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This paper presents a contactless system for automatic gauge blocks calibration based on combination of laser interferometry and low-coherence interferometry. In the presented system, the contactless measurement of the absolute gauge block length is done as a single-step operation without any change in optical setup during the measurement. The optical setup is combined with compact gauge block changer with a capacity of 126-ga blocks, which makes the resulting system fully automatic. The paper also presents in detail a set of optimization steps which have been done in order to transform the original experimental setup into the automatic system which meets secondary length metrology requirements. To prove the measurement traceability, we conducted a set of gauge block length measurement comparing data from the optimized system and the established reference systems TESA NPL A.G.I. 300 and TESA–UPC operated in Czech Metrology Institute Laboratory.

The air refractive index is an important parameter in interferometric length measurements, since it substantially affects the measurement accuracy. We present a refractive index of air measurement method based on monitoring the phase difference between the ambient air and vacuum inside a permanently evacuated double-spaced cell. The cell is placed in one arm of the Michelson interferometer equipped with two light sources—red LED and HeNe laser, while the low-coherence and laser interference signals are measured separately. Both phase and group refractive indices of air can be calculated from the measured signals. The method was experimentally verified by comparing the obtained refractive index values with two different techniques.

This paper presents a method implemented in a system for automatic contactless calibration of gauge blocks designed at ISI ASCR. The system combines low-coherence interferometry and laser interferometry, where the first identifies the gauge block sides position and the second one measures the gauge block length itself. A crucial part of the system is the algorithm for gauge block alignment to the measuring beam which is able to compensate the gauge block lateral and longitudinal tilt up to 0.141 mrad. The algorithm is also important for the gauge block position monitoring during its length measurement.

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 present an interferometric technique based on a differential interferometry setup for measurement under atmospheric conditions. The key limiting factor in any interferometric dimensional measurement are fluctuations of the refractive index of air representing a dominating source of uncertainty when evaluated indirectly from the physical parameters of the atmosphere. Our proposal is based on the concept of an over-determined interferometric setup where a reference length is derived from a mechanical frame made from a material with a very low thermal coefficient. The technique allows one 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 in an experimental setup.

In this paper, a novel principle of contactless gauge block calibration is presented. The principle of contactless gauge block calibration combines low-coherence interferometry and laser interferometry. An experimental setup combines Dowell interferometer and Michelson interferometer to ensure a gauge block length determination with direct traceability to the primary length standard. By monitoring both gauge block sides with a digital camera gauge block 3D surface measurements are possible too. The principle presented is protected by the Czech national patent No. 302948.

The influence of the refractive index of air has proven to be a major problem on the road to improvement of the uncertainty in interferometric displacement measurements. We propose an approach with two counter-measuring interferometers acting as a combination of tracking refractometer and a displacement interferometer referencing the wavelength of the laser source to a mechanical standard made of a material with ultra-low thermal expansion. This technique combines length measurement within a specified range with measurement of the refractive index fluctuations in one axis. Errors caused by different position of the interferometer laser beam and air sensors are thus eliminated. The method has been experimentally tested in comparison with the indirect measurement of the refractive index of air in a thermal controlled environment. Over a 1 K temperature range an agreement on the level of 5 × 10(-8) has been achieved.

This paper presents a theoretical analysis and an experimental verification of a direct method for a refractive index of air measurement combining low-coherence interferometry and laser interferometry. The method is based on monitoring optical path changes in a measuring arm of the Michelson interferometer caused by the different optical environment in a double-spaced glass cell. This article presents a set of experimental results in comparison with the results obtained by a couple of reference techniques and proves the ability of the designed method to measure the refractive index of air with accuracy in the order of 10sup-8/sup.

This paper deals with interference fringe center detection techniques used in low-coherence interferometry for contactless 3D inspection of macroscopic objects. It presents a complex analysis of several frequently used detection techniques and shows their impact on the measurement accuracy. The analysis compares those techniques in terms of computational complexity, measurement accuracy, and resistance to optical dispersion caused by wedge-shaped optical components.

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.

This paper presents a method for active angular alignment of gauge block implemented in a system for automatic contactless calibration of gauge blocks designed at ISI ASCR. The system combines low-coherence interferometry and laser interferometry, where the first identifies the gauge block sides position and the second one measures the gauge block length itself. A crucial part of the system is the algorithm for gauge block alignment to the measuring beam which is able to compensate the gauge block lateral and longitudinal tilt up to 0.141 mrad. The algorithm is also important for the gauge block position monitoring during its length measurement.

