A miniature monolithic spatial heterodyne spectrometer for remote raman and libs, 1d raman imaging, and in-situ raman using a drone camera detector

Overview: Raman spectroscopy is a vibrational technique and is a good candidate for planetary exploration, because it can be used to make remote geochemical measurements and to identify organic and inorganic biomarkers of life [1]. In previous work we described a spatial heterodyne Raman spectrometer (SHRS) that is small with no moving parts, and ideally suited for planetary spacecraft and rovers. The SHRS is based on a fixed grating interferometer and has high spectral resolution and high light throughput. The resolution of the SHRS is not dependent on a slit, so miniature systems can be made without sacrificing resolution. A miniature SHRS we recently described used a cell-phone detector and imaging optics with 2.5 mm sized diffraction gratings, and the performance was compared to a standard laboratory instrument [2]. In this paper we describe an extension of this idea, using monolithic construction techniques to make a solid state SHRS (mSHRS), which is very stable and better suited to space applications. The mSHRS spectrometers are about 35x35x25mm in size, weigh about 80g, with a 3500 cm-1 spectral range and 4-5 or 8-9 cm-1 resolution, depending on the device. Experimental: Fig. 1 shows a monolithic SHRS next to a US Quarter for scale. The mSHRS interferometer consists of two 15 mm by 15 mm diffraction gratings, a BK7 50:50 cube beam splitter, and two angled BK7 spacers, cemented together with UV cured epoxy into a solid piece. The device pictured was designed for 532 nm and has ~8-9 cm-1 resolution with a 3500 cm-1 spectral range using a 2048 pixel CCD. Results and Discussion: The basic design and operation of the SHRS has been discussed previously. In the interferometer, collimated light is passed through a 50/50 beam splitter, diving the beam into two parts which are directed onto tilted diffraction gratings. After being diffracted off the gratings, the beams recombine at the beamsplitter as crossing wave fronts. The gratings are titled at an angle, θθLL, such that a particular wavelength, the Littrow wavelength, λλLL, is retro-reflected and recombined so that no interference pattern is produced. For any wavelength other than Littrow, the crossed wave fronts will generate a fringe pattern, which is imaged onto the CCD to produce a fringe image. The high throughput and multiplex advantage of the mSHRS allows Raman spectra to be measured using low-cost, uncooled CMOS, such as those used as cameras on commercial drones. To test this idea, we used a drone (Mavic Platinum Pro 2) to image the fringes produced by the gratings of the mSHRS. Figure 2 shows Raman spectra of (a) sulfur, (b) potassium perchlorate, and (c) ammonium nitrate using the mSHRS pictured in Fig. 1 using the drone camera (shown as inset, Fig. 2). The inserts in Figure 2 show the Raman interferograms for each spectrum. The SNR of the measured Raman spectra were comparable to similar measurements using a scientific grade, cooled CCD.