Spatial Heterodyne Research Articles

A deep convolutional neural network for blind element error correction of spatial heterodyne spectrometer using line selective convolutional blocks

The "GF Special Project" is a massive remote sensing technology initiative including a number of satellites and various observation platforms. GF-5 is the satellite with the most payloads, the highest spectral resolution, and the most difficulty in development, and it can monitor a variety of environmental elements using spatial heterodyne spectroscopy (SHS) technology, including atmospheric aerosols, carbon dioxide, methane, terrestrial vegetation, straw burning, and urban heat islands. In this study, a novel blind element error correction technique based on deep learning network is investigated and developed for spatial heterodyne interferograms, as well as the formation mechanism and distribution characteristics of the SHS interferometric data. LSConv-Net, a new CNN model, was created and trained to denoise in the presence of high-density and ultra-high-density blind element errors. We do this by introducing a new line-selective convolutional (LSConv) block. Simultaneously, experimental validation of blind element error correction utilizing laboratory water vapor interferometric data and atmospheric CO2 absorption interferometric data from GF-5, and the change in FWHM before and after the experiment was tested using potassium lamp interferograms. Experiments show that the Deep neural networks trained with this model may successfully suppress the effect of blind element noise on spectra, recover spectra that have been overwhelmed by high-density blind element noise without any effect on other non-blind pixels, and surpass all similar techniques in terms of spectral recovery.

Spatial phase distortion compensation technology based on combinatorial optimization method in heterodyne detection

The spatial phase distortion caused by turbulent atmosphere, target speckle, or optical system aberration causes decoherence of heterodyne signals. The decoherence effect is the key problem that imposes restrictions on the application of laser heterodyne techniques. We propose an approach for spatial phase distortion correction based on a genetic algorithm by establishing a one-to-one correspondence between combination of spatial phase compensation amounts and heterodyne efficiency of the system. The spatial phase distortion compensation problem is converted into a combinatorial optimization problem. The experiment results show that the method achieves excellent compensation performance for the spatial phase distortion, significantly contributing to the applications of heterodyne techniques.

Structural design and cryogenic stability analysis for Long-wave Infrared Spatial Heterodyne Spectrometers

Structural design and cryogenic stability analysis for Long-wave Infrared Spatial Heterodyne Spectrometers

On-orbit spectral characteristics analysis of the greenhouse gases monitoring instrument: Spectral wavelength stability and instrumental line shape stability

The Greenhouse Gases Monitoring Instrument (GMI) is a carbon satellite payload developed based on the principle of spatial heterodyne spectroscopy technology, which is specifically designed for the global analysis of greenhouse gases (CO2 and CH4) “sources” and “sinks”. Due to the low concentration and minimal gradient variations of CO2 and CH4 in the atmosphere, higher precision is required for their retrieval. Vibrations during satellite launch and harsh space environments during on-orbit operation may cause changes in the performance of various payload components, resulting in decreased quality of spectrum products and retrieval precision. This paper proposes a set of on-orbit spectral characteristic evaluation methods tailored for the GMI to monitor the quality of its spectrum. It also develops an adaptive blind pixel detection and correction algorithm specifically for the GMI and optimizes the interferogram processing flow algorithm. On-orbit spectral calibration is performed, and methods to evaluate spectral characteristic parameters have been established. The analysis focuses on the variations of the spectral characteristics in the CO2-1 bands (1.575 μm) of the GMI during one-year of on-orbit calibration spectrum. The initial spectral wavenumber offset is 0.133 cm−1 after entering orbit, and the average spectral wavenumber offset is 0.0313 cm−1 during stable operation. During the one-year period, the maximum variation in the instrumental line shape function of the GMI is 0.006 cm−1. The observed spectrum obtained in nadir observation mode, underwent to accuracy verification. The results demonstrate consistency between the spectral peak wavenumbers and the relative trend changes of the observed and theoretical spectrum, with an average relative residual of 0.987%.

