Spatial Heterodyne Raman Spectrometer Research Articles

High-resolution, broad-spectral-range Raman measurement using a spatial heterodyne spectrometer with separate filters and multi-gratings

We propose a high-resolution, broad-spectral-range spatial heterodyne Raman spectrometer (SHRS) having separate filters and multi-gratings (SFMG). A prototype of the SFMG-SHRS is built using multi-gratings with four sub-gratings having groove densities of 320, 298, 276, and 254 gr/mm and separate filters with filter bands corresponding to the sub-gratings. We use the SFMG-SHRS to measure the Raman spectra of inorganic and organic compounds with various integration times, laser power, and transparent containers, compare measurements of microplastics with and without the separate filters, and measure mixtures of inorganic powders and organic solutions. The designed SFMG-SHRS makes high-resolution, broad-spectral-range Raman measurements with improved signal-to-noise ratios and visibility of weak Raman peaks even in the presence of fluorescence.

Development of a high-resolution, broadband spatial heterodyne Raman spectrometer based on field-widened grating-echelle structure

We propose a spatial heterodyne Raman spectrometer (SHRS) based on a field-widened grating-echelle (FWGE). A normal grating is combined with an echelle grating in a conventional spatial heterodyne spectrometer to eliminate ghost images without using masks, and prevents interference among the spatial frequencies of different diffraction orders. Mathematical expressions and derivation processes are given for the spectral parameters in the FWGE-SHRS and a verification breadboard system is fabricated. The FWGE-SHRS measures Raman spectra of single chemicals and mixed targets with different integration times, laser powers, concentrations, and transparent containers. The results of the experiments demonstrate that the FWGE-SHRS is suitable for high-resolution, broadband Raman measurements for a wide range of applications.

Design and evaluation of spatial heterodyne Raman spectrometer at 785 nm excitation wavelength

Design and evaluation of spatial heterodyne Raman spectrometer at 785 nm excitation wavelength

Enhancement of the signal-to-noise ratio by compressing the illumination of a spatial heterodyne Raman spectrometer

Improving the spectral signal-to-noise ratio is crucial for obtaining high-quality Raman spectra of target substances. To achieve a higher instrument signal-to-noise ratio in the development of spatial heterodyne Raman spectrometer, the effect of spatially compressed illumination on the spectral signal-to-noise ratio was theoretically analyzed. A spatial heterodyne Raman spectroscopic experimental setup with compressed illumination was constructed. The experimental results with ethanol validate the significant enhancement of the signal-to-noise ratio achieved through compressed illumination. The ethanol spectra exhibited signal-to-noise ratio enhancements ranging from 1.38 to 7.98 times under various excitation powers and integration times. These findings provide valuable guidance for further improving the performance of spatial heterodyne Raman spectrometers.

Quantitative Analysis of Mixtures Based on Portable Spatial Heterodyne Raman Spectrometer

Spatial heterodyne Raman spectroscopy has been widely applied in various fields due to its non-contact, nondestructive, fast, high stability, and high spectral resolution characteristics. This article integrated spatial heterodyne Raman spectroscopy with chemometric methodologies to assess the feasibility of peak-to-peak ratio regression, partial least squares regression, support vector machine regression, and non-negative matrix factorization for the quantitative analysis of mixtures. Chemometrics methods were used to model and analyze the interference data in the interferogram domain, and variational mode decomposition was used to extract features from the interference data, further improving the interference data’s modeling and prediction accuracy. The results demonstrate that modal intensities obtained through variational mode decomposition of interference data improve the prediction accuracy of regression analysis. Support vector regression exhibited the most favorable predictive performance among the tested models. The root mean square error of cross-validation was reduced from 5.55% to 2.64%, and the prediction root mean square error was reduced from 5.08% to 1.5%. The improved model utilizing interference data showed higher fitting accuracy and more precise sample predictions compared to spectral data modeling.

Single-Grating Monolithic Spatial Heterodyne Raman Spectrometer: An Investigation on the Effects of Detector Selection.

