The Development of a Spatial Heterodyne Spectrometer for Raman Spectroscopy

Raman spectroscopy is a valuable tool for molecular analysis and is currently being investigated for a wide range of applications, including standoff planetary measurements. NASA has stated a desire for small, space-flight capable instruments with minimal or no moving parts that can be used to analyze minerals and detect organic molecules and biomarkers on other planets. Raman spectroscopy is an ideal technique for this application and a standoff instrument would allow measurements to be made at a much greater range than is now possible. Existing Raman systems are not optimal for standoff planetary measurements because they are too large and heavy, contain moving parts, have low light throughput and have a limited ability to measure extended objects. This thesis discusses the development of a new type of Raman spectrometer for planetary applications called the Spatial Heterodyne Raman Spectrometer (SHRS). The SHRS is based on a Michelson interferometer, but the SHRS uses stationary diffraction gratings in place of mirrors and requires no moving parts. The SHRS does not require long focal length optics to achieve high spectral resolution, which makes the SHRS intrinsically small. The SHRS measures all wavelengths simultaneously and can be coupled with a pulsed laser source and gated detector for measurements in high ambient light conditions. The SHRS system boasts an expanded etendue because it does not have an entrance slit and has a large acceptance angle (up to 10 degrees). This allows the system to maintain high sensitivity when making standoff measurements of large area samples using defocused laser excitation. This is important for photosensitive samples where a larger laser spot size might reduce photodegradation. It is also useful for heterogeneous samples where measuring a larger area would be more representative of the sample as a whole. This dissertation will introduce the theories of Raman spectroscopy and spatial heterodyne spectroscopy, discuss the use of Raman spectroscopy for planetary studies, describe the development of the SHRS, and show how the SHRS can be used for wide area standoff measurements.