Spatial Heterodyne 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.

Scalable integrated two-dimensional Fourier-transform spectrometry

Integrated spectrometers offer the advantages of small sizes and high portability, enabling new applications in industrial development and scientific research. Integrated Fourier-transform spectrometers (FTS) have the potential to realize a high signal-to-noise ratio but typically have a trade-off between the resolution and bandwidth. Here, we propose and demonstrate the concept of the two-dimensional FTS (2D-FTS) to circumvent the trade-off and improve scalability. The core idea is to utilize 2D Fourier transform instead of 1D Fourier transform to rebuild spectra. By combining a tunable FTS and a spatial heterodyne spectrometer, the interferogram becomes a 2D pattern with variations of heating power and arm lengths. All wavelengths are mapped to a cluster of spots in the 2D Fourier map beyond the free-spectral-range limit. At the Rayleigh criterion, the demonstrated resolution is 250 pm over a 200-nm bandwidth. The resolution can be enhanced to 125 pm using the computational method.

Analysis of Velocimetry Processing Method Based on Doppler Asymmetric Spatial Heterodyne Spectrometer

Analysis of Velocimetry Processing Method Based on Doppler Asymmetric Spatial Heterodyne Spectrometer

Su8-waveguide based phase corrected fourier transform spectrometer chip with low side ripples

Integrated Fourier transform spectrometer (FTS) chip is of great interest recently due to its small size, low cost, and inherent high throughput. Phase and intensity errors caused by fabrication imperfections can give rise to severe side ripples in the retrieving spectrum of an FTS chip. To suppress the side ripples poses a great challenge, especially for the spatial heterodyne spectrometer (SHS) chip constituted by numerous Mach-Zehnder interferemoters (MZI) and having a relatively large size. Pseudoinverse algorithm combined with apodization function is considered to be able to compensate the phase and intensity errors efficiently. However, recent researches show that it still has its limitations, some results illustrate the side ripples of the retrieving spectra can be as large as −6 dB. In this article, we design, fabricate and characterize an Su8-waveguide based SHS chip with 51 MZIs, each MZI is accompanied with a pair of heating electrodes to correct the phase error. Different factors that contribute to the side ripples of the retrieved spectrum are analyzed and simulated. Our experimental result shows that by means of physically heat-tuning the metal electrodes and mathematically compensating the transmissivity fluctuations, the side ripples can be suppressed to be in the range from −19 dB to −15 dB for monochromatic and doublet light in our Su8-waveguide based SHS chip.

Algebraic ray trace analysis of spatial heterodyne spectrometers.

Algebraic ray traces of various configurations of spatial heterodyne spectrometers are developed to derive general, approximate, formulas for resolving the power, fringe localization plane, and admissible off-axis angle for each configuration. Michelson, all-reflective, and field-widened configurations are considered separately. The derived formulas for each configuration are tested against exact numerical ray traces using optical design software and in general found to be in good agreement.

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.

Faint but Not Forgotten. I. First Results from a Search for Astrospheres around AGB Stars in the Far-ultraviolet

Using the GALEX archive, we have discovered extended structures around ten asymptotic giant branch (AGB) stars (out of a total 92 searched) emitting in the far-ultraviolet (FUV) band. In all but one, we find the typical morphology expected for a spherical wind moving relative to, and interacting with, the interstellar medium (ISM) to produce an astrosphere. The exception is V Hya whose mass ejection is known to be highly aspherical, where we find evidence of its large parabolic outflows interacting with the ISM, and its collimated, extreme velocity outflows interacting with the circumstellar medium. For eight objects with relatively large proper motions, we find (as expected) that the termination-shock region lies in a hemisphere that contains the proper motion vector. Radial intensity cuts for each source have been used to locate the termination shock and the astropause’s outer edge. In a few objects, the cuts also reveal faint emission just outside the astropause that likely arises in shocked ISM material. We have used these data, together with published mass-loss rates and wind expansion velocities, to determine the total mass lost and duration for each source—we find that the duration of and total mass in the shocked wind are significantly larger than their corresponding values for the unshocked wind. The combination of FUV and far-IR data on AGB astrospheres provides a unique database for theoretical studies (numerical simulations) of wind–ISM interactions. We show that a Cyclical Spatial Heterodyne Spectrometer on a small space-based telescope can provide high-resolution spectra of astrospheres to confirm the emission mechanism.

Design and theoretical analysis of a conical-diffraction-based cross-dispersed spatial heterodyne spectrometer

A conical-diffraction-based cross-dispersed spatial heterodyne spectrometer (CDCDSHS) is presented. Conical diffraction is introduced, and one grating in the SHS is rolled around the central normal of its surface to expand the spectral range. The two-dimensional fringes are generated by the longitudinal-dispersion grating and the lateral-dispersion gratings through conical diffraction. Mathematical expressions are given for the parameter of an interferogram with varying spectral features and for the intensity distribution. Two examples are numerically simulated to demonstrate the performance of the spectrometer. The simulations show that the designed spectrometer can limit the interferogram of each spectral feature to be recorded by certain given rows of pixels on the charge-coupled device to avoid multiplexing disadvantages, and it can obtain clear features in each detailed spectrum with a higher signal-to-noise ratio than the conventional SHS while eliminating the need for filters or moving parts. Replacing the lateral-dispersion grating in the designed spectrometer with a mosaic grating can increase the spectral resolution and broaden the detectable spectral range. The designed spectrometer is also compact, very stable, and high-throughput. All mentioned advantages make the CDCDSHS a good prospect in broadband, high-resolution, high-sensitivity spectral measurement.