Spatial heterodyne Raman spectrometers (SHRSs) are modified forms of Michelson interferometers, except the mirrors in a Michelson interferometer are replaced with stationary diffraction gratings. This design removes the need for an entrance slit, as is the case in a dispersive spectrometer, and removes the need to scan the spectrum by using a moving mirror in a modern Michelson interferometer. In previous studies, various SHRS variants, such as free-standing two-grating SHRS, single-grating SHRS (1g-SHRS), monolithic SHRS (mSHRS), and single-grating mSHRS (1g-mSHRS), have been evaluated. However, the present study exclusively focuses on the 1g-mSHRS configuration. The 1g-mSHRS and 1g-SHRS increase the spectral range at fixed grating line density while trading off spectral resolution and resolving power. The mSHRS benefits from increased rigidity, lack of moving parts, and reduced footprint. In this study, we investigate how the choice of detector impacts the performance of the 1g-mSHRS system, with a specific focus on evaluating the performance of three types of cameras: charged-coupled device (CCD), intensified CCD (ICCD), and complementary metal-oxide-semiconductor (CMOS) cameras. These systems were evaluated using geological, organic, and inorganic samples using a 532 nm continuous wave laser for the CMOS and CCD cameras, and a 532 nm neodymium-doped yttrium aluminum garnet pulsed laser for the ICCD camera. The footprint of the 1g-mSHRS was 3.5 × 3.5 × 2.5 cm3 with a mass of 272 g or 80 g, depending on whether the monolith housing is included or not. We found that increasing the number of pixels utilized along the x-axis of the camera increases fringe visibility (FV) and optimizes the resolution (by capturing the entirety of the grating and magnifying the fringes). The number of pixels utilized in the y-axis, chip size, and dimensions, affect the signal-to-noise ratio of the systems. Additionally, we discuss the effect of pixel pitch on the recovery of Fizeau fringes, including the relationship between the Nyquist frequency, aliasing, and FV.

Broadband, high-resolution spatial heterodyne Raman spectroscopy measurement based on a multi-Littrow-angle multi-grating.

This paper proposes a spatial heterodyne Raman spectrometer (SHRS) based on a multi-Littrow-angle multi-grating (MLAMG). Compared with a conventional multi-grating, the MLAMG not only provides higher spectral resolution and a broader spectral range, but is also easier to produce. A verification breadboard system is built using the MLAMG combined with four sub-gratings with a groove density of 300 gr/mm and Littrow angles of 4.6355°, 4.8536°, 5.0820°, and 5.3253°. This MLAMG-SHRS is used to obtain the Raman spectra of inorganic solids and organic solutions for different integration times, laser powers, suspension contents, and containers. The Raman spectra of mixed targets and minerals are also presented. The experiments demonstrate that the MLAMG-SHRS is suitable for broadband measurements at high spectral resolution in a wide range of potential applications.

Remote Raman Sensing Using a Single-Grating Monolithic Spatial Heterodyne Raman Spectrometer: A Potential Tool for Planetary Exploration.

Advances in Raman instrumentation have led to the implementation of a remote dispersive Raman spectrometer on the Perseverance rover on Mars, which is used for remote sensing. For remote applications, dispersive spectrometers suffer from a few setbacks such as relatively larger sizes, low light throughput, limited spectral ranges, relatively low resolutions for small devices, and high sensitivity to misalignment. A spatial heterodyne Raman spectrometer (SHRS), which is a fixed grating interferometer, helps overcome some of these problems. Most SHRS devices that have been described use two fixed diffraction gratings, but a variance of the SHRS called the one-grating SHRS (1g-SHRS) replaces one of the gratings with a mirror, which makes it more compact. In a recent paper we described monolithic two-gratings SHRS, and in this paper, we investigate a single-grating monolithic SHRS (1g-mSHRS), which combines the 1g-SHRS with a monolithic setup previously tested at the University of South Carolina. This setup integrates the beamsplitter, grating, and mirror into a single monolithic device. This reduces the number of adjustable components, allows for easier alignment, and reduces the footprint of the device (35 × 35 × 25 mm with a weight of 80 g). This instrument provides a high spectral resolution (∼9 cm-1) and large spectral range (7327 cm-1) while decreasing the sensitivity to alignment with a field of view of 5.61 mm at 3m. We discuss the characteristics of the 1g-mSHRS by measuring the time-resolved remote Raman spectra of a few inorganic salts, organics, and minerals at 3 m. The 1g-mSHRS makes a good candidate for planetary exploration because of its large spectral range, greater sensitivity, competitively higher spectral resolution, low alignment sensitivity, and high light throughput in a compact easily aligned system with no moving parts.

