Acousto-optic Tunable Filter Imaging Research Articles

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.

Blurred spectral images restoration technology for AOTF imaging spectrometer based on dual-path architecture.

The acousto-optic tunable filter (AOTF) imaging spectrometer is a staring-type instrument that acquires spectral images of different bands asynchronously and sequentially, and is susceptible to platform jitter and target motion. When the integration time of the detector increases, serious spectral images degradation occurs along with a large band-to-band misregistration. The dual-path optical configuration, capable of simultaneously acquiring diffracted and undiffracted beams of AOTF, has been shown to be effective in solving the problem of band-to band misregistration. Hence by solving the remaining spectral image degradation problems, the application range of AOTF will transcend the previous limitation. Therefore, this paper presents a new, to the best of our knowledge, method of capturing a series of short-exposure undiffracted beam images to calculate the blur kernel of the diffracted spectral images, since the high-intensity characteristics of the undiffracted beam can also be exploited in the dual-path system. The paper starts with analyzing the impact of the platform on spectral images, and then demonstrates the development and calibration of a dual-path based spectral image deblurring method. Finally, laboratory experiment verifies that the resulting blur kernel can be effectively used for spectral image restoration, thus demonstrating the robustness of this technique.

Ray tracing method for removing sidelobe laser interference in AOTF-based hyperspectral imaging.

The sidelobe phenomenon of acousto-optic tunable filter (AOTF)-based hyperspectral imaging is the result of the phase mismatching condition, which will reduce the quality of hyperspectral imaging data. This phenomenon can be observed in two specified scenes, namely, high brightness panchromatic target imaging and high brightness monochromatic target imaging. In this paper, the acousto-optic interaction in these two scenes is discussed using the phase matching condition. As a follow-up work of the previous related research, this paper studies laser interference in AOTF imaging, which is a typical example of the second case. A method to remove laser sidelobes in AOTF-based hyperspectral imaging is proposed, which combines a series of procedures including laser point extraction, reverse imaging before AOTF, ray tracing in AOTF, forward imaging process after AOTF, and laser elimination. Experimental results have verified the effectiveness of the proposed method.

A novel dual-path high-throughput acousto-optic tunable filter imaging spectropolarimeter

It is highly demanding to identify healthy and non-healthy species in a heterogeneous environment such as human tissues. In such a case, one identifier, such as a spectral fingerprint, might be inadequate. Therefore, additional identification is required, for instance, a polarisation measurement. In view of that, the development of a spectropolarimeter that captures two cross-polarised arrays of spectral images is a key requirement. To meet this requirement, an imager optical setup has been designed to provide spatial, spectral and polarisation preference information for species that exist in a heterogeneous environment, such as in medical tissue samples. The spectral and polarisation information is obtained employing an acousto-optic tunable filter and a polarising beam splitter, respectively. The optical imager is designed to operate in the visible-near infrared range (450–850 nm) with a spectral resolution of 3 nm. The spectropolarimeter design along with optical characterisation results are reported.

SWIR AOTF Imaging Spectrometer Based on Single-pixel Imaging.

An acousto-optic tunable filter (AOTF) is a new type of mono-wavelength generator, and an AOTF imaging spectrometer can obtain spectral images of interest. However, due to the limitation of AOTF aperture and acceptance angle, the light passing through the AOTF imaging spectrometer is weak, especially in the short-wave infrared (SWIR) region. In weak light conditions, the noise of a non-deep cooling mercury cadmium telluride (MCT) detector is high compared to the camera response. Thus, effective spectral images cannot be obtained. In this study, the single-pixel imaging (SPI) technique was applied to the AOTF imaging spectrometer, which can obtain spectral images due to the short-focus lens that collects light into a small area. In our experiment, we proved that the irradiance of a short-focus system is much higher than that of a long-focus system in relation to the AOTF imaging spectrometer. Then, an SPI experimental setup was built to obtain spectral images in which traditional systems cannot obtain. This work provides an efficient way to detect spectral images from 1000 to 2200 nm.