An Efficient Calibration System of Optical Interferometer for Measuring Middle and Upper Atmospheric Wind

Detection of the Doppler shift of airglow radiation in the middle and upper atmosphere is one of the most important methods for remote sensing of the atmospheric wind field. Laboratory and routine field calibration of an optical interferometer for wind measurement is very important. We report a novel calibration system that simulates a frequency shift of airglow emission lines introduced by wind in the middle and upper atmosphere for calibrating passive optical interferometers. The generator avoids the shortcomings of traditional motor-driven Doppler-shift generators in terms of stability and security while improving accuracy and simplifying assemblies. A simulated wind speed can be determined simultaneously using the light-beat method. The wind error simulated by the generator mainly comes from the light source, which is about 0.63 m/s. An experimental demonstration was conducted using a calibrated Fabry–Perot interferometer and showed that the root mean square of the measurement uncertainty is 0.91 m/s. The novel calibration system was applied to calibrate an asymmetric spatial heterodyne spectrometer (ASHS)-type interferometer successfully. The results demonstrate the feasibility of the system.

Spatial heterodyne spectroscopy for fast local magnetic field measurements of magnetized fusion plasmas.

A novel spectroscopy diagnostic for measuring internal magnetic fields in high temperature magnetized plasmas has been developed. It involves spectrally resolving the Balmer-α (656nm) neutral beam radiation split by the motional Stark effect with a spatial heterodyne spectrometer (SHS). The unique combination of high optical throughput (3.7 mm2sr) and spectral resolution (δλ ∼ 0.1 nm) allows these measurements to be made with time resolution ≪1ms. The high throughput is effectively utilized by incorporating a novel geometric Doppler broadening compensation technique in the spectrometer. The technique significantly reduces the spectral resolution penalty inherent to using large area, high-throughput optics while still collecting the large photon flux provided by such optics. In this work, fluxes of order 1010s-1 support the measurement of deviations of <5 mT (ΔλStark ∼ 10-4 nm) in the local magnetic field with 50 µs time resolution. Example high time resolution measurements of the pedestal magnetic field throughout the ELM cycle of a DIII-D tokamak plasma are presented. Local magnetic field measurements give access to the dynamics of the edge current density, which is essential to understanding stability limits, edge localized mode generation and suppression, and predicting performance of H-mode tokamaks.

Greenhouse gases monitoring instrument on a GF-5 satellite-II: correction of spatial and frequency-dependent phase distortion.

Interference data obtained by a spatial heterodyne spectrometer (SHS) is subject to various error factors and suffers from complex phase distortion. Traditional phase correction methods, such as the Amplitude, Merzt, and Forman methods, extract phase distortion in the spectral domain and correct it, which cannot effectively correct spatial phase distortion. Through theoretical derivation and numerical simulation, the spatial phase distortion is firstly determined and corrected in the interference domain. The frequency-dependent phase distortion is then extracted in the spectral domain and corrected. This novel phase distortion correction method named the phase decomposition method was applied to the in-orbit interference data of Greenhouse gases Monitoring Instrument-II (GMI-II). Compared with traditional phase correction methods, the root-mean-square error of the spectrum corrected using the phase decomposition method is reduced by 81.37%.

Determination of broadband-light atomic absorption through interferometric spectrometry with a spatial heterodyne spectrometer

A spatial heterodyne spectrometer (SHS) was combined with a flame atomic absorption (FAA) setup to reveal the analytical potential of SHS to be used as a tool for high-resolution atomic absorption studies.

Local measurements of the pedestal magnetic field profile throughout the ELM cycle on DIII-D

New high speed localized measurements of the pedestal magnetic field during the edge localized mode (ELM) cycle of a DIII-D High confinement mode (H-mode) discharge indicate a temporally and spatial complex redistribution of the edge current density profile, jedge. The measurement technique extracts the magnetic field magnitude, B, via the spectral separation of Stark-split neutral beam radiation in the pedestal. Single spatial channel measurements from a novel spatial heterodyne spectrometer are validated in discharges with core current profile changes. The technique measures Stark-splitting changes that imply B changes as small as 1 mT with high time resolution (50 μs). At normalized poloidal flux ψn=1.0, B appears saturated in the inter-ELM period and then rapidly decreases in &amp;lt;200 μs by ∼1%, before edge recycling emission begins to increase. Radially inboard of jedge, B increases at the ELM crash. The behavior is consistent with a rapid collapse of jedge at the ELM crash and subsequent pedestal recovery. In some discharges, at ψn&amp;lt;0.96, changes in B are observed throughout the ELM cycle. In others, B recovers and is relatively stable until a few ms leading up to the next crash. Measurements of B during the H-mode transition show a large increase at ψn=1 with little change at ψn=0.9, consistent with the formation of the edge bootstrap current density peak. The ψn=0.9 spectrum is complicated by predicted changes to the Stark component intensities with density at the L–H transition.

