Review of acousto-optic spectral systems and applications

Acousto-optic devices represented by acousto-optic tunable filters (AOTFs), have the advantages of wide wavelength range from the ultraviolet to the long-wave infrared and fast wavelength switching speed. Nowadays, acousto-optic spectral systems have become very important scientific instruments in laboratory. There are many factors to be considered when we choose different solutions for acousto-optic spectral systems, but there is no comprehensive analysis and summary of them. This paper explains the working principle of the acousto-optic devices and summarizes the most common optical schemes for acousto-optic spectral systems. We also analyzed their characteristics of application conditions. In addition, specific applications of acousto-optic spectral systems in some common fields are presented.

1 Introduction

The study of acousto-optic interactions began in the 1920s and was limited to isotropic media such as water and glass [1, 2]. When ultrasonic waves pass through the medium, the refractive index of the medium changes periodically by the modulation of the strain. The acousto-optic medium is equivalent to a dislocation grating, and diffraction occurs when light passes through it. With the emergence of lasers and high-performance acousto-optic crystals, the study of acousto-optic devices has broadened from isotropic to anisotropic media and from normal to anomalous interactions [3–5]. As the research of acousto-optic theory continues to progress, the principles of isotropic and anisotropic acousto-optic interactions need to be unified. We can consider the acousto-optic effect as a parametric interaction process, which is described by the relationship between the non-linear polarization vector and the strain [6–8].

The first AOTF was proposed by Harris and Wallace utilizing the collinear acousto-optic interaction [9]. In 1974, I. C. Chang proposed the idea of a non-collinear acousto-optic tunable filter design, which laid a solid foundation for the development of acousto-optic devices [10]. When an excitation RF signal of a certain frequency is applied to the transducer, the piezoelectric crystal transducer converts it into an ultrasonic signal of the corresponding frequency and couples it into the birefringent crystal. The refractive index of the crystal then changes periodically, which is equivalent to the formation of a bit-phase grating in the crystal, and the grating constant is the wavelength of ultrasonic waves [11–14]. The acousto-optic interaction wave vector diagram, which takes place in AOTF, is shown in Figure 1A. The incident light wave vector ${\mathbf{k}}_{\mathbf{i}}$, diffracted light wave vector ${\mathbf{k}}_{\mathbf{d}}$, and the acoustic wave vector ${\mathbf{k}}_{\mathbf{a}}$ are strictly matched to the momentum triangle closure condition:

$\overrightarrow{{\mathit{k}}_{\mathit{d}}}=\overrightarrow{{\mathit{k}}_{\mathit{a}}}\pm \overrightarrow{{\mathit{k}}_{\mathit{i}}}(1)$

${\mathit{k}}_{\mathit{i}}=\frac{2\mathit{\pi }{\mathit{n}}_{\mathit{i}}}{\mathit{\lambda }},{\mathit{k}}_{\mathit{d}}=\frac{2\mathit{\pi }{\mathit{n}}_{\mathit{d}}}{\mathit{\lambda }},{\mathit{k}}_{\mathit{a}}=\frac{2\mathit{\pi }{\mathit{f}}_{\mathit{a}}}{{\mathit{\nu }}_{\mathit{a}}}(2)$

Where ${\mathit{n}}_{\mathit{i}}$ is the refractive index of the crystal to the incident light, and ${\mathit{n}}_{\mathit{d}}$ is the refractive index of the crystal to the diffracted light. The incident angle ${\mathit{\theta }}_{\mathit{i}}$ and diffraction angle ${\mathit{\theta }}_{\mathit{d}}$ are shown in Figure 1A. λ is the optical wavelength in vacuum, ${\mathit{f}}_{\mathit{a}}$ is the acoustic frequency, and ${\mathit{\upsilon }}_{\mathit{a}}$ is the speed of the ultrasonic wave.

FIGURE 1

FIGURE 1. (A) Non-collinear acousto-optic interaction wave vector diagram in TeO₂, (B) collimating scheme, (C) confocal scheme, (D) tandem scheme, (E) double-pass single-crystal AOTF based spectral scheme, (F) double-path scheme (Obj, object; L, lens; P, polarizer; R, retroreflector; BS, beam splitter; CCD, charge-coupled device).