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 10− 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.

Although the laser interferometry represents the most precise class of techniques in the field of precise measurement of geometrical quantities, its wide use in measurement systems is still accompanied by many unresolved challenges. One of these challenges is the complexity of underlying optical systems. We present a novel approach to the interference phase detection - fringe subdivision - in the homodyne laser interferometry that aims at reduction of the optical complexity while the resolution is preserved. Our method employs a series of computational steps to infer a pair of signals in quadrature that allows to determine the interference phase with a sub-nanometre resolution from an interference signal from a non-polarising interferometer sampled by a single photodetector. The complexity trade-off is the use of laser beam with frequency modulation capability. The method was experimentally evaluated on a Michelson interferometer-based free-space setup and its performance has been compared to a traditional homodyne detection method. The results indicate the method is a feasible al­ ternative for the traditional homodyne detection since it performs with comparable accuracy (< 0.5nm standard deviation), especially where the optical setup complexity is principal issue and the modulation of laser beam is not a heavy burden, for instance in multi-axis measurement systems or laser diode based systems.

At present time the DFB (Distributed FeedBack), laser diodes are the most suitable laser sources from semiconductor laser sources for using in laser interferometry. The DFB laser diodes have narrow frequency linewidth, mode hop free tuning range up to 2 nm and sufficient optical power. In addition to the other types of laser diodes are available in the package with optical fiber at the output and the DFB laser diodes with 1541 nm wavelength has optical isolator inside. Unfortunately the DFB laser diodes with 760 nm wavelength have no optical isolator inside package. This induces a back reflection from the fiber connections at the output to the laser chip. Than the mode hop free tuning range is decreasing rapidly. We present our experience with adaptation of the DFB laser diodes to laser interferometer and methods to decrease back reflection. We used frequency stabilization by frequency lock to F-P (Fabry – Perot) resonator for VCSEL laser diode but the DFB laser diodes can be better frequency stabilized using absorption to absorption spectrum of gases (760 nm, 1542 nm, 1552 nm) to compare achieved frequency stability of the 760 nm DFB laser diodes.

The work presents a method where the change of the length of the measuring arm of the Michelson interferometer is monitored by the cavity length of the optical resonator. The resonator works as distance-to-optical frequency converter with ultimate linearity. It has locked a tunable laser to certain cavity mode. The optical frequency of the laser is heterodyned with immediate “tooth” of the femtosecond comb spectrum. The results of measured scale linearity of the homodyne interferometer for corrected and non-corrected scale are presented for measuring range of the homodyne interferometer in order of one micrometer and more.

This paper presents a novel principle for contactless gauge block measurement using a combination of low-coherence interferometry and laser interferometry. The experimental setup combines a Dowell interferometer and a Michelson interferometer to ensure a gauge block length determination with direct traceability to the primary length standard. This setup was designed for contactless complex gauge block analysis providing information about gauge block length, gauge block faces surface profile (e.g., indication of scratches) and by analysis of the interference fringes shape, also about the gauge block edge flatness distortion. The designed setup is supplemented by an automatic handling system designed for a set of 126 gauge blocks (0.5 mm to 100 mm) to allow the automatic contactless calibration of the complex gauge block set without a human operator.

The presented work is focused on digital processing of beat note signals from a femtosecond optical frequency comb. The levels of mixing products of single spectral components of the comb with CW laser sources are usually very low compared to products of mixing all the comb components together. RF counters are more likely to measure the frequency of the strongest spectral component rather than a weak beat note. Proposed experimental digital signal processing system solves this problem by analyzing the whole spectrum of the output RF signal and using software defined radio (SDR) algorithms. Our efforts concentrate in two main areas: Firstly, using digital servo-loop techniques for locking free running continuous laser sources on single components of the fs comb spectrum. Secondly, we are experimenting with digital signal processing of the RF beat note spectrum produced by f–2f 1 technique used for assessing the offset and repetition frequencies of the comb, resulting in digital servo-loop stabilization of the fs comb. Software capable of computing and analyzing the beat-note RF spectrums using FFT and peak detection was developed. A SDR algorithm performing phase demodulation on the f– 2f signal is used as a regulation error signal source for a digital phase-locked loop stabilizing the offset frequency of the fs comb.