Research on Near-infrared Spatial Heterodyne Raman Spectrometer

Research on Near-infrared Spatial Heterodyne Raman Spectrometer

A Monolithic Spatial Heterodyne Raman Spectrometer: Initial Tests.

A monolithic spatial heterodyne Raman spectrometer (mSHRS) is described, where the optical components of the spectrometer are bonded to make a small, stable, one-piece structure. This builds on previous work, where we described bench top spatial heterodyne Raman spectrometers (SHRS), developed for planetary spacecraft and rovers. The SHRS is based on a fixed grating spatial heterodyne spectrometer (SHS) that offers high spectral resolution and high light throughput in a small footprint. The resolution of the SHS is not dependent on a slit, and high resolution can be realized without using long focal length dispersing optics since it is not a dispersive device. Thus, the SHS can be used as a component in a compact Raman spectrometer with high spectral resolution and a large spectral range using a standard 1024 element charge-coupled device. Since the resolution of the SHRS is not dependent on a long optical path, it is amenable to the use of monolithic construction techniques to make a compact and robust device. In this paper, we describe the use of two different monolithic SHSs (mSHSs), with Littrow wavelengths of 531.6 nm and 541.05 nm, each about 3.5 × 3.5 × 2.5 cm in size and weighing about 80 g, in a Raman spectrometer that provides ∼3500 cm-1 spectral range with 4-5 cm-1 and 8-9 cm-1 resolution, for 600 grooves/mm and 150 grooves/mm grating-based mSHS devices, respectively. In this proof of concept paper, the stability, spectral resolution, spectral range, and signal-to-noise ratio of the mSHRS spectrometers are compared to our bench top SHRS that uses free-standing optics, and signal to noise comparisons are also made to a Kaiser Holospec f/1.8 Raman spectrometer.

Hyperspectral Raman Imaging Using a Spatial Heterodyne Raman Spectrometer with a Microlens Array.

A new hyperspectral Raman imaging technique is described using a spatial heterodyne Raman spectrometer (SHRS) and a microlens array (MLA). The new technique enables the simultaneous acquisition of Raman spectra over a wide spectral range at spatially isolated locations within two spatial dimensions (x, y) using a single exposure on a charge-coupled device (CCD) or other detector types such as a complementary metal-oxide semiconductor (CMOS) detector. In the SHRS system described here, a 4 × 4 mm MLA with 1600, 100 µm diameter lenslets is used to image the sample, with each lenslet illuminating a different region of the SHRS diffraction gratings and forming independent fringe images on the CCD. The fringe images from each lenslet contain the fully encoded Raman spectrum of the region of the sample "seen" by the lenslet. Since the SHRS requires no moving parts, all fringe images can be measured simultaneously with a single detector exposure, and in principle using a single laser shot, in the case of a pulsed laser. In this proof of concept paper, hyperspectral Raman spectra of a wide variety of heterogeneous samples are used to characterize the technique in terms of spatial and spectral resolution tradeoffs. It is shown that the spatial resolution is a function of the diameter of the MLA lenslets, while the number of spatial elements that can be resolved is equal to the number of MLA lenslets that can be imaged onto the SHRS detector. The spectral resolution depends on the spatial resolution desired, and the number of grooves illuminated on both diffraction gratings by each lenslet, or combination of lenslets in cases where they are grouped.

One‐mirror, one‐grating spatial heterodyne spectrometer for remote‐sensing Raman spectroscopy

AbstractRecently, we evaluated a new type of Fourier transform Raman spectrometer, the spatial heterodyne Raman spectrometer (SHRS), which provides high‐resolution Raman spectra without the need for an entrance slit. An SHRS is a variant of a Michelson interferometer in which the mirrors in the arms of a Michelson interferometer are replaced by two stationary diffraction gratings. Instead of sampling path length differences temporally, as in the case of a Michelson interferometer, SHRS samples path length differences spatially at a two‐dimensional array detector. Applying a Fourier transform to the resulting interferogram recovers the desired spectrum. In the modified SHRS (mSHRS) instrument used in the present work, one of the diffraction gratings has been replaced by a stationary λ/10 mirror. This modification has a few effects. First, the detector records a greater number of photons, as spectral light is not lost into unused diffraction orders in the mirror arm of mSHRS. Second, the free spectral range (wavelength coverage) is doubled, whereas the spectral resolution is cut in half. In this work, the authors present remote‐sensing Raman spectra of minerals, organics, and biomarkers using this mSHRS for the first time.

