A linear mode photon counting focal plane array using HgCdTe mid-wave infrared (MWIR) cutoff electron initiated avalanche photodiodes (e-APDs) has been designed, fabricated, and characterized. The broad spectral range (0.4 to 4.3 μm) is unique among photon counters, making this a “first of its kind” system spanning the visible to the MWIR. The low excess noise [F(M)≈1] of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500–1000. The readout integrated circuit (ROIC) design included a very high gain-bandwidth product resistive transimpedance amplifier (3×1013 Ω-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high-bandwidth analogs and 16 low-voltage differential signaling digital outputs. The 2×8 array was integrated into an LN2 Dewar with a custom leadless chip carrier and daughter board design that preserved high-bandwidth analog and digital signal integrity. The 2×8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured avalanche photodiode gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse signal-to-noise ratio of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a photon background limited false event rate of about 1 MHz. The measured jitter was in the range of 550–800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.
A linear mode photon counting FPA using HgCdTe MWIR cutoff e-APDs has been designed, fabricated, and characterized. The broad spectral range (0.4 μm to 4.3 μm) is unique among photon counters, making this a "first of its kind" system spanning the visible to the MWIR. The low excess noise ((F(M) ≈ 1) of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500 to 1000. The ROIC design included
a very high gain-bandwidth product RTIA (3x1011 Ohm-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high bandwidth analog and 16 LVDS digital outputs. The 2x8 array was integrated into an LN2 Dewar with a custom LCC and daughter board design that preserved high bandwidth analog and digital signal integrity. The 2x8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were
approximately 1 pA at 13 V bias where the measured APD gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse SNR of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a background limited false event rate (FER) of about 1 MHz. The measured jitter was in the range of 550 ps to 800 ps. The demonstrated minimum time
between distinguishable events was less than 10 ns.
Linear-Mode Photon Counting (LMPC) detection requires a combined system consisting of a
semiconductor avalanche photodiode (APD), a high-gain low-noise amplifier, and a comparator
circuit. Modeling these aspects of the system requires a combination of semiconductor detector
theory, electronics circuit modeling, and classic decision theory. Because of the disparate skills
involved, it is difficult to both model and build such devices. In this paper, we present an end-to-end
model of the LMPC detector that contains all the required theory. As part of the decision theory
aspect of LMPC technology, we present a three-dimensional Receiver Optimization Characteristic
(ROC) curve that contains the key performance aspects of the LMPC as a function of the comparator
threshold setting. We present nomenclature and specification methods that provide for unambiguous
definitions of the combined-system detector performance for both the fabricators and users of LMPC
technology. Finally, we apply the model to a noiseless-gain HgCdTe APD, ROIC, and comparator
device being developed by DRS and GEOST in order to demonstrate the photon counting end result,
as well as several key intermediate values in the signal chain.
A multi-dimensional laser radar sensor is developed to perform 3D imaging polarimetry. The imaging polarimeter is an extension of the existing, well-developed Streak Tube Imaging Lidar technology. With polarization optics added to the transceiver system, simultaneous capture of 3D high- resolution polarimetric and reflectance imagery is achieved over wide fields of view. Laboratory and field experiments exhibit the capability of the imaging polarimeter for enhancing clutter reduction, image segmentation, and target discrimination for low contrast and camouflaged targets.
An imaging lidar system is developed for capturing laser- induced fluorescence imagery. With minor changes to the transmitter and receiver optics, the system operates in a number of different modes including 3D multispectral, hyperspectral, multi-excitation hyperspectral, and fluorescence-lifetime hyperspectral. All of these sensor functions provide discriminating capabilities for targets exhibiting spectral fluorescence signatures.
Multiple-Slit Streak Tube Imaging Lidar (MS-STIL) represents a new method for using streak tubes in a laser radar configuration. The MS-STIL approach uses several slits instead of the usual single slit to provide a number of additional capabilities over conventional laser radar systems. System configurations for providing 3D single-laser-pulse scannerless imaging, 3D multispectral imaging, 3D multispectral fluorescence imaging, 3D polarimetry, and combined 3D spectro- polarimeters are presented. In addition, a very high-speed laser diagnostic system capable of measuring laser wavefront changes at GHz rates is discussed.
SEACAM is a combination of a digital camera and an underwater sonar range finder that allows the precise range to the target to be measured simultaneously with the image. Given this measured range and the known field of view of the camera, the `plate scale' of the image (i.e., the number of millimeters/pixel) can be precisely determined, allowing for accurate estimates of the target dimensions. Two prototype systems have been developed: a `functional' camera that has all the functional capability of the final camera, but which is not packaged in the final form; and an `ergonomic' prototype that represents the first attempt at the final package design, but which is non-functional.