We present a concept of suppression of the influence of variations of the refractive index of air in displacement measuring interferometry. The principle is based on referencing of wavelength of the coherent laser source in atmospheric conditions instead of traditional stabilization of the optical frequency and indirect evaluation of the refractive index of air. The key advantage is in identical beam paths of the position measuring interferometers and the interferometer used for the wavelength stabilization. Design of the optical arrangement presented here to verify the concept is suitable for real interferometric position sensing in technical practice especially where a high resolution measurement within some limited range in atmospheric conditions is needed, e.g. in nanometrology.

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.

Determination of the interference phase, i.e. the mutual phase shift between interfering waves is a principal issue in the interferometric measurement, especially on the nanometre scale. Our goal was to develop a novel interference phase detection technique that employs a computational approach and a frequency modulation of the laser source to achieve comparable performance with a homodyne detection with an optical phase shift generation. We have used a Michelson setup with polarizing optics that allowed to compare both the homodyne detection method and our novel method side by side while both methods shared the optical setup. Our method also comprises error compensation that deals e.g. with residual amplitude modulation and the scale non-linearities. The experiments revealed that the novel method achieves a periodic error less than 0.16 angular degrees and a standard deviation of less than 1.5 degrees, compared against the reference. The operational distance was 600mm. The method had proven it is suitable replacement for traditional homodyne detection techniques since it has comparable performance and significantly lower demands on the optical setup.

The presented work is focused on digital processing of beat note signals from a femtosecond optical frequency comb. The levels of mixing products of single spectral components of the comb with CW laser sources are usually very low compared to products of mixing all the comb components together. RF counters are more likely to measure the frequency of the strongest spectral component rather than a weak beat note. Proposed experimental digital signal processing system solves this problem by analyzing the whole spectrum of the output RF signal and using software defined radio (SDR) algorithms. Our efforts concentrate in two main areas: Firstly, we are experimenting with digital signal processing of the RF beat note spectrum produced by f–2f 1 technique and with fully digital servo-loop stabilization of the fs comb. Secondly, we are using digital servo-loop techniques for locking free running continuous laser sources on single components of the fs comb spectrum. Software capable of computing and analyzing the beat-note RF spectrums using FFT and peak detection was developed. A SDR algorithm performing phase demodulation on the f– 2f signal is used as a regulation error signal source for a digital phase-locked loop stabilizing the offset and repetition frequencies of the fs comb.

The work presents measurements of the length stability of Zerodur glass ceramic with temperature change. Measurement of this thermal characteristic is necessary for determination of the optimal temperature at which the Zerodur glass ceramic has a coefficient of thermal expansion close to zero. The principle of the measurement is to monitor the length changes using an optical resonator with a cavity mirror spacer made from the Zerodur material to be studied. The resonator is placed inside a vacuum chamber with a temperature control. A tunable laser diode is locked to a certain optical mode of the resonator to monitor the optical frequency of this mode. A beat-note signal from optical mixing between the laser and a stabilized femtosecond frequency comb is detected and processed. The temperature dependence of the glass ceramics was determined and analyzed. The resolution of the length measurement of the experimental set-up is on the order of 0.1 nm.

This paper presents a novel principle for contactless gauge block measurement using a combination of low-coherence interferometry and laser interferometry. The experimental setup combines a Dowell interferometer and a Michelson interferometer to ensure a gauge block length determination with direct traceability to the primary length standard. This setup was designed for contactless complex gauge block analysis providing information about gauge block length, gauge block faces surface profile (e.g., indication of scratches) and by analysis of the interference fringes shape, also about the gauge block edge flatness distortion. The designed setup is supplemented by an automatic handling system designed for a set of 126 gauge blocks (0.5 mm to 100 mm) to allow the automatic contactless calibration of the complex gauge block set without a human operator.

The work presents measurements of the length stability of Zerodur glass ceramic with temperature change. Measurement of this thermal characteristic is necessary for determination of the optimal temperature at which the Zerodur glass ceramic has a coefficient of thermal expansion close to zero. The principle of the measurement is to monitor the length changes using an optical resonator with a cavity mirror spacer made from the Zerodur material to be studied. The resonator is placed inside a vacuum chamber with a temperature control. A tunable laser diode is locked to a certain optical mode of the resonator to monitor the optical frequency of this mode. A beat-note signal from optical mixing between the laser and a stabilized femtosecond frequency comb is detected and processed. The temperature dependence of the glass ceramics was determined and analyzed. The resolution of the length measurement of the experimental set-up is on the order of 0.1 nm.