Spatial Heterodyne Raman Spectrometer (SHRS) for In Situ Chemical Sensing Using Sapphire and Silica Optical Fiber Raman Probes.

A spatial heterodyne Raman spectrometer (SHRS), constructed using a modular optical cage and lens tube system, is described for use with a commercial silica and a custom single-crystal (SC) sapphire fiber Raman probe. The utility of these fiber-coupled SHRS chemical sensors is demonstrated using 532 nm laser excitation for acquiring Raman measurements of solid (sulfur) and liquid (cyclohexane) Raman standards as well as real-world, plastic-bonded explosives (PBX) comprising 1,3,5- triamino- 2,4,6- trinitrobenzene (TATB) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) energetic materials. The SHRS is a fixed grating-based dispersive interferometer equipped with an array detector. Each Raman spectrum was extracted from its corresponding fringe image (i.e., interferogram) using a Fourier transform method. Raman measurements were acquired with the SHRS Littrow wavelength set at the laser excitation wavelength over a spectral range of ∼1750 cm-1 with a spectral resolution of ∼8 cm-1 for sapphire and ∼10 cm-1 for silica fiber probes. The large aperture of the SHRS allows much larger fiber diameters to be used without degrading spectral resolution as demonstrated with the larger sapphire collection fiber diameter (330 μm) compared to the silica fiber (100 μm). Unlike the dual silica fiber Raman probe, the dual sapphire fiber Raman probe did not include filtering at the fiber probe tip nearest the sample. Even so, SC sapphire fiber probe measurements produced less background than silica fibers allowing Raman measurements as close as ∼85 cm-1 to the excitation laser. Despite the short lengths of sapphire fiber used to construct the sapphire probe, well-defined, sharp sapphire Raman bands at 420, 580, and 750 cm-1 were observed in the SHRS spectra of cyclohexane and the highly fluorescent HMX-based PBX. SHRS measurements of the latter produced low background interference in the extracted Raman spectrum because the broad band fluorescence (i.e., a direct current, or DC, component) does not contribute to the interferogram intensity (i.e., the alternating current, or AC, component). SHRS spectral resolution, throughput, and signal-to-noise ratio are also discussed along with the merits of using sapphire Raman bands as internal performance references and as internal wavelength calibration standards in Raman measurements.

Raman Spectroscopic Investigation of Sulfates Using Mosaic Grating Spatial Heterodyne Raman Spectrometer

We introduce a field-widened spatial heterodyne Raman spectrometer with a mosaic grating structure to investigate the broadband Raman spectra of sulfates. The broadband static spatial heterodyne Raman spectrometer configuration employs two mosaic gratings instead of the diffraction gratings to divide the broadband Raman spectrum into two bands. It is able to record high-resolution, broadband Raman spectrum range, covering 5740 cm -1 with 3.061 cm -1 spectral resolution using a regular CCD in a single-shot measurement. Raman spectra of mineral sulfates (celestine, gypsum) are investigated. This is the first time to conduct this Raman spectroscopic experiments to investigate the sulfates using the spatial heterodyne Raman spectroscopy. The sodium, potassium and ammonium sulfates at solid state and in aqueous solutions had been detected and analyzed. The effects of metal ions (K + , Na + , NH4 + , Ca 2+ , Sr 2+ ) on vibration of sulfate anion SO4 2- were discussed. The main v1 mode can be probed by the spatial heterodyne Raman spectroscopy to determine the concentration of sulfate and unambiguous identification of different sulfate minerals, solid sulfates.