In the case of collinear acousto-optic interaction, the incident light wave vector, the diffracted light wave vector and the ultrasonic wave vector are in the same direction. The momentum matching triangle at this point is simplified to a straight line. The geometric relationship between the vectors can be turned into an algebraic sum. Bringing Eq. 2 into Eq. 1, the tuning equation of the common-linear acousto-optic tunable filter can be obtained as

$\mathit{\lambda }{\mathit{f}}_{\mathit{a}}=\pm {\mathit{v}}_{\mathit{a}}\left({\mathit{n}}_{\mathit{d}}-{\mathit{n}}_{\mathit{i}}\right)(3)$

In the non-collinear case, the diffracted light and the incident light propagate in different intrinsic modes in the crystal, and the incident light wave vector and the diffracted light wave vector are not parallel. The incident light wave vector, the ultrasonic wave vector and the diffracted light wave vector are in a vector triangle relationship (i.e., momentum matching condition). Usually, in the design of AOTF, the tangents of the incident and diffracted light wavevectors are parallel to each other at the corresponding wavevector surface in order to have a large incident angle aperture. In this case, the design parameters are related as follows

${\mathit{n}}_{\mathit{i}}={\left[\frac{{\mathbf{cos}}^2{\mathit{\theta }}_{\mathit{i}}}{{\mathit{n}}_{\mathit{o}}^2}+\frac{{\mathbf{sin}}^2{\mathit{\theta }}_{\mathit{i}}}{{\mathit{n}}_{\mathit{e}}^2}\right]}^{-1∕2}(4)$

${\mathit{n}}_{\mathit{d}}={\mathit{n}}_{\mathit{o}}(5)$

$n_o$ and $n_e$ are the refractive indices of ordinary ray and extraordinary ray which are perpendicular to the optical axis, and they are a function of the wavelength of light. According to the tangential parallelism condition, the relationship between the diffracted light polar angle and the incident light polar angle is

$\mathit{tan}{\mathit{\theta }}_{\mathit{d}}={\left({\mathit{n}}_{\mathit{o}}/{\mathit{n}}_{\mathit{e}}\right)}^2(6)$

The tuning relationship between optical wavelength and ultrasonic frequency can be expressed by

${\mathit{f}}_{\mathit{a}}=\frac{{\mathit{v}}_{\mathit{a}}}{\mathit{\lambda }}{\left({\mathit{n}}_{\mathit{i}}^2+{\mathit{n}}_{\mathit{d}}^2-2{\mathit{n}}_{\mathit{i}}{\mathit{n}}_{\mathit{d}}\mathit{cos}\left({\mathit{\theta }}_{\mathit{i}}-{\mathit{\theta }}_{\mathit{d}}\right)\right)}^{\frac{1}{2}}(7)$

Combining Eqs 1–3, we can simplify Eq. 4 to

${\mathit{f}}_{\mathit{a}}=\frac{{\mathit{v}}_{\mathit{a}}}{\mathit{\lambda }}\left({\mathit{n}}_{\mathit{i}}-{\mathit{n}}_{\mathit{d}}\right){\left({\mathit{sin}}^{2}2{\mathit{\theta }}_{\mathit{i}}+{\mathit{sin}}^4{\mathit{\theta }}_{\mathit{i}}\right)}^{\frac{1}{2}}(8)$

Exceptionally, when ${\theta }_i=90°$, the equation reduces to the tuning relationship for the common-linear interaction. Therefore, the collinear interaction can be viewed as a special case of non-collinear interaction. The non-collinear AOTF keeps the tangents of the incident light and the diffracted light on the wave vector trajectory parallel to each other, so that when there is a small change for the incident angle, the momentum matching condition still holds.

There are many applications based on acousto-optic action, such as acousto-optic modulators, deflectors, frequency shifters, and tunable filters. The AOTFs are becoming a widely used tool for these applications. Spectral imagers based on AOTF have a wide range of applications in science and engineering [15–17]. Although there have been many reports on the generation and application of AOTF acousto-optic spectral instruments, there is no comprehensive summary of the characteristics of different schemes. In this paper, we summarize in detail the commonly used optical schemes for AOTF-based acousto-optic spectral systems and compare them with examples, and finally present specific applications in major representative fields.

2 Acousto-optic spectral system schemes

AOTFs have a variety of applications, and researchers have used a variety of different optical solutions for purposes [18, 19]. Different schemes differ in image quality, the number of coupling components, size, and alignment complexity [20–22]. To properly select the optical system for an AOTF-based spectral system, many factors must be considered [23–25]. Although various acousto-optic filtering schemes have been tested and discussed in various articles, a summary of them is not available so far. In this section, the four most common optical schemes based on the AOTF module as shown in Figure 1 are presented: collimating scheme, confocal scheme, tandem scheme, and double-path scheme. We will compare and analyze their main features with some examples. Although these schemes are derived from both collimation and confocal schemes, this division makes them easier to be summarized as well as to be understood.

