Scott T. McCain

Multi-excitation Raman spectroscopy technique for fluorescence rejection

Multi-excitation Raman spectroscopy filters out Raman signals from a fluorescent background by sequentially using multiple excitation frequencies. The filtering method exploits the shift of the Raman spectra with excitation frequency and the static response of the fluorescent background. This technique builds upon previous work which used two slightly shifted excitations, Shifted Excitation Raman Difference Spectroscopy (SERDS), in order to filter the Raman signal. An Expectation-Maximization algorithm is used to estimate the Raman and fluorescence signals from multiple spectra acquired with slightly shifted excitation frequencies. In both simulation and experiment, the efficacy of the algorithm increases with the number of excitation frequencies even when holding the total excitation energy constant, such that the signal to noise ratio is inversely proportional to the number of excitation frequencies. In situations where the intense fluorescence causes significant shot noise compared to the weak Raman signals, the multi-excitation approach is more effective than non-iterative techniques such as polynomial background subtraction.

Coded Aperture Raman Spectroscopy for Quantitative Measurements of Ethanol in a Tissue Phantom

Coded aperture spectroscopy allows for sources of large étendue to be efficiently coupled into dispersive spectrometers by replacing the traditional input slit with a patterned mask. We describe a coded aperture spectrometer optimized for Raman spectroscopy of diffuse sources, (e.g., tissue). We provide design details of the Raman system, along with quantitative estimation results for ethanol at non-toxic levels in a lipid tissue phantom. With 60 mW of excitation power at 808 nm, leave-one-out and blind cross-validation of partial least squares (PLS) regression models achieve r2 > 0.98. Leave-one-out cross-validation demonstrates prediction errors of < 15% at the common legal limit for intoxication (17.4 mmol/L = 0.08% by vol) and the best blind cross-validation achieves < 12% error at this concentration.

Coded-aperture Raman imaging for standoff explosive detection

This paper describes the design of a deep-UV Raman imaging spectrometer operating with an excitation wavelength of 228 nm. The designed system will provide the ability to detect explosives (both traditional military explosives and home-made explosives) from standoff distances of 1-10 meters with an interrogation area of 1 mm x 1 mm to 200 mm x 200 mm. This excitation wavelength provides resonant enhancement of many common explosives, no background fluorescence, and an enhanced cross-section due to the inverse wavelength scaling of Raman scattering. A coded-aperture spectrograph combined with compressive imaging algorithms will allow for wide-area interrogation with fast acquisition rates. Coded-aperture spectral imaging exploits the compressibility of hyperspectral data-cubes to greatly reduce the amount of acquired data needed to interrogate an area. The resultant systems are able to cover wider areas much faster than traditional push-broom and tunable filter systems. The full system design will be presented along with initial data from the instrument. Estimates for area scanning rates and chemical sensitivity will be presented. The system components include a solid-state deep-UV laser operating at 228 nm, a spectrograph consisting of well-corrected refractive imaging optics and a reflective grating, an intensified solar-blind CCD camera, and a high-efficiency collection optic.

Multimodal multiplex Raman spectroscopy optimized for in vivo chemometrics

We have designed and constructed a multimodal multiplex Raman spectrometer which uses multi-wavelength excitation to better detect signals in the presence of fluorescence by taking advantage of the shift-variance of the Raman signal with respect to excitation frequency. Coupled with partial-least-squares (PLS) regression, the technique applied to ethanol estimation in a tissue phantom achieves root-mean-squared-cross-validation errors (RMSCVE) of 9.2 mmol/L with a model formed with 2 principal components, compared to a single wavelength data set with equivalent energy where 7 principal components were used to achieve an RMSCVE of 39.1 mmol/L.

Multimodal optical spectrometers for remote chemical detection.

We have developed a class of aperture coding schemes for Remote Raman Spectrometers (RRS) that remove the traditional trade-off between throughput and spectral resolution. As a result, the size of the remote interrogation region can be driven by operational, rather than optical considerations. We present theoretical arguments on the performance of these codes and present data from where we have utilized these codes in other spectroscopy efforts.

Coded-Excitation Raman Spectroscopy for Raman Signal Estimation in Highly Fluorescent Media

Raman signal estimation in highly fluorescent media is investigated using multiple excitation lasers and an iterative EM spectral reconstruction algorithm. Results from an 8-laser system show estimation performance increases with the number of excitation lasers.

Large-Aperture Raman Spectroscopy for Quantitative Chemometrics

One of the most selective techniques for identifying chemical unknowns has been limited by low cross- sections. Now we have techniques for enhancing the way we collect the photons and generate a signal from the vibrational manifold.

Multimodal, Multiplex, Raman Spectroscopy of Alcohol in Diffuse, Fluorescent Media

Optical diagnostics in biological materials are hindered by fluorescence and scattering. We have developed a multimodal, multiplex, coded-aperture Raman spectrometer to detect alcohol in a lipid tissue phantom solution.