Odd Lovhaugen

Dedicated spectrometers based on diffractive optics: Design, modelling and evaluation

Abstract The described design of diffractive optical elements for low cost IR-spectrometers gives a built-in wavelength reference and allows ‘spectral arithmetic’ to be implemented in the optical performance of the DOE. The diffractive element combines the function of the lenses and the grating and eliminates the need for alignment of those components in the standard scanned grating spectrometer design. The element gives out a set of foci, each with one spectral component, which are scanned across a detector, thus relaxing the demands for scan angle control. It can thus be regarded as an alternative solution to a beam splitter and band pass filter instrument. Software tools have been designed to ease the adaptation of the design to different applications. To model the performance of the spectrometers we have implemented a scalar Rayleigh-Sommerfeldt diffraction model. The gold-coated elements are produced by injection moulding using a compact disc (CD) moulding technique and mould inlays mastered by e-beam lithography. The optimized selection of wavelength bands and the classification of the measured signal use a combination of principal component analysis and robust statistical methods. Typical applications will be material characterization of recycled plastics and gas monitoring. Spectrometers for two different applications have been built and tested. Comparisons between the design goals and the measured performance have been made and show good agreements.

Reconfigurable near-infrared optical filter with a micromechanical diffractive Fresnel lens

We have fabricated and characterized a micromechanical Fresnel lens that serves as a reconfigurable filter for near-infrared spectrometry, for the analysis of gas, liquids, or solids. The lens consists of silicon segments that can be individually actuated. Moving the segments vertically controls the spectrum of the light that is focused on a detector. The holographic pattern on top of the segments permits both focusing of the light and spectral control. We have demonstrated experimentally the desired spectral control of the filter.

Configurable spectral filter with an array of diffraction grating

This paper presents the design, fabrication, and characterization of a configurable diffractive filter capable of high optical resolution within a narrow spectral range. Based on measurements of spectral reflectance from this device, and from corresponding set of fixed diffractive elements, this type of spectral filter is well suited for spectroscopy applications.

Discrete dipole approximation: a numerical tool for subsurface sensing

We present a method to simulate scattering of microwaves in realistic soil representations. Our results will be used by the designers of a new microwave antenna for humanitarian mine clearance being developed within the European Project PICE. We aim to identify scenarios giving false alarm signals and search for means to avoid these conditions. We use the Discrete Dipole Approximation (DDA), a method originally developed to study the scattering of light by particles in interstellar dust. The DDA simulates the scattering objects by a lattice of electric dipoles and calculates the total scattering as the contribution of all the dipoles in the lattice to the total field. It is a very suitable method to study electromagnetic scattering by objects of irregular shape and by clusters of them. We have successfully applied DDA to the analysis of light scattering by clusters of many thousands of particles, simulating pigments in paper coatings.

A micro-mechanical fresnel lens for spectral filtering

In this paper, there has been a recent interest for MEMS-based adjustable or re-configurable diffraction filters. Reconfigurable diffraction filters are well adapted to NIR spectroscopy, with applications such as identification of polymers and detection of carbon monoxide in air. A cost-effective implementation of a NIR spectrometer was proposed by Lovhaugen. The system uses a focusing static Diffractive Optical Element (DOE). The set-up minimizes the number of elements in the system, as no lens is needed

Low-cost spectrometers based on diffractive optics

This presentation describes the developments done in spectroscopic instrumentation using diffractive optics. The aim of the work has been to produce durable, low-cost multiwavelength sensors for, for instance, material classification and gas level monitoring. The primary example is a spectrometer developed for classification of plastic material in recycling. The production cost should be a few 10s USD.