Acousto-optic Tunable Filter Research Articles

Development of a mathematical model of a collinear filter based on Quartz (SiO2) and its practical implementation

Purpose of the work. Development and testing of a theoretical model of an acousto-optic quasi-collinear tunable filter on crystalline quartz, operating in the spectral range of 0.25-0.4 microns with a spectral resolution of 0.2 nm.Methods. The paper analyzes the basic properties of acousto-optic tunable filters based on crystals of various classes. Based on literature analysis and calculations, an experimental AOTF model was proposed and practically implemented. The study of the physical properties of light filtration on an α-SiO2 crystal was carried out using an experimental method.Practical studies of the spectral tuning of the device at experimental optical frequencies were carried out in the proposed ultrasonic standing wave system in accordance with the tuning characteristic. Based on known experimental data, a theoretical model for describing the passband taking into account piezoelectric effects is proposed.Results. The influence of crystal anisotropy on their acoustoelectric properties is shown. The optimal crystallographic parameters for the operation of filters on a silicon oxide crystal have been determined. A computer model of AOTF has been developed, implemented on the basis of numerical calculations in the Wolfram Mathematica environment. It was experimentally revealed that in the red part of the spectrum under study, the maximum intensity of the frequencies of diffracted light is observed, which corresponds to those closest to the natural frequencies of the piezoelectric transducer, while at the same time, in the blue and violet spectra, the lowest transmission was observed. Practical studies have confirmed that the broadening of the filter passbands occurs due to an increase in the divergence of light.Conclusions. A mathematical model of an acousto-optic quasi-collinear tunable filter on crystalline quartz was substantiated and developed, on the basis of which the calculation of the AOTF operating in the spectral range of 0.25 - 0.4 μm with a spectral resolution of about 0.2 nm was performed.

Spatial-Dependent Spectral Response of Acousto-Optic Tunable Filters with Inhomogeneous Acoustic Distribution.

The spectral response of an acousto-optic tunable filter (AOTF) is crucial for an AOTF based spectral imaging system. The acousto-optic (AO) interaction within the spatial-distributed area of the acoustic field determines the spectral response of the light incidence. Assuming an ideally uniform acoustic field distribution, phase-matching geometries can be applied to calculate the anisotropic Bragg diffraction in AO interactions, determining the wavelength and direction of the diffracted light. In this ideal scenario, the wavelength of the diffracted light depends solely on the direction of the incident light. However, due to the non-ideal nature of the acoustic field, the wavelength of the diffracted light exhibits slight variations with incident position. In this paper, an analytical model is proposed to calculate the spatial-dependent spectral response of the diffracted light under non-uniform acoustic field distribution. The study computes the variation pattern of the diffracted light amplitude caused by the inhomogeneous acoustic distribution. The theoretical considerations and computational model are confirmed by AOTF frequency scanning experiments. The study demonstrates that the distribution of the acoustic field leads to non-uniform spatial-spectral response in the AOTF, and the spatial AO interaction computational model can provide data support for calibrating AOTF systems in imaging applications.

Transportable hyperspectral imaging setup based on fast, high-density spectral scanning for in situ quantitative biochemical mapping of fresh tissue biopsies.

Histopathological examination of surgical biopsies, such as in glioma and glioblastoma resection, is hindered in current clinical practice by the long time required for the laboratory analysis and pathological screening, typically taking several days or even weeks to be completed. We propose here a transportable, high-density, spectral scanning-based hyperspectral imaging (HSI) setup, named HyperProbe1, that can provide in situ, fast biochemical analysis, and mapping of fresh surgical tissue samples, right after excision, and without the need for fixing, staining nor compromising the integrity of the tissue properties. HyperProbe1 is based on spectral scanning via supercontinuum laser illumination filtered with acousto-optic tunable filters. Such methodology allows the user to select any number and type of wavelength bands in the visible and near-infrared range between 510 and 900nm (up to a maximum of 79) and to reconstruct 3D hypercubes composed of high-resolution (4 to ), widefield images ( ) of the surgical samples, where each pixel is associated with a complete spectrum. The HyperProbe1 setup is here presented and characterized. The system is applied to 11 fresh surgical biopsies of glioma from routine patients, including different grades of tumor classification. Quantitative analysis of the composition of the tissue is performed via fast spectral unmixing to reconstruct the mapping of major biomarkers, such as oxy-( ) and deoxyhemoglobin (HHb), as well as cytochrome-c-oxidase (CCO). We also provided a preliminary attempt to infer tumor classification based on differences in composition in the samples, suggesting the possibility of using lipid content and differential CCO concentrations to distinguish between lower and higher-grade gliomas. A proof of concept of the performances of HyperProbe1 for quantitative, biochemical mapping of surgical biopsies is demonstrated, paving the way for improving current post-surgical, histopathological practice via non-destructive, in situ streamlined screening of fresh tissue samples in a matter of minutes after excision.

