Development and performance evaluation of a high mass resolution time of flight laser ablation ionisation mass spectrometer

This thesis contributes to the field of analytical chemistry of solid samples by introducing a new instrument with unprecedented performance characteristics and analytical capabilities. The instrument is based on the laser ablation and ionisation time-of-flight (LI-TOF-MS) technique. This technique has the potential to provide standard free quantitative analysis with sensitivities in the ppm range and below over a wide mass range of several hundred amu and more. LI-TOF-MS was initially developed and commercialised in the 1980ies and 1990ies by a series of companies. However, the interest in the technique faded soon because the laser and computer technology at the time was not sufficient to meet the requirements of reliable quantitative analysis and alternative techniques were preferred. In this thesis, a new instrument, called LMS- GT, is presented. By using modern computer and laser- technology, this instrument achieves mass resolutions of m/Dm = 10000 and sensitivities in the ppm range and below. These performance characteristics open new perspectives for the LIMS technique to rival the dominant analytical techniques. At the beginning of this project was a miniature LIMS prototype, from which plenty of know-how on laser ablation/ionisation mass spectrometry was collected, an empty laboratory space and the vision that is possible to build a high performance LIMS instrument for element and isotope analysis of solid samples at the micrometre scale. This thesis documents the technology transfer from the miniature prototype to a reliable full-scale laboratory instrument that was designed from scratch. A series of pre-developments on the miniature prototype allowed to test the potential of a UHV compatible microscope, the application of a high speed switch to blank highly abundant species in the mass spectra and advanced data processing. All of these pre-developments were published in three first author papers and are included in the thesis. Also, a thorough literature research was conducted to find the best instrument layout for the new system and initial parameters for the subsequent modelling of the ion system. The extensive simulation was concurrently carried out in SIMION, ZEMAX and CATIA for ion optics, light optics and mechanical design respectively. After successful completion of the design phase, over 4000 parts and several hundreds of metres of cables were manufactured and assembled. The complete development phase is documented in this thesis. Now, the initially empty laboratory space hosts an instrument setup of more than 2x2x2.5 m3 size. The commissioning phase showed that the design and assembly was performed correctly: Mass spectra were recorded since the first laser shot. The thesis concludes with the presentation of the first mass spectra recorded with the new instrument and a detailed discussion of the performance parameters. It was also possible to show good agreement between the simulation and the measurement.

Self-tracking Solar Concentration

Volker Günter Zagolla

Solar concentration is intended to decrease the cost of expensive solar cell material by replacing the solar cell with a focusing optical system as the initial receiving aperture. This reduces the amount of solar cell material needed but adds the need of tracking to the entire system in order to keep the smaller solar cell at the focal spot. In state of the art concentrators, precise tracking (less than 1°) is needed in two directions to cover diurnal and seasonal variations. This adds to the cost of the system, diminishing the savings from the smaller solar cell, and consumes energy which in turn decreases the overall system efficiency. A self-tracking concentrator has the potential to increases the tolerances of the system (less precise tracking needed) and cover seasonal variations (only tracking in one dimension remains). Self-tracking makes use of the energy of the spectrum that is typically not utilized in PV, thus self-tracking does not consume an outside surplus of energy. During the thesis such a self-tracking system was developed. This thesis describes two concepts for the realization of a self-tracking concentrator, one of which is then developed into a working demonstration device with an angular acceptance of 32° (±16°). The work covers both the development of these two different mechanisms as well as an analysis of their performance as a single element and a discussion of their performance as part of a much larger device. The first concept, the micro-fluidic concentrator, uses vapor bubbles to efficiently couple light into a liquid waveguide (chapter 3), while the second concept, based on a phase change actuation uses the thermal expansion of paraffin wax to create a coupling feature at a waveguide (chapter 4). Based on the findings, the phase-change concentrator was then expanded into a self-tracking concentrator with a possible effective concentration of 10x (chapter 5). As a result this thesis shows the feasibility of a self-tracking concentrator and describes in detail the processes and techniques for its realization. Future work based on the designs presented in this thesis can optimize the concepts. Simulations show that a system based on this approach can achieve 150x effective concentration by scaling the system collecting area to reasonable dimensions (30 x 1 cm²). An optimized optical system can then also increase the acceptance angle of the self-tracking concentrator to the ±23° needed in order to cover seasonal variations. In addition, the presented concept is based on earth abundant, inexpensive materials, making it technologically and economically viable to extend it to much larger formats.

