Monte Carlo simulation of the field back-scattered from rough surfaces

A novel approach for the simulation of the field back-scattered from a rough surface is presented. It takes into account polarization and multiple scattering events on the surface, as well as diffraction effects. The validity and usefulness of this simulation is demonstrated in the case of surface topology measurement. (C) 2012 Optical Society of America

On the Limits of Precision Glass Molding for Diffractive Optical Elements

Karin Prater

Diffractive optical elements (DOEs) consist of surface reliefs with dimensions in the micrometer range and nanometer precision. Two technologies dominate: Elements replicated in plastic and directly microfabricated elements in fused silica. Plastic DOEs are mostly used in mass production because they can be fabricated very cost efficient by replication technologies such as plastic injection molding and hot embossing. Glass DOEs are only used when its superior characteristics e.g. higher temperature stability, higher form accuracy due to a low reaction to humidity and stress are necessary for the specific application. Today, glass DOEs are fabricated by cleanroom technology based on direct structuring of fused silica. In this thesis we investigate the possibility to use precision glass molding to fabricate glass DOEs. Up to now, precision glass molding is used only for continuous surfaces like in aspherical lenses or freeform elements. Diffractive optical elements with more complex structures including surfacediscontinuities (steps) are not found. One reason is the lack of a suitable mold material that can withstand the high molding temperatures and can be microstructured with the necessary accuracy. One potential candidate to close this technology gap is glassy carbon. Glassy carbon is a fullerene like carbon with extreme temperature resistance and unmatched chemical inertness. The key factor of applying glassy carbon is the possibility to structure its surface with the suitable dimensions and conformity. We tested and developed microstructuring processes to overcome limitations such as process compatibility, etch selectivity, structural integrity and surface roughness. Our major objective of this work aimed to investigate the limits of precision glass molding for DOEs. Of special interest are the minimal feature size and maximal aspect ratio that can be obtained by keeping the optical quality of surfaces. We showed that precision glass can replicate features down to 800 nm and the process is stable. With this newly established processes the whole fabrication chain could be tested for the first time including lifetime tests of the stamp. For each step extensive characterization was done. Measuring the optical performance was the last step that allowed us to develop a complete guideline for the fabrication of diffractive optics with precision glass molding. Beamsplitting elements were chosen as test designs, because they allow a rigorous evaluation of the optical performance by measuring the diffraction efficiencies and uniformity distribution. As the main results we could show that precision glass molding of DOEs can reach a comparable optical performance as directly etched fused silica DOEs, which is the state-of-the-art technology. We could confirm that glassy carbon is an excellent mold material for precision glass molding of complex optical surfaces.

EPFL2017

Contactless optical trapping and manipulation of nanoparticles utilizing SIBA mechanism and EDL force

Mahdi Sahafi

On-chip optical tweezers based on evanescent fields overcome the diffraction limit of the free-space optical tweezers and can be a promising technique for developing lab-on-a-chip devices. While such trapping allows for low-cost and precise manipulation, it suffers from unavoidable contact with the device surface, which eliminates one of the major advantages of the optical trapping. Here, we use a 1D photonic crystal cavity to trap nanoparticles and propose a novel method to control and manipulate the particle distance from the cavity utilizing a self-induced back-action (SIBA) mechanism and electrical-double-layer (EDL) force. It is numerically shown that a 200 nm radius silica particle can be trapped near the cavity with a potential well deeper than 178k(B)T by 1 mW of input power without any contact with the surface and easily moved vertically with nanometer precision by wavelength detuning. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

