Arrays of independently tunable MEMS Fabry-Perot filters have been developed that enable spectral tuning over the
range of 11 - 8 microns with a filter bandwidth of ~ 120 nm. Actuation is provided using a MEMS driver IC that is
hybridized to the MEMS chip. Combining the filter array with an IR FPA enables spatially-resolved spectral tuning in a
compact architecture. Tunable spectral response data from the first integrated tunable filter / FPA device are presented.
Many natural materials produce polarization signatures, but man-made objects, typically having more planar or smoother
surfaces, tend to produce relatively strong polarization signatures. These signatures, when used in combination with
other means, can significantly aid in the detection of man-made objects. To explore the utility of polarization signatures
for target detection applications we have developed a new type of polarimetric imaging sensor based on tunable liquid
crystal components. Current state-of-the-art polarimetric sensors employ numerous types of imaging polarimeters, the
most common of which are aperture division, micropolarizer, and rotating polarizer/analyzer. Our design uses an
electronically tunable device that rotates the polarization of incoming light followed by a single fixed oriented linear
polarizer. Its unique features include: 1) sub-millisecond response time switching speed, 2) ~75% transmission
throughput, 3) no loss of sensor resolution, 4) zero mechanical moving parts, 5) broadband (~75% of center wavelength),
6) ~100:1 contrast ratio, 7) wide acceptance angle (±10°), and 8) compact and monolithic architecture (~10 inch3). This
paper summarizes our tunable liquid crystal polarimetric imaging sensor architecture, benefits of our design, analysis of
laboratory and field data, and the applicability of polarization signatures in target detection applications.
Creating intelligent integrated microsystems, devices that incorporate photonics, electronics, MEMS, and embedded intelligence, presents multiple challenges. The three device technologies have been largely developed independently and have established their own sets of design and process rules that have led to highly stable, high yield processes. In combining these technologies to achieve a desired functionality, constraints are placed on each technology to avoid adverse impacts on the others. Finding a common path towards achieving a single end objective requires process reoptimization and the development of new processes. This paper discusses the dual band adaptive focal plane array (AFPA) that is currently under development, with an emphasis on technology integration and the resulting functional benefits that can be realized. The AFPA device is a dual band IR imaging sensor that enables simultaneous collection of high-resolution MWIR imagery, with spatially independent spectrally tuned imaging in the LWIR for enhanced target detection and classification.
The effectiveness of infrared imaging sensors for target detection and identification can be greatly enhanced by adding spectral analysis capability. Unfortunately, this usually comes with the penalties of increased size, weight, and significantly increased computational requirements, which limit the rate at which information can be made available to the user. By integrating MEMS, photonics and electronics technologies, a new type of staring spectral imager can be realized. An Adaptive Focal Plane Array (AFPA) device is being developed under DARPA sponsorship that consists of an array of MEMS tunable Fabry-Perot filters, hybridized with a dual band IR focal plane array. The MEMS filters will provide narrowband tuning in the LWIR (8.0-10.7 mm) and simultaneous broadband imaging in the MWIR (3-5 mm). Individual filter elements will be on the size scale of a small number of detector pixels. Each filter will be independently electrically addressable, enabling tailored spectral analysis of different regions in the scene. Rather than collecting the complete hyperspectral cube, work is focusing on methods that will enable selection of spatially optimized spectral band sets for a variety of targets and materials that are selected "on-the-fly" to maximize the contrast between the local background and the target or material to be identified.
This paper discusses the structure and status of the AFPA device and highlights some unique challenges inherent in the integration of MEMS, photonics and electronics technologies.
An Adaptive Focal Plane Array (AFPA) device that enables a "chip scale" imaging spectrometer is being developed. The AFPA device consists of an array of MEMS tunable filters that is intimately coupled to a dual band IR FPA. The MEMS filters provide narrowband tuning in the LWIR and simultaneous broadband imaging in the MWIR. Each filter element can be independently tuned. In the current design, each filter tunes the wavelength of pixel subarrays. Ultimately, filter size will be reduced to achieve independent wavelength tunability for each pixel element.
This unique architecture of an AFPA device enables adaptive spectral analysis of the scene. Rather than collecting the complete hyperspectral cube, methods being developed will enable selection of spatially optimized spectral band sets for a variety of targets and materials that are selected "on-the-fly" to maximize the contrast between the local background and the target or material to be identified. The analyzed LWIR spectral information may then be overlaid with a pixel registered high resolution MWIR image.