Raman measurements using a field‐widened spatial heterodyne Raman spectrometer

AbstractSpatial heterodyne Raman spectroscopy has become a useful spectroscopic detection technique that is particularly suitable for Raman measurements. This method uses the Fourier transform of the interferogram imaged on the detector by stationary diffraction gratings. Spatial heterodyne Raman spectroscopy has the same characteristic of conventional Fourier‐transform spectroscopy, which is that the field of view is limited. We propose a two‐dimensional spatial heterodyne Raman spectrometer (SHRS) that uses a field widening prism to effectively increase the throughput or sensitivity without sacrificing the spectral resolution for Raman measurements while broadening the bandpass. The signal‐to‐noise ratio is measured under different integrations or laser powers and shows that the system exhibits good stability and fair repeatability. Raman spectra obtained from the field‐widened SHRS and a commercialized Raman instrument are compared, and the field‐widened SHRS and the SHRS using the same instrumental parameters without field widening are also compared. A higher signal‐to‐noise ratio can be achieved using the field‐widened SHRS. In our measurements, the field‐widened SHRS can be operated under the slightly more complex conditions required for wide‐field measurements with short integration times and low laser power. Raman spectra of a pure inorganic target or a target contained in glass and a plastic bottle are observed. A sulfate is investigated in two states: solid salt and aqueous solution. Several mixture liquids, mixture solids, minerals, and anti‐Stokes Raman shifts are also investigated. The results show that the field‐widened SHRS exhibits a good performance to successfully perform wide‐field Raman measurements.

Backscattering Raman measurements using a broadband and high-resolution mosaic grating spatial heterodyne Raman spectrometer

Backscattering Raman measurements using a broadband and high-resolution mosaic grating spatial heterodyne Raman spectrometer

Broadband, high-resolution Raman observations from a double-echelle spatial heterodyne Raman spectrometer.

A new broadband Raman spectrometer has been developed, to the best of our knowledge, using a double-echelle spatial heterodyne Raman spectrometer (DESHRS). The instrument is constructed by using two echelle gratings. Masks are used to remove the shadow ghosts caused by the different orders of the two echelle gratings. Raman spectra of inorganic solid targets and methanol are given, and Raman shifts of up to 3000 cm-1 are obtained by the DESHRS. The instrument has shown that a broadband coverage and high resolution can be achieved simultaneously to meet the requirements of Raman measurements, covering 3590 cm-1 with 1.21 cm-1 spectral resolution.

Broadband transmission Raman measurements using a field-widened spatial heterodyne Raman spectrometer with mosaic grating structure.

A field-widened spatial heterodyne Raman spectrometer with a mosaic grating structure is developed for the simultaneous sensitivity enhancement and broadband transmission Raman measurements. We optimize the etendue to maximize the signals collected from the samples by using field-widening prisms and employ two mosaic gratings to achieve broadband operation, covering 5638 cm-1 with 2.865 cm-1 spectral resolution. The signal-to-noise ratios are improved by a factor of more than 11 and show a good stability and fair repeatability. We investigate the effects of the sample thickness and outer layer depth and observe liquids, solids, mixed targets, and anti-Stokes shifts. The instrument exhibits good performance for wide-field, high-resolution broadband transmission Raman measurements.

Development of a spatial heterodyne Raman spectrometer with echelle-mirror structure.

Spatial heterodyne Raman spectroscopy is a spectroscopic detection technique that is particularly suitable for Raman measurements. The spectral range of traditional spatial heterodyne Raman spectrometer (SHRS) is limited by its spectral resolution and the number of detector elements. We propose an SHRS with an echelle-mirror structure that employs multiple diffraction orders to achieve a broad spectral coverage and high spectral resolution simultaneously. This SHRS is used to obtain the Raman spectra of organic liquids, inorganic solid targets, and mixed targets. Observations of aqueous solutions, and minerals are presented. In addition, anti-Stokes Raman shifts are also presented. The proposed SHRS technique shows good performance for broadband, high-resolution Raman measurements.

Optimizing Data Reduction Procedures in Spatial Heterodyne Raman Spectroscopy with Applications to Planetary Surface Analogs.

A spatial heterodyne Raman spectrometer (SHRS) is a variant of a Michelson interferometer in which the mirrors of a Michelson are replaced with two stationary diffraction gratings. When light enters the SHRS, it is reflected off of diffraction gratings at frequency-dependent angles that produce crossed wavefronts in space that can be imaged using a plane array detector. The crossed wavefronts, which represent a superposition of interference fringes, are converted to a Raman spectrum upon applying a Fourier transform. In this work, a new approach to intensity calibration is discussed that originates from modeling the shot noise produced by the SHRS and converting the real noise to idealized white noise as predicted by theory. This procedure has two effects. First, the technique produces Raman spectra with white noise. Second, when the mean of the noise is normalized to one, the technique produces Raman spectra where the intensity axis is equivalent to signal-to-noise ratio. The data reduction technique is then applied to the measurement of materials of interest to the planetary science community, including minerals and inorganic salts, at a distance of 5 m from the collecting optic.