Modelling of standard and specialty fibre-based systems using finite element methods

We report on the investigation of an approach for modelling light transmission through systems consisting of several jointed optical fibres, in which the analytical modelling of the waveguides was replaced by Finite Element Modelling (FEM) simulations. To validate this approach we first performed FEM analysis of standard fibres and used this to evaluate the coupling efficiency between two singlemode fibres under different conditions. The results of these simulations were successfully compared with those obtained using classical analytical approaches, by demonstrating a maximum loss deviation of about 0.4 %. Further, we performed other more complex simulations that we compared again to the analytical models. FEM simulations allow addressing any type of guiding structure, without limitations on the complexity of the geometrical waveguide cross section and involved materials. We propose as example of application the modelling of the light transmitted through a system made of a hollow core photonic crystal fibre spliced between two singlemode standard optical fibres, and qualitatively compare the results of the simulation with experimental results.

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

Direct observation of longitudinal spatial hole burning in semiconductor optical amplifiers with injection

Benoît Marie Joseph Deveaud, Marc-André Dupertuis, Lars Kappei, Daniel Marti

Measurements of spontaneous emission from InGaAsP semiconductor optical amplifiers provide information on both the carrier density and temperature. By spatially resolving the light emitted along the active layer of the device, we find evidence of longitudinal spatial hole burning which results from amplified spontaneous emission in the structure and is modified by the injected optical signal. Under injection, we also observe pronounced asymmetry of the amplified spontaneous emission intensity from the two facets which we relate to the carrier density profile. The experimental results are in good agreement with numerical simulations. An analysis of the measured spectra reveals an unexpected very high temperature (400 K) and its decrease by at least 35 K in the middle of the device when light is injected. (C) 2001 American Institute of Physics.

2001

Ultra low quantum decoherence nano-optomechanical systems

Mohammadjafar Bereyhi

Thermal motion of a room-temperature mechanical resonator typically dominates the quantum backaction of its position measurement. This is a longstanding barrier for exploring cavity optomechanics at room temperature. In order to enter the quantum regime of the optomechanical interaction, we need to be in the "backaction-dominated" regime, where we can study the limits of quantum measurement and how to circumvent them. To create a system which can enter the quantum regime, the optomechanical transducer has to satisfy certain criteria. A quantum-enabled optomechanical system consists of an optical cavity and a mechanical resonator with ultra low quantum decoherence, which are strongly coupled to each other via the optomechanical interaction.High stress silicon nitride has enabled nanomechanical resonators with exceptionally low dissipation at room temperature via several soft-clamping techniques. Achieving high optomechanical coupling to these coherent nanobeams results in high optomechanical cooperativities, thus alleviating the thermal motion barrier for room temperature quantum optomechanics. Monolithic integration of high-Q nanobeam resonators with optical cavities has been limited to doubly-clamped nanobeams due to the complexity of the device fabrication and long device sizes required for conventional soft-clamping using phononic crystals. In addition, the previous demonstrations of such systems showed limited optomechanical couplings thus a limited single photon cooperativity.I present the design, fabrication and characterization of three different classes of nanomechanical resonators clamp-tapered, fractal-like, and polygon resonators, which support perimeter modes, with Q factors exceeding 3 billion at room temperature and their optical readout using an integrated nearfield nano-optomechanical transducer using high stress silicon nitride. Our transducer features a one-dimensional Fabry-Perot optical cavity integrated with a high-Q nanomechanical resonator. The Fabry-Perot optical cavity is formed by patterning two photonic crystal reflectors on a silicon nitride waveguide designed for high-Q optical modes. Our approach allows individual optimization of optical and mechanical resonators, while maintaining a high optomechanical coupling rate due to large optomechanical mode overlap. Our best performing devices show on-chip optomechanical transducers with single photon cooperativities as high as 123 with mechanical quality factor of 120 million at room temperature. The developed system is of great interest to the optomechanics and sensing community. In quantum optomechanics, it will serve as a platform for quantum feedback control of the nanomechanical resonators to achieve motional ground state of a macroscopic resonator and generation of squeezed light at room temperature. Owing to their record value mechanical quality factors, the room temperature force sensitivity of our highest Q perimeter modes is on par with atomic force microscopy cantilevers at millikelvin temperature.Complete documentation of a nanofabrication process is the key to its reproducibility. Process-specific details of the fabrication techniques are usually missing in journal publications. To bridge this gap, I developed NanoFab-net.org. An online open-access tool that allows sharing process-specific fabrication reports, extracting the metadata and obtaining a unique digital object identifier (DOI) for each report.