Remote Pulsed–laser Raman Spectroscopy System for Mineral Analysis

Introduction: There is a need for an instrument that can be used for remote in situ identification of biogenic and a-biogenic minerals, various types of ices, organic materials etc. on planetary surfaces. The potential for measuring remoteRaman spectra of organic [1] and inorganic materials and minerals [2] with a small telescope and CW lasers have been demonstrated. Pulsed-laser Raman spectroscopy, or simply pulsed-Raman spectroscopy, is a very powerful technique for identifying both inorganic and organic materials from their unique vibrational signatures. Pulsed Raman techniques offer two important benefits: namely, (i) discrimination against unwanted ambient light, and (ii) discrimination against long lived fluorescence emission from the sample [3]. With pulsed Raman techniques high quality Raman spectra of minerals and melts at high temperatures have been measured for investigating phase transitions and anharmonicity of various vibrational modes [4]. Advancements in solid-state lasers and gated ICCD detectors have made it possible to develop a compact and lightweight Raman system for remote analysis of mineral surfaces. In this paper, we present results of our pulsed-Raman measurements at a distance of 10’s of meters in the laboratory as well as in an outdoor environment. Experimental: The remote pulsed-Raman system consists of a 5-inch telescope, a 20 Hz frequency doubled NdYAG laser source and a 1⁄4 meter spectrometer and gated intensified detector. Samples were placed at a distance of 10 to 66 meters from the telescope and were excited with a 532-nm laser pulse , with < 35 mJ/pulse, from the frequency doubled NdYAG laser (Model ULTRA CFR, Big Sky Laser). The width of the laser pulses was 8 nanosec (ns), and the beam divergence was <8 mrad. The scattered light from the sample was collected using the telescope (Meade ETX-125). This Maksutov-Cassegrain telescope has a 127-mm clear aperture and a focal length of 1900 mm. The ULTRA CFR laser head was mounted along the length of the telescope tube. The laser beam was made coaxial with the telescope axis using two small dichroic mirrors. Raman spectra were recorded with a Spex 0.25 meter imaging spectrograph (Model 270M) equipped with a ruled grating of 1800 grooves/mm blazed at 500 nm, and with a thermoelectrically cooled (-20 C), gated and intensified ICCD detector (Model IMax ICCD Princeton Instruments, Inc.). We used an optical fiber of 1-meter length and 200-μm diameter to couple the telescope output with the spectrometer. The output of the telescope was collimated with a lens. A holographic super-notch filter was inserted between the collimating lens and a 10X microscopic objective that focussed the light at the end of the fiber. The super-notch filter was adjusted to attenuate the elastically (Rayleigh and Mie) scattered and diffuse reflected light. We used benzene as a test sample to measure the performance of this system. We recorded an excellent Raman spectra of benzene with high signal to noise ratio to a distance of ~66 meters using as few as a single laser pulse. Measuring the remote-Raman spectra of a marble (CaCO3) and silicate and hydrous minerals further verified the system performance. Results and Discussion: Figure 1 shows the fingerprint region of the Raman spectra of benzene recorded with the pulsed-Raman system in an out