Acousto-optic tunable filter chromatic aberration analysis and reduction with auto-focus system

An acousto-optic tunable filter (AOTF) displays optical band broadening and sidelobes as a result of the coupling between the acoustic wave and optical waves of different wavelengths. These features were analysed by wave-vector phase matching between the optical and acoustic waves. A crossed-line test board was imaged by an AOTF multi-spectral imaging system, showing image blurring in the direction of diffraction and image sharpness in the orthogonal direction produced by the greater bandwidth and sidelobes in the former direction. Applying the secondary-imaging principle and considering the wavelength-dependent refractive index, focal length varies over the broad wavelength range. An automatic focusing method is therefore proposed for use in AOTF multi-spectral imaging systems. A new method for image-sharpness evaluation, based on improved Structure Similarity Index Measurement (SSIM), is also proposed, based on the characteristics of the AOTF imaging system. Compared with the traditional gradient operator, as same as it, the new evaluation function realized the evaluation between different image quality, thus could achieve the automatic focusing for different multispectral images.

MWIR thermal imaging spectrometer based on the acousto-optic tunable filter.

Mid-wavelength IR (MWIR) thermal imaging spectrometers are widely used in remote sensing, industrial detection, and military applications. The acousto-optic tunable filter (AOTF)-based spectrometer has the advantages of fast tuning, light weight, and no moving parts, which make it ideally suited for MWIR applications. However, when designing an AOTF imaging spectrometer, the traditional method uses a refractive grating or parallel glass model in optical design software to simulate the AOTF, lowering the imaging performance of the optical system. In this paper, an accurate simulating model for an actual MWIR AOTF using the user-defined surface function in ZEMAX is presented, and an AOTF-based MWIR thermal imaging spectrometer is designed and tested successfully. It is based on a MWIR tellurium dioxide (TeO2) AOTF with an operational spectral range from 3.0 to 5.0μm and a spectral resolution of 30.8nm at 3.392μm. The optical system employs a three-mirror off-axis afocal telescope with a 2.4°×2.0° field of view. The operation of the MWIR thermal imaging spectrometer and its image acquisition are computer controlled. Furthermore, the imaging spectrometer is tested in the laboratory, and several experiments are also presented. The experimental results indicate that the proposed AOTF model is efficient, and also show that the imaging spectrometer has the ability to distinguish the real hot target from the interfering target effectively.

Effects of sidelobes on acousto-optic tunable filter imaging

An acousto-optic tunable filter (AOTF) is used in the development of hyperspectral imagers from the ultraviolet to the longwave infrared. The spectral response of the transmitted intensity from an AOTF with a rectangular transducer has a sinc2(x) distribution and so far the light leakage from the sidelobes is ignored in hyperspectral imagers. When unpolarized white light is incident on an AOTF, two orthogonally polarized diffracted beams at a specific wavelength with a narrow bandpass filter are transmitted for an applied radio frequency (RF), and an image cube is obtained by tuning the applied RF. We carried out a detailed study of light transmitted through the sidelobes of a TeO2 AOTF operating in the shortwave infrared region from 0.9 to 1.7 μm to image a scene containing a laser. The AOTF imaging system used telecentric confocal optics that compensate for AOTF aberrations. We used a 16-channel RF driver with independent amplitude and frequency control. By switching off specific RF signals applied to the AOTF, the detailed sidelobe structure for the transmitted intensity was measured and compared with theory. We found that close to 10% of the transmitted light is leaked through the sidelobes. Here, we present our experiment and analysis of the results.

Calibration of AOTF-based 3D measurement system using multiplane model based on phase fringe and BP neural network.

A specifically designed imaging system based on an acousto-optic tunable filter (AOTF) can integrate hyperspectral imaging and 3D reconstruction. As a result of the complicated optical structure, the AOTF imaging system deviates from the traditional pinhole model and lens distortion form, causing difficulty to achieve precise camera calibration. The influencing factors leading to the deviation are discussed and a multiplane model (MPM) is proposed with phase fringe to produce dense mark points and a back propagation neural network to obtain subpixel calibration. Experiments show that MPM can reduce the back projection error efficiently compared with the pinhole model. A 3D reconstruction process is conducted based on the calibration result to verify the feasibility of the proposed method.

Notch filtering using a multiple passband AOTF in the SWIR region.

