Acousto-optic Tunable Filter Spectrometer Research Articles

Real-Time Registration of Unmanned Aerial Vehicle Hyperspectral Remote Sensing Images Using an Acousto-Optic Tunable Filter Spectrometer

Differences in field of view may occur during unmanned aerial remote sensing imaging applications with acousto-optic tunable filter (AOTF) spectral imagers using zoom lenses. These differences may stem from image size deformation caused by the zoom lens, image drift caused by AOTF wavelength switching, and drone platform jitter. However, they can be addressed using hyperspectral image registration. This article proposes a new coarse-to-fine remote sensing image registration framework based on feature and optical flow theory, comparing its performance with that of existing registration algorithms using the same dataset. The proposed method increases the structure similarity index by 5.2 times, reduces the root mean square error by 3.1 times, and increases the mutual information by 1.9 times. To meet the real-time processing requirements of the AOTF spectrometer in remote sensing, a development environment using VS2023+CUDA+OPENCV was established to improve the demons registration algorithm. The registration algorithm for the central processing unit+graphics processing unit (CPU+GPU) achieved an acceleration ratio of ~30 times compared to that of a CPU alone. Finally, the real-time registration effect of spectral data during flight was verified. The proposed method demonstrates that AOTF hyperspectral imagers can be used in real-time remote sensing applications on unmanned aerial vehicles.

Image blurring and spectral drift in imaging spectrometer system with an acousto-optic tunable filter and its application in UAV remote sensing

ABSTRACT In the design and research of an unmanned aerial vehicle-borne hyperspectral imager with an acousto-optic tunable filter (AOTF), the depth of the field range of the imaging objective lens is fixed, the spectral range of 400–1000nm is wide, and the AOTF crystal has different deflection angles for diffracted light of different wavelengths. These features all cause problems such as image blurring and spectral drifts. Therefore, we propose a solution using a tunable zoom lens and multi-modal spectrum registration to solve these problems. We also propose an improved image definition evaluation function using the zoom lens to obtain clear horizontal lines and blurred vertical lines and, thereby, hope to overcome image blurring in an AOTF spectrometer. The zoom parameters corresponding to the clear image in each spectral segment of the AOTF spectrometer can be determined rapidly and accurately. However, it is difficult to establish a mathematical model of the drift, using wavelength as a variable, to compensate the drift due to the clear image of each spectrum obtained using the zoom lens. The calibration method completes the registration of the spectral data cube composed of clear images, solving the problem of spectral band drift. Moreover, it has been verified through UAV-borne hyperspectral imaging that image blurring and spectral drift of the AOTF imaging spectrometer were significantly reduced, providing opportunities for the application and research of AOTF-based hyperspectral imagers in more fields, such as remote sensing.

Acousto-optic tunable filter spectrometers in space missions [Invited

Spectrometers employing acousto-optic tunable filters (AOTFs) rapidly gain popularity in space, and in particular on interplanetary missions. They allow for reducing volume, mass, and complexity of the instrumentation. To date, space operations of 11 AOTF spectrometers are reported in the literature. They were used for analyzing ocean color, greenhouse gases, atmospheres of Mars and Venus, and for lunar mineralogy. More instruments for the Moon, Mars, and asteroid mineralogy are in flight, awaiting launch, or in the state of advanced development. The AOTFs are used in point (pencil-beam) spectrometers for selecting echelle diffraction orders, or in hyper-spectral imagers and microscopes. We review the AOTF-employing devices flown in space or ready to set off. The paper considers basic principles of the AOTF and science applications of the AOTF spectrometers, and describes developed instruments in some detail. We also address some advanced developments for future missions and plans. In addition, we discuss lessons learned during instrument design, build, calibration, and exploitation, and advantages and limitations in implementing the AOTF-based systems in space instrumentation.

Spectral characterization and calibration of AOTF spectrometers and hyper-spectral imaging systems

The goal of this article is to present a novel method for spectral characterization and calibration of spectrometers and hyper-spectral imaging systems based on non-collinear acousto-optical tunable filters. The method characterizes the spectral tuning curve (frequency–wavelength characteristic) of the AOTF (Acousto-Optic Tunable Filter) filter by matching the acquired and modeled spectra of the HgAr calibration lamp, which emits line spectrum that can be well modeled via AOTF transfer function. In this way, not only tuning curve characterization and corresponding spectral calibration but also spectral resolution assessment is performed simultaneously over the whole imaging plane. The obtained results indicated that the proposed method is efficient, accurate and feasible for routine calibration of AOTF spectrometers and hyper-spectral imaging systems and thereby a highly competitive alternative to the existing calibration methods.

High-throughput acousto-optic-tunable-filter-based time-resolved fluorescence spectrometer for optical biopsy

Time-resolved fluorescence spectroscopy has been studied to perform minimally invasive optical biopsy in clinical diagnostics. A critical barrier preventing current time-resolved techniques from clinical studies is their impractically long data acquisition time. We have developed an acousto-optic-tunable-filter (AOTF)-based time-resolved fluorescence spectrometer, which is capable of near real-time data acquisition. Both first-order diffraction beams are collected in this AOTF spectrometer, which results in significantly improved overall throughput. Using standard fluorescence dyes, we have demonstrated that this spectrometer can acquire 200 nm time-resolved spectra within 4 s, while its throughput is comparable to a grating-based spectrometer.

