Zdeněk Buchta

Suppression of Air Refractive Index Variations in High-Resolution Interferometry

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.

Refractive Index Compensation in Over-Determined Interferometric Systems

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.

Displacement interferometry with stabilization of wavelength in air

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.

Small displacement measurements with subatomic resolution by beat frequency measurements

In this paper a novel method for high-resolution measurement of displacements with sub-atomic resolution is described. With this method, a length change of an optical resonator is directly transformed to a radio-frequency signal. A tunable He?Ne laser is locked to a mode of the resonator using a digital signal processing technique. Heterodyne mixing of this locked laser with an iodine-stabilized He?Ne laser converts the frequency of the laser locked to the cavity into the radio-frequency region. A HF counter measures the beat frequency from which the displacement can be derived directly. This method delivers inherent linearity and sub-nanometre resolution of the displacement over a range of several micrometres. An example of the capabilities of this system is given in this paper, where it is used for checking periodic deviations of a laser interferometer system. Emphasis is put on the construction of the optical resonator, on how its narrow resonance line-width is achieved, and how the required mechanical stability is achieved. The measurement range and the scale linearity are discussed in detail. Possible applications of this method are the calibration of nano-position systems based on PZT transducers, as well as inductive and capacitive sensors.

White-light fringe detection based on a novel light source and colour CCD camera

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 the white-light spectra, the 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 parts 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.

Novel Principle of Contactless Gauge Block Calibration

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.

Study of the thermal stability of Zerodur glass ceramics suitable for a scanning probe microscope frame

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.

Detection techniques in low-coherence interferometry and their impact on overall measurement accuracy

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.

Air refractive index measurement using low-coherence interferometry.

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 10-8.

High-power extended cavity laser optimized for optical pumping of Rb

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 HpXe (hyperpolarized xenon) 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 coinciding with the absorption line of desire. At the optimum emission linewidth was reduced approximately ten times with only half of the total optical power loss.