EPFL2015

Endoscopic low coherence interferometry applied to tri-dimensional contouring of upper airways

Yves Delacrétaz

This thesis presents a novel approach for optical tri-dimensional contouring, requiring only one acquisition to extract a contour depth. It is based on an interferometric setup, using a reduced coherence light source, and is fully adaptable to endoscopic procedure commonly in used in ear-nose-throat (ENT) medicine. It has been demonstrated that the method can be successfully applied to imaging of porcine upper airways. Although this method uses interferometry to obtain depth range measurement, the phase signal is not directly evaluated, rather the location of the coherent superposition of two waves is extracted, and thus provides a robust technique even in harsh environment. The key feature is to extract the coherent area on the image by locating intentionally induced fringes created by off-axis superposition of a smooth reference wave and an object wave coming from a rough or diffusing sample through an optical system with limited aperture. A model for the propagation of the light through the optical system has been developed. It takes into account the broadened emission spectrum of the light source, as well as the statistical speckle effect induced by the illumination of a rough surface with partially coherent light. Simulated results have been carefully compared with measurements, both in the spatial domain and in the Fourier domain (spatial frequencies response), which has permitted to better understand the processes involved in the creation of the interference fringes, and improve the design of the device. Different filters have been investigated in order to extract the depth information from acquired interferograms. By using Gabor filterbanks, which do not need any a priori knowledge of the signal, selectivity of the extracted signal has been enhanced by a factor of 1.3 compared to a simple local Fourier spectrum evaluation, and time of the extraction procedure divided by 6, even when compared to fast iterative calculation in the Fourier spectrum. Moreover, by imposing the frequency and/or orientation of the signal to detect, which can be known by a simple calibration procedure, it has been proved that the selectivity can be further enhanced by a factor of 2.5, keeping the calculation time as short or even shorter. A compact prototype has been designed and built, with a specific multiple fibers arrangement for illumination. The system is separated in two parts: the first one is directly attached to the rigid endoscope, and contains imaging as well as recombining object/reference wave optics, and the detector. It fits into a box with overall dimensions of 115 × 83 × 94 mm. The second part contains the laser source, the scanning arm and the injection optics for the fibered system. It is designed to be left on a small cart. This device can potentially be brought to the bedside, and used during routine endoscopic check-up. Associated with the device, a complete framework has been elaborated in order to simplify the acquisition procedure as well as the signal processing. This results in a fully automated environment taking in charge hardware control, data storage and signal processing, up to the tri-dimensional scene rendering.

EPFL2009

Arbitrarily high time bandwidth performance in a nonreciprocal optical resonator with broken time invariance

Hatice Altug, Camille Sophie Brès, Ivan Cardea, Simon Julien Fabbri, Davide Grassani, Kosmas Tsakmakidis

Most present-day resonant systems, throughout physics and engineering, are characterized by a strict time-reversal symmetry between the rates of energy coupled in and out of the system, which leads to a trade-off between how long a wave can be stored in the system and the system’s bandwidth. Any attempt to reduce the losses of the resonant system, and hence store a (mechanical, acoustic, electronic, optical, or of any other nature) wave for more time, will inevitably also reduce the bandwidth of the system. Until recently, this time-bandwidth limit has been considered fundamental, arising from basic Fourier reciprocity. In this work, using a simple macroscopic, fiber-optic resonator where the nonreciprocity is induced by breaking its time-invariance, we report, in full agreement with accompanying numerical simulations, a time-bandwidth product (TBP) exceeding the ‘fundamental’ limit of ordinary resonant systems by a factor of 30. We show that, although in practice experimental constraints limit our scheme, the TBP can be arbitrarily large, simply dictated by the finesse of the cavity. Our results open the path for designing resonant systems, ubiquitous in physics and engineering, that can simultaneously be broadband and possessing long storage times, thereby offering a potential for new functionalities in wave-matter interactions.