Mid-infrared photonic imaging strategies

Abstract

Imaging at mid-infrared (MIR) wavelengths between from 3–12 µm can provide unique insights and contrast mechanisms because of the low scattering of MIR light and the chemical specificity of MIR absorption. Consequently, new light sources and spectroscopic methods applied in the MIR offer previously unavailable capabilities for MIR hyperspectral (HS) or depth-resolved imaging. In this thesis, I report the development of three new MIR imaging techniques, configured in particular for applications in heritage science. Current MIR HS imaging technologies are expensive, as they employ complex cooled MIR detectors, or slow, in terms of acquisition rates. These factors limit widespread use in certain applications such as pigment mapping of paintings for cultural heritage. Here, I demonstrate two relatively inexpensive and fast HS imaging systems which utilise novel compressive sensing strategies. The first system is an imaging Fourier-transform spectromter (IFTS) based on recording a set of microbolometer camera frames of a sample’s response to illumination by uniquely spectrally structured blackbody radiation from a Michelson interferometer. The developed non-uniform sampling strategy was, to my knowledge, the first practical implementation of compressive sensing in Fourier-transform spectroscopy and enabled a sampling rate as low as 15% Nyquist-limited sampling with a generic prior. The instrument was used in a campaign at the Hunterian Museum on the artwork “Uplands in Lorne” by David Young Cameron. A second system utilised a fast digital micromirror device (DMD) to arbitrarily shape MIR spectra, providing sample illumination by optimised spectral structures for material identification. OCT has seen extensive work in the visible and near-infrared (NIR) but not as much in the MIR. This is, in part, due to the limitation of suitable MIR ultrafast sources. MIR OCT has the advantage of greater sample penetration depth which could find applications in the security sector. Results are presented from an MIR time-domain OCT system with an ultrafast orientation-patterned gallium phosphide (OP-GaP) optical parametric oscillator (OPO).