Acousto-optic Tunable Filter Research Articles

A multi-spectral polarimetric imager for atmospheric profiling of aerosol and thin cloud: Prototype design and sub-orbital performance

We report on the development of a novel multi-spectral polarimetric imager for atmospheric remote sensing of aerosol and cloud properties. The instrument concept, called the Aerosol Limb Imager (ALI), is ultimately intended for satellite measurements from a low Earth orbit. It utilizes a coupling of a dual transducer acousto-optic tunable filter and a liquid crystal rotator to provide dual linear polarization observations over a wide spectral range covering 600 nm-1500 nm. In the limb, or side-viewing, geometry, these measurements provide the capability to resolve vertical and horizontal distributions of aerosol and cloud properties such as extinction coefficient, optical depth, and particle distribution parameters. Here, we present the design and performance of an ALI prototype. Lab characterization of the instrument is used to develop a mathematical instrument model to predict signal levels under various atmospheric conditions. Results from a sub-orbital flight of the ALI prototype on a stabilized high-altitude stratospheric balloon gondola are presented that show the first known polarimetric, multi-spectral images of the limb radiance. The signal levels obtained agree reasonably well with those predicted by the instrument model using radiative transfer calculations for typical atmospheric conditions.

Optimization of noncollinear AOTF design for laser beam shaping.

Optimization of a wide-angle paratellurite acousto-optic tunable filter (AOTF) is performed for applications in laser beam shaping systems. The AOTF configuration with annular transfer function is analyzed. It is demonstrated that the optimal AOTF design for single-frequency operation as a narrow-band spatial frequency filter is obtained at acoustic propagation angle of 5.6° relative to the [110] axis. The optimal design for maximization of AOTF resolution in multifrequency laser beam shaping operation mode is obtained at acoustic propagation angle of 13.8°.

Novel acousto-optical tunable filter (AOTF) based spectropolarimeter for the characterization of auroral emission

Auroras can be observed at the north and south pole of the Earth. At these locations, the Earth magnetic field enters the atmosphere, causing ionization of the atmospheric constituents. Due to this phenomenon, different optical wavelengths are emitted from these constituents, causing the typical spectacular displays. Several optical wavelengths are representative for the auroral emission. The purpose of the described instrument is to measure the degree of linear polarization of the northern lights, in order to detect and confirm previous observations in which only a low level of polarization was seen. The instrument is capable of resolving the polarization measured at any wavelength in the visible region, using two optical chains, each containing an acousto-optical tunable filter (AOTF). The latter is responsible for the selection of the desired optical wavelength and projects the two polarized beams onto an optical detection system. The driving of the AOTF, using a radio-frequency (RF) signal, is a key-element in establishing trustworthy results. The design of this double RF chain, consisting of a generator, an amplifier, and the necessary circumjacent electronics, is described in the study. In addition, the optical setup of the instrument, together with the detection system is explained. The instrument will be used in the harsh environment of the pole regions, so it must be able to withstand these weather conditions. Hence, a thermal compensation system is necessary and is also described.

Generation of widely wavelength tunable Q-switched pulses in an ytterbium doped all-fiber laser

We report an all-fiber, broadband wavelength-tunable, acousto-optic Q-switched ytterbium-doped fiber laser. The tunable lasing operation was achieved by fiber-based acousto-optic tunable filter with a wavelength range spanning to more than 70 nm from 1008.6–1080.3 nm. Pulse shortening is achieved by spectral filtering and reducing the duty cycle of the acousto-optic modulator (AOM) used for Q-switching. At a 345 mW pump power and 1030 nm laser operation, the duration of Q-switched pulses is reduced from 110 ns (without spectral filtering) to 65 ns with 2.2 μJ pulse energy at a 10 kHz repetition rate and 10% duty cycle of AOM. The Q-switched pulse duration is further shortened to ∼10 ns by reducing the duty cycle of AOM to 0.12%. We believe that this all-fiber integrated Q-switched laser system with wide wavelength tunability and shorter duration pulses will be useful in many scientific applications.