2.1 Collimating scheme

For conventional acousto-optic spectral systems, the collimating scheme is the most common and structurally simple scheme, and the optical path diagram is shown in Figure 1B. The adopted scheme is to filter the light directly using a single AOTF, which is also the basis of other schemes [26, 27]. The light entering the optical system from the object under test is collimated by L0, the mutually parallel light is filtered by the AOTF, and then be focused on the CCD by the focusing lens L1. Non-uniformity of the central wavelength of the filtered light across the field of view can lead to specific image spectral distortions [28].

The conventional acousto-optic spectral system can satisfy the needs of many tasks. But, the optimization of this system is necessary, and the integration of the optimized AOTF units into other schemes can multiply the efficiency. In 2021, proposed a method for optimizing the size of piezoelectric transducers of quasi-collinear AOTF [29]. In this scheme, they used an AOTF with a large interaction length. From the experimental results, it is shown that the variation of the transducer size can minimize the RF power consumption of the AOTF. Comparing the optimized transducer dimensions with those commonly used ones in quasi-collinear AOTF, the optimized AOTF energy efficiency can be improved about twice.

2.2 Confocal scheme

Confocal optics can compensate for almost any degree of AOTF diffraction aberration. Unlike wedge compensation, this optical system does not require dispersion and can be used with any type of AOTF [30–32]. The confocal optics scheme is shown in Figure 1C. The use of telecentric confocal optics can eliminate errors caused by inaccurate focus. In addition, the system has the advantage of uniform image field with the same resolution and diffraction efficiency over the scene [33, 34]. However, the focal length of the lens in the system must be shot enough to prevent diffraction from limiting resolution.

In 2019, combined AOTF with a rigid borescope, a flexible fiberscope, and a video endoscope, and designed an acousto-optic spectral imaging endoscopic analysis system for observing cancerous tissue [35]. This scheme uses a confocal optical design that can provide high-quality spectral images. This solution can be very effective in solving different specific tasks in biomedical and industrial fields. The main aberration in the confocal scheme is from the presence of longitudinal color focus shift and lateral chromatic image drift. However, this color difference can be almost completely compensated by adjusting the tilt of the output side of the acousto-optic unit to the input side. While, in the collimating scheme, the main aberration is the transverse chromatic aberration, which can be eliminated by choosing a different tilt angle [36, 37].

Collimating and confocal optical schemes both have their own advantages and disadvantages, the confocal scheme is not the optimization of the collimated scheme. In 2021, a super-angular aperture scheme was proposed by [38]. They used both schemes to quantify the change in radiation flux caused by the super-angular aperture scheme and the response of the AOTF element at tilted incidence. They analyzed the system response of the collimated and confocal optical schemes and verified the simulation results. The collimated optical path was found to be more suitable for the super-angular aperture scheme by comparing the two optical schemes. This is because its spectral bandwidth is better than that of the confocal optical scheme, and the central wavelength shift can be corrected by calibration.

2.3 Tandem scheme

The most common and straightforward application of the double filter structure is to connect two AOTFs in series. In 2018, Lei Shi et al. designed a series of double filtering schemes [39]. They compared the spectral widths at different frequencies by analyzing the experimental data. It was finally found that the double-filter structure reduced the spectral width by an average of 37% and improved the spectral resolution by an average of 57% compared to the single filter. By analyzing and comparing the theoretical calculations and experimental measurements of the properties of single and double filter structures, we can find that the spectral width of the double filter structure is smaller than that of the single filter structure for equal central wavelengths. This situation illustrates the superiority of the double-filter technique in improving the spectral width and in increasing the spectral resolution [40].

In 2019, Vitoid E. Pozhar et al. designed a system architecture to address the problem of creating hyperspectral optoelectronic systems for unmanned aerial vehicles [41]. The developed hyperspectrometer uses a dual compact AO monochromator as a spectral element. It consists of two identical AO cells, deployed by 180°, which provides compensation for most spatial spectral aberrations. The device’s small size, low power consumption, and ability to obtain both spectral and color images with high spectral (∼5 nm) and spatial (600–500 elements) resolution over a sufficiently wide wavelength range (450–850) nm make it possible to use it effectively on unmanned aerial vehicles. In fact, back in 2005, Pozhar and other researchers proposed a double-AOTF spectral imaging system for microscopic analysis in the visible and near-infrared range, and it was shown that double AOTF monochromator ensures improved image quality than single imaging AOTF [42].

