Spatial Heterodyne Research Articles

Dependence of interferogram phase on incident wavenumber and phase stability of Doppler asymmetric spatial heterodyne spectroscopy**Project supported by the National Natural Science Foundation of China (Grant No. 41005019), the Fund from the Chinese Academy of Scieneces for West Yong Scientists (Grant No. XAB 2016A07), and the Natural Science Basic Research Program of Shaanxi Province, China (Grant No. 2019JQ-931).

Instrument drifts introduce additional phase errors into atmospheric wind measurement of Doppler asymmetric spatial heterodyne spectroscopy (DASH). Aiming at the phase sensitivity of DASH to instrument drifts, in this paper we calculate the optical path difference (OPD) and present an accurate formula of DASH interferogram. By controlling variables in computational ray-tracing simulations and laboratory experiments, it is indicated that initial phase is directly determined by incident wavenumber, OPD offset and field of view (FOV). Accordingly, it is indicated that retrieved phase of DASH is sensitive to slight structural change caused by instrument drift, which provides the proof of necessary-to-track and -correct phase errors from instrument drifts.

First Level 1 Product Results of the Greenhouse Gas Monitoring Instrument on the GaoFen-5 Satellite

The Greenhouse Gas Monitoring Instrument (GMI) is a short-wavelength infrared (SWIR) hyperspectral-resolution spectrometer onboard the Chinese satellite GaoFen-5 that uses a spatial heterodyne spectroscopy (SHS) interferometer to acquire interferograms. The GMI was designed to measure and study the source and sink processes of carbon dioxide and methane in the troposphere where the greenhouse effect occurs. In this study, the processing and geometric correction algorithms of the GMI Level 1 product (radiance spectrum) are introduced. The spectral quality and greenhouse gas (GHG) inversion ability of the Level 1 products are analyzed, and the results illustrate that the specifications meet the mission's requirements. An initial evaluation of the resolution, signal-to-noise ratio (SNR), and stability of the radiance spectrum reveals that the overall function and performance are within the design objectives. A comparison between our Level 1 products and the theoretical spectrum shows that the root mean square (rms) of the residual is approximately 0.8%, and the Level 1 products of the GMI captured within five months after observations have good spectral stability characteristics (less than 0.005 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for Band 1, 0.003 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for Band 2, 0.002 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for Band 3, and 0.004 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> for Band 4). These results demonstrate that the GMI payload and the processing algorithm all work well and reliably. Furthermore, based on the Level 1 products, a GHG retrieval experiment is carried out, and the results are compared with data from Total Column Carbon Observing Network (TCCON) stations. The initial comparison of the XCO <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> results yields a value of 0.869 for R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (goodness of fit), 0.51 ppm for bias (mean of absolute error), and 0.53 ppm for σ (standard deviation of error). Similarly, the XCH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> comparison yields values of 0.841 for R <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , 4.64 ppb for bias, and 4.66 ppb for σ.

Ultra-high resolution and broadband chip-scale speckle enhanced Fourier-transform spectrometer.

Recent advancements in silicon photonics are enabling the development of chip-scale photonics devices for sensing and signal processing applications, among which on-chip spectrometers are of particular interest for precision wavelength monitoring and related applications. Most chip-scale spectrometers suffer from a resolution-bandwidth trade-off, thus limiting the uses of the device. Here we report on a novel passive, chip-scale, hybrid speckle-enhanced Fourier transform device that exhibits a two order-of-magnitude improvement in finesse (bandwidth/resolution) over the state-of-the art chip-scale speckle and Fourier transform spectrometers. In our proof-of-principle device, we demonstrate a spectral resolution of 140 MHz with 12-nm bandwidth for a finesse of 104 that can operate over a range of 1500-1600 nm. This chip-scale spectrometer structure implements a typical spatial heterodyne discrete Fourier transform interferometer network that is enhanced by speckle generated from the wafer substrate. This latter effect, which is extremely simple to invoke, superimposes the high wavelength resolution intrinsic to speckle generated from a strongly guiding waveguide with a more broadband but lower resolution discrete Fourier transform modality of the overarching waveguide structure. This hybrid approach signifies a new pathway for realizing chip-scale spectrometers capable of ultra-high resolution and broadband performance.

