Jan Hrabina

Multidimensional interferometric tool for the local probe microscopy nanometrology

This work reports on the measurement at the nanoscale using local probe microscopy techniques, primarily atomic force microscopy. Recent applications using the atomic force microscope as a nanometrology tool require that not only the positioning of the tip has to be based on precise measurements but also the traceability of the measuring technique has to be ensured up to the primary standard. Thus, in our experimental work, laser interferometric measuring methods were employed. In this paper, a new design of the six-axis-dimensional interferometric measurement tool for local probe microscopy stage nanopositioning is presented.

Absolute frequency shifts of iodine cells for laser stabilization

We present an investigation of iodine cell purity and influence of contaminations upon frequency shifts of iodine-stabilized frequency-doubled Nd : YAG lasers. The study combines measurements of laser-induced fluorescence and evaluation through the Stern–Volmer formula, with direct measurement of frequency shifts referenced by means of an optical comb to a radiofrequency clock etalon. These indirect and direct approaches are compared and provide feedback on the cell manufacturing procedure. Significant improvement of the apparatus for the measurement of induced fluorescence is reported, leading to better repeatability of the results. The ultimate precision that can be achieved in measurements of the absolute frequency of a stabilized laser is discussed in terms of the cell quality. (Some figures in this article are in colour only in the electronic version)

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.

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.

Multiaxis interferometric displacement measurement for local probe microscopy

We present an overview of design approaches for nanometrology measuring setups with a focus on interferometry techniques and associated problems. The design and development of a positioning system with interferometric multiaxis monitoring and control is presented. The system 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.

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.

Standing Wave Interferometer with Stabilization of Wavelength on Air

Abstract 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. This approach represents a contribution primarily to high-resolution and high-precision dimensional metrology in the nanoscale. Zusammenfassung Ein Interferometer wird vorgestellt, welches die vollständige Kompensation des zeitlich variierenden Brechungsindex der Luft entlang des Messpfades erreicht. Das Konzept basiert auf der Idee die Laserwellenlänge nicht optisch zu stabilisieren, sondern von einer bekannten Distanz abzuleiten, welche über ein Material mit niedrigem thermischem Ausdehnungskoeffizienten realisiert wird. Die hier eingesetzte kommerziell verfügbare Glaskeramik erreicht thermische Ausdehnungskoeffizienten in der Größenordnung von 10−8 / °C. Dies ist weit unterhalb jener Grenze die erreicht werden kann, wenn der Brechungsindex der Luft mithilfe der Beziehung von Edlen korrigiert wird. Somit ergeben sich Anwendungen in der präzisen Bestimmung von geometrischen Größen auch im Nanometerbereich.