A notch filtering operation was accomplished using a TeO2 acousto-optic tunable filter (AOTF) with 16 simultaneous overlapping passbands in the shortwave infrared wavelength region. By switching off specific radio frequency signals applied to the AOTF, laser wavelengths corresponding to the inactive passbands are rejected, providing see-through capability with the remaining wavelengths. The rejection level was determined by leakage through the sidelobes of adjacent passbands, as was shown by theory and corresponding measurements. By switching off multiple passbands near the laser wavelength, the rejection level can be increased at the expense of reduced see-through capability. The AOTF imaging system used telecentric confocal optics that compensate for AOTF aberrations, which are severe at high sidelobe operation.

Accurate band-to-band registration of AOTF imaging spectrometer using motion detection technology

This paper concerns the problem of platform vibration induced band-to-band misregistration with acousto-optic imaging spectrometer in spaceborne application. Registrating images of different bands formed at different time or different position is difficult, especially for hyperspectral images form acousto-optic tunable filter (AOTF) imaging spectrometer. In this study, a motion detection method is presented using the polychromatic undiffracted beam of AOTF. The factors affecting motion detect accuracy are analyzed theoretically, and calculations show that optical distortion is an easily overlooked factor to achieve accurate band-to-band registration. Hence, a reflective dual-path optical system has been proposed for the first time, with reduction of distortion and chromatic aberration, indicating the potential of higher registration accuracy. Consequently, a spectra restoration experiment using additional motion detect channel is presented for the first time, which shows the accurate spectral image registration capability of this technique.

Photoelastic and Acousto-optic Properties of KDP Crystal Applied in Wide Angle Tunable Filters

The paper presents results on calculation and measurement of acousto-optic properties of KDP crystals applied in imaging tunable acousto-optic filters operating in the ultraviolet region of electromagnetic light. Effective photoelastic coefficients and acousto-optic figures of merit are evaluated in the mode of wide angle diffraction. The carried out analysis is based on results of measurements of the efficiency of Bragg diffraction at the same frequency of ultrasound but at two different angles of light incidence in the KDP crystal. We used the crystal in an imaging acousto-optic tunable filter operating at the wavelengths of light 405, 532 and 633nm. The filters apply a slow shear acoustic wave propagating at the angles α=90 and α=120 relatively to the [100] axis in (010) plane of the crystal. Differences between the measured data on the photoelastic coefficients and those found in literature are discussed in the presentation.

Three-surface model for the ray tracing of an imaging acousto-optic tunable filter

A three-surface model is proposed for the ray tracing of an imaging acousto-optic tunable filter (AOTF) in the optical design of an AOTF imaging system. The first and last surfaces are two refractive planes corresponding to the incident and exit facets of the AOTF, while the property of the second surface is defined particularly to describe the change of the ray trace owing to the interaction of the acoustic and optic waves. One parameter, the acoustic angle, is first corrected using the test tuning relation to compensate for the nonideality of the acoustic wave. The model has been verified with a two-piezotransducer AOTF to show its usefulness. The differences between the measured diffracted angles and the modeling value are below 0.01°. The comparison demonstrates the accuracy and the efficiency of the three-surface model.

The visible to the near infrared narrow band acousto-optic tunable filter and the hyperspectral microscopic imaging on biomedicine study

Based on the parallel tangents momentum-matching condition, a narrow band noncollinear acousto-optic tunable filter (AOTF) using a single TeO2 crystal is designed with the consideration of the birefringence and the rotatory property of the material. An effective setup is established to evaluate the performance of the designed AOTF. The experimental observed spectrum pattern of the diffracted light is nearly the same with the theoretical result. The measured tuning relationship between the diffracted central optical wavelength and acoustic frequency is in accordance with the theoretical prospect. The optical bandwidth of the diffracted light is as narrow as 1.88 nm when the central wavelength is 556.75 nm. The high spectral resolution is significant in practical applications of imaging AOTF. Additionally, the AOTF based hyperspectral microscopic imaging system is established. The stability and the image resolution of the designed narrow band AOTF are satisfying. Finally, the study of the biologic samples indicates the feasibility of our system on biomedicine.