Practical high-resolution spectroscopy with a spatial heterodyne spectrometer: Determination of instrumental function for lineshape recovery

The spatial heterodyne spectrometer (SHS) is a well-recognized platform for its high resolving power in various use cases of spectroscopy. Same as other spectrometer topologies, the SHS, unfortunately, also suffers from classical challenges such as distorted lineshape due to the instrumental function. The goal of this work is to tackle this persisting issue through a simple numerical approach. With the inherent characteristics of an SHS interferogram, we report the direct extraction and determination of the instrumental function in its numerical representation from an SHS interferogram; this instrumental function was further used for spectral data processing that enables significant improvements in spectral resolution through deconvolution algorithms.Here, we systematically discuss the recognition of the embedded instrumental function among various ingredients within an interferogram. To verify the numerical approach, lithium was chosen as the model sample, resembling the use of SHS in an isotopic analysis application. Specifically, the resonance transition of lithium D-lines (2P1/2,3/2 ← 2S1/2) was selected to assess the performance of the spectral processing. With the spectral deconvolution, the spectral features that represent the 6Li and 7Li were nearly baseline-separated, allowing for the accurate measure of the isotopic abundance without external references or algorithm adjustments (e.g., curve fitting).

Deep learning-based denoising of spatial heterodyne spectroscopy interferograms without clear images

A newly developed spectroscopic method called spatial heterodyne spectroscopy (SHS) characterized by grating space diffraction, compact size, high etendue, larger optical tolerance, and no moving parts. Spatial heterodyne spectroscopy has many uses, including atmospheric remote sensing, atmospheric composition research, and so on. However, due to the features of the sensor material and the operating environment, various noise will be introduced throughout the CCD image acquisition process. The imaging performance of the device will be reduced or possibly disabled as a result of these noise effects. We demonstrate how deep learning techniques may be used to denoise SHS images. Furthermore, we propose the use of N2N (noise2noise, noise to noise) method for the difficulty of deep learning to obtain a large number of target-noise pair. This method trains the deep learning network using only noisy images, which greatly reduces the difficulty of acquiring training data. We analyzed denoising data for monochromatic and continuous light. In terms of interferograms, the experimental results show that the deep neural network trained using this method can effectively recover interferogram image information, suppress noise, and maintain the continuity of the interference fringe edge. In terms of spectrograms, the deep neural network trained by this approach can suppress noise in monochromatic spectra and retrieve spectral signals damaged by noise in continuous spectra. As a result, the technique is suitable for spatial heterodyne spectroscopy denoising.

Compact Spatial Heterodyne Spectrographs for Future Space-Based Observations: Instrument Modeling and Applications.

High-resolution spectroscopy employing spatial heterodyne spectrographs (SHS) holds significant promise for forthcoming space missions, building upon its established track record in science applications. Notably, it offers exceptional performance and cost- effectiveness in the ultraviolet-visual (UV-Vis) region compared to contemporary instruments. SHS instruments provide high-resolution capabilities and substantially larger etendues than similar resolving power instruments. This study introduces a comprehensive Python-based SHS model integrated with a user-friendly web scraping interface for target star selection, parameter generation, and 2D interferogram creation. Our SHS model demonstrates double the resolving power of a grating spectrometer and a throughput comparable to a Fourier transform spectrometer (FTS) but without moving parts, enhancing robustness for deployment in space. The interferogram processing algorithm includes flat-fielding, bias removal, apodization, and an inverse Fourier transform (IFT) for accurate spectrum retrieval. Despite bandwidth limitations due to resolving power constraints, SHS models excel in applications requiring high spectral resolution over narrow wavelength ranges, such as studying isotopic emission lines. The model provides optimization results and trade-offs for system parameters, ensuring precise spectral recovery with realistic signal-to-noise ratio (SNR) values. SHS is versatile and effective for various scientific applications, including investigating atomic and molecular emissions from comets, planetary atmospheres, the Earth's atmosphere, the Sun, and the interstellar medium (ISM). This research significantly contributes to expediting the development and deployment of SHS instruments, demonstrating their potential across numerous scientific domains.

Interference correction for polarization spectral intensity modulation (PSIM) with spatial heterodyne spectroscopy (SHS)

Spectropolarimetry, capable of measuring both the polarization parameters and spectral information of light, is widely utilized in astrophysical research and atmospheric remote sensing. Spatial Static Polarization Modulation Interferometric Spectroscopy merges polarization spectral intensity modulation (PSIM) with spatial heterodyne spectroscopy to achieve high spectral resolution and static synchronous measurement of continuous band polarization spectral information. This article is based on spatial static polarization modulation interferometric spectroscopy principles and describes correction techniques for non-uniform response and phase errors observed in interferograms. A quantitative relationship was established through simulation between the non-uniform response, phase errors in the interferograms, and the demodulation accuracy of polarization parameters. An experimental setup based on these principles was constructed to validate the effectiveness of the correction. The experimental results demonstrated that by using the reported data processing workflow the error in the degree of polarization was reduced from 0.1354 to 0.0095.

