Miroslava Holá

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.

Frequency Noise Properties of Lasers for Interferometry in Nanometrology

In this contribution we focus on laser frequency noise properties and their influence on the interferometric displacement measurements. A setup for measurement of laser frequency noise is proposed and tested together with simultaneous measurement of fluctuations in displacement in the Michelson interferometer. Several laser sources, including traditional He-Ne and solid-state lasers, and their noise properties are evaluated and compared. The contribution of the laser frequency noise to the displacement measurement is discussed in the context of other sources of uncertainty associated with the interferometric setup, such as, mechanics, resolution of analog-to-digital conversion, frequency bandwidth of the detection chain, and variations of the refractive index of air.

Investigation of Short-term Amplitude and Frequency Fluctuations of Lasers for Interferometry

One of the limiting factors of accuracy and resolution in laser interferometry is represented by noise properties of the laser powering the interferometer. Amplitude and especially frequency fluctuations of the laser source are crucial in precision distance measurement. Sufficiently high long-term frequency stability of the laser source must be achieved especially in applications in fundamental metrology. Furthermore, the short-term frequency variations are also important primarily for measurements done at high acquisition speeds. This contribution presents practical results of measurements of short-term amplitude and frequency noises of a set of laser sources commonly used in laser interferometry. The influence of the interferometer design and electrical parameters of the detection system are also discussed.

Spectral properties of molecular iodine in absorption cells filled to specified saturation pressure

We present the results of measurement and evaluation of spectral properties of iodine absorption cells filled at certain saturation pressure. A set of cells made of borosilicate glass instead of common fused silica was tested for their spectral properties in greater detail with special care for the long-term development of the absorption media purity. The results were compared with standard fused silica cells and the high quality of iodine was verified. A measurement method based on an approach relying on measurement of linewidth of the hyperfine transitions is proposed as a novel technique for iodine cell absorption media purity evaluation. A potential application in laser metrology of length is also discussed.

Comparison of Molecular Iodine Spectral Properties at 514.7 and 532 nm Wavelengths

Abstract We present results of investigation and comparison of spectral properties of molecular iodine transitions in the spectral region of 514.7 nm that are suitable for laser frequency stabilization and metrology of length. Eight Doppler-broadened transitions that were not studied in detail before were investigated with the help of frequency doubled Yb-doped fiber laser, and three of the most promising lines were studied in detail with prospect of using them in frequency stabilization of new laser standards. The spectral properties of hyperfine components (linewidths, signal-to-noise ratio) were compared with transitions that are well known and traditionally used for stabilization of frequency doubled Nd:YAG laser at the 532 nm region with the same molecular iodine absorption. The external frequency doubling arrangement with waveguide crystal and the Yb-doped fiber laser is also briefly described together with the observed effect of laser aging.

Contribution of the Refractive Index Fluctuations to the Length Noise in Displacement Interferometry

Abstract We report on investigations of how fast changes of the refractive index influence the uncertainty of interferometric displacement measurements. Measurement of position within a limited range is typical for precise positioning of coordinate measuring systems, such as nanometrology standards combined with scanning probe microscopy (SPM). The varying refractive index of air contributes significantly to the overall uncertainty; it plays a role especially in case of longer-range systems. In our experiments we have observed that its fast variations, seen as length noise, are not linearly proportional to the measuring beam path and play a significant role only over distances longer than 50 mm. Thus, we found that over longer distances the length noise rises proportionally. The measurements were performed under conditions typical for metrology SPM systems

Air flow and length noise in displacement interferometry

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.

Deformable mirror for high power laser applications

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.

Using spatial light modulator for correction of wavefront reflected from optically rough surface

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.