Visible and Uv Standoff Raman Measurements in Ambient Light Conditions Using a Gated Spatial Heterodyne Raman Spectrometer

Overview: Raman spectroscopy is ideally suited for planetary exploration, because Raman spectra provide detailed molecular and structural information very useful for geochemical measurements as well as to measure organic and inorganic biomarkers in the search for past or present life on other planets [1]. Raman scattering, a very weak process, can be relatively strong at deep UV wavelengths because of the 1/λ dependence of Raman scattering and the possibility of large resonance enhancement for molecules that absorb strongly at UV wavelengths. However, the traditional grating based dispersive monochromators are relatively large and require very narrow entrance slits to provide the high resolution needed for deep UV wavelengths, thus limiting light throughput. Recently, we developed a new type of Fourier transform (FT) Raman spectrometer; the spatial heterodyne Raman spectrometer (SHRS), which provides high spectral resolution in a very small system without limiting light throughput. The SHRS addresses many of the issues related to the use of UV Raman for planetary exploration and standoff measurements. The SHRS is based on a stationary diffraction grating interferometer that uses an imaging detector to record an interferogram (see Fig. 1). The interferometer has no moving parts, and provides high spectral resolution in a very small package. Also, the SHRS has a large entrance aperture with a wide acceptance angle or field of view (FOV), which allows Raman measurements to be made using a large laser spot on the sample with low laser irradiance. This allows the SHRS to be used to measure highly absorbing photosensitive samples that might otherwise be damaged by the laser. Since there are no moving parts and all wavelengths are measured simultaneously, the SHRS is compatible with a pulsed laser and gated detectors allowing the measurements to be made in ambient light conditions, important for daylight measurements [2]. The use of gated detection also minimizes the need for special vibration isolation. Moreover, the SHRS is relatively easy to couple with a telescope with fewer laser pointing stability issues. The SHRS also has a relatively large Raman spectral band pass in the deep UV. All of these characteristics together make the SHRS a potentially powerful system for planetary exploration. Here, we present the first UV standoff Raman measurements using a SHRS. High spectral resolution Raman spectra are measured using a pulsed laser and gated detector in ambient light conditions with both visible and UV laser excitation, and without the use of a vibration isolation table. Experimental: A schematic of the standoff SHRS instrument is shown in Fig. 1. The SHRS interferometer consists of a 20 mm fused silica beamsplitter and a pair of 300 grooves/mm, 25 mm square diffraction gratings. The gratings were mounted in piezo motor driven optical mounts for precisely setting the grating angle. Interference fringes formed at the grating plane are imaged onto an intensified CCD (ICCD) detector with 1024×256 pixels using a 105 mm focal length, f/4.5 lens. A Q-switched Nd:YAG laser with fundamental frequency at 1064 nm was used to generate 532 nm and 266 nm laser pulses. The path of laser was collinear with the field of view of telescope. Light collected by the telescope was collimated and passed into the 20-mm aperture of the interferometer. Two laser-blocking filters, 266 nm or 532 nm (RazorEdge, Semrock) were used to block laser light. Spectra were produced from cross sections of the fringe images using a Fast Fourier transform (FFT) function.