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.
An acousto-optic tunable filter (AOTF) is an all solid-state robust device with no-moving parts that has been used in the development of hyperspectral imagers from the ultraviolet to the longwave infrared. Such a device is fabricated by bonding a piezoelectric transducer on a specially cut prism in a birefringent crystal. When a radio frequency (RF) signal is applied to the transducer, a diffraction grating is set up in the crystal with grating period determined by the applied RF and the diffracted wavelength depends upon the applied RF and the prism parameters. When broadband white light is incident on the prism input facet, two orthogonally polarized diffracted beams at a specific wavelength with a narrow bandpass are transmitted for each applied RF. The transmitted wavelength can be tuned by varying the applied RF and this is used in a hyperspectral imager design. The spectral response of the transmitted intensity from an AOTF with a rectangular transducer has a sinc2 distribution with a main lobe and a large number of sidelobes. In general, in hyperspectral imaging applications the light leakage from the sidelobes is not taken into account. When imaging is carried out for a scene with a laser or an intense source of light, the spectral images are affected by the light transmitted through these sidelobes away from the main lobe. We have carried out a detailed study of light transmitted through the sidelobes of a TeO2 AOTF with a rectangular transducer operating in the shortwave infrared (SWIR) region from 0.9 to 1.7 μm. The AOTF imaging system used a telecentric confocal optics that compensates for AOTF aberrations, which are severe at high sidelobe operation. We used a 16-channel RF driver with independent amplitude and frequency control. By switching off specific RF signals applied to the AOTF, we measured the detailed sidelobe structure for the transmitted intensity by analyzing the spectral images and compared it with theoretical prediction. Here we present our experimental setup and results.
We studied the growth and design of solid-solution crystal of mercurous chloride (Hg2Cl2) and mercurous bromide (Hg2Br2). The lattice parameters of the mixtures obey Vagard’s law in the studied composition range. The study demonstrates that properties are very anisotropic with crystal orientation, and performance achievement requires extremely careful fabrication to utilize theoretical AO figure of merit. In addition, some parameters such as crystal growth fabrication, processing time, resolution, field of view and efficiency will be described for imagers based on novel solid solution materials. It was predicted that very similar to the mercurous chloride and mercurous bromide solid solutions also have very large anisotropy, and acousto-optic figure of merit decreases significantly as function of the crystal orientation.
We have developed and fabricated a Tl3AsSe3 (TAS) crystal based acousto-optic tunable filter
(AOTF) for operation between the 8 to 12.0 μm wavelength regions. We have demonstrated
peak efficiency greater than 60% with a 10.6 μm source and 2 watts of RF input power. This
high efficiency should enable high resolution and large throuput for AOTF based imaging and
spectroscopic systems.
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.
Device-quality single crystals of mercurous bromide were grown by the physical vapor transport method. Crystals transmitted light wavelengths up to 30 µm and did not show any absorption bands. Detailed x-ray Laue and x-ray diffraction studies were used to characterize and orient the crystals. Optical evaluation was performed by fabricating slabs of crystals. A design was developed to fabricate acousto-optic tunable filters with 10-deg off-axis orientation operating in the mid- and long-wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 16-mm optical aperture for the 10-deg design. A theoretical tuning curve for a mercurous bromide crystal-based AOTF using this design was also computed for the first time. Experimentally measured data on frequency matching agreed well with the theoretical predictions, and the transducer thickness was suitable for filtering 7.58 µm with the fabricated AOTF.
Northrop Grumman has designed, developed and demonstrated acousto-optic tunable filter (AOTF) based hyperspectral imager to cover visible to long wavelength infrared (LWIR) spectral region. We have experimentally demonstrated and report the results of crystal growth, fabrication, design, development and performance for the long wavelength infrared (LWIR) acousto-optic tunable filter (AOTF)-based hyperspectral imager based on an efficient crystal thallium arsenic selenide. The results on the growth of 40 mm diameter and 15 cm long crystals, 4.0 cm long AOTF fabrication, and system design and performance are presented. 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 SF6. The object was placed ~20" in front of the AOTF. The camera was aligned to the first order diffracted beam of the AOTF. The AOTF was tuned to 10.6 um wavelength by applying a 13.9 MHz RF signal on the transducer. The results on the growth of crystals, AOTF fabrication, and systems design and performance are presented.
High-quality crystals of gallium selenide (GaSe) and thallium arsenic selenide (Tl3AsSe3) were successfully grown. The refractive indices were measured in the subterahertz spectral region using time-domain spectroscopy. GaSe has a refractive index of 3.2 and an absorption coefficient of 1 cm–1, along with an absorption peak at 0.6 THz. Tl3AsSe3 clearly shows birefringence, where the refractive indices are 5.0 and 5.4 along the fast and slow axes, respectively. The absorption coefficient is over 3 cm–1 at 0.3 THz, increasing steadily with frequency.
We carried out studies to identify, synthesize, purify and grow crystals of a novel class of halides for nonlinear optical applications. Tl3PbBr5, Tl4PbI6, Tl4HgI6 and Tl3PbI5, were synthesized by reacting binary halides and crystals were grown. Optical quality was evaluated by fabricating cm size crystals. The homogeneity of bulk crystal was evaluated by studying transparency, etchpit and X-ray rocking curve and 2θ-ω scans. These halides have transparency from visible to far-IR wavelength region. The material of the compounds of Tl3PbBr5 composition showed self-poling during the growth.
