We investigate the optical performance of anti-reflective coatings subject to high temperatures and the presence of iodine gas to determine the most suitable technological process for iodine-filled absorption cell production. Two unwanted effects need to be avoided or mitigated. One is the settling of iodine gas in the coating structure, which reduces the transmittance of the cell windows. The other is the thermally induced spectral shift which limits the effect of the antireflective coating. We investigate the thermal resistance of TiO2 and Ta2O5 based coatings produced by electron beam evaporation (e-beam) and plasma ion-assisted deposition (PIAD). We prepared sets of quartz samples subject to a range of temperatures and the presence of iodine gas. We show that the choice of the coating technique and annealing procedure can mitigate the aforementioned effects.
KEYWORDS: Calibration, Interferometers, Field programmable gate arrays, Interferometry, Sensors, Monte Carlo methods, Homodyne detection, Distance measurement, Signal processing, Signal detection, Laser interferometry
We report on a traceable calibration system for a 3500mm-long console that carries a measurement system for inspecting the diameter of a circular reactor chassis. The system uses two single-pass laser interferometers with homodyne fringe detection for measurement in two degrees of freedom. The hybrid FPGA-microcontroller control module carries out the fringe detection together with the application-specific scale linearization approach and the compensation of environmental influences such as thermal elongation and the refractive index of air fluctuations. We demonstrated the system feasibility with an accuracy of a few microns and translation velocity higher than 0:1 metre per second.
We present an experimental study of the method using a spatial light modulator for correction of the wavefront reflected from the optically rough surface. This method is based on the detection of the mutual phase differences between different regions of the wavefront that correspond to the constructive interference. We study the capabilities of this method from the metrological point of view for the ground glass samples characterized by several different levels of roughness. The resulting wavefront correction is tested in dependence on the measurement parameters settings and is verified by analyzing two specific patterns generated by the spatial light modulator.
We present method for the improvement of the measurement accuracy in the optical frequency spectra
measurements based on tunable optical filters. The optical filter was used during the design and realization of the
measurement system for the inspection of the fiber Bragg gratings. The system incorporates a reference block for the
compensation of environmental influences, an interferometric verification subsystem and a PC - based control software
implemented in LabView. The preliminary experimental verification of the measurement principle and the measurement
system functionality were carried out on a testing rig with a specially prepared concrete console in the UJV Řež. The
presented system is the laboratory version of the special nuclear power plant containment shape deformation
measurement system which was installed in the power plant Temelin during last year.
On the base of this research we started with preparation other optical fiber sensors to nuclear power plants
measurement. These sensors will be based on the microstructured and polarization maintaining optical fibers. We started
with development of new methods and techniques of the splicing and shaping optical fibers. We are able to made optical
tapers from ultra-short called adiabatic with length around 400 um up to long tapers with length up to 6 millimeters.
We developed new techniques of splicing standard Single Mode (SM) and Multimode (MM) optical fibers and
splicing of optical fibers with different diameters in the wavelength range from 532 to 1550 nm. Together with
development these techniques we prepared other techniques to splicing and shaping special optical fibers like as
Polarization-Maintaining (PM) or hollow core Photonic Crystal Fiber (PCF) and theirs cross splicing methods with focus
to minimalize backreflection and attenuation. The splicing special optical fibers especially PCF fibers with standard
telecommunication and other SM fibers can be done by our developed techniques. Adjustment of the splicing process has
to be prepared for any new optical fibers and new fibers combinations. The splicing of the same types of fibers from
different manufacturers can be adjusted by several tested changes in the splicing process. We are able to splice PCF with
standard telecommunication fiber with attenuation up to 2 dB. The method is also presented.
Development of these new techniques and methods of the optical fibers splicing are made with respect to using
these fibers to another research and development in the field of optical fibers sensors, laser frequency stabilization and
laser interferometry based on optical fibers. Especially for the field of laser frequency stabilization we developed and
present new techniques to closing microstructured fibers with gases inside.
The absorption cells represent an unique tool for the laser frequency stabilization. They serve as irreplaceable optical frequency references in realization of high-stable laser standards and laser sources for different brands of optical measurements, including the most precise frequency and dimensional measurement systems. One of the most often used absorption media covering visible and near IR spectral range is molecular iodine. It offers rich atlas of very strong and narrow spectral transitions which allow realization of laser systems with ultimate frequency stabilities in or below 10-14 order level. One of the most often disccussed disadvantage of the iodine cells is iodine’s corrosivity and sensitivity to presence of foreign substances. The impurities react with absorption media and cause spectral shifts of absorption spectra, spectral broadening of the transitions and decrease achievable signal-to-noise ratio of the detected spectra. All of these unwanted effects directly influence frequency stability of the realized laser standard and due to this fact, the quality of iodine cells must be precisely controlled. We present a comparison of traditionally used method of laser induced fluorescence (LIF) with novel technique based on hyperfine transitions linewidths measurement. The results summarize advantages and drawbacks of these techniques and give a recommendation for their practical usage.