Tunable Acousto-Optic Filter for the 450–900 and 900–1700 nm Spectral Range

The problem of creating an acousto-optic (AO) tunable filter, which allows us to obtain high-quality spectral images both in the visible and near-infrared ranges, is studied. Based on the analysis of the wide-aperture noncollinear geometry of AO diffraction in paratellurite, the AO cell configuration is determined that is optimal in terms of minimizing chromatic shift and is suitable for both ranges. Two identical cells are manufactured and matched for the ranges 450–900 and 900–1700 nm. Experimental verification of the results of optical modeling showed that, over the entire range of 450 to 1700 nm, the calculated AO filter provides high-quality spectral images.

Accuracy improvement of a white-light spectral interferometer using a line-by-line spectral calibration method

Smile distortion caused by aberration of the optical system is the key factor that affects the spectral calibration of a white-light spectral interferometer. To improve the accuracy of surface metrology, this paper proposes a novel calibration approach based on a line-by-line method. An acousto-optic tunable filter (AOTF) is adopted for the wavelength scanning process during calibration. By fitting sufficient calibration data, a more accurate relationship between wavelength and pixel position can be obtained. The simulation results show that the accuracy of surface metrology has different sensitivities to the coefficients of the calibration equation, and that the effect of smile distortion becomes more severe as the optical path difference increases. The presence of smile distortion is confirmed in the calibration experiment of a home-built white-light spectral interferometer. Subsequently, a silicon wafer and a standard step of 1.806 ± 0.011 μm are tested using the calibrated metrology system. The measurement results demonstrate that the line-by-line calibration method performs well in correcting spectral distortion and can improve the measurement accuracy of surface profile.

Measurement of thermal conductivity in laser-heated diamond anvil cell using radial temperature distribution

ABSTRACT Thermal conductivities of planetary materials under extreme conditions are important input parameters for modeling planetary dynamics such as accretion, geodynamo and magnetic field evolution, plate tectonics, volcanism-related processes etc. However, direct experimental measurements of thermal conductivity at extreme conditions remain challenging and controversial. Here we propose a new technique of thermal conductivity measurement in laser-heated diamond anvil cell (LH-DAC) based on radial temperature distribution around laser focal spot, mapped by imaging tandem acousto-optical tunable filter (TAOTF). The new technique provides much more information about heat fluxes in the laser-heated sample than existing static heating setups, and does not require dynamic numerical modeling using heat capacities in contrast to dynamic pulsed heating setups. In the test experiment, thermal conductivity of γ-Fe at conditions relevant to cores of terrestrial planets was measured.

Determination of the effective emissivity distribution of a heated specimen using tandem acousto-optical tunable filter

We present the first experimental demonstration of a new imaging system for in-situ measurement the two-dimensional distribution of the effective emissivity and temperature of a heated specimen. In this work, we use the model of a gray body, assuming that the emissivity is constant over the entire wavelength range from 600 to 800 nm. Data acquisition was done using the laser heating (LH) system developed at the STC UI RAS. The LH system allows us to reach extremely high temperatures up to 6000 K at high pressures. The main component of the system is an imaging tandem acousto-optical tunable filter synchronized with a video camera. The maximal error of the emissivity measurement of the tungsten sample was found to be 13%, whereas the maximal error of the temperature measurements did not exceed 2%. An influence of different factors on the error of the emissivity determination is also discussed.

FEM simulation analysis of TiO2/ZnO/SiO2/Si multilayer structure for CMOS compatible acousto-optic tunable filter

In this paper, TiO2/ZnO/SiO2/Si layer structures with different thickness of ZnO and TiO2 is discussed and explained. The study is carried out in order to obtain optimal normalized thickness of the multilayer structure for fabricating acousto-optic tunable filter with high figure of merit (FOM). Based on simulation and study of SAW propagation characteristics like, electromechanical coupling co-efficient, figure of merit and phase velocity, it is inferred that this structure is suitable for the desired application.

