Birefringence can be a crucial problem for many optical instruments operating with laser beams. Even for birefringence free optical elements, birefringence can be introduced due to mounting forces and torques. Highly sensitive polarimeters need to be used to reveal the limit values of linear or circular retardance introduced into the optical system. The correct assessment of polarimeter’s performance needs to be done with an appropriate test sample. We propose a birefringence test sample based on a basic load case of a circular plate loaded with torsion stress. Such a test sample has many advantages for a birefringence measurement testing. There is always present a zero level of stress in the middle of the circular sample. The stress level linearly increases towards the perimeter and its slope can be set arbitrarily. There is no change of either the sample volume or its shape under torsion stress. In the paper we evaluate the birefringence of a sample under torsion stress with Jones matrix calculus. We used a torsion stressed sample to verify reliability of our proposed setup for birefringence measurement based on polarizing Mach-Zehnder interferometer.
In the applications of computer graphics, bidirectional texture function (BTF) is used for realistic and predictive rendering. The goal of current research is to get a surface representation indistinguishable from the real world. We developed, built, and tested a portable instrument for BTF acquisition based on kaleidoscopic imaging. We discuss the color issues we experienced after the initial tests. We show that the same color balance cannot be applied to the whole image as the spectral response of the instrument varies depending on the position within the image. All optical elements were inspected for their contributions to the spectral behavior of the instrument. A software simulator of a mathematical model of the device was implemented. We found a way to implement all these contributions into the image processing pipeline. In this way, the correct white balance for each individual pixel in the image is found and applied, allowing for a more faithful color representation. Also proposed is an optimized dielectric protective layer for the kaleidoscope’s mirrors causing the least possible color aberration.
The measurement of spatially varying surface reflectance is required for faithful reproduction of real world to allow for predictive look of computer generated images. One such proposed method uses a rotational kaleidoscopic imaging, where illumination and imaging paths are realized by subimages on kaleidoscopic mirrors and illumination is carried out by a DLP projector. We describe a novel geometric calibration method for a rotational kaleidoscope that is necessary to get aligned and accurate data from measurement. The calibration has two stages. The first stage mechanically adjusts the camera, the projector, and the autocollimator against the kaleidoscope mirrors. The second stage is based on the software. By random perturbation of camera and projector in corresponding mathematical model of the kaleidoscope we estimate better real positions of camera and projector in a physical setup, comparing the computed images from the software simulator and the acquired images from the physical setup.
In computer graphics and related fields, bidirectional texture function (BTF) is used for realistic and predictive rendering. BTF allows for the capture of fine appearance effects such as self-shadowing, inter-reflection and subsurface scattering needed for true realism when used in rendering algorithms. The goal of current research is to get a surface representation indistinguishable from the real world. We developed, produced and tested a portable instrument for BTF acquisition based on kaleidoscopic imaging. Here we discuss the colour issues we experienced after the initial tests. We show that the same colour balance cannot be applied to the whole picture as the spectral response of the instrument varies with the position in the image. All optical elements were inspected for their contributions to the spectral behaviour of the instrument. The off-the-shelf parts were either measured or the manufacturer’s data were considered. The custom made mirrors’ spectral reflectivity was simulated. The mathematical model of the instrument was made. We found a way how to implement all these contributions to the image processing pipeline. In this way, a correct white balance for each individual pixel in the image is found and applied, allowing for a more faithful colour representation. Also proposed is an optimized dielectric protective layer for the kaleidoscope’s mirrors.
Large aperture composite adaptive optics for laser applications is investigated in cooperation of Institute of Plasma Physic, Department of Instrumentation and Control Engineering FME CTU and 5M Ltd. We are exploring opportunity of a large-size high-power-laser deformable-mirror production using a lightweight bimorph actuated structure with a composite core. In order to produce a sufficiently large operational free aperture we are developing new technologies for production of flexible core, bimorph actuator and deformable mirror reflector. Full simulation of a deformable-mirrors structure was prepared and validated by complex testing. A deformable mirror actuation and a response of a complicated structure are investigated for an accurate control of the adaptive optics. An original adaptive optics control system and a bimorph deformable mirror driver were developed. Tests of material samples, components and sub-assemblies were completed. A subscale 120 mm bimorph deformable mirror prototype was designed, fabricated and thoroughly tested. A large-size 300 mm composite-core bimorph deformable mirror was simulated and optimized, fabrication of a prototype is carried on. A measurement and testing facility is modified to accommodate large sizes optics.