Acousto-optic femtosecond laser beam shaping for bottle-beam generation

Abstract This study explores the properties of a laser beam shaping system comprising an acousto-optic dispersive delay line (AODDL) and an acousto-optic tunable filter (AOTF) for ultrashort laser pulse shaping. The system demonstrates the AODDL’s ability to generate signals with arbitrarily positioned transmission windows and investigates the impact of spectral width on field distributions. The AOTF’s frequency scanning and simultaneous control of two and three spectral components are presented. The experimental setup utilizes femtosecond laser radiation, and the results highlight the system’s versatility for programmable spectral and spatial filtering, offering potential applications in advanced optical manipulation.

Temperature impact on acoustic wave reflection in quasi-collinear acousto-optic devices.

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that applies the acoustic wave reflection from the AO cell input optical face and provides an extremely large interaction length for achieving abnormally high spectral resolution of AO tunable filters. As a result, it becomes possible to implement the multifrequency diffraction which has found important applications for laser pulse shaping. The operation of quasi-collinear AO devices in the multifrequency diffraction regimen is accompanied by the appearance of the longitudinal and transverse temperature gradients in the crystal, mainly due to the acoustic power absorption. Temperature changes the AO cell material stiffness moduli, affecting the characteristics of the incident and reflected acoustic waves (propagation velocities and walk-off angles), and the reflection condition in general. On the example of paratellurite crystal is shown that the AO cell heating near the reflecting facet leads to a deviation of the reflected acoustic beam propagation direction from that specified during the AO cell manufacturing. The deviation magnitude depends on the reflection geometry choice and, in the paratellurite, may exceed several degrees, which adversely affects the AO diffraction characteristics, reducing the AO interaction efficiency and distorting the transmission function shape. The reflected beam deviation may be compensated by means of choosing the angle between the AO cell reflecting face and the piezoelectric transducer face, taking into account the operating AO device thermal regimen.

Evaluation of the Tellurium Dioxide Crystal Shear Acoustic Wave Attenuation at 40-140 MHz Frequency.

The attenuation of slow shear acoustic waves in the (110) plane of tellurium dioxide (TeO2) crystals was investigated. The strong acoustic anisotropy of TeO2 crystals results in a non-uniform acoustic power distribution, which can introduce errors in conventional acousto-optic testing methods. In this study, we propose a general method to measure the acoustic power distribution along the propagation direction of acoustic waves in non-collinear acousto-optic tunable filters (AOTFs). Additionally, we analyze the errors introduced by the non-uniform acoustic field resulting from strong acoustic anisotropy in acousto-optic testing methods. The measurements were carried out for a crystal cutoff angle of 6.5° from the [110] axis, for the ultrasound frequency range from 40 to 140 MHz. The attenuation coefficients were determined and their quadratic dependence on ultrasound frequency was confirmed.

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.

Stacking spectral lines with multiplexed Bragg gratings in an acousto-optical tunable filter.

Volume Bragg gratings are a useful tool for spectral manipulation in a variety of settings. In a previous paper [Astron. & Astrophys. to be published, (2024)] we simulated an astronomical instrument that detects molecules on exoplanets by optically creating matched filters using multiplexed Bragg gratings, which optically stack spectral lines. Here, we confirm that we can freely modify and multiplex Bragg gratings, with an acousto-optical tunable filter (AOTF) and that we can stack spectral features in a lab setup using a commercially available AOTF. We find that we are able to multiplex an additional grating or modify an existing grating without altering other multiplexed gratings, so long as the bandpasses do not overlap. We can also successfully stack emission and absorption lines, producing comparable results when optically overlaying spectral lines and computationally summing images of separate lines.

Phase-tuned acousto-optic spatial filter.

An acousto-optic (AO) tunable filter with a phase-controlled dual-section piezoelectric transducer is designed and created for laser beam shaping (LBS). Owing to the acoustic beam steering effect, we experimentally observe splitting of the two-dimensional transfer function. As a result, we demonstrate generation of tunable bottle laser beams and dual-ring intensity distributions for the diffracted beam.