In this contribution we present an approach to incremental interferometric measurements of displacements over a limited range where the atmospheric wavelength of the coherent laser source is either directly stabilized to a mechanical reference or is corrected to fit to the reference. The idea comes from the possibility to use a highly stable material for a reference frame, material which can perform thermal expansion coefficients on the level 10 -8 /K within a large temperature range up to 50K. Over a range of several K which may be the practical range for displacement measurements the coefficient is even smaller. This may outperform the best techniques of correction for the variations of the refractive index of air. The mechanics is always a part of the measurement setup and represents one of the sources of uncertainty. A link of the refractive to the mechanical reference can practically eliminate another source of uncertainty.

We describe in this paper a pilot experiment of optimization of a white-light source for a low-coherence interferometry. The white-light source combines the light beams generated with colour LEDs. By modelling of the white light spectra, a contrast of a white light interference fringe could be changed and set to the maximal value. The second part of this paper is a description of a white-light fringe analysis ensured with a low-cost colour CCD camera. The used detection technique employs a phase-crossing algorithm which identifies a zero optical path difference as the point where the phase difference between the red, green and blue part of the white-light interference fringe becomes equal to zero. The optimized white light source is designed to be a crucial part of an experimental setup for the surface diagnostics and automatic calibration of gauge blocks.

The use of an ultra low expansion cavity plays a crucial role in laser stabilization, and in atomic or ion clocks. We propose an easy method of precise monitoring of optical path distance in Fabry-Perot interferometer. The spacing of mirrors of the Fabry-Perot interferometer in ambient air represents the optical path distance referenced to stable optical frequency of the femtosecond mode-locked laser. With the help of highly selective optical filter it is possible to get only a few of separate spectral components of laser comb. Optical path distance is transfered to optical frequency of the comb component and through the repetition frequency of the laser to the radio-frequency domain. Repetition frequency of the laser can be monitored with the uncertainty referenced to the any local oscillator or through the GPS to the atomic clock standard. By using this mehod we are able to measure and lock the Fabry-Perot cavity to a selected single component of optical frequency comb an to measure the optical path distance directly in rf domain.

We present an experimental arrangement of an interferometric system designed to operate with full compensation for varying refractive index of air in the measuring axis. The concept is based on a principle where the wavelength of the laser source is derived not from an optical frequency of the stabilized laser but from a fixed length being a base-plate or a frame of the whole measuring setup. This results into stabilization of the wavelength of the laser source in atmospheric conditions to mechanical length of suitable etalon made of a material with very low thermal expansion. The ultra-low thermal expanding glass ceramic materials available on the market perform thermal expansion coefficients on the level 10 -8 which significantly exceeds the limits of uncertainty posed by indirect evaluation of refractive index of air through Edlen formula. To verify the concept a tracking of laser frequency following the drift of refractive index has been performed.

We describe in this paper a pilot experiment of a white-light fringe analysis with a low-cost color CCD camera. The used detection technique employs the phase-crossing algorithm which identifies the zero optical path difference as the point where the phase difference between the red, green and blue part of the white-light interference fringe becomes equal to zero. An experimental arrangement is based on superluminescent LED diode. The experimental setup is designed to be a crucial part of the complex system for automatic contactless diagnostic and calibration of gauge blocks.

In metrology applications of laser interferometers like a scale calibration of capacitive or inductive sensors, strictly linear positioning of the measuring mirror of the interferometer is necessary. It is maintained usually by a stage, which is based on principle of linear guide ways with ball carrier bearings. But possible imperfection of guides of the travel stage can cause deviations of the mirror plane from the right angle to the axis of traveling. Mentioned angle deviations lead to distortion of interference fringes in the output of the interferometer and by other words it causes non-linearity of the interferometer scale. Because the phenomenon is very random for this type of the travel stage the uncertainty of calibration of sensors is higher. In the work we present a method, which eliminates this usual problem by two ways. The first of them utilizes a special configuration of the laser interferometer where possible angle deviation of the mirror plane is compensated by second pass of the laser beam in the measuring arm of the interferometer. The next way is based on continual monitoring of spatial position of laser beams in the interferometer when the measuring mirror is positioned. It works with condition that the mirror can be slightly tilted by piezoelectric actuators in servo-loop mode with respect to detected spatial position.