Compact spatially heterodyned static interferometer.

This paper presents a novel, to the best of our knowledge, and simple technique for achieving a higher spectral resolution in classical static Fourier transform spectrometers. This is achieved by heterodyning the frequency of a standard interferogram to a lower spatial frequency by placing a single transmission grating at the image plane of two mutually coherent beams produced by the interferometer. The grating splits the beams into diffraction orders, which overlap to produce the heterodyned interferogram, similar to that seen in techniques such as spatial heterodyne spectroscopy. The increase in spectral resolution for such a system is shown to be related to the angle between the beams and the groove period of the transmission grating. The theoretical performance of this design is compared with a proof-of-concept system built using off-the-shelf components and tested at visible wavelengths. The experimental results agree well with those produced from a theoretical simulation.

Extending low-coherence interferometry dynamic range using heterodyne detection

Low-coherence interferometry (LCI) technique is generating considerable interest in industrial applications where there is a need for larger measurements with high resolution. Conventional Fourier domain systems reach a limiting depth of around 3 mm, mainly due to the spectrometers used as detectors. In this work, we present an optical detection system that performs the Fourier transform of the LCI signals, based on a spatial heterodyne spectrometer. This device avoids the fall-off effect of the spectrometer, allowing to reach measurable optical depths of almost 5 cm without losing resolution. We describe the theory underlying this detection system and present experimental results which are in great accordance.

On-chip polarization-insensitive Fourier transform spectrometer.

Chip-scale monolithic Fourier transform spectrometers (FTSs) offer great potential for inexpensive, high-resolution, and robust spectroscopic applications in a wide variety of scenarios. Having attracted considerable attention, spatial heterodyne FTSs (SH-FTSs) are featured with a simple and stable configuration composed of an array of Mach-Zehnder interferometers (MZIs) with linearly increasing optical path differences. Owing to the strong waveguide birefringence, MZIs on the popular silicon-on-insulator platform are polarization-sensitive, raising the challenge of polarization control of incident light. We propose and demonstrate a polarization-insensitive SH-FTS using a two-dimensional grating coupler to split an arbitrary state of polarization into two orthogonal polarization components that are both coupled into the TE mode but propagate in opposite directions in the arrayed MZIs. The two orthogonal polarization components are finally recombined in photodetection without polarization-dependent losses. An edge-coupling configuration using a polarization splitter-rotator is also proposed.

Evaluation of a Spatial Heterodyne Spectrometer for Raman Spectroscopy of Minerals

Spatial heterodyne spectroscopy (SHS) is a novel spectral analysis technique that is being applied for Raman spectroscopy of minerals. This paper presents the theoretical basis of SHS and its application for Raman measurements of calcite, quartz and forsterite in marble, copper ore and nickel ore, respectively. The SHS measurements are done using a broadband (518–686 nm) and resolving power R ≈ 3000 instrument. The spectra obtained using SHS are compared to those obtained by benchtop and modular dispersive spectrometers. It is found that SHRS performance in terms of resolution is comparable to that of the benchtop spectrometer and better than the modular dispersive spectrometer, while the sensitivity of SHRS is worse than that of a benchtop spectrometer, but better than that of a modular dispersive spectrometer. When considered that SHS components are small and can be packaged into a handheld device, there is interest in developing an SHS-based instrument for mobile Raman spectroscopy. This paper evaluates the possibility of such an application.

Khayyam, a tunable, cyclical spatial heterodyne spectrometer on Mt. Hamilton

We describe the design considerations, installation, and technical challenges of coupling a cyclical spatial heterodyne spectrometer (SHS) with the Cassegrain telescope at the Lick Observatory on Mt. Hamilton, California. The SHS instrument (named Khayyam after the mathematician) is mounted to a fixed focal plane shared by the 0.6-m Cassegrain Coude Auxiliary Telescope (CAT) and has the field-of-view of ∼4 arc min, on the sky, spectral resolving power (R) of 48,000 and a tunable wavelength bandpass range over ΔλB ∼ 150 Å. This instrument–telescope pairing is optimal for temporal observations of extended astronomical targets, e.g., cometary coma, when significant observing time is available since it provides high-resolution spectra from small input apertures. Khayyam’s approach contrasts with traditional high spectral resolution spectrometers that need to be coupled to large aperture telescopes to compensate for their low throughput. Based on our reference lamp results, we were able to formulate the undesirable impact of the spider pattern on the SHS data that prohibited us from acquiring spectra from our sky targets. However, more analysis is needed to investigate if we can create a framework to systematically eliminate the diffracted spider pattern shadow from the fringe pattern without compromising the integrity and quality of the data.