Tandem AOTF is only one way to realize double filtering, in addition, there are different ways such as the single-crystal double filtering technique. The optical scheme design diagram is shown in Figure 1D. Double filtering is realized using a single crystal, but structurally it is similar to double filtering using two AOTFs. It is simpler and more economical to realize double filtering using a single crystal. Therefore, the scheme of the series connection is less used in practical applications, and nowadays, the single crystal double filtering technique is more often used.

In 2019, Xiaofa Zhang et al. designed a single-crystal double filtering hyperspectral microscopic imaging system [43]. By analyzing the experimental results of diffracted light spectrograms of single-crystal double-filtering scheme, we can find that the comparison yielded a double-filter structure with an average 32% reduction in spectral width compared to single-filtering at a fixed ultrasound frequency of 120 MHz. In the visible range, the spectral resolution can be improved by 37.08%–59.95%. In addition, in 2021, Vladislav Batshev et al. similarly devised a method to improve the spectral resolution of a single AOTF by using a single-crystal for secondary filtering [44]. The structure is similar to Figure 1E. From the results, the transmission spectral width at the 0.5 level (FWHM) is about 1.3 times smaller than that of the classical single-pass scheme.

Among the three schemes mentioned above, the series double-filtering system designed by Shi Lei et al. works best from the results. But unfortunately, the specific series filtering method does not appear in the article. The double-filtering technique can effectively improve the spectral resolution of the acousto-optic filters. In addition, the primary filtered signal after the AOTF is accompanied by an obvious side flap phenomenon, which comes from the acousto-optic interaction [45–48]. There is no way to eliminate the side flaps by using the primary filtering technique. After double-pass filtering, the side flaps are suppressed very significantly, which can improve the spectral purity of the diffracted light [49].

2.4 Double-path scheme

For double-path acousto-optic spectroscopy systems or multi-path analysis systems, they are often used in special scenarios to meet specific requirements [50, 51].

Khoptyar et al. published two articles back and forth in 2012 and 2013 on the use of double-path optical schemes to fabricate novel photon time-of-flight spectrometers for the analysis of turbid media [52, 53]. The wide spectral range of the instrument helps to characterize the structure of the sample and to obtain excellent accuracy in the measurement of absorption and scattering coefficients. Photon time-of-flight spectrometers are used for the evaluation of pharmaceutical chemical composition analysis with proven results. Therefore, double-path optical systems and triple-path spectral systems tend to demand high precision as well as wide spectrum.

In 2019, Ramy Abdlaty and Qiyin Fang both designed an AOTF-based hyperspectral imaging system [54]. Object illumination is provided from both sides to provide uniformly distributed illumination and to avoid shadowing problems. The light reflected from the object is captured by the zoom lens and beam shaping optics. Polarization beam splitter (PBS) splits the collimated beam into two orthogonally polarized beams of transmitted and reflected light. The polarization of the PBS reflected beam is rotated using a half-wave plate to match the PBS transmitted beam. The two PBS beams have the same polarization matched to the AOTF crystal, and this polarization matching allows them to come to the maximum diffraction efficiency.

In 2020, Alexander Machikhin et al. proposed a new concept of spectral stereo imaging [55]. The stereo imaging optical system is shown in Figure 1F. This stereo imaging system is based on simultaneous wide-aperture acousto-optic diffraction of two beams through a conventional AOTF. Experimental results have shown that the quality of the spectral images is quite high, which is necessary for the stereo reconstruction process. In 2021, A. A. Naumov proposed an optical stereo system [56], and this system is very similar to the structure of Figure 1E. The characteristics of such optical systems depend to a large extent on three parameters: the focal length of the incident lens, the focal length of the matrix sensor lens, and the diameter of the incident optical pupil of the acousto-optic filter. The variability of these parameters allows the optical system to be adapted to different tasks [57].