Birefringent optical compensators containing layers with substantially inclined optic axes can improve not only the contrast but also the gray scale and chromatic stability of 90 degrees twisted nematic LCDs over a large field of view. We present the detailed architecture of such a compensator. It consists of multiple birefringent layers, including one with an in-plane optic axis, one with its optic axis normal to the plane, and two with optic axes inclined at about 40 degrees from the plane. The in-plane and inclined layers are fabricated by photopolymerization of oriented liquid crystal monomers to form anisotropic networks. The precise thicknesses and azimuthal orientations of the various layers are determined by computer optimization. Laboratory measurements of compensated display units show good contrast, gray level, and chromatic stability over a large field of view. The performance is suitable for demanding avionics applications. These compensators are currently being fabricated at the Rockwell Science Center.
Fabrication of refractive microlens arrays on several infrared (IR) transmissive materials was studied. The fabrication process consists of forming photoresist microlenses by thermal reflow of photoresist islands, and transferring this pattern into the IR substrate by ion milling. Microlens arrays having a wide range of F-numbers (F/0.3 - F/12) and sizes were fabricated using a modified ion milling process, where background oxygen and ion energy were used to control the photoresist and substrate erosion rates, respectively. This approach enabled a large range of milling selectivity (e.g. 0.6 - 12 for CdTe) and hence accurate control of lens sag heights. This is important since photoresist microlenses can be successfully fabricated only within a limited range of F-numbers. The etch selectivity also enabled fabrication of nonspherical shapes, starting from spherical photoresist preforms, by judicious control of sputter selectivity during the milling process. Microlens arrays were fabricated in several IR materials, including CdTe, ZnS, Ge, Si, GaAs, InP, GaP and Al2O3. Among these materials GaP and ZnS are also attractive visible and near IR wavelength microlens materials, where their high refractive index results in much lower sag heights than quartz lenses of comparable F-number.
Fabrication issues of microlens arrays, made by first forming photoresist microlenses, by patterning and reflowing photoresist islands under temperature, and then transferring this into the substrate by a dry etch process, were studied. Photoresist microlenses were reliably fabricated within a range of aspect ratios. The desired sag of the microlenses in the substrate was controllably achieved by adjusting the etch selectivity. Etching behavior of fused silica in mixtures of fluoroform with oxygen or sulfur hexafluoride was studied in detail. High quality microlens arrays were fabricated in fused silica, silicon and germanium, and selected lenses were characterized.
Optical beam scanners are critical components for airborne and space-based laser radar, on-machine-inspection systems, factory automation systems, and optical communication systems. We describe here a laser beam steering system based on dithering two complementary (positive and negative) microlens arrays. When the two microlens arrays are translated relative to one another in the plane parallel to their surfaces, the transmitted light beam is scanned in two directions. We
have demonstrated scanning speeds up to 300 Hz with a pair of 6-mm-aperture microlens arrays designed for input from a HeNe laser. The output beam covers a discrete 16 x 16 spot scan pattern with about 3.6 mrad separation and only 400 μrad of beam divergence, in close agreement with design predictions. This demo system is relatively compact; less than 2 in. on a side. We also describe several near-term applications, some critical design trade-offs, and important fabrication and design issues.
Variable angle transmission ellipsometry has been used to characterize the various elements of the liquid crystal display (LCD) architecture. Ellipsometric data, which are in the form of polarization ellipses as a function of incident angle, are analyzed using the 2 X 2 extended Jones matrix formalism. Information which can be deduced from the ellipsometric data includes the birefringence, cell gap, twist angle, and pretilt angle of the liquid crystal cell, polarization efficiency of the polarizers, as well as the retardation values of birefringent compensators. The ellipsometric method is capable of complete characterization of the polarization state of the transmitted light.
The twisted nematic liquid crystal display (TN-LCD) is the leading technology for high performance flat panel displays. However, the region of high contrast for TN-LCD's is limited. Birefringent elements, of compensators, may be used to provide improved contrast at high viewing angles. The phenomenon of form birefringence has been used to design a compensator that can be fabricated by physical vapor deposition of silica and titania, two common coating materials. Improved viewing angle characteristics, particularly in the horizontal direction, have been demonstrated using the compensator. The 20:1 isocontrast region has been extended to +/- 50 degree(s) in displays incorporating the compensator, an improvement of 10 degree(s) or more relative to uncompensated displays. The compensator design includes integrated antireflection coatings to reduce glare. In this way, display legibility is maximized in the presence of both high and low ambient illumination.