Mercurous bromide crystals with very good optical transparency were grown by the physical vapor transport method. A design was developed to fabricate 10-degree orientation acousto-optic tunable filters operating in the mid and long wavelength regions. An acousto-optic tunable filter (AOTF) was fabricated using a crystal with a 13-15 mm diameter. A theoretical tuning curve for a mercurous bromide crystal based AOTF using this design was also computed for the first time.
A Tl3AsSe3 (TAS) acousto-optic tunable filter (AOTF) hyperspectral imaging system is designed and demonstrated that enables real-time imaging of ambient temperature scenes at wavelengths around 10 µm. The predicted sensitivity improvement is a factor of 46 compared to previous TAS AOTF systems, and is accomplished by increasing the spectral passband, improving the AOTF diffraction efficiency, and using a higher sensitivity camera. Optical system designs are also developed and tested for reducing AOTF aberrations and producing pixel-limited resolution.
The Army Research Laboratory has a program to develop and characterize compact field-portable hyperspectral and polarization imagers using electronically tunable spectral filters—acousto-optic tunable filters (AOTFs)—that are polarization sensitive. A spectropolarimetric imager has been designed that combines a liquid-crystal retardation plate with an AOTF and an off-the-shelf charge coupled device (CCD) camera. The imager uses a tellurium dioxide (TeO2) AOTF that operates from the visible to the near-infrared region. The imager is relatively compact, lightweight, and programmable. We used this imager to collect spectral and polarization data from various objects and backgrounds, both in the laboratory and in field tests. The spectral images were collected from 450 to 1000 nm at 10- or 20-nm intervals, at two or four polarization settings for each spectral interval. We analyzed a portion of these data to assess the effectiveness of this system for target detection and identification. Here we present and discuss our measurements and analysis results.
Acousto-optic tunable filter (AOTF) based imagers hold great promise for the emergence of the next generation of compact, lightweight, low-cost, programmable hyperspectral imaging systems that can solve the problem of data bottleneck as well as provide polarization signatures for better target detection and identification. At the U.S. Army Research Laboratory, we have been developing such imagers that can operate from the visible to the long infrared wavelengths. Some of these imagers have been used in the laboratory and in the field for the collection of hyperspectral images. During the past year, we have worked on the design of higher sensitivity more compact visible and infrared imagers. We have designed imagers with tellurium dioxide (TeO2) AOTF cells up to 4.5 micrometers and with a thallium arsenic selenide (Tl3AsSe3, TAS) AOTF cell up to 11.5 micrometers . These imagers use focal plane arrays (FPAs)--Si CCD, InGaAs, InSb, and HgCdTe--as needed for the spectral region of interest. In this paper, we will describe the latest advances in our AOTF imager research and present the results obtained from these imagers.
The acousto-optic tunable filter (AOTF) has a narrow passband and a large angular acceptance angle, which allows for imaging at a given wavelength without having to assemble an image cube, as with grating based imagers. It is also possible to use an acousto-optic dispersive filter (AODF), which has a small acceptance angle and a broad spectral passband to form spatial images similar to the grating imager. Although the processing is more complex, the advantages of the AODF are that pixel registration and temporal fluctuations of the spectra are greatly reduced compared to the AOTF. Both the AOTF and AODF can operate in a birefringent mode, allowing for the use of high efficiency materials such as Tl3AsSe3 (TAS) in the infrared region. They can both be multiplexed to increase the sensitivity, and reduce the spectral fluctuation problem of the AOTF. The AODF can also operate in an isotropic mode, which allows for the use of deflector materials such as Ge. The issues of complexity, fluctuations, efficiency, and multiplexing are compared for AOTFs and AODFs operating in the infrared. A comparison is also made for both systems using TAS, along with AODFs using Ge.
KEYWORDS: Calibration, Signal to noise ratio, Sensors, Modulation transfer functions, Black bodies, Crystals, Acoustics, Target detection, Electrons, Cameras
An advanced infrared imaging spectroradiometer for hot targets (100 to 1500 C) has been designed to give real-time spectrally resolved images in the 2-5 micrometers band and to measure irradiances with high accuracy. A tripod-mounted off-axis afocal telescope with a 2.5 degree diagonal field of view directs the input radiation through a Tl3AsSe3 acousto-optic tunable filter (AOTF). The AOTF deflects away from the main beam a narrow spectral bandwidth beam with its central wavelength determined by the acoustic frequency of the AOTF. The AOTF is electronically controlled and can change the wavelength of the deflected beam within approximately 25 microsecond(s) to any other wavelength. The deflected beam is focused onto an 128 X 128 InSb focal plane array which has a frame rate electronically adjustable from 1 to 217 frame/s. With this versatility: 1) key wavelength discriminators of potential targets can be rapidly accessed, 2) the signal-to-noise ration can be improved by increasing the integration time for point targets, and 3) detector saturation can be avoided by reducing the integration time and AOTF diffraction efficiency for very hot targets. Calculations indicate that irradiance measurement errors should usually be less than 1% and often less than 0.1%.
The spatial resolution limit of noncollinear acousto-optic tunable filters used for imaging with incoherent light is derived based on
phase-matching requirements. It is shown that the noncollinear filter can achieve near diffraction limited imaging. This conclusion was verified experimentally.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.