On-chip NIR optical spectrometer based on polymeric waveguide and metallic nano-structures

In this work, we report about optical spectrometry using gold nano-structures printed on a polymer based integrated optical waveguide. The optical waveguide is a single mode buried waveguide, having dimensions of 3x2.2 mu m(2). It is made from a combination of photo-polymerizable materials and is fabricated by photolithography on a glass substrate. To sense the electric field inside the waveguide, a gold nano-coupler array of thin lines (50 nm thick and 8 mu m length) is embedded on top of the aforementioned waveguide. They are produced by E-beam lithography. The array pitch is 2.872 mu m and the number of lines 564, which yields an array of 1.619 mm length. The device is enclosed with a glass superstrate to prevent it from dust and destruction. Both waveguide ports are polished and the output port in particular, is coated with a thin gold layer to assimilate a mirror and hence, it enables the creation of stationary waves inside the structure. The measurement procedure involves light injection using a single mode fiber carrying both visible light (658nm) and infrared light (785nm), used for alignment and measurement purposes respectively. Stationary waves generated inside the guide constitute the spatial interferogram. Locally, light is out-coupled using the nano-couplers and allows measuring the interferogram structure. The resulting pattern is imaged by a vision system involving an optical microscope with a digital camera mounted on-top of it. Signal processing, mainly based on Fast Fourier transform is performed on the captured signal to extract the spectral content of the measured signal.

Hans Peter Herzig, Maurine Malak Karam Awadallah, Toralf Scharf

In this work, we report about optical spectrometry using gold nano-structures printed on a polymer based integrated optical waveguide. The optical waveguide is a single mode buried waveguide, having dimensions of 3x2.2µm 2 . It is made from a combination of photo-polymerizable materials and is fabricated by photolithography on a glass substrate. To sense the electric field inside the waveguide, a gold nano-coupler array of thin lines (50 nm thick and 8 µm length) is embedded on top of the aforementioned waveguide. They are produced by E-beam lithography. The array pitch is 2.872 µm and the number of lines 564, which yields an array of 1.619 mm length. The device is enclosed with a glass superstrate to prevent it from dust and destruction. Both waveguide ports are polished and the output port in particular, is coated with a thin gold layer to assimilate a mirror and hence, it enables the creation of stationary waves inside the structure. The measurement procedure involves light injection using a single mode fiber carrying both visible light (658nm) and infrared light (785nm), used for alignment and measurement purposes respectively. Stationary waves generated inside the guide constitute the spatial interferogram. Locally, light is out-coupled using the nano-couplers and allows measuring the interferogram structure. The resulting pattern is imaged by a vision system involving an opticalmicroscope with a digital camera mounted on-top of it. Signal processing, mainly based on Fast Fourier transform is performed on the captured signal to extract the spectral content of the measured signal.

2014

On-chip near-infrared optical spectrometer based on single-mode Epo waveguide and gold nanoantennas

Joab Di Francesco, Maurine Malak Karam Awadallah, Irène Philipoussis Fernandez, Toralf Scharf

We report about optical spectrometry using gold nanostructures printed on top of an integrated optical waveguide. The optical waveguide is a single-mode buried waveguide made from a combination of photo-polymerizable materials and is fabricated by photolithography on a glass substrate. To detect the electric field inside the waveguide, a gold nanocoupler array of thin lines (50 nm thick and 8 mu m in length) is embedded on top of the aforementioned waveguide. They are produced by e-beam lithography. Both waveguide ports are polished, and the output port, in particular, is coated with a thin gold layer to assimilate a mirror and hence, it enables the creation of stationary waves inside the structure. Stationary waves generated inside the guide constitute a spatial interferogram. Locally, light is out-coupled using the nanocouplers and allows measuring the interferogram structure. The resulting pattern is imaged by a vision system involving an optical microscope with the objective lenses of different magnifications and a digital camera mounted on top of the microscope. The 5x objective lens demonstrates a superior performance in retrieving the investigated spectrum compared to 20x and 100x objectives. Fast Fourier transform is performed on the captured signal to extract the spectral content of the measured signal. (C) 2014 Society of Photo-Optical Instrumentation Engineers (SPIE)

Spie-Soc Photo-Optical Instrumentation Engineers2014

Sampling the Multiple Facets of Light

Gilles Baechler

The theme of this thesis revolves around three important manifestations of light, namely its corpuscular, wave and electromagnetic nature. Our goal is to exploit these principles to analyze, design and build imaging modalities by developing new signal processing and algorithmic tools, based in particular on sampling and sparsity concepts. First, we introduce a new sampling scheme called variable pulse width, which is based on the finite rate of innovation (FRI) sampling paradigm. This new framework enables to sample and perfectly reconstruct weighted sums of Lorentzians; perfect reconstruction from sampled signals is guaranteed by a set of theorems. Second, we turn to the context of light and study its reflection, which is based on the corpuscular model of light. More precisely, we propose to use our FRI-based model to represent bidirectional reflectance distribution functions. We develop dedicated light domes to acquire reflectance functions and use the measurements obtained to demonstrate the usefulness and versatility of our model. In particular, we concentrate on the representation of specularities, which are sharp and bright components generated by the direct reflection of light on surfaces. Third, we explore the wave nature of light through Lippmann photography, a century-old photography technique that acquires the entire spectrum of visible light. This fascinating process captures interferences patterns created by the exposed scene inside the depth of a photosensitive plate. By illuminating the developed plate with a neutral light source, the reflected spectrum corresponds to that of the exposed scene. We propose a mathematical model which precisely explains the technique and demonstrate that the spectrum reproduction suffers from a number of distortions due to the finite depth of the plate and the choice of reflector. In addition to describing these artifacts, we describe an algorithm to invert them, essentially recovering the original spectrum of the exposed scene. Next, the wave nature of light is further generalized to the electromagnetic theory, which we invoke to leverage the concept of polarization of light. We also return to the topic of the representation of reflectance functions and focus this time on the separation of the specular component from the other reflections. We exploit the fact that the polarization of light is preserved in specular reflections and investigate camera designs with polarizing micro-filters with different orientations placed just in front of the camera sensor; the different polarizations of the filters create a mosaic image, from which we propose to extract the specular component. We apply our demosaicing method to several scenes and additionally demonstrate that our approach improves photometric stereo. Finally, we delve into the problem of retrieving the phase information of a sparse signal from the magnitude of its Fourier transform. We propose an algorithm that resolves the phase retrieval problem for sparse signals in three stages. Unlike traditional approaches that recover a discrete approximation of the underlying signal, our algorithm estimates the signal on a continuous domain, which makes it the first of its kind. The concluding chapter outlines several avenues for future research, like new optical devices such as displays and digital cameras, inspired by the topic of Lippmann photography.

EPFL2018

Sampling the Multiple Facets of Light

Gilles Baechler

EPFL2018