Effects in Wavelength Converters based on Semiconductor Optical Amplifiers

Martin Martin, Raul January 2007 (has links)
Polarization Effects in Wavelength Converters based on Semiconductor Optical Amplifiers
30

Quantum Cascade Laser for Spectroscopic Gas Detection

Sletbakk, Bjørn January 2007 (has links)
In this project it has been focused on the use of a 7.42 um, 4 mW Quantum Cascade Laser in trace gas detection spectroscopy. Norsk Elektro Optikk (NEO) is in possession of a Nanoplus G2102/DFB2/5-12 QCL laser, that can be used in spectroscopic detection of H2O, CO2 and SO2 It has been attempted to construct a setup that can be used for spectroscopic measurements using a self constructed current driver module to produce current pulses for the QCL. The QCL is operated in pulsed mode, with pulses of &lt;500 ns width, and a 1 kHz pulse repetition frequency. Temperature control of the laser has been conducted with a LaserGas II SP monitor module, and temperature has been kept at 280 K during testing. Initially, various test have been conducted to ensure the general functionality of the driver module, and to avoid damage to the QCL. Two different solutions for the transmission line from the driver to the QCL have also been examined, and it has been concluded that the current pulses supplied to the laser are of good quality with minor broadening and short rise/fall times. Furthermore an I-V characteristic for the laser has been produced by incrementing the laser driving current, and measuring the corresponding laser voltage. Measurements of the laser output power have been conducted using a CaF2 collimating lens to focus the laser beam onto a PVI-2TE-8 Vigo photovoltaic detector from Vigo Systems S.A. It has proved difficult to obtain a good signal from the detector, with the maximum voltage measured across a 50Ω shunt resistor being 3 mV. This is 100-200 mV less than what should be expected. Various methods of noise reduction have been applied to improve the detected signal, none have however provided any noticeable improvements. Several possible reasons for the generally low output have been examined.

Page generated in 0.0988 seconds