This research deals with preparation of an optical frequency references based on hollow-core photonic crystal fibers (HC-PCF). This fiber-based type of absorption cells represents a effiecient way how to replace classic bulky and fragile glass made tubes references with low-weight and low-volume optical fibers. This approach allows not only to increase possible interaction length between incident light and absorption media but it also carries a possibility of manufacturing of easy-operable reference which is set up just by plugging-in of optical connectors into the optical setup. We present the results of preparation, manufacturing and filling of a set of fiber-based cells intended for lasers frequency stabilization. The work deals with setting and optimalization of HC-PCF splicing processes, minimalization of optical losses between HC-PCF and SMF fiber transitions and finishing of HC-PCF spliced ends with special care for optimal closing of hollow-core structure needed for avoiding of absorption media leakage.
The homodyne detection with only a single detector represents a promising approach in the interferometric application which enables a significant reduction of the optical system complexity while preserving the fundamental resolution and dynamic range of the single frequency laser interferometers. We present the design, implementation and analysis of algorithmic methods for computational processing of the single-detector interference signal based on parallel pipelined processing suitable for real time implementation on a programmable hardware platform (e.g. the FPGA - Field Programmable Gate Arrays or the SoC - System on Chip). The algorithmic methods incorporate (a) the single detector signal (sine) scaling, filtering, demodulations and mixing necessary for the second (cosine) quadrature signal reconstruction followed by a conic section projection in Cartesian plane as well as (a) the phase unwrapping together with the goniometric and linear transformations needed for the scale linearization and periodic error correction. The digital computing scheme was designed for bandwidths up to tens of megahertz which would allow to measure the displacements at the velocities around half metre per second. The algorithmic methods were tested in real-time operation with a PC-based reference implementation that employed the advantage pipelined processing by balancing the computational load among multiple processor cores. The results indicate that the algorithmic methods are suitable for a wide range of applications [3] and that they are bringing the fringe counting interferometry closer to the industrial applications due to their optical setup simplicity and robustness, computational stability, scalability and also a cost-effectiveness.
We present a new self-referenced interferometer for the form measurement of hollow cylindrical tubes that provides the accuracy in the micrometer range. It is based on a simple and robust setup where the reference and object waves are represented by the central and peripheral parts, respectively, of the conical wave generated by a single axicon. The inteferogram is characterized by a closed-fringe pattern with a circular carrier which allows for the interference phase demodulation using spatial synchronous detection. The misalignment aberrations are corrected by an iterative computational procedure. The measurement capabilities of the interferometer are experimentally tested for the glass and stainless steel tubes with lengths from 220 mm to 600 mm.
We report on a design of an interferometric position measuring system for control of a sample stage in an e-beam writer with reproducibility of the position on nanometer level and resolution below nanometer. We introduced differential configuration of the interferometer where the position is measured with respect to a central reference point to eliminate deformations caused by thermal and pressure effects on the vacuum chamber. The reference is here the electron gun of the writer. The interferometer is designed to operate at infrared, telecommunication wavelength due to the risk of interference of stray light with sensitive photodetectors in the chamber. The laser source is here a narrow-linewidth DFB laser diode with electronics of our own design offering precision and stability of temperature and current, low-noise, protection from rf interference, and high-frequency modulation. Detection of the interferometric signal relies on a novel derivative technique utilizing hf frequency modulation and phase-sensitive detection.
This work is oriented towards our research in the field of splicing and ending of optical frequency references based on hollow core photonics crystal fibers (HC-PCF). This type of references is very promising optical element to replacing classic bulky absorption cells for laser frequency stabilization. We prepared and present methods of splicing HC-PCF to standard telecommunication fiber by a fiber splicer. A special care was taken to optimize the splicer setting and to find and obtain a splice with minimal optical losses between HC-PCF and SMF. The manufactured fiber cell was closed at one side by connecting to SMF and second fiber end was prepared for placing into the vacuum chamber with the help of vacuum-tightened connection to be used as a optical frequency reference based on the acetylene gas for frequency stabilization of the laser standards.
We report on the results of the common collaborative project of applied research where the Institute of Scfientific Instruments (ISI) of the Academy of Sciences of the Czech Republic and a company Meopta – optika joined their effort in development of high-precision interferometric systems for dimensional metrology and nanometrology. This research exploits previous results in the field of laser standards of optical frequencies and the methodology of interferometric metrology of length together with detection systems of interference signals and their processing at the ISI and the production technology of precise optical components at Meopta – optika. Within this project we developed a compact, solid-state frequency stabilized laser referenced to iodine transitions and technology of iodine cells for laser frequency stabilization. A fundamental setup of the laser interferometer has been arranged and tested. The company Meopta – optika contributes with development of new technology for processing and polishing of high-precision flat-surface optical components.
