Aleš Babnik

Optical fiber reflection refractometer

An optical fiber sensor for measurement of the refractive index of liquids is described. Three parallel fibers and a mirror are used. One fiber is used to launch the light to the mirror and the other two capture the reflected light. The operation principle is based on the output light angle change due to the refractive index of the liquid into which the optical fiber probe is dipped. The paper contains the theoretical consideration as well as the experimental results. Aqueous solutions of NaCl and LiBr are used as testing media.

Optodynamic monitoring of the laser drilling of through-holes in glass ampoules

We present an accurate and reliable method for the detection of wall perforation during the excimer-laser micro-drilling of glass ampoules and vials. The method is based on the detection of shock waves generated in the air during the drilling process using a laser-beam deflection probe. An analysis of the detected optodynamic signals gives important information about the progress of the drilling process, and we take this as the basis for the presented online process-monitoring method. We show that a significant change in the signals amplitude is observed when the wall of a liquid-filled ampoule is perforated and the exit process point is in contact with the liquid, and that this signal change can serve as an indicator of wall perforation. We have verified this optodynamic method by examining the processed holes using optical and electron microscopy as well as with a non-destructive gas-leakage test method. The described method was employed for the production of test ampoules used for the adjustment of a high-voltage leak-detection device in a pharmaceutical production line. Holes with a diameter of less than 10 µm were produced in the walls of 0.5 mm thick glass ampoules using a XeCl excimer laser.

Measurement of the bending vibration frequencies of a rotating turbo wheel using an optical fibre reflective sensor

An optical fibre reflective sensor was used to analyse the vibrations of a rotating turbo wheel up to 20 400 rpm. The measured signal required correction because of the natural unevenness of the turbo wheel and because of the variable deflection. Because the turbo wheel was rotating the signal became distorted and so we used a special method to extract the frequencies of the vibrations from the power spectra. The analysis showed increased intensity of the first three natural frequencies with an increased speed of rotation. The experimental results match very well with those obtained by numerical computation.

From laser ultrasonics to optical manipulation.

During the interaction of a laser pulse with the surface of a solid object, the object always gains momentum. The delivered force impulse is manifested as propulsion. Initially, the motion of the object is composed of elastic waves that carry and redistribute the acquired momentum as they propagate and reflect within the solid. Even though only ablation- and light-pressure-induced mechanical waves are involved in propulsion, they are always accompanied by the ubiquitous thermoelastic waves. This paper describes 1D elastodynamics of pulsed optical manipulation and presents two diametrical experimental observations of elastic waves generated in the confined ablation and in the radiation pressure regime.

Improved probe geometry for fluorescence-based fibre-optic temperature sensor

Abstract A fluorescent fibre probe with high collection efficiency for fluorescence-lifetime measurements is presented. A numerical model for calculating the collection efficiency for different probe designs has been employed. The position and number of fibres, crystal geometry, crystal-fibre gap and the backside crystal reflectivity are used as evaluation parameters for probe optimization. A possible three-fold increase in collected fluorescent light intensity is predicted in comparison with the classical two-fibre configuration. The model has been confirmed by constructing a seven-fibre probe with alexandrite crystal as the sensing element. An accuracy of ±0.3 K is achieved in the 20–90°C temperature range.

Interference effects at a dielectric plate applied as a high-power-laser attenuator

The interference effects caused by the Fresnel reflections of a Gaussian beam on the boundaries of a dielectric plate, which can be considered as a Fabry-Perot etalon, were theoretically and experimentally investigated. In addition to the incident angle and the polarization of the incident light, two additional parameters--the plates parallelism and the temperature--which are often neglected, were analyzed. Based on the theoretical predictions and the measured behavior of the transmittance of the dielectric plate a new, temperature-controlled variable high-power-laser attenuator is proposed. Unwanted changes in the plates transmittance caused by the absorption of laser pulses within the plate are also presented. These phenomena are important in many applications where dielectric plates are used for a variety of purposes.

Refractive index measurement with optical fiber Mach-Zehnder interferometer

An interferometric technique for determination of the refractive index of liquids is described. The method is based on measurements of phase variations caused by the relative movement of an optical fiber tip in a liquid sample. The apparatus consists of two independent interferometers. A two-frequency Michelson interferometer is used to measure the liquid sample displacement in the air, while an optical fiber Mach-Zehnder interferometer measures the optical path length difference in the moving sample. The liquid sample refractive index is then derived by dividing the fringe counts obtained by both interferometers. The measurements have been performed in different liquids. With the distilled water sample, the statistical error of this method was found to be 5 X 10-5.

Isolated detection of elastic waves driven by the momentum of light

Electromagnetic momentum carried by light is observable through the mechanical effects radiation pressure exerts on illuminated objects. Momentum conversion from electromagnetic fields to elastic waves within a solid object proceeds through a string of electrodynamic and elastodynamic phenomena, collectively bound by momentum and energy continuity. The details of this conversion predicted by theory have yet to be validated by experiments, as it is difficult to distinguish displacements driven by momentum from those driven by heating due to light absorption. Here, we have measured temporal variations of the surface displacements induced by laser pulses reflected from a solid dielectric mirror. Ab initio modelling of momentum flow describes the transfer of momentum from the electromagnetic field to the dielectric mirror, with subsequent creation/propagation of multicomponent elastic waves. Complete consistency between predictions and absolute measurements of surface displacements offers compelling evidence of elastic transients driven predominantly by the momentum of light.The exact mechanism of momentum conversion from light to an object has varied descriptions in the literature and experimental verifications are difficult. Here the authors do an in-depth experimental and numerical study of the momentum dynamics of elastic waves in a dielectric mirror hit by a pulsed laser beam.

Reabsorption effects on calibration of fluorescence-lifetime-based sensors

Intrinsic advantages of fluorescence lifetime based optical sensors, as referred in the literature, are their stability towards source fluctuations and self-reference. The latter can be interpreted also as non-dependence on geometrical design and other properties of the constructed sensor. In the article presented it is shown this is not the case. The measured fluorescence decay time, which should be unambiguously connected to the measured quantity (temperature, for example), shows an intricate dependence on source, geometrical properties and signal processing technique. Numerical calculations together with theoretical discussion are employed to quantize and interpret this effects, mainly caused by reabsorption. Different sensor designs are compared, an optimal sensor construction suggested and alternative digital signal processing technique introduced.

Design aspects of an alexandrite fluorescence lifetime fiber optic thermometer

Fluorescence lifetime based sensors have long since proven their advantages in several measurement fields, including thermometry[l]. Inherent stability to the source intensity fluctuations and possible reference-free design give them a unique position among optical sensors. However, certain issues, such as experimental design (measurement type, dynamic range) and signal processing to decode information from the fluorescence lifetime, still have to be addressed