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.

Detection of microplastics via a confocal-microscope spatial-heterodyne Raman spectrometer with echelle gratings.

Microplastic pollution has become a global environmental problem that cannot be ignored. Raman spectroscopy has been widely used for microplastics detection because it can be performed in real-time and is non-destructive. Conventional detection techniques have had weak signals and low signal-to-noise ratios (SNR). Here, an efficient and reliable detection method is demonstrated. Specifically, a confocal microscope combined with an echelle-grating spatial-heterodyne Raman spectrometer (CM-ESHRS) was constructed. The confocal microscopy and the characteristics of the echelle grating enabled high optical throughput, high SNR, high spectral resolution, and a wide spectral detection band. After spectral calibration, the resolution approached 0.67 cm-1, moreover, the spectral detection range for a single order was 1372.16 cm-1. We detected and analyzed nineteen kinds of microplastics, such as polyamide, polypropylene, and polymethylmethacrylate, and the main vibrational spectral bands were categorized. Compared with commercial dispersive spectrometers, CM-ESHRS has a higher optical throughput. In addition, we examined microplastics with various particle sizes, microplastics mixed in flour, and microplastic particles of different materials under mixed conditions, all of which yielded complete spectral information. Overall, CM-ESHRS exhibits good potential applications for the detection of microplastics.

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.

White-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer.

A Stokes white-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer (IPSPPSI) is presented, which provides an effective solution to the problem of channel aliasing in broadband polarimeters. The expression for the light intensity distribution and a method to reconstruct polarization information are derived, and an example design for an IPSPPSI is given. The results reveal that a complete measurement of the Stokes parameters in broad band can be achieved with a snapshot on a single detector. The use of dispersive elements like gratings suppresses broadband carrier frequency dispersion so the channels in the frequency domain do not affect each other, ensuring the integrity of information coupled across the channels. Furthermore, the IPSPPSI has a compact structure and does not employ moving parts or require image registration. It shows great application potential in remote sensing, biological detection, and other fields.

Theoretical analysis of a multi-grating-based cross-dispersed spatial heterodyne spectrometer.

This paper presents a multi-grating-based cross-dispersed spatial heterodyne spectrometer (MGCDSHS). The principle of generation of two-dimensional interferograms for two cases, where the light beam is diffracted by one sub-grating or two sub-gratings, is given and equations for the interferogram parameters in these two cases are derived. An instrument design with numerical simulations is presented that demonstrates the spectrometer's ability to simultaneously record separate interferograms corresponding to different spectral features with high resolution over a broad spectral range. The design solves the mutual interference problem caused by overlapping of the interferograms, and also provides the high spectral resolution and broad spectral measurement range that cannot be achieved using conventional SHSs. Additionally, by introducing cylindrical lens groups, the MGCDSHS solves the throughput loss and light intensity reduction problems caused by direct use of multi-gratings. The MGCDSHS is compact, highly stable, and high-throughput. These advantages make the MGCDSHS suitable for high-sensitivity, high-resolution, and broadband spectral measurements.