KEYWORDS: Mirrors, Deformable mirrors, Actuators, High power lasers, Laser cutting, 3D scanning, 3D metrology, Optical testing, Manufacturing, Heat treatments
The modern trend in high power laser applications such as welding, cutting and surface hardening lies in the use of solid-state lasers. The output beam of these lasers is characterized by a Gaussian intensity distribution. However, the laser beams with different intensity distributions, e.g. top-hat, are preferable in various applications. In this paper we present a new type of deformable mirror suitable for the corresponding laser beam shaping. The deformation of the mirror is achieved by an underlying array of actuators and a pressurized coolant that also provides the necessary cooling. We describe the results of the surface shape measurement using a 3D scanner for different settings of actuators. Further, we show the achieved intensity distributions measured by a beam profiler for a low power laser beam reflected from the mirror.
We report on an evaluation of the influence that fast changes of the refractive index has on the uncertainty of interferometric displacement measurement. Measurement of position within a limited range is typical for coordinate measuring systems such as nanometrology standards combining scanning probe microscopy (SPM) with precise positioning. For long-range systems the varying refractive index of air contributes the most to the overall uncertainty. We proposed to extend the principle of compensation of the fluctuations of the refractive index of air through monitoring the optical length within the measuring range of the displacement measuring interferometer. In this contribution we evaluate the level of uncertainty associated with the nature of the fluctuations of the refractive index of air in laser interferometry. We have observed that its fast variations, seen as length noise, are not linearly proportional to the measuring beam path but play a significant role only over distances longer than 50 mm. Over longer distances the length noise rises proportionally. The measurements were performed under conditions typical for metrology SPM systems.
We report on the results of the common collaborative project of applied research where the Institute of Scientific Instruments (ISI) of the Academy of Sciences of the Czech Republic and a company Meopta - optika joined their effort in development of high-precision interferometric systems for dimensional metrology and nanometrology. This research exploits previous results in the field of laser standards of optical frequencies and the methodology of interferometric metrology of length together with detection systems of interference signals and their processing at the ISI and the production technology of precise optical components at Meopta – optika.
The main aim of the project is a design of a complex interferometric measuring system in a form of a prototype serving as a master for further production. It concept is a modular family of components configurable for various arrangements primarily for multi-axis measurements in nanotechnology and surface inspection. Within this project we developed a compact, solid-state frequency stabilized laser referenced to iodine transitions and technology of iodine cells for laser frequency stabilization. A fundamental setup of the laser interferometer has been arranged and tested. The company Meopta – optika contributes with development of new technology together with a design of a machine for processing and polishing of high-precision flat-surface optical components.
The main aim of this work is oriented towards preparation and spectral properties evaluation of optical frequency references for laser standards – molecular iodine cells. These references represent the crucial part of setups for practical realization of the meter unit – highly stable laser standards. Furthermore, not only in the most precise laboratory instruments, but also in less demanding interferometric measuring setups the frequency stabilization of the lasers throught the absorption in suitable media ensure the direct traceability to the fundamental standard of length. A set of absorption cells filled with different amounts of molecular iodine (different saturation pressure point of absorption media) was prepared and an agreement between expected and resulting spectral properties of these cells was observed and evaluated. The usage of borosilicate glass instead of common fused silica as a material for cells bodies represents an approach to simplify the manufacturing technology process and also reduces the overall cell costs. A great care must be taken to control/avoid the risk of absorption media contamination by impurities releasing from the cell walls. We introduce an iodine purity and spectral properties evaluation method based on measurement of linewidth of the hyperfine transitions. The proposed method was used for verification of great iodine purity of manufactured cells by comparison of spectral properties with cells traditionally made of fused silica glass with well known iodine purity. The results confirmed a great potential of proposed approaches.
We present a measuring technique for displacement and position sensing over a limited range with detection of standingwave pattern inside of a passive Fabry-Perot cavity. The concept considers locking of the laser optical frequency and the length of the Fabry-Perot cavity in resonance. Sensing of the interference maxima and minima within the cavity along the beam axis has been tested and proven with a low loss photoresistive photodetector based on a thin polycrystalline silicon layer.