Laser beam scanners are used to modulate the direction of laser light, and are critical components of airborne and space-based LIDAR and optical communications systems. We report here a laser beam steering device design based on dithering two complementary (positive and negative) binary optic microlens arrays. When the two microlenses are translated relative to one another in the plane parallel to their surfaces, a light beam can be scanned and controlled in two directions. The first demonstration of this device concept was reported by Lincoln Laboratory. We have demonstrated a miniaturized version of this concept consisting of a pair of 6-mm-aperture binary optic microlens arrays designed for HeNe laser wavelength.
This paper proposes that wavelet construction of the gradient-index refractive index be used to design optical interference coatings. The basic wavelet used is a localized apodized rugate. By incorporating scaled and shifted copies of the basic wavelet, which are other elements of a basis set according to wavelet theory, a variety of interference coatings can be designed.
Binary optics can produce microlenses and lens arrays with theoretical diffraction efficiency as high as 95% for eight-phase level devices. Due to shadowing, mask misalignment, and etching errors that accumulate during fabrication, the actual diffraction efficiency can be reduced to less than 70%. Advances in mask design and e-beam writing have reduced mask misalignment errors to less than 0.2 micrometers but the major issue is the accuracy of the RIE process that is used to transfer a lithographic pattern into the substrate. RIE has two limitations for binary optic applications. First, it cannot be readily employed for the wide range of possible optical substrates of interest (Al2O3 for example), and second, since the pattern is etched directly into the substrate, there is no simple means to calibrate the etch depth during the process. Thin film deposition of the binary structure addresses both of these limitations. It is applicable to a wide range of materials, and accurate in process monitoring of the deposit permits precise control of the feature height. In this paper, we report on eight-phase level binary optic microlenses processed by deposition of SiO2 on fused silica and Al2O3 on sapphire using a projection lithography system. Photoresist processing was achieved by image reversal and lift-off technique. The microlens arrays (in a square format) were designed for (lambda) equals 0.632 micrometers with two microlens sizes of 120 micrometers X 120 micrometers and 240 micrometers X 240 micrometers having speeds of F/12 and F/6 (at the corners), respectively. Optical characterization has demonstrated that the microlens arrays are near diffraction limited and diffraction efficiency is in excess of 80%.
An automated visible-infrared optical measurement system has been used to characterize the imaging performance of binary optic microlenses. Measurements of the point spread function (PSF) were made, from which the modulation transfer functions (MTFs) were derived. Diffraction efficiencies may also be measured using the same system. PSF measurements on both infrared and visible microlenses are in close agreement with theoretical predictions. Data on both IR and visible microlens arrays are presented.
We report the development of binary optic microlens arrays in GaAs. The application of this microlens array is for (gamma) -hardening of HgCdTe focal plane arrays. We intend to reduce the effective spot size of the picture elements and provide significant nuclear hardening for the focal plane array by considerable volume reduction of the detector elements. The microlens design is an eight-phase level approximation to an ideal kinoform lens. The lenses are designed to focus into the GaAs or air with a focal length of 480 micrometers or 148 micrometers respectively, at (lambda) equals 9 micrometers . Arrays of square lenses and individual circular lenses were fabricated. The square lens dimensions and f-numbers are 120 micrometers X 120 micrometers , f/1.23; 240 micrometers X 240 micrometers , f/0.62; and 480 micrometers X 480 micrometers , f/0.31, respectively. Designs include correction for spherical aberration. A set of four 10X projection masks was designed using graphic language (GPL) interfaced to computer-generated binary optics elements. The binary optic pattern was etched into the 3'' diameter GaAs substrate by reactive ion etching. Highly anisotropic etch profiles were obtained with feature heights in excess of 2 micrometers . Measured microlens efficiency for f/1.23 microlenses was as high as 60. The average measured value for a whole array was 55. Measurement of the point spread function at (lambda) equals 10.6 micrometers demonstrates optical concentration. This demonstration of binary optic microlenses in GaAs is of considerable importance to the future integration of purely optical and optoelectronic functions on a single wafer.