The GRIN rod lenses are favorable optical elements for the image transmission in applications where transverse dimensions of the optical system are limited such as rigid borescopes or endoscopes. Flexible versions of these instruments usually use imaging bundles of optical fibers to assure both the mechanical elasticity and the optical transmission. The image resolution of this solution suffers from being limited by minimum fiber core diameter allowable for imaging and total number of fibers in a given cross-section area. GRIN rod lens allows for a higher image resolution for small diameters. While the optical properties of the GRIN rod lens are well known, there is no information about its allowed mechanical stress. In this paper we present a simple system for load and deformation measurement of a GRIN rod lens aimed to be used for a miniature endoscope for eye surgery operation. The system allows for loading force measurement with a deformation strain gauge cell and deformation measurement with an optical triangulation sensor. The paper presents deformation limits of the tested GRIN rod lens loaded by a three-point bending test. We also evaluated the Young modulus of the GRIN rod lens material from the measured data. Our data shows that tested GRIN lenses can be statically deformed by bending down to minimum radius 105 mm without braking.
For quick, efficient and accurate alignment and characterization of focused short-wavelength (i.e., extreme ultraviolet, soft x-ray, and x-ray) laser beams directly in the vacuum interaction chambers, an instrument has to be developed and implemented. AbloCAM should represent such a handy tool looking at ablation imprints of the beam in a suitable material without breaking vacuum and need for a liberation of exposed samples from the chamber to analyse them ex situ. First steps we made in this direction can be found in ref. [1] The technique of the fluence scan (F-scan method; for details see [2,3]), proven at several FEL facilities, e.g., FLASH (Free-electron LASer in Hamburg) and LCLS (Linac Coherent Light Source), makes possible to characterize the beam utilizing just an outer contour of the damage pattern. It is not necessary to measure a crater profile for the beam reconstruction. Not only lateral, but also a longitudinal distribution of irradiance can be determined in the focused beam by its imprinting (z-scan method [4]). Technically, the AbloCAM tool consists of a vacuum compatible motorized positioning system executing a series of well-defined irradiations of a chosen slab target according to algorithms fulfilling requirements of the combined F(z)-scan procedure. Damage patterns formed in that way should then be visualized in situ by means of Nomarski (DIC – Differential Interference Contrast) microscope equipped with the software which indicates and processes pattern outer contours. There is a feedback established between positioning and inspecting components and functions of the tool. The software helps to align and characterize any focused beam in the interaction chamber semi-automatically in a reasonable time.
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.
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.
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.
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 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.
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.
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