Near-infrared acousto-optic tunable filter based instrumentation for the measurement of dynamic spectra of polymers

The use of an acousto-optic tunable filter spectrometer can simplify the instrumental requirements for the collection of time resolved spectra to measure the difference in absorbance of polarized radiation due to chemical functional group orientation of a polymer as it undergoes repetitive oscillatory strain. The rapid data collection achieved in the near-infrared spectral region rivals that of current interferometric instruments. Achieving a good signal to noise ratio requires only a singly modulated optical signal, and the use of digital filtering eliminates the requirement of phase sensitive detection. The usefulness of this instrument for routine testing of polymers is demonstrated by differentiating polymers of the same chemical composition but different rheology and by determining the effect of varying the concentration of monomers in a copolymer.

Parabolic Concentration of Diffusely Transmitted near Infrared Radiation in an Acousto-Optic Tunable Filter Spectrometer

The research model near infrared (NIR) spectrometer described in this paper, worthy of quantitative use and suited for industrial applications, is the result of stepwise refinement of the acousto-optic tunable filter (AOTF)-based monochromator and other optical components. A sensitive and rapid response extended-range indium gallium arsenide (InGaAs) detector (covering the 1100–2500 nm range) was selected to match the high speed of acousto-optic TFS electronic wavelength switching. Excellent quantitative performance has been demonstrated in comparison with commercial grating monochromator and Fourier transform (FT)-NIR instruments for multi-component liquids, films, pumped fluids and even supercritical fluids. Software-controlled wavelength switching, data acquisition and programmed or interactive time averaging at each wavelength demonstrated analytically-enhanced performance. Throughput is not limited by slits, and exposure of the sample to white light, as with a diode array or an FT instrument, is avoided. Although focusing optics between sample and detector were found to work well for concentrating radiation on a small surface area InGaAs detector, detector size limited performance in the case of optically difficult samples because of the low solid angle of collection. Use of an experimental non-focusing multistage parabolic concentrator immediately after the sample vessel and before the 1 mm2 detector was found to offer an advantage for highly scattering specimens. Although the reflection optics of the locally fabricated concentrator directed a lower intensity of light transmitted by clear specimens than from the standard lens arrangement, successful analysis was extended to include highly scattering samples. The percentage of incident radiation attenuated via scatter was reduced, and for scatter beyond a point, the absolute intensity of radiation falling on the detector was greater, even though the absolute maximum of non-scattered intensity was less, thus establishing proof of principle. We anticipate that further improvement could be attained for highly scattering samples with a commercial optics manufacturer's version of the homemade concentrator.

A compact collinear AOTF Raman spectrometer

A compact, lightweight, completely packaged, uncooled, fully-automated collinear acousto-optic tunable-filter (AOTF) based spectrometer has been used to measure Raman spectra of three organic energetic materials (NQ, HMX, and TNT) using argon-ion laser excitation. Even though the resolution of the AOTF spectrometer is modest (7.4 cm −1) and it was not specifically designed for measuring Raman spectra, it has performed impressively. Such an instrument is specially useful for remote sensing and field measurements. In this paper, we will describe this instrument, present the measured Raman spectra and their comparison with the corresponding FT-IR spectra.

Near-Infrared Acousto-Optic Tunable Filter Hadamard Transform Spectroscopy

The acousto-optic tunable filter (AOTF) is a digitally accessible, compact, solid-state spectrometer that is well suited to high-frequency optical switching and wavelength selection. With injection of a combination of radio-frequency signals into its transducer, the AOTF acts as an electronically controllable, multiplexing spectrometer. The multiplexing AOTF in this study is employed in two distinct fashions. The first of these utilizes the multiplexing AOTF as a matched filter whose intended spectral profile (i.e., bandpass and band symmetry) is controlled almost at will, providing unprecedented flexibility and high throughput. Second, the multiplexing AOTF is employed for the first time as a Hadamard transform spectrometer. In operation, the integrated intensity on the detector measures combinations of the diffracted wavelengths. The light encodement is performed without the use of physical masks and is governed by HT mathematics, which allow efficient recovery of the optical spectrum. Appreciable signal-to-noise enhancement is demonstrated with the HT AOTF spectrometer.

Determination of film-coated tablet parameters by near-infrared spectroscopy

Near-infrared (near-IR) spectroscopy was used in the determination of three parameters of theophylline tablets film-coated with ethylcellulose. Spectra of individual intact tablets were collected on two near-IR spectrometers: a grating-based spectrometer, and an acousto-optic tunable filter spectrometer. Calibrations were developed for the prediction of the time to 50% dissolution ( t 50%) of theophylline for tablets of varying coat thickness, for the determination of the thickness of the ethylcellulose coat applied, and for the prediction of the hardness of coated tablets. Principal component analysis was performed on the spectra prior to calibration development. The standard errors of calibration (SEC) and prediction (SEP) for determination of dissolution rates were 2.8 and 6.6 min, respectively. The SEC for the coating thickness calibration was 0.0002 inches, with an SEP of 0.00024 inches, and the SEC and SEP for the determination of tablet hardness were 0.54 and 0.62 kilopons, respectively.