Tunable semiconductor laser with two acousto-optic tunable filters in its external cavity

We report a laser with a linear external cavity containing a near-IR injection semiconductor optical amplifier as a gain element and two acousto-optic tunable filters (AOTFs) with quasi-collinear interaction of acoustic and light waves. At identical frequencies of RF signals that control the filters and ensure compensation for the Doppler frequency shift of light passing through the AOTFs, the steady-state laser emission linewidth can be reduced to about 25 MHz, which is almost three orders of magnitude smaller than that in the case of one filter (20 GHz). The position of the emission line, which remains narrow, slowly fluctuates within a spectral range about 3 GHz wide, which seems to be due to the large length of the external cavity and insufficient thermal stabilization of its components. This does not prevent us from obtaining light with a large coherence length. In emission wavelength sweep mode, the instantaneous emission linewidth increases with tuning rate, reaching 0.022 nm (8.8 GHz) at the highest tuning rate: 104 nm s−1. In the case of automatic control over the output optical power at a level of 3 mW, the tuning range is 815 – 875 nm. Such instruments are of practical interest for optical coherence tomography, spectroscopy, optical metrology, and other application areas.

An Investigation of Spectral Band Selection for Hyperspectral LiDAR Technique

Hyperspectral LiDAR (HSL) has been widely discussed in recent years, which attracts increasing attention of the researchers in the field of electronic information technology. With the application of supercontinuum laser source, it is now possible to develop an HSL system, which can collect spectral and spatial information of targets simultaneously. Meanwhile, eye-safety and miniature HSL device with multiple spectral bands are given more priorities in on-site applications. In this paper, we tempt to investigate how to select spectral bands with a selection method. The proposed method consists of three steps: first, the variances among the classes based on hyperspectral feature parameters, termed inter-class variances, are calculated; second, the channels are sorted based on corresponding variances in descending order, and those with the two highest values are adopted as the initial input of classification; finally, the channels are selected successively from the rest of the sorted sequence until the classification accuracy reaches 100%. To test the performance of the proposed method, we collect 91/71-channel hyperspectral measurements of four different categories of materials with 5 nm spectral resolution using an acousto-optic tunable filter (AOTF) based HSL. Experimental results demonstrate that the proposed method could achieve higher classification accuracy than a random band selection method with different classifiers (naïve Bayes (NB) and support vector machine (SVM)) regardless of classification feature parameters (echo maximum and reflectance). To reach 100% accuracy, it demands 8–9 channels on average by echo maximum and 4–5 channels on average by reflectance based on NB classifier; these figures are 3–4 by echo maximum and 2–3 by reflectance with SVM classifier. The proposed method can complete classification task much faster than the random selection method. We further confirm the specific channels for the classification of different materials, and find that the optimal channels vary with different materials. The experimental results prove that the optimal band selection of HSL system for classification is reliable.

Single-volume dual-channel acousto-optical tunable filter.

In this paper, we present a novel approach to spectral stereoscopic imaging. It is based on simultaneous spectral filtration of two light beams with a tunable acousto-optical filter (AOTF) of original design. It does not require large crystals and complicated optical relay systems, because two beams diffract in the same volume of the crystal medium but at different angles. We show that this geometry can be composed of a common-type AO cell and two triangular prisms of the same material. We derive equations, which specify the prism angles ensuring the necessary orientation of beams trajectories inside the crystal medium as well as parallel propagation of input and output beams. Some angles were additionally optimized for aberrations minimization by means of ray-tracing simulation. Experimental testing demonstrates rather high quality of spectral images, which is necessary for stereoscopic reconstruction procedure. The proposed approach makes possible development of spectral stereo-imaging components based on different types of previously developed AOTFs.