Realistic reproduction of appearance of real-world materials by means of computer graphics requires accurate measurement and reconstruction of surface reflectance properties. We propose an interactive software simulation tool for modeling properties of a kaleidoscopic optical system for surface reflectance measurement. We use ray tracing to obtain fine grain simulation results corresponding to the resolution of a simulated image sensor and computing the reflections inside this system based on planar mirrors. We allow for a simulation of different geometric configurations of a kaleidoscope such as the number of mirrors, the length, and the taper angle. For accelerating the computation and delivering interactivity we use parallel processing of large groups of rays. Apart from the interactive mode our tool also features batch optimization suitable for automatic search for optimized kaleidoscope designs. We discuss the possibilities of the simulation and present some preliminary results obtained by using it in practice.
This paper gives short overview of laser-based experiment OSQAR at CERN which is focused on search of axions and
axion-like particles. The OSQAR experiment uses two experimental methods for axion search – measurement of the
ultra-fine vacuum magnetic birefringence and a method based on the “Light shining through the wall” experiment.
Because both experimental methods have reached its attainable limits of sensitivity we have focused on designing a
vacuum laser resonator. The resonator will increase the number of convertible photons and their endurance time within
the magnetic field. This paper presents an opto-mechanical design of a two component transportable vacuum laser
resonator. Developed optical resonator mechanical design allows to be used as a 0.8 meter long prototype laser resonator
for laboratory testing and after transportation and replacement of the mirrors it can be mounted on the LHC magnet in
CERN to form a 20 meter long vacuum laser resonator.
Imaging of surface textures requires many combinations of incident illumination angles and detector angles of view.
Kaleidoscope is one of the means for measurement of bidirectional texture function of various sample surfaces.
An optical system featuring the kaleidoscope is proposed in the paper. Optical parameters of such an imaging system are
described and evaluated. We also discuss the optimization process of these parameters which influences the overall
imaging performance of a kaleidoscope device. We provide the visualization of various kaleidoscope designs.
This paper covers a description and a technique of a possible optical method of mode locking within a laser resonator.
The measurement system is a part of instrumentation of laser-based experiment OSQAR at CERN. The OSQAR
experiment aims at search of axions, axion-like particles and measuring of ultra-fine vacuum magnetic birefringence.
It uses a laser resonator to enhance the coupling constant of hypothetical photon-to-axion conversion. The developed
locking-in technique is based on differential interferometry. Signal obtained from the measurement provide crucial
information for adaptive control of the locking-in of the resonator in real time. In this paper we propose several optical
setups used for measurement and analysis of mutual position of the resonator mirrors. We have set up a differential
interferometer under our laboratory conditions. We have done measurements with hemi-spherical cavity resonator
detuned with piezo crystals. The measurement was set up in a single plane. Laser light was directed through half-wave
retarder to a polarizing beam splitter and then converted to circular polarization by lambda/4 plates. After reflection
at the mirrors, the beam is recombined in a beam splitter, sent to analyser and non-polarizing beam splitter and then
inspected by two detectors with mutually perpendicular polarizers. The 90 degrees phase shift between the two arms
allows precise analysis of a mutual distance change of the mirrors. Because our setup was sufficiently stable, we were
able to measure the piezo constant and piezo hysteresis. The final goal is to adapt the first prototype to 23 m resonator
and measure the displacement in two planes.
Two optical methods are used in the laser-based experiment OSQAR at CERN for the search of axions and axion-like
particles. The first method looks as light shining through the wall. The second one wants to measure the ultra-fine
vacuum magnetic birefringence. Both methods have reached its attainable limits of sensitivity. Present work is focused
on increasing the number of photons and their endurance time within the magnetic field using a laser cavity. Presented
paper covers recent state of development of a prototype of a 1 meter long laser cavity which is the prerequisite of further
development of the experiment.
This paper presents a short analysis of possible techniques for fusion targets tracking in rep-rate regime. Target tracking
solution is limited with necessity of high speed, high precise and long-distance measurement combined with a harsh
environment of the vacuum fusion chamber. The only optical measurement seems to be usable to meet required
conditions to measurement system. Few standards and less traditional methods are presented in this paper. Its possibility
to meet the target goal resolution is discussed. Preparation of experimental techniques for verification of measurement
conditions of suggested methods is shown too.