We present a compact Extended Cavity Laser (ECL) system based on a high-power laser diode optimized for maximum efficiency of the Rb optical pumping process. The system represents the crucial part of the HpG (hyperpolarized gasses) production process. We concentrated on the ECL system optimization - linewidth matching and frequency stabilization - for the optical pumping process. We show that the intensity of optical feedback in the ECL laser influences linewidth and output power and it is possible to find an optimum value for the highest power spectral density at the absorption line of desire. The emission linewidth was reduced approximately 10 times with only half of the total optical power loss. The ECL system is controlled by electronic servo-loop for laser frequency stabilization.

We present a compact extended cavity laser (ECL) system based on a high-power laser diode with reduced linewidth and stabilized to the rubidium D1 line at the 794.74 nm. The laser system was derived from an arrangement designed for optical pumping of Rb and production of hyperpolarized gasses. We concentrated on the ECL system optimization - linewidth narrowing effects with the broad-area power laser diode. We show that the intensity of optical feedback in the ECL laser influences linewidth and output power and it is possible to find an optimum value for the highest power spectral density and eventually adjust the linewidth over wide range. The emission linewidth was reduced approximately 10 times with only half of the total optical power loss. The ECL system is controlled by electronic servo-loop for laser frequency stabilization.

We present the experimental results of generation and of detection of hyperpolarized xenon obtained by 4.7 T, 200 MHz magnetic resonance (MR) system. Xenon 129 was hyperpolarized by the spin-exchange with Rubidium pumped first by Ti:Sa laser and also by extended cavity laser system based on a high-power laser diode optimized for maximum efficiency of the optical pumping process. The application of the arrangement with semiconductor laser system is oriented medicine, especially to continuous production of hyperpolarized Xenon (HpXe) which serves as an contrast media in MR imaging systems.

Laser interferometers are even more precise distance measurement devices with resolution up to sub-nanometer region. If the measurements are carry out under atmospheric conditions (usual situation in an industry), the interferometers measure optical path length of an unknown distance instead of its true geometrical value. It is caused by an index of refraction of air that introduces a multiplicative constant to measured results. If we want correct values of the distance measurement the knowledge of the instantaneous value of the index is necessary. In the work, we present design and the first experimental results of method of the direct measurement of the index, where a Fabry-Perot (F.-P) interferometer is used as a detection system. The method employs a differential setup of two F.-P interferometers, where the cavity of the first is permanently evacuated and the other is on the air. The ultimate resolution of the measurement and the operating regime without need of a vacuum pump stay the method very advantage. The work includes comparison of the method with conventional refractometer where evacuatable cell is inserted into the measuring arm of Michelson interferometer. The comparison of the method with indirect measurement of the index with using Edlen formula is presented too.

We present an extended cavity laser system based on a tunable high-power laser diode optimized for maximum efficiency of the optical pumping process of Rb atoms. The aplication of the laser system is orineted to employment in an experimental arrangement for production of hyperpolarized gasses (HpG), namely Xenon. It is designed to operate in medical and industrial applications to come. We concentrated on the laser diode emission linewidth reduction because of the efficiency of the optical pumping process. The emission linewidth was reduced approximately from 1 THz to 69 GHz with only half of the total optical power loss and quadruple increase of the power spectral density at the wavelength of desire.

We present a new interferometric method for shape measurement of hollow cylindrical tubes. We propose a simple and robust self-referenced interferometer where the reference and object waves are represented by the central and peripheral parts, respectively, of the conical wave generated by a single axicon lens. The interferogram detected by a digital camera is characterized by a closed-fringe pattern with a circular carrier. The interference phase is demodulated using spatial synchronous detection. The capabilities of the interferometer are experimentally tested for various hollow cylindrical tubes with lengths up to 600 mm.

We present a laser system based on a tunable Titanium:Sapphire (Ti:Sa) laser optimized for maximum efficiency of the optical pumping process of Rb atoms. The system represents the first and crucial part of the HpG (hyperpolarized gasses) production process. It was designed for laboratory purposes where the expensive and complicated Ti:Sa laser is justified because of its variability. narrow linewidth and easy tuning of the wavelength. The system consists of a modified commercial Ti:Sa laser pumped by a Nd:YAG laser, beam-forming and polarizing optics, and heated interaction cell attached to a vacuum manifold and gas containers placed in homogeneous magnetic field generated by a set of Helmholz coils.