Analysis of signal-to-noise ratio of spatial heterodyne spectroscopy

We have established a novel spatial heterodyne spectroscopy (SHS) signal-to-noise ratio (SNR) model, relating the spectral SNR to the spectral band and resolution, which helps guide the instrument's design and optimization and assess the spectral quality. The experimental and simulation results show that the spectral resolution affects the SNR differently under different noise types and spectral characteristics of the measured targets, which fully validates the novel SHS SNR model. Despite the disadvantage of multiplexing in SHS, we determine the SNR advantage of SHS over the grating spectroscopy (GS) through rigorous theoretical derivations. In 94.34 % of the polychromatic light detections, the average SNR of SHS is 2–31 times higher than that of GS. In emission spectra detections, the SNR gain of SHS relative to GS is up to 1–2 orders of magnitude. In additive noise domination, the SNR gain reaches 1–3 orders of magnitude.

Broadband spectropolarimeter based on spatial heterodyne spectroscopy and channeled polarimetric technique

A spectropolarimeter based on spatial heterodyne spectroscopy and the channeled polarimetric technique (SSHSCP) is introduced. The SSHSCP uses a polarization grating for channel modulation, meaning that the Stokes parameter is no longer dependent on the wavenumber; this suppresses channel aliasing considerably and improves the polarization parameter reconstruction accuracy. The SSHSCP’s optical path structure is based on the Mach–Zehnder interferometer, ensuring efficient energy use. The spectral detection and polarization detection principles of the SSHSCP are derived, and a specific design example is presented. Simulations indicate that the SSHSCP can restore the target’s spectral and complete Stokes parameters with high accuracy for a broadband source and narrowband lines. The reconstructed Stokes parameter errors are of the order of 10−3 or less. The SSHSCP acquires spectral and polarization parameters in a single step and has no moving parts in a compact structure.

Research on Calculation Method of On-Orbit Instrumental Line Shape Function for the Greenhouse Gases Monitoring Instrument on the GaoFen-5B Satellite

The Greenhouse Gases Monitoring Instrument is based on the spectroscopic principle of spatial heterodyne spectroscopy technology and has the characteristics of no moving parts, a hyperspectral resolution, and a large luminous flux. The instrumental line shape function is one of the most important parameters characterizing the features of the instrument, and it plays a vital role in the system error analysis of the instrument’s measurements. To accurately obtain the instrumental line shape function of a spatial heterodyne spectrometer during the on-orbit period and improve the accuracy of gas concentration retrieval, this study develops a method to model and characterize the characteristics of the instrumental line shape function, including modulation loss and phase error. This study employs the solar calibration spectrum in the 1.568–1.583 μm bands to conduct iterative calculations of the instrumental line shape function error model. After the instrumental line function is updated, the average relative deviation is reduced from 1.83% to 0.756% between the theoretical and measured solar spectra. Additionally, the average relative deviation is reduced from 7.049% to 2.106% between the GMI nadir and theoretical nadir spectra. The findings demonstrate that updating the instrumental line shape function mitigates the impact of variations in the spectrometer’s instrumental line shape due to alterations in the orbital environment. This study offers a dependable reference for both the enhancement and oversight of a spectrometer’s instrumental line shape function, along with an investigation of shifts in instrument parameters.

Half-Inch Monolithic Spatial Heterodyne Raman Spectrometer: A Study of Polarized Raman Spectra of Organic Liquids and Instrumental Performance.

Raman spectroscopy allows for the unambiguous identification of materials through the inelastic scattering of light. This technique has a great many uses in various aspects of society from academic, scientific, and industry. This paper explores a specific type of Raman spectrometer called a spatial heterodyne Raman spectrometer (SHRSy), which is a variation of an interferometric spectrometer. It utilizes a Michelson interferometer and replaces the mirrors with gratings that transform it from a time-domain spectrometer to a spatial-domain spectrometer, allowing for the entirety of the spectrum to be captured at once. This study specifically tests a half-inch two-grating monolithic SHRS (½-in. 2g-mSHRS), which has a weight of <60 g and a size of 2.2 × 2.2 × 1.3 cm. To do this we excite a variety of organic liquids with a 532 nm neodymium-doped yttrium aluminum garnet (Nd:YAG) pulsed laser, using an excitation energy of 6.5 mJ/pulse and distance of 3 m in conjunction with an intensified charge-coupled device camera. This is the first time that the SHRS has been used for investigating polarized Raman spectra of liquids. We discuss and contrast the instrumental properties such as resolution, spectral range, étendue, and field of view with previously tested mSHRS to give context to the instrument's performance.