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.

冲击噪声对多普勒差分干涉仪相位反演精度影响分析及校正方法

冲击噪声对多普勒差分干涉仪相位反演精度影响分析及校正方法

Optical Simulation and Design of Spatial Heterodyne Spectrometers for Remote Sensing Applications

We present the optical simulation and design concept of a spatial heterodyne spectrometer (SHS) for mobile applications. A framework using Python and Zemax OpticStudio was developed for the optical system design and automated tolerance analysis from the incoming light, the spectrometer and the imaging lens system to the 2D detector. The spectrometer design and the fabrication methods were validated using a test setup in the VIS spectral range for a future SHS fabrication in the LWIR spectral band operating on small unmanned aerial vehicles (SUAV) for remote sensing applications.

冲击噪声对多普勒差分干涉仪相位反演精度影响分析及校正方法

冲击噪声对多普勒差分干涉仪相位反演精度影响分析及校正方法

A propagation of interferogram signal-to-noise (SNR) and phase uncertainty in Doppler asymmetric spatial heterodyne spectrometer

Passive atmospheric wind detection technique retrieves atmospheric wind profile by measuring the Doppler shift of airglow emissions. Doppler asymmetric spatial heterodyne spectrometer (DASH), which is a Fourier transform spectrometer(FTS), retrieves the Doppler shift information of airglow emissions by detecting the phase shift of interferograms, and the measured phase accuracy directly affects the retrieved wind speed precision. The signal-to-noise (SNR) ratio is one of the significant indexes for evaluating the performance of wind-measuring interferometers in engineering applications. Studying the quantitative relationship between retrieved phase uncertainty and original interferogram SNR that is based on observations is quite essential for the DASH design, performance evaluation and wind profile applications. In this paper, the study is based on the noise propagation theory in FTS and DASH phase retrieval model. According to the Fourier transform relationship between time and frequency domain, we start from original interferogram expression, then we conduct the Fourier transforming, single frequency extracting, inverse Fourier transforming, phase calculating and first-order Taylor expanding, and finally we establish a theoretical relationship model between original interferogram SNR and retrieved phase uncertainty. In order to verify the theoretical relationship model, firstly, we generate 20 groups of interferograms (each group with 1000 frames) randomly with varying the 30–250 times SNR value. After removing the low frequency baseline, we calculate the phase of each interferogram by DASH phase retrieval model, and obtain the phase uncertainty by calculating standard deviation of the 512th sampling of each group interferogram. Another phase retrieval uncertainty is obtained by using the theoretical relationship model between SNR and retrieved phase uncertainty derived from this paper. Secondly, a total of 23 groups of experimental interferograms (each group with 100 frames) with different intensities are collected through the self-developed DASH with a center wavelength of 632.8 nm, basic optical path difference of 50 mm, spectral resolution of 0.78 cm&lt;sup&gt;−1&lt;/sup&gt;. Combining physical characteristics of shot noise and DASH parameters, interferogram SNR of each frame is calculated. We calculate phase uncertainty of experimental data through the two methods mentioned above. The results from the two different calculation methods are compared with each other to determine whether the conclusion is correct. In order to improve the accuracy of phase calculation, three lines are averaged as input to reduce the random error. The average residual between the two methods is only 0.03 mrad, the high consistency of the results indicates that the theoretical relationship model between SNR and retrieved phase uncertainty for DASH is correct. The phase uncertainty can be evaluated by interferogram SNR directly in engineering, which provides a theoretical basis for optimizing the interferometer design.

Doppler radial velocity detection based on Doppler asymmetric spatial heterodyne spectroscopy technique for absorption lines**Project supported by the National Basic Research Program of China (Grant No. 2014CB744204).