2019

Multiple scattering in wide-field optical coherence tomography

Boris Karamata

Optical Coherence Tomography (OCT), a well-established imaging method based on low-coherence interferometry, provides cross-sectional images of the internal structure of biological samples with a resolution in the micrometer range. OCT was successfully applied on various tissues such as for instance the retina, the skin or a tooth. In highly scattering tissues like the skin, probing depth is limited to approximately 2mm, mainly due to insufficient rejection of multiply scattered light. Presently, the contribution of multiple scattering in OCT is not fully understood. Therefore, there is a strong and urgent need to develop models allowing a reliable evaluation of the system's limitations as well as the improvement of the imaging capabilities. It is generally believed that a relevant model should account for loss of correlation between the reference and the sample field due to multiple scattering. We developed a new comprehensive model of OCT. Our preliminary study revealed that the reference and sample fields are actually fully correlated. This important result allowed us to model the OCT signal as a sum of stationary random phasors and treated it as a statistical signal. The mean of this signal can be calculated thanks to classical results of statistical optics and to a Monte Carlo simulation. Unlike other existing models, our model accounts for the source autocorrelation function. The model proved to be in excellent agreement with a whole range of experimental data gathered in a comprehensive study of cross-talk in wide-field OCT. Moreover, our results put in question the applicability of widely used models of OCT based on the "extended Huygens-Fresnel principle", which assume a partial correlation between interfering fields due to multiple scattering. The construction of conventional OCT images is based on lateral scanning of a beam focused within the sample. To increase image acquisition speed and eliminate the need for lateral scanning, wide-field OCT was recently developed. Our experimental and theoretical investigations of the potential and limitations of wide-field OCT revealed the crucial role played by the spatial coherence of the light source. Spatially coherent illumination generates considerable coherent optical cross-talk, which prevents shot-noise-limited detection and diffraction-limited imaging in scattering samples. The dependence on several parameters of the optical system and of the sample properties was investigated in a comprehensive study. Cross-talk increases with the wide-field diameter, numerical aperture, source coherence length, and sample optical density; and strongly depends on sample anisotropy. We showed that spatially incoherent illumination realized with a thermal light source permits cross-talk suppression in wide-field OCT, i.e. rejection of multiply scattered light to a level comparable to that of point scanning OCT. We performed a theoretical study which revealed that the power per spatial mode radiated by thermal light sources is too low to permit a high signal-to-noise ratio while maintaining a fast acquisition speed. Therefore, wide-field OCT realized with either spatially coherent or spatially incoherent illumination suffers from inherent fundamental limitation. This led us to investigate the possibility of exploiting a spatially incoherent light source brighter than a thermal light source. We came to the conclusion that such a "pseudothermal" light source can potentially lead to wide-field OCT systems devoid of cross-talk and an image acquisition speed higher than that of a point scanning OCT system. However, the attractive properties of pseudothermal light sources could be gained at the expense of the simplicity and the economical advantages offered by thermal light sources. Furthermore, fast acquisition speed also relies on a performing "smart pixel detector array". Presently, such detectors do not have sufficient sensitivity and their frequency readout is too low as shown in our feasibility study. We performed a theoretical investigation of the potential of thermal light sources in terms of axial resolution and power per mode. The former revealed that the maximum power per mode is radiated at a wavelength higher than the spectral peak of a blackbody radiator. This led to the important practical conclusion that, at 6000 K, the maximum power is collected in the therapeutic spectral window in OCT (600 - 1300 nm), while at 3000 K this peak is shifted out of the therapeutic window leading to significant power losses. More generally, our work provides a design tool for choosing the optimal thermal light source for a given therapeutic window in terms of signal-to-noise ratio. Currently available sources at 6000 K consist of high pressure gas arc lamps providing a spectrum endowed with spectral lines deleterious for OCT. By suppressing a portion of the spectrum devoid of spectral lines of a mercury arc lamp, we obtained amongst the highest axial resolution so far reported in OCT. Furthermore, the importance of the speckle statistics in OCT incited us to clarify the origin of a difference between two theoretical results reported in the literature. Indeed, two calculations of the amplitude distribution of speckles in OCT, each of them based on a different mathematical formulation, yield different results. We showed that a modification of an initial assumption in one of the formulation leads to equivalent results. In conclusion, this thesis provides a deeper understanding of the potential and limitations of widefield OCT, leading to important design rules. Moreover, it presents a new comprehensive model of OCT putting in question other widely used models.