High-Repetition-Rate 2.3–2.7 µm Acousto-Optically Tuned Narrow-Line Laser System Comprising Two Master Oscillators and Power Amplifiers Based on Polycrystalline Cr2+:ZnSe with the 2.1 µm Ho3+:YAG Pulsed Pumping

High-average-power narrow-linewidth tunable solid-state lasers in the wavelength region between 2 and 3 μm are attractive light sources for many applications. This paper reports a narrow-linewidth widely tunable laser system based on the polycrystalline Cr2+:ZnSe elements pumped by repetitively pulsed 2.1 µm Ho3+:YAG laser operating at a pulse rate of tens of kilohertz. An advanced procedure of ZnSe element doping and surface improvement was applied to increase the laser-induced damage threshold, which resulted in an increase in the output power of the Cr2+:ZnSe laser system. The high-average-power laser system comprised double master oscillators and power amplifiers: Ho3+:YAG and Cr2+:ZnSe laser oscillators, and Ho3+:YAG and Cr2+:ZnSe power amplifiers. The output wavelength was widely tuned within 2.3–2.7 µm by means of an acousto-optical tunable filter inside a Cr2+:ZnSe master oscillator cavity. The narrow-linewidth operation at the pulse repetition rate of 20–40 kHz in a high-quality beam with an average output power of up to 9.7 W was demonstrated.

Photon counting Raman spectroscopy: a benchmarking study vs surface plasmon enhancement

We demonstrate a single-photon counting Raman spectroscope and benchmark it against conventional and surface-enhanced Raman spectroscopy. For direct comparison without ambiguity, we use the same solutions of Rhodamine 6G and a common optical setup with either a spectrometer or an acousto-optic tunable filter, whereas the surface enhancement is realized with immobilized Ag nanoparticles. Our results find that the single photon counting significantly elevates the detection sensitivity by up to eight orders of magnitude, arriving at a comparable level of surface-enhanced Raman spectroscopy. Another significant advantage is with the time-resolving measurement, where we demonstrate time-gated and time-correlated single-photon counting with sub-nanosecond resolution. It offers insights into the samples’ transient responses and enables the isolation of Raman scattering from fluorescence signals.

Development of a visible to 1600 nm hyperspectral imaging rigid-scope system using supercontinuum light and an acousto-optic tunable filter

In this study, we developed a rigid-scope system that can perform hyperspectral imaging (HSI) between visible and 1600 nm wavelengths using a supercontinuum light source and an acousto-optic tunable filter to emit specific wavelengths. The system optical performance was verified, and the classification ability was investigated. Consequently, it was demonstrated that HSI (490–1600 nm) could be performed. In addition, seven different targets could be classified by the neural network with an accuracy of 99.6%, recall of 93.7%, and specificity of 99.1% when the wavelength range of over 1000 nm (OTN) was extracted from HSI data as train data.

Analysis of Phase Mismatch for Mercurous Bromide-Based Non-Collinear AOTF Design in Spectral Imaging Applications.

The spectral and spatial characteristics of Acousto-Optic Tunable Filters (AOTFs), such as a tuning curve, spectral resolution, angular aperture, and diffraction efficiency, are determined by the device's acousto-optic crystal configuration and piezoelectric transducer. For high-throughput spectral imaging applications, it is essential to enlarge the spectral bandwidth and angular aperture during the design phase of AOTFs. Thus, phase mismatch due to incident angle or wavelength was studied analytically using phase diagrams in this paper. Additionally, a performance parameter analysis model was established based on the use of mercurous bromide crystals for large angular aperture AOTF device design, and the impact of crystal and transducer design parameters on the spectral bandwidth and angular aperture was evaluated. This also experimentally validates the diffraction capability of AOTFs made from mercurous bromide crystal, which possess a broad spectral transmission ability ranging from visible to long-wave infrared.

Formation of contrasting polychromatic object images based on multi-window acousto-optical filtering

The article presents a technique for acousto-optical multispectral imaging based on choosing the most informative spectral channels and the formation of a multi-window transmission function of an acousto-optical tunable filter. The developed algorithm allows one to select combinations of spectral channels that provide high contrast for a given set of objects and backgrounds with known spectra. The method is tested on the contrast visualization of 20 different object-background pairs with a similar color tone. The results of the experiment demonstrate the effectiveness of the proposed method and algorithm.

Design and testing of a multi-electrode apodized acousto-optic filter for arbitrary polarized light.

We present an original acousto-optic tunable filter that is able to filter visible light from 400 to 650 nm and is designed to interact simultaneously with two polarizations. The filter shows an adjustable optical bandwidth and apodization capabilities. These features make it suitable for practical spectroscopic applications. Experimental validation is also presented.

Design of a broadband acousto-optic filter using bulk acoustic wave beam steering with an interdigital transducer

Acousto-optic tunable filters are photonics devices whose operation is fundamentally based on crystal acoustics. One of the phenomena used to improve performances of bulk acousto-optic devices is acoustic beam steering. Here we present a complete analytical model, which covers bulk acoustic wave beam steering in crystals with phased-array excitation and Bragg phase matching of anisotropic acousto-optic diffraction. An interdigital phased-array transducer is proposed for laterally excited bulk acoustic wave generation enabling the effect of beam steering. The design of a paratellurite-based acousto-optic filter with extended transmitted optical wavelength range from 0.4 to 1.1μm is discussed and supported by numerical simulations.

