Design and performances of an optical metrology system to test position and tilt accuracy of fiber positioners

This paper describes the design of an optical metrology system for fiber positioners. The system can be used for accurate calibration and verification of fiber positioners with SCARA-like RR planar kinematics. It is capable of measuring accurately the absolute position and tilt of the fiber tip over the whole workspace of the positioner. The metrology system works by back illuminating the optical fiber of the positioner with a laser. The position and tilt of the exiting cone at the tip of the fiber is measured with two optical cameras.

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

Mid-infrared surface plasmon resonance sensing applied to refractive index measurements of liquids

Sylvain Herminjard

There has been a very strong development of the sensors based on surface plasmon resonance during the last thirty years, mostly for biological and biomedical applications. If the first experiments in this field were carried out at the beginning of the 20t century, it would have been necessary to wait for the end of the sixties to see the first devices performing accurate measurements of the refractive indices of gas and liquid samples. Indeed, the surface plasmon resonance phenomenon allows to measure refractive index variations with a high sensitivity that is hardly obtained with other optical measurement techniques. The basic principle consists of exciting the free electrons of a metallic layer with light, which induces the formation of a surface wave called surface plasmon. This resonant phenomenon depends on the wavelength of the excitation light and the refractive indices of the metallic layer and the material located close to the metallic layer. A variation of the refractive index of the latter induces a variation of the resonance condition, which allows to realize optical sensors designed to measure determined samples. An appropriate calibration allows to set a relationship between the variation of the concentration of the sample and the measured refractive index value. Among the various parameters that describe the performances of the sensor, one of the most important is the sensitivity. It is possible to demonstrate that the excitation wavelength has an impact on the sensitivity of the system : the latter increases if the measured sample has an absorption peak at a wavelength close to the excitation wavelength. The main goal of this thesis is to explore this sensitivity enhancement technique based on the absorption properties of the probed medium. A proof-of-principle is experimentally demonstrated with an optical setup constituted of laser sources working in the visible range of the electromagnetic spectrum. However, many materials have absorption peaks located in the mid-infrared range due to their fundamental molecular vibrations. Therefore, the realization of a plasmonic sensor working in the mid-infrared could allow to measure these materials with an increased sensitivity. To the best of our knowledge, surface plasmon resonance based sensors have never been developed above the near-infrared. This thesis presents for the first time the design and realization of a plasmonic sensor working in the mid-infrared range able to measure refractive index variations of gas and liquids samples. Even if the optical sources, components and detectors designed for the mid-infrared are clearly different from the ones designed for the visible range, it has been possible to successfully realize two systems working in the mid-infrared range during this project. The first is able to perform gas measurements. As the excitation wavelength depends on the measured sample, it was necessary to select a laser source that had a wavelength located close to the absorption peak of the gas under measurement. For this purpose, we designed a plasmonic sensor working at a 4.4 µm wavelength able to measure carbon dioxide (CO2) absorbing at 4.3 µm. It has been demonstrated that the experimental sensitivity of this system is five times higher than the theoretical sensitivity obtained with a system working in the visible range at a 633 nm wavelength. The second has been conceived to measure liquid samples in order to measure variations of concentration of CO2 dissolved in water. If measurements of refractive index variations (without using the sensitivity enhancement technique) could be performed successfully with variations of concentration of alcohol in water, further work is necessary to get decisive results for CO2 dissolved in water. This project was carried out in collaboration with an industrial partner in order to take into account some industrial constraints during the development of the sensor. Therefore, particular care has been taken in order to design a system that is compact, mechanically robust, without any mechanical moving parts, cheap and easily reproducible. The technological development presented in this thesis finds many applications mainly in the chemical, biological and biomedical fields. Indeed, many proteins show absorption peaks in the mid-infrared. It could be possible to measure their concentration or the modification of their properties – like their composition or structure – with an increased sensitivity. The interest of surface plasmons is that the measurement is not a transmission-based technique, but rather a "reflection-based technique", as the light does not go through the gas or liquid sample. This measurement technique is also adapted for high volume samples that are present in industrial production processes like water or soft drink quality control.

EPFL2010

Brillouin dynamic gratings in optical fibres for distributed sensing and advanced optical signal processing

Andrey Denisov

This thesis presents results of a research on applications of Brillouin dynamic gratings - distributed reflectors that can be dynamically created in an optical fibre by two optical waves. A basic theory of stimulated Brillouin scattering (SBS) is introduced, on a level enough for understanding the processes that govern SBS. A major part of this thesis is dedicated to studies of distributed Brillouin sensors based on phase correlation. First, the concept of correlation-based sensors is introduced - the commonly used Brillouin optical correlation-domain analysis (BOCDA) and the phase-correlation technique. It is described how the Brillouin interaction between two waves can be localised, creating a permanent reflector confined to a centimetre-scale section of the fibre. This allows creating a distributed sensing system with very a high spatial resolution. A detailed theoretical model for the phase-correlation technique is presented, showing how the gain response of a system can be calculated and how the system resolution can be determined. For an ideal case an analytical solution is derived, while for real experimental conditions the expected behaviour is found via numerical simulation. Results of numerical modelling are compared with experimentally obtained data, showing a good agreement. A spatial resolution of 1 cm is demonstrated over a 200 m distance representing 20000 separate points. The concept of time gating is introduced, extending the measurement distance from 200 m to 17.5 km while retaining a sub-centimetre spatial resolution. This technique allows for a two order of magnitude increase in the number of points that a system is capable to resolve. An absolute record for distributed fibre sensors is achieved, demonstrating a system capable to resolve 2100000 separate points. Limitations for a further increase in number of points are discussed as well as possible ways to overcome them. An issue related to the temperature dependency of the refractive index is discussed in details, since it can lead to significant errors in spatial accuracy. An algorithm is presented that is capable of using the measured temperature change to account for the change in the refractive index and correctly determine the positioning of measured data. In the last chapter potential applications of Bragg dynamic gratings in signal processing are investigated. A theoretical model of BDG's in polarisation-maintaining fibres is presented capable of calculating reflection of a probe wave - continuous or pulsed. The model includes the case of non-uniform birefringence along the fibre. Dynamic gratings are applied to create a flip-flop - an all-optical memory, turned on and off by a light pulse. A working system is demonstrated in a 1 m long fibre, corresponding to a 10 ns storage time. Using the theoretical model the birefringence variation is measured along the fibre. A preliminary study of spectral properties of dynamic gratings is presented, along with a model predicting the spectrum of a uniform BDG. It is demonstrated that the spectral properties of a BDG can be manipulated by changing the spectra of optical waves used in the generation process. This work is concluded by a discussion, summing up the above-mentioned theoretical the experimental work. Potential applications of the presented research are proposed along with the most promising direction of further research activities.

EPFL2015