Doppler asymmetric spatial heterodyne spectroscopy (DASH) technique has developed rapidly in passive Doppler-shift measurements of atmospheric emission lines over the last decade. With the advantages of high phase shift sensitivity, compact, and rugged structure, DASH is proposed to be used for celestial autonomous navigation based on Doppler radial velocity measurement in this work. Unlike atmospheric emission lines, almost all targeted lines in the research field of deep-space exploration are the absorption lines of stars, so a mathematical model for the Doppler-shift measurements of absorption lines with a DASH interferometer is established. According to the analysis of the components of the interferogram received by the detector array, we find that the interferogram generated only by absorption lines in a passband can be extracted and processed by a method similar to the approach to studying the emission lines. In the end, numerical simulation experiments of Doppler-shift measurements of absorption lines are carried out. The simulation results show that the relative errors of the retrieved speeds are less than 0.7% under ideal conditions, proving the feasibility of measuring Doppler shifts of absorption lines by DASH instruments.

On-chip Fourier-transform spectrometers and machine learning: a new route to smart photonic sensors.

Miniaturized silicon photonics spectrometers capable of detecting specific absorption features have great potential for mass market applications in medicine, environmental monitoring, and hazard detection. However, state-of-the-art silicon spectrometers are limited by fabrication imperfections and environmental conditions, especially temperature variations, since uncontrolled temperature drifts of only 0.1°C distort the retrieved spectrum precluding the detection and classification of the absorption features. Here we present a new strategy that exploits the robustness of machine learning algorithms to signal imperfections, enabling recognition of specific absorption features in a wide range of environmental conditions. We combine on-chip spatial heterodyne Fourier-transform spectrometers and supervised learning to classify different input spectra in the presence of fabrication errors, without temperature stabilization or monitoring. We experimentally show the differentiation of four different input spectra under an uncontrolled 10°C range of temperatures, about $ 100\times $100× increase in operational range, with a success rate up to 82.5% using state-of-the-art support vector machines and artificial neural networks.

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.

Analysis and Classification of Liquid Samples Using Spatial Heterodyne Raman Spectroscopy

Spatial heterodyne spectroscopy (SHS) is used for quantitative analysis and classification of liquid samples. SHS is a version of a Michelson interferometer with no moving parts and with diffraction gratings in place of mirrors. The instrument converts frequency-resolved information into a spatially resolved one and records it in the form of interferograms. The back-extraction of spectral information is done by the fast Fourier transform. A SHS instrument is constructed with the resolving power 5000 and spectral range 522-593 nm. Two original technical solutions are used as compared to previous SHS instruments: the use of a high-frequency diode-pumped solid-state laser for excitation of Raman spectra and a microscope-based collection system. Raman spectra are excited at 532 nm at the repetition rate 80 kHz. Raman shifts between 330 cm-1 and 1600 cm-1 are measured. A new application of SHS is demonstrated: for the first time, it is used for quantitative Raman analysis to determine concentrations of cyclohexane in isopropanol and glycerol in water. Two calibration strategies are employed: univariate based on the construction of a calibration plot and multivariate based on partial least squares regression. The detection limits for both cyclohexane in isopropanol and glycerol in water are at a 0.5 mass% level. In addition to the Raman-SHS chemical analysis, classification of industrial oils (biodiesel, poly(1-decene), gasoline, heavy oil IFO380, polybutenes, and lubricant) is performed using the Raman-fluorescence spectra of the oils and principal component analysis. The oils are easily discriminated showing distinct non-overlapping patterns in the principal component space.

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.

On the assembly and calibration of a spatial heterodyne interferometer for limb sounding of the middle atmosphere

Spatial heterodyne interferometers are an enabling technology to build highly miniaturized optical instruments for the observation of faint emissions in the atmosphere. They are particularly suited for the deployment on nano- or micro-satellite constellations. One application of the SHI technology is a middle atmosphere temperature sounder based on the measurement of relative intensities emitted by the O2 atmospheric band system. Beside basic design considerations, aspects of the opto-mechanical design and assembly of a monolithic SHI for a space application are addressed. For the characterization of such an instrument, a light stimulus based on a Köhler illuminator is presented.