High dynamic range spatial heterodyne one-dimensional imaging spectroscopy based on a digital micromirror device.

Spatial heterodyne one-dimensional imaging spectrometer (SHIS) can simultaneously acquire hyperspectral information from different fields of view (FOVs). However, the dynamic range of SHIS is limited by the detector's performance. We propose a high dynamic range spatial heterodyne one-dimensional imaging spectroscopy (HD-SHIS) based on a digital micromirror device (DMD), which can control the exposure time of each FOV signal by adjusting the flip time of micromirrors on an M-bit DMD, realizing the simultaneous detection of strong and weak signals in FOVs with a theoretical improvement of the dynamic range by dB. Meanwhile, HD-SHIS utilizes a DMD to realize the Hadamard modulation of interference data in the spectral dimension, which can be used with the linear array detector to complete the detection of the imaging spectrum. We have built an HD-SHIS principle prototype and carried out dynamic range experiments. The experimental results show that HD-SHIS can achieve 48 dB dynamic range improvement by utilizing an 8-bit display width DMD.

Detection of microplastics based on splicing grating spatial heterodyne Raman spectroscopy

As a new type of persistent pollutant, microplastics pose a serious threat to the earth’s ecological environment and human health. Efficient and reliable microplastic detection technology is of great significance in the management of microplastic pollution. Aiming at the problems of low signal-to-noise ratio (SNR), narrow spectral range and low spectral resolution in traditional microplastic detection technology, a splicing grating spatial heterodyne Raman spectroscopy (SG-SHRS) is proposed in this paper. The splicing grating is composed of four sub-gratings with groove densities of 320, 298, 276 and 254 gr / mm, respectively. Each sub-grating has an independent sub-filter to improve the SNR of the system. The system is simulated, built and calibrated. The actual resolution of the SG-SHRS system is 0.7 cm−1, and the spectral detection range of a single sub-grating is 2947.2 cm−1. Four kinds of microplastics, polyamide (PA), polystyrene (PS), polycarbonate (PC), and polyphenylene sulfide (PPS), were detected by the SG-SHRS system. The complete Raman spectral information of microplastics was obtained, and the peak assignment of Raman characteristic peaks of the four kinds of microplastics was analyzed. By comparing the test results with a commercial dispersion spectrometer, it has been proven that the SG-SHRS system has the advantages of high spectral resolution, wide spectral range, and high SNR, and has good application prospects in the field of microplastic detection.

Research on asymmetric spatial heterodyne spectroscopy for velocimetry navigation

The further development of deep space exploration contributes to the exploration of the universe and the origin and evolution of life on Earth, and is a prerequisite and foundation for the development and utilization of space resources. Autonomous navigation of deep space probes, one of the key technologies for deep space exploration, can significantly reduce ground support costs and improve the autonomous operation, management, and on-orbit survivability of deep space probes. Among them, autonomous astronomical navigation methods based on velocity measurements directly measure velocity information, effectively avoiding the impact of using calculus to solve velocity on response time and navigation accuracy in navigation methods based on angle and distance measurements. The passive radial velocity measurement technique of asymmetric spatial heterodyne spectroscopy has the advantages of compact structure, large luminous flux, and multiple spectra detected simultaneously. Taking the Sun as the navigation target source, we carried out the selection of observational spectral lines and the parameter design of the velocimetry navigation system, designed the calculation of phases for the absorption characteristic line under the complex polychromatic strong background, carried out the simulation analysis of velocimetry under the typical relative velocimetry.

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

A study on denoising with deep convolutional neural networks in spatial heterodyne spectroscopy

Spatial heterodyne spectroscopy is a hyperspectral remote sensing technology. With the continuous improvement of signal detection accuracy, new methods are urgently needed to further reduce the interference of noise on the information contained in spatial heterodyne interferograms. Convolutional neural networks, as an emerging field, have achieved good results in image denoising in recent years. This paper applies convolutional neural networks to the field of spatial heterodyne spectroscopy, constructs and trains four deep convolutional neural network models for denoising spatial heterodyne interferograms under different brightness and Gaussian noise conditions, and compares their performance with other algorithms. The results show that deep convolutional neural networks have significant advantages in denoising spatial heterodyne interferograms. Finally, a comparison of four neural networks was conducted to explore how to reasonably select network models. This work provides new ideas and effective solutions for reducing the interference of noise on spatial heterodyne spectral information and improving signal detection accuracy.

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.