The photon time-of-flight spectrometer utilizes an AOTF module with only partial overlap of the two spectral bands, and the results of the two spectral bands are stitched together to obtain broad-spectrum information. In contrast, in the hyperspectral imaging system, the information of both line polarized beams is processed and retained to obtain the complete spectrum. The two cases of stereo imaging have the same principle, both of which collect the spectral information observed in different orientations and then reorganize them. From the previous analysis, it can be known that both double-path acousto-optic spectral analysis systems aim to obtain more complete spectral information, only the way and results are different. The double-path scheme allows for a wider spectral range than other optical schemes as well as a more complete spectral information from different angles of the same object. At the same time, the flexible placement of the device may also bring more possibilities for spectral system detection. The double pathway acousto-optic spectroscopy system has been used in practical devices for stereoscopic imaging, drug characterization, and multi-directional evaluation after continuous experimentation and is still being improved.

Each scheme have different features and resolutions, the comparison is shown in Table 1.

TABLE 1

TABLE 1. Comparison of acousto-optic spectral analysis system schemes.

3 Applications

Acousto-optic devices are used more frequently in representative fields such as medical and healthcare, aerospace, and food safety. Due to the small mass, small size, absence of removable elements, and compact construction of the AOTF, acousto-optic filters are ideally suited for use with devices used to view hard-to-reach objects, such as rigid lenses and flexible fiberoptic endoscopes [58–61]. The application of acousto-endoscopic imaging spectroscopy will significantly reduce the cost of laboratory testing and increase the information density of research [62–64]. In addition, the development of space instruments based on AOTFs has enhanced the detection capabilities of various space probes. In 2019, He et al. summarized the acousto-optic spectrometers used by China in recent years for lunar exploration [65]. The study include the infrared imaging spectrometer on Chang’e 3 and Chang’e 4 lunar rovers, and the lunar mineral spectrometry analyzer on the Chang’e 5 and Chang’e 6 lunar landers. Acousto-optic devices are becoming widely used in planetary observation, laser observation, surface positioning, and remote sensing [66–69].

Acousto-optic hyperspectral imaging detection technology has also been applied to the detection of microorganisms. In 2018, Y. Seo et al. used acousto-optic spectral image processing techniques for extracting information related to morphological characteristics of 15 different foodborne bacterial species and serotypes [70]. This study achieved a cost-effective classification of foodborne bacteria. In addition, acousto-optic spectroscopy detection systems are also used to test meat, grains and even liquid foods [71–77]. For example, in 2021, I. Baek et al. proposed a short-wave infrared hyperspectral imaging system for the detection of total volatile basic nitrogen content in fresh pork. This system can be used for rapid non-destructive assessment of pork freshness and can be an effective alternative to traditional methods for assessing pork freshness [78].

In addition, acousto-optic spectroscopy systems are also widely used in agriculture, forestry, pharmaceutical analysis, and environmental monitoring [79]. Hong Liu et al. designed a drone-based hyperspectral imaging remote sensing system in 2021 [80], which can be used for activities such as water surface remote sensing, imaging, and spectral analysis. In agriculture, a commercial AOTF-based near-infrared spectrometer is already available for the non-destructive detection of agricultural products such as dried apples and olive fruit [81]. Acousto-optic spectroscopy systems are becoming more and more relevant to our lives.

With the development of various new technologies, the miniaturization and intelligence of spectrometers equipped with acousto-optic spectral analysis systems are becoming more obvious. Like the applications of acousto-optic spectral analysis instruments in various fields described above, acousto-optic devices are now used in a large number of fields. The application of acousto-optic spectral systems is likely to be associated with the development of new materials and new energy sources in the future. Not only for the test of new materials, but also for improving the performance of acousto-optic spectral systems in combination with new materials. The acousto-optic spectral analysis systems are expected to get more long-term development.

4 Conclusion

This article focuses on review of the basic construction of four different optical schemes based on AOTF. Due to the simplicity of the acousto-optic spectral systems, these four optical schemes can be easily embedded in different application scenarios. We summarize the advantages and disadvantages of the different schemes by analyzing the characteristics of the four optical schemes. At the same time, we compare the different schemes with each other and provide theoretical references for the application in different scenarios. Acousto-optic spectral systems have great potential for development in various fields, but in order to adapt to different application scenarios, the requirements for each parameter of the acousto-optic device become higher. The relevant summary and the analysis of characteristics described are expected to provide references for further applications of acousto-optic spectral technology.

Author contributions

YP: Investigation, writing—original draft, writing—review and editing, supervision. KZ: Investigation, writing—original draft. LL: Investigation, writing—review and editing.

Funding

This work was supported by the National Natural Science Foundation of China (61905063), Natural Science Foundation of Hebei Province (F2020202055).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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