We propose to extend the principle of compensation of the fluctuations of the refractive index of air through monitoring the optical length within the measuring range of the displacement measuring interferometer. The concept is derived form a tracking refractometry evaluating the refractive index of air in the beam axis coinciding with the positioning interferometer. Application of this approach in multi-axis positioning and measurement mans to compromise the principle of spatial unity of the displacement measuring laser beam and the beam of the tracking refractometer. In this contribution we evaluate the level of uncertainty associated with the spatial shift of these two beams. Consequently the nature of the fluctuations of the refractive index of air in laser interferometry is investigated and discussed with the focus on potential applications in coordinate measuring systems and long-range metrological scanning probe microscopy systems.
We present a measuring technique for displacement and position sensing over a limited range with detection of standingwave pattern inside of a passive Fabry-Perot cavity. The concept considers locking of the laser optical frequency and the length of the Fabry-Perot cavity in resonance. Fixing the length of the cavity to e.g. a highly stable mechanical reference allows to stabilize wavelength of the laser in air and thus to eliminate especially the faster fluctuations of refractive index of air due to air flow and inhomogeneities. Sensing of the interference maxima and minima within the cavity along the beam axis has been tested and proven with a low loss photoresistive photodetector based on a thin polycrystalline silicon layer. Reduction of losses was achieved thanks to a design as an optimized set of interference layers acting as an antireflection coating. The principle is demonstrated on an experimental setup.
We present an overview of approaches to the design of nanometrology coordinates measuring setup with a focus on
methodology of nanometrology interferometric techniques and associated problems. The design and development of a
nanopositioning system with interferometric multiaxis monitoring and control involved for scanning probe microscopy
techniques (primarily atomic force microscopy, AFM) for detection of the sample profile is presented. Coordinate
position sensing allows upgrading the imaging microscope techniques up to quantified measuring. Especially imaging
techniques in the micro- and nanoworld overcoming the barrier of resolution given by the wavelength of visible light are
a suitable basis for design of measuring systems with the best resolution possible. The practical measurement results of
active compensation system for positioning angle errors suppression are presented as well as the analysis of overall
achievable parameters. The system is being developed in cooperation with the Czech metrology institute and it is
intended to operate as a national nanometrology standard combining local probe microscopy techniques and sample
position control with traceability to the primary standard of length.
We present an interferometric technique based on differential interferometry setup for measurement in the subnanometer scale in atmospheric conditions. The motivation for development of this ultraprecise technique is coming from the field of nanometrology. The key limiting factor in any optical measurement are fluctuations of the refractive index of air representing the greatest source of uncertainty. Our proposal is based on the concept of combining overdetermined interferometric setup where a reference wavelength is derived from a mechanical reference for compensation of fast fluctuations and traditional indirect evaluation of the refractive index compensating on the other hand the long-term drift of the mechanical reference caused by thermal expansion. The technique allows to track the variations of the refractive index of air on-line directly in the line of the measuring beam and to compensate for the fluctuations. The optical setup consists of three interferometers sharing the same beam path where two measure differentially the displacement while the third evaluates the changes in the measuring range acting as a tracking refractometer. The principle is demonstrated on an experimental setup and a set of measurements describing the performance is presented.
We present an interferometric technique based on differential interferometry setup for measurement in the subnanometer
scale in atmospheric conditions. One of the important limiting factors in any optical measurement are fluctuations of the
refractive index of air representing a source of uncertainty traditionally compensated when the index is evaluated
indirectly from the physical parameters of the atmosphere. Our proposal is based on the concept of overdetermined
interferometric setup where a reference length is derived from a mechanical frame made from a material with very low
thermal coefficient on the 1*E-8 level. The technique allows to track the variations of the refractive index of air on-line
directly in the line of the measuring beam and to compensate for the fluctuations. The optical setup consists of three
interferometers sharing the same beam path where two measure differentially the displacement while the third evaluates the changes in the measuring range acting as a tracking refractometer. The principle is demonstrated on an experimental setup and a set of measurements describing the performance is presented.
We present a concept combining traditional displacement incremental interferometry with a tracking refractometer following the fluctuations of the refractive index of air. This concept is represented by an interferometric system of three Michelson-type interferometers where two are arranged in a counter-measuring configuration and the third one is set to measure the changes of the fixed length, here the measuring range of the overall displacement. In this configuration the two counter-measuring interferometers have identical beam paths with proportional parts of the overall one. The fixed interferometer with its geometrical length of the measuring beam linked to a mechanical reference made of a high thermal-stability material (Zerodur) operates as a tracking refractometer monitoring the atmospheric refractive index directly in the beam path of the displacement measuring interferometers. This principle has been demonstrated experimentally through a set of measurements in a temperature controlled environment under slowly changing refractive index of air in comparison with its indirect measurement through Edlen formula. With locking of the laser optical frequency to fixed value of the overall optical length the concept can operate as an interferometric system with compensation of the fluctuations of the refractive index of air.
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