Milton S. Gottlieb

Acousto-optic tunable filters in imaging applications

Milton S. GottliebBoris KaminskyCarnegie Mellon Research Institute700 Technology DrivePittsburgh, Pennsylvania 15230E-mail: ld2s@andrew.cmu.eduAbstract. We examine various physical characteristics of current-artacousto-optic tunable filters (AOTFs) for spectrally filtered imaging appli-cations. Imaging spectroscopy, where the goal is to obtain simulta-neously spatial and spectral information, is a topical application that ef-fectively utilizes the attractive features of the AOTF. Key measures ofperformance include image blur, relative contribution from sidelobes,contribution from background illumination, broadband scattering and dif-fraction efficiency.

Object detection with a field-portable spectropolarimetric imager

A relatively compact, lightweight, and programmable spectropolarimetric imager was used to collect spectral and polarization data from various objects and backgrounds, both in the laboratory and in field tests. This imager uses a tellurium dioxide (TeO2) acousto-optic tunable filter and a liquid-crystal retardation plate with a CCD camera. The spectral images were collected 450-1000 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 object detection. We present our measurements and discuss the analysis results.

Crystal growth, fabrication, and design of mercurous bromide acousto-optic tunable filters

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.

Spectropolarimetric imaging for object recognition

We have built an all-electronic spectro-polarimetric imaging camera utilizing an acousto-optic tunable filter and a liquid crystal variable retardation plate. This combination of rapidly adjustable parameters allows operations at 30/sec. frame rate, and near real time adaptability to changing target signatures. The spectral capability of the AOTF permits us to apply simultaneous, multiple wavelength filtering which greatly increases selectivity. Electronically agile polarization analysis adds a valuable signature feature for many scenarios. The adjustable retardation gives the capability to analyze and display not only linear polarization, but more generally, elliptical polarization as well. We have developed background suppression algorithms based on spectral and polarization signatures so that a wide variety of targets may be displayed with greatly enhanced contrast.

Properties of nonlinear optical crystals in the terahertz wavelength region

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.

Spectro-polarimetric imager for intelligent transportation systems

We have built a portable spectro-polarimetric machine vision system that operates at video frame rates. Our system contains only electronically controllable components, including an imaging acousto-optic tunable filter (AOTF), a phase retarder, acceptance and imaging optics, and a standard CCD-based camera. The device operates like an ordinary camera, except that a computer controls the spectral and polarization content of light to be viewed. For example, by sweeping the wavelength over the AOTFs range, one can obtain a spectral signature for each pixel in an image. Alternately, the camera can switch between two wavelengths, allowing for high-speed discrimination of closely matched colors in a scene. In addition to digitally controlling the wavelength, our imager uses a liquid crystal retarder to filter images based on polarization signatures of objects. We have implemented a number of algorithms to take advantage of the unique capabilities of our sensor, some of which can be applied to problems specific to transportation systems. We present two image processing applications that highlight the different methods we use to analyze scenes with our system. One application uses spectral processing to locate vegetation in a scene; the second uses polarization signatures to detect glare from hazardous road conditions such as water and ice.

Long-wave infrared spectral imager with an 8-cm−1 passband acousto-optic tunable filter

A Tl 3 AsSe 3 (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.

AOTF polarization difference imaging

Recent work has indicated that polarization difference imaging has the potential to enhance the image quality of objects viewed in the presence of scattering media, such as fog and turbid water. We have utilized an AOTF spectro- polarimeter to implement this concept, and to expand its usefulness by incorporating real-time, adaptive, complex polarization and spectral filtering techniques into the system.

Hyperspectral imaging using acousto-optic tunable filters

Hyperspectral imaging holds great promise for object detection and recognition due to the richness of the spectral content in images from such objects. Ordinary broad-band imagers integrate the spectral information over the entire spectral band of coverage when used without any spectral filtering. In general, the spectral details in the images can be obtained by using an optical filtering element such as a filter wheel, a grating, or an acousto-optic tunable filter (AOTF). Since each task of detection and object recognition may require only a limited set of specific spectral bands based on the object as well as the background, it is best to choose a filtering optical element that has high-speed spectral selectivity with high resolution. Of all the optical filtering elements available, only an AOTF offers this capability. Such capability greatly reduces the amount of data collection and processing. In this paper, we present hyperspectral images obtained in the laboratory and from field tests, using visible-to-near-IR (VNIR) AOTF imagers. The imagers use a tellurium dioxide, TeO2, AOTF cell that covers the spectral band from 450 to 1000 nm with a spectral resolution of 10 nm at 600 nm, a charged coupled device (CCD) camera, image-forming optics, frame grabber board, rf electronics, and control and processing software. The imager used for outdoor testing is equipped with a variable phase retardation plate to obtain images with polarimetric signatures (patent pending). The spectral and polarimetric imaging capabilities of the AOTF imager were successfully tested to discriminate targets and backgrounds in various environments.

Image processing using acousto-optical tunable filtering

We describe the construction and performance of an acousto- optic tunable filter (AOTF) camera system for obtaining images in the range of 450 to 1000 nm. A combination of a 10-bit digital video camera and a high speed frame grabber board allows continuous display of high-resolution, filtered images on a computer monitor at 30 frames per second. Ability for target recognition is significantly enhanced by processing filtered images. A typical speed in a basic operation that requires two frame grabs at two different filter settings and image processing is currently limited to 6 frames per second. The pre-processing of the target image by the AOTF simplifies subsequent image processing and is nearly real time.