Feasibility study of forest wood-boring pest detection base on hyperspectral LiDAR

ABSTRACT Forest pests impose significant threats to tree productivity and ecosystem stability. Most studies focussed on foliage water, tree defoliate and mortality, which lag behind pest outbreak. To address this issue, we developed hyperspectral LiDAR (HSL) system based on an acousto-optic tunable filter (AOTF) to examine the echo intensity of tree trunks. First, we demonstrated the echo power of the cylindrical surface is approximately equivalent to that of the plate surface within 60° incidence angle, which provide the theoretical reference for trunk surface echo intensity calibration with a white panel. Second, we propose a two-step method to detect the attacked parts from barks. The first step is rapid pest detection conducted by the echo intensity ratio of damaged area and normal area. While the ratio is around one, the second step is pest detection based on support vector machine classifier with reflectance. When the reflectivity of five spectral channels is used as input parameters, 100% rapid detection accuracy of damage area is achieved. We also use the radiance under seven channels to achieve 100% classification of tree species and attack features. The results show that this HSL is effective in wood-boring pest detection and contributes to forest applications.

Wide-field mid-infrared hyperspectral imaging beyond video rate

Mid-infrared hyperspectral imaging has become an indispensable tool to spatially resolve chemical information in a wide variety of samples. However, acquiring three-dimensional data cubes is typically time-consuming due to the limited speed of raster scanning or wavelength tuning, which impedes real-time visualization with high spatial definition across broad spectral bands. Here, we devise and implement a high-speed, wide-field mid-infrared hyperspectral imaging system relying on broadband parametric upconversion of high-brightness supercontinuum illumination at the Fourier plane. The upconverted replica is spectrally decomposed by a rapid acousto-optic tunable filter, which records high-definition monochromatic images at a frame rate of 10 kHz based on a megapixel silicon camera. Consequently, the hyperspectral imager allows us to acquire 100 spectral bands over 2600-4085 cm−1 in 10 ms, corresponding to a refreshing rate of 100 Hz. Moreover, the angular dependence of phase matching in the image upconversion is leveraged to realize snapshot operation with spatial multiplexing for multiple spectral channels, which may further boost the spectral imaging rate. The high acquisition rate, wide-field operation, and broadband spectral coverage could open new possibilities for high-throughput characterization of transient processes in material and life sciences.

Angular-Spectral Characteristics of Acousto-Optic Tunable Filters Based on Mercurous Halide Crystals.

The angular and spectral properties crucial for the functionality of acousto-optic (AO) devices are determined by phase-matching geometries in AO interactions. In applications such as spectral imagers based on acousto-optic tunable filters (AOTFs), systematic throughput is constrained by the angle separating diffracted and transmitted light. This research introduces an analytical model that elucidates the angular-spectral properties of diffracted beams in mercurous halide crystals. These crystals possess a wide transmissive spectral range, from visible light to long-wave infrared light. The study computes and confirms correlations between the separation angle and parameters including incident angle, polarization, acoustic angle, and crystal birefringence. Experimental validation conducted on mercurous halide and tellurium dioxide crystals demonstrates that higher birefringence in crystals significantly amplifies the separation angle, augmenting the device's performance. The study contributes to the development of devices with large separation angles during the design phase, enhancing systematic throughput in spectral imaging applications.

Double acousto-optic spectral filtering in off-axis multi-wavelength digital holography

Multi-wavelength digital holography (MDH) is a label-free technique for mapping spectral and spatial properties of samples in scientific and industrial applications. Light sources for MDH often include acousto-optic tunable filters (AOTFs) for sequential or simultaneous selection of narrow spectral bands in broadband radiation. We theoretically and experimentally evaluated the influence of the spectral transmission function of single and double AOTFs on the quality of the acquired holograms and proposed ways to improve their characteristics. We analyzed the spatio-spectral properties of the acousto-optic interaction and changed the incident angle of light to reduce the spectral bandwidth and increase the width of the high-contrast area in the interference image. Double spectral filtering using two AOTFs significantly improves the visibility distribution of the interference pattern with peak spectral transmission close to 100% and provides a good compromise between an increase in the coherence length and a decrease in the peak intensity by shifting driving frequency of the second filter. The results of the study provide valuable information for the development of AOTF technology and can be used for both optimal design and adaptive control of AOTF-based tunable light sources in MDH.