Jan Hrabina, Ph.D.

The dawn of nanotechnology created a great number of new challenges and demands for metrologists and even created a new branch of metrology - the nanometrology dealing with the nanoscale. It combines imaging techniques with precise nanoscale measuring techniques to characterize nanostructures. Imaging of the nanoworld together with the development of microscopy techniques is a field where a huge effort has been invested and there are various techniques available including electron microscopy, confocal optical microscopy and most recently a family of local probe microscopy techniques, from which the most common is the atomic force microscopy (AFM). The AFM seems to be the most sensitive tool to show us what is happening "down there" and quickly became the drafter of nanometrology laboratories. I have been interested in problems of multi-axis laser interferometric measuring systems, especially nanopositioning systems for high-resolution microscopy. From the metrological point of view, the main problems of these systems are traceability to fundamental etalon of length. I work on investigation of noise spectral density distribution of used laser source, optimal lasers frequency stabilization techniques, Abbe angular errors, thermal dilatation problems and other influences to accurancy, resolution and uncertanity of measured dimensions.

Photo: Nanometrologic multi-axis laser interferometric system for AFM microscope.

Next scope of my interest is oriented on frequency stabilization of the laser etalons, one of the most important thing in fundamental metrology of lengths. Since 2001, when I started to work on frequency stabilization of He-Ne-I2 primary etalons, I have realized some stabilization systems primarily based on saturated subdoppler spectroscopy with 3f detection technique. During the experiments with iodine cells verification when I was working on my PhD thesis, I realized frequency comparison experiment of two frequency-doubled iodine-stabilized Nd:YAG lasers. The measuring laser system had four-pass in-line configuration. The modulation signals of both lasers (reference and measuring) were synchronized for improvement of signal-to-noise ratio of the beat-signal.

Photo: Frequency comparison of two iodine-stabilized Nd:YAG lasers.

Development and manufacturing of iodine cells at our institute started in 1985, when the first high-coherent He-Ne-I2 etalons were designed here. From this point, we have been manufacturing and offering absorption iodine cells for various purposes to laboratories worldwide.

After realization of experimental methods for iodine cells purity evaluation, we offer testing of iodine cells for frequency shifts by induced fluorescence method. For more information about iodine cells and its verification, please contact me.

Photo: Some examples of our absorption iodine cells.

Absolute frequency shifts of the iodine-based laser etalons are determined by the absorption iodine cell itself once other technical sources of shifts have been eliminated. Once filled, the iodine cell has its own characteristic shift. The purity of iodine determines the value of this shift and depends on the care taken during the preparation and filling of the cell.

The introduction of frequency-doubled Nd:YAG lasers operating at 532 nm brought a narrow-linewidth and low-noise laser source with a frequency in coincidence with much stronger iodine hyperfine components. The detection of iodine transitions can be optimized to obtain the best relative stability through a compromise between signal-to-noise ratio and linewidth, which can be reduced by reducing the pressure of the iodine vapour by cooling the cold finger. The stability of Nd:YAG iodine-stabilized lasers approaches 1 part in 10e-14, which is quite close to the long-term stability of primary radiofrequency etalons. At this level the absolute frequency shift associated with the iodine cell becomes more critical here, as does the cell manufacturing process. This necessitates an improved means of evaluating the iodine cell quality and leads to questions about the limits of the preparation-filling-evaluation chain.

To enable a study of the iodine cell quality as a function of the manufacturing process we assembled an experimental set-up to measure induced fluorescence and allow evaluation by the Stern-Volmer formula as has been used by the International Bureau of Weights and Measures (BIPM). The main aim here was to compare the results obtained by fluorescence measurements and calculation of the Stern-Volmer coefficient with direct beat-frequency measurements performed with the help of iodine-stabilized Nd:YAG lasers. Better relative stability of the Nd:YAG-based laser etalons makes them the appropriate source for the evaluation of absolute frequency shifts with higher precision.

For more informations about our experiments, please see the section "Publications".

Nowdays I am planning to start experiments of mass spectrometer technique on iodine cells. I expect these experiments to expand further the field of the iodine cells quality and bring more knowledge of cells preparation and manufacturing technology. I plan to combine the results of these experiments with the results of induced fluorescence technique and frequnecy comparison of iodine-stabilized laser technique for the next improvement of our iodine cells absorption technology.