Многооконные акустооптические фильтры для корреляционной спектроскопии

We consider the problem of development of optical filters with synthesized transmission function. We demonstrate experimentally that acousto-optical tunable filters, which high-frequency control signal is generated digitally, is capable to synthesize various transmission functions: multi-band with adjustable location and bandwidth including asymmetric. And this fact open ways for complicated transmission function synthesizing.

High-Resolution Hyperspectral Microscopic Imaging With Single Acousto-Optic Tunable Filter Based on Double Filtering

Hyperspectral imaging is a technique that integrates multiple spectral bands and image information. Its applications range from improving the accuracy of cancer diagnosis to testing the quality of products. Here, we introduce a double-filtering technique that provides high-resolution diagnostic histological images within minutes. The hyperspectral microscopic imaging system is built based on an acousto-optic tunable filter (AOTF). The optimized system is analyzed from the perspective of spectrum and imaging. The spectral resolution can be improved by 37.08 % to 59.95% in the visible light range. The side lobe is obviously inhibited and the purity of spectrum is improved. Furthermore, the example of hyperspectral microscopic imaging is demonstrated with unstained gastric cancer tissue sections to assess the ability of the system in terms of its spectral performances and image quality. The microscopic imaging results of single filtering optical path system and single crystal double filtering optical path system are compared. In general, the optimized double filtering system achieves excellent performance in bandwidth compression and side lobe suppression, especially the first application of hyperspectral microscopy combined with microscope in the visible light range.

Feasibility Study on Hyperspectral LiDAR for Ancient Huizhou-Style Architecture Preservation

Huizhou-style ancient architecture was one of the most important genres of architectural heritage in China. The architecture employed bricks, woods, and stones as raw materials, and timber frames were significant structures. Due to the drawback that the timbers were vulnerable to moisture and atmospheric agents, ancient timber buildings needed frequent protective interventions to maintain its good condition. Such interventions unavoidably disrupted the consistency between the original timber components. Besides this, the modifications brought about difficulty in correctly analysing and judging the state of existing ancient buildings, which, in current preservation practices, mainly rely on the expertise of skilled craftsmen to classify wood species and to identify the building-age of the timber components. Therefore, the industry and the research community urgently need a technique to rapidly and accurately classify wood materials and to discriminate building-age. In the paper, we designed an eye-safe 81-channel hyperspectral LiDAR (HSL) to tackle these issues. The HSL used an acousto-optic tunable filter (AOTF) as a spectral bandpass filter, offering the HSL measurements with 5 nm spectral resolution. Based on the HSL measurements, we analysed the relationship between the surface and cross-section spectral profiles of timber components from different ancient architectures built in the early Qing dynasty (~300 years), late Qing dynasty (~100 years), and nowadays, and confirmed the feasibility of using surface spectra of timber components for classification purpose. We classified building-ages and wood species with multiple Naive Bayes (NB) and support vector machine (SVM) classifiers by the surface spectra of timber components; this also unveiled the possibility of classifying gnawed timber components from its spectra for the first time. The encouraging experimental results supported that the AOTF-HSL is feasible for historic timber building preservation.

Mode-locked and tunable fiber laser at the 3.5 µm band using frequency-shifted feedback

We report on a mid-infrared mode-locked fiber laser that uses an acousto-optic tunable filter to achieve frequency-shifted feedback pulse generation with frequency tuning over a 215 nm range. The laser operates on the 3.5 µm transition in erbium-doped zirconium fluoride-based fiber and utilizes the dual-wavelength pumping scheme. Stable, self-starting mode locking with a minimum pulse duration of 53 ps was measured using a two-photon absorption autocorrelator. The longest wavelength demonstrated was 3612 nm, and the maximum average powers achieved were 50 and 167 mW in fundamental and multi-pulse mode-locking regimes, respectively. To the best of our knowledge, this is the longest wavelength rare-earth-doped mode-locked fiber laser demonstrated. The broad tunability promises potential uses for environmental sensing applications.