Improvement in the synchronization between the radio frequency signal and the image detector in an acousto-optic tunable filter imaging spectrometer.

An improved synchronization between the radio frequency (RF) signal and the image detector in an acousto-optic tunable filter (AOTF) imaging spectrometer is proposed to optimize power consumption and eliminate image smear. The RF signal is controlled on and off alternately to match the exposure of the image sensor. This scheme reduces the RF power and rejects the light illumination on the image sensor in the interval of charge transfer. An experiment using a visible AOTF, a frame transfer charge-coupled device camera, and an incandescent lamp is conducted for demonstration. The average RF power decreases 7.6%, and the image smear is eliminated.

Development of double-filtering imaging acousto-optic tunable filter with increased spectral resolution

The spectral resolution is a key index of evaluating the performance of the designed acousto-optic tunable filter (AOTF). We present a special double-filtering method of obviously enhancing the spectral resolution. We have discussed the main technique functional in the double-filtering method in detail. It is confirmed that the double-filtering method can effectively enhance the spectral resolution with good imaging quality. This study is significant in the imaging applications of AOTF.

Spectral resolution enhancement of acousto-optic tunable filter by double-filtering

The development of the large angular aperture noncollinear acousto-optic tunable filter (AOTF) is based on the parallel tangents momentum-matching condition. In this letter, we have introduced a special double-filtering method that leads to an enhancement of the spectral resolution. And the birefringence together with the rotatory property of the interaction material has been considered to ensure the accuracy of designing an AOTF. The principle and availability of double-filtering are discussed in detail. It is confirmed that double-filtering method is effective to enhance the spectral resolution on the condition of keeping the quality of imaging, which is significant in practical applications of imaging AOTF.

Operational characteristics of a long-wavelength IR multispectral imager based on an acousto-optic tunable filter

We have experimentally demonstrated and report on the results of crystal growth, fabrication, design, development, and performance for the long-wavelength infrared (LWIR) hyperspectral imager based on an acousto-optic tunable filter (AOTF) utilizing an efficient crystal, thallium arsenic selenide (Ti3AsSe3 TAS). Results on the growth of 40-mm-diameter, 15-cm-long crystal boules, to fabricate 4.0-cm-long AOTF devices, and on the system design and performance are presented. To achieve an 8-cm−1-resolution AOTF, we developed a design utilizing growth at 10.6 deg off from the c axis of the crystal and achieved >37% efficiency. A system concept was developed with high efficiency, resolution, and throughput utilizing this TAS AOTF. The test setup consisted of an LWIR camera (microbolometer), the AOTF, and a blackbody radiative source (hot filament), and represents the first time AOTF imaging has been achieved with a microbolometer camera. The filament was placed 25 cm in front of the AOTF, and the camera was aligned to the first-order diffracted beam of the AOTF. The AOTF was tuned to 10.6-µm wavelength by applying a 13.9-MHz rf signal to the transducer. Preliminary experimental results obtained for SF6 gas utilizing this system are reported.

Acousto-optic tunable filter imaging spectrometer with full Stokes polarimetric capability

Polarization is an important addition to spectral imaging in detecting and identifying objects of interest, and simple linear polarization measurements are often inadequate. Full polarization analysis can give additional information for discrimination where the polarization state is completely described by the Stokes parameters. An acousto-optic tunable filter (AOTF) imaging system was built incorporating two liquid-crystal variable retarders (LCVRs) that can provide complete spectral-polarimetric analysis, and it is believed to be the first demonstration of a full Stokes polarimetric AOTF spectral imaging system with no moving parts. It is also shown that a single LCVR cannot provide all the Stokes parameters.

Analysis of the optimum optical incident angle for an imaging acousto-optic tunable filter

The selection of the incident polar angle is very important in the entire design of the noncollinear acousto-optic tunable filter (AOTF). The authors discussed how the factors, including tuning range of wavelength, the acoustic frequency, the acousto-optic figure of merit, the spectral bandwidth, the spread of filtered beam and the wavelength dependence, influence the selection of the optimum incident polar angle. By an accurate theoretical analysis, a method of selecting the optimum incident angle was presented. The analysis was significant for improving the performance of the imaging AOTF from the visible to the infrared.