Detection of microplastics based on spatial heterodyne Raman spectroscopy

As a new type of pollutant, microplastics have attracted extensive attention due to its characteristics such as small size and degradation difficulty under natural circumstance. Though there are many detection methods nowadays, there are still some limitations like long detection time, high false detection rate and expensive detection equipment, which make the detection of microplastics exists in natural environment hard to carry out. To improve the environmental conditions, rapid, efficient, and accurate detection methods for microplastics are needed urgently. In this paper, a method combined the Spatial heterodyne technology and Raman spectroscopy was proposed to detect the microplastics. A spatial heterodyne spectrometer (SHS) was built, and the algorithms used to process the spectrum data were introduced. An optical system used to collect the Raman signal of the sample was built and connected with the SHS. Four different kinds of microplastic samples were detected by the complete system and identification can be achieved according to the Raman spectrum. The samples were also detected by using a normal spectrometer for comparison. A comparison of Raman spectrum using conventional dispersion spectrometer was carried out and the result shows that SHS is of higher signal-to-noise ratio. The results indicate that SHRS technology has good performance and development prospects in the field of microplastics detection.

Demonstration of a compact deep UV Raman spatial heterodyne spectrometer for biologics analysis.

Monoclonal antibodies and antibody fragments are increasingly important classes of biotherapeutics. However, these products are both challenging and expensive to manufacture. New process analytical technologies used to monitor these products during their manufacture are of significant interest. Deep UV Raman spectroscopy promises to provide the required specificity and accuracy, however instruments, have historically been large and complex. In this paper, a new deep UV Raman instrument is described using a solid-state laser and a spatial heterodyne spectrometer. The instrument overcomes practical limitations of the technique and could readily be used for online measurement. A series of observations have been made of biopharmaceutical products, including immunoglobulin G and domain antibodies. Where high levels of both specificity and linearity when measuring samples of different concentration with a precision of better than 0.05 mg/mL has been demonstrated.

Design study of a cross-dispersed spatial heterodyne spectrometer.

A cross-dispersed spatial heterodyne spectrometer (CDSHS) that integrates a spatial heterodyne spectrometer (SHS), a reflection grating, and a cylindrical lens is presented. Expressions for the width, height, and location of the cross-dispersed interferograms corresponding to narrow spectral regions are given. An example CDSHS design, including numerical simulations of the interferogram and the spectrum, is provided to illustrate the designed system. The results show that the CDSHS can simultaneously disperse longitudinally and laterally to record interferograms corresponding to different narrow spectral regions with different rows on a charge-coupled device, and obtain independent detailed spectra simultaneously with a high signal-to-noise ratio. Additionally, high-intensity light rays at a specific wavelength in the CDSHS do not interfere with the detailed spectra of the other wavelengths. Simultaneously, the CDSHS offers advantages including high resolution, high throughput, broadband operation, compactness, and zero moving parts. The CDSHS shows great application potential in fields including multiple spectral feature measurement, weak spectral measurements.

Charge exchange spectroscopy using spatial heterodyne spectrometer in the large helical device.

In this study, the use of a spatial heterodyne spectrometer (SHS) to measure the toroidal flow velocity (Vf) and the ion temperature (TC6+ ) of the C6+ impurity ion by charge exchange spectroscopy was explored. The instrumental width (IW) of the SHS (aperture size = 16.77 mm2, etendue = 2.9867 mm2sr) was extrapolated to be 0.09nm, which is half of the 0.17nm IW extrapolated for a conventionally used dispersive spectrometer (DS) (aperture size = 2.6 mm2, etendue = 0.2605 mm2sr). The resulting Vf and TC6+ measurements were found to be in good agreement with those measured using the DS.

Design of Orthogonal Tunable Spatial Heterodyne Spectrometer Based on Prism Dispersion

Design of Orthogonal Tunable Spatial Heterodyne Spectrometer Based on Prism Dispersion

Optical modeling of the characteristics of dual reflective grating spatial heterodyne spectrometers for use in laser-induced breakdown spectroscopy

In the present work, we performed a combination of non-sequential ray tracing and numerical interferometric simulations to study the effect of the most important optical and experimental parameters on the performance of spatial heterodyne interferometric spectrometers, relevant to their use in laser-induced breakdown spectroscopy (spatial heterodyne laser-induced breakdown spectroscopy, SH-LIBS). We provide a detailed, numerical assessment of the spectral bandpass, tuning characteristics, spectral resolution, sensitivity and temporal gating achievable in such an instrument. These modeling results can pave the way for the construction of improved SH-LIBS spectrometers for elemental sensing.