Experimental study of an in-fiber acousto-optic tunable bandpass filter for single- and dual-wavelength operation in a thulium-doped fiber laser.

A tunable single- and dual-wavelength thulium-doped all-fiber laser is demonstrated based on the implementation of an in-fiber acousto-optic tunable bandpass filter (AOTBF). The AOTBF is fabricated to be operated in the 1.9 µm region, and takes advantage of the intermodal coupling effect produced by traveling flexural acoustic waves in an optical fiber. It exhibits a 3-dB bandwidth of 2.04 nm with an insertion loss of 4.75 dB. The tuning properties of the AO device allows a continuous-wave operation with characteristics of wide tuning range (211.5 nm), narrow linewidth (50 pm) and high signal-to-noise ratio (60 dB). In the dual-wavelength regime, the laser is capable of independent tuning of each of the laser lines, achieving a tunable dual-wavelength emission that extends from 1802.67 to 1932.75 nm. A controllable wavelength spacing with minimum and maximum separations of 1.04 and 130.08 nm is obtained.

In-depth Investigation of Hg2Br2 Crystal Growth and Evolution

Physical vapor transport (PVT) has frequently been adopted for the synthesis of mercurous bromide (Hg2Br2) single crystals for acousto-optic modulators. However, thus far, very few in-depth studies have been conducted that elucidate the growth process of the Hg2Br2 single crystal. This paper reports an in-depth investigation regarding the crystal growth and evolution behavior of the Hg2Br2 crystal with facet growth mode. Based on the experimental and simulation results, the temperature profile conditions concerning the seed generation and seed growth could be optimized. Next, the PVT-grown Hg2Br2 crystals (divided into single crystal and quasi-single crystal regions) were characterized using various analysis techniques. The single-crystal Hg2Br2 was found to possess a more uniform strain than that of the quasi-single crystal through a comparison of the X-ray diffraction data. Meanwhile, the binding energy states and electron backscatter diffraction images of the as-synthesized Hg2Br2 crystals were similar, regardless of the crystal type. Furthermore, Raman spectroscopy and transmission electron microscopy analyses provided information on the atomic vibration mode and atomic structures of the two kinds of samples. The synergistic combination of the simulation and experimental results used to verify the growth mechanism facilitates the synthesis of high-quality Hg2Br2 crystals for potential acousto-optic tunable filter device applications.

Multi-spectral phase imaging based on acousto-optic selection of light in common-path interferometer

Multispectral interference techniques are widely used in biomedicine and science for quantitative characterization of morphological, chemical and other properties. These techniques, for example, multispectral digital holography and spectral-domain optical coherence tomography, require wavelength tuning in the interferometer. In this paper, we discuss the effectiveness and potential applications of acousto-optic light filtration for this purpose. We show experimentally that acousto-optical tunable filters are effective for simultaneous spectral and phase data collection not only in conventional dual-path interferometers but also in a common-path interferometer. Scheme of a compact spectral interferometric module based on such interferometer is presented.

Analysis of the statistical errors of the emissivity measurement of a laser heated surface by acousto-optical tunable filter

The aim of this report is to conduct a rigorous data analysis of the emissivity and temperature measurements on the surface of a solid specimen during laser heating (LH). Measurements of the emissivity and temperature is based on the use of a tundem acousto-optic filter (tandem acousto-optical tunable filter or TAOTF) connected with a high-resolution video camera. The set of spectral images obtained with the help of TOTF in the range of 750-800 nm makes it possible to calculate the dependence of the radiation intensity of the heated surface on the wavelength at each point of the surface. The distribution of the temperature and the emissivity of the heated body surface is obtained by fitting the experimental spectral dependence of the radiation intensity at each point of the heated object to the Planck distribution using the least squares method (LSM). A development of the mathematical analysis of the statistical errors of the emissivity determination by the LSM is described.