The SEAL camera (SEACAM) is a small underwater imaging and ranging device that will provide a number of useful functions for the divers, such as intelligence gathering, limited nigh vision, and providing ranging information for countermining. SEACAM is designed for low-visibility missions, where 'low-visibility' refers to low diver observability, and is not a reference regarding water clarity. The camera, by virtue of the laser wavelength, is specifically made for the low-visibility mission where emission of visible light is undesirable, and is capable of working in conditions of low ambient lighting. SEACAM is a combination of a digital camera and an underwater laser range finder that allows the process range to the target to be measured simultaneously with the image. Given this measured range and the known field of view of the camera, the 'plate scale' of the image can be precisely determined, allowing for accurate estimates of the target dimensions. Upon return from the mission, the image and range data can be downloaded into a computer for rapid distribution. The camera will be magnetically and acoustically qualified for the Mine Countermeasures environment.
The status and goals of the W. M. Keck Observatory adaptive optics (AO) program are reviewed. The overall system design is discussed, followed by a more detailed presentation of the optical design. Included is a description of a method for focusing the wavefront sensor (WFS) on either natural guide stars (NGS) or laser guide stars (LGS). Also, a combination tilt and focus sensor is described that corrects both the global tilt ambiguity common to LGSs, but also for instability in the focus of the LGS due to changes in the altitude of the sodium layer.
The first astronomical results using a seven segment edge-matched adaptive mirror are presented. The mirror is a prototype for adaptive secondary mirrors, consisting of seven flat hexagonal mirrors, with inductive edge sensors to measure piston errors, and piezoelectric actuators. A Hartmann- Shack wavefront sensor uses the visible light for centroiding information while the IR image is formed on a 58 X 62 InSb array at 2.2 microns. In addition, a number of mathematical models are presented that illustrate how well the system should perform when correcting atmospheric turbulence.
We discuss issues in optimizing the design of adaptive optics and laser guide star systems for the Keck Telescope. The initial tip-tilt system will use Keck's chopping secondary mirror. We describe design constraints, choice of detector, and expected performance of this tip-tilt system as well as its sky coverage. The adaptive optics system is being optimized for wavelengths of 1 - 2.2 micrometers . We are studying adaptive optics concepts which use a wavefront sensor with varying numbers of subapertures, so as to respond to changing turbulence conditions. The goal is to be able to `gang together' groups of deformable mirror subapertures under software control, when conditions call for larger subapertures. We present performance predictions as a function of sky coverage and the number of deformable mirror degrees of freedom. We analyze the predicted brightness of several candidate laser guide star systems, as a function of laser power and pulse format. These predictions are used to examine the resulting Strehl as a function of observing wavelength. We discuss laser waste heat and thermal management issues, and conclude with an overview of instruments under design to take advantage of the Keck adaptive optics system.
The status and plans for a multi-phase program to build adaptive optics (AO) user facilities for one of the Keck telescopes is presented. The planned facilities include (1) fast tip/tilt correction, (2) near infrared AO with natural stars, and potentially (3) a near infrared AO facility with a single laser beacon. Description of these facilities and their implementation on the telescope are described. In addition, descriptions of the current and future suite of scientific instruments that would take advantage of adaptive optics are provided. Problems and concerns associated with implementing adaptive optics facilities at Keck (e.g., a segmented primary, a 10 meter baseline, rotation of a non-symmetric pupil, etc.) are discussed.
The Phased Array Mirror, Extendible Large Aperture telescope has been fully assembled and testing has started. The telescope is the first to have a fully adaptive primary mirror, which consists of 36 hexagonal injection-molded Pyrex segments that are seven centimeters flat-to- flat. The segments are mounted on three long-throw voice-coil actuators for tip, tilt, and piston motion. The segment tiles are measured with a Hartmann-Shack wavefront sensor and the piston errors between adjacent segments are measured via inductive edge-sensors. The personnel at NASA MSFC are performing a significant amount of testing in the area of controls/structure interactions; therefore, in addition to a description of the optical performance and aberration correction capability of the telescope, a description of the plan to model the mechanical structure with emphasis on how this will interact with the adaptive optics system is presented.
We propose an optimized design for the redeployment of the Multiple Mirror Telescope (MMT) primary optics as an Adaptive Steerable Imaging Array (ASIA) providing direct focal plane image formation. The design incorporates concepts of active telescope alignment and adaptive optics proven in the existing MMT. Existing 25 m radio telescope mount technology could easily be adapted to provide support of the optics with tracking stability, against wind loading, sufficient to allow optical rigidity to be obtained using off-axis stellar references, a possibility only feasible with comounted MMT-like interferometer designs which mitigate the field-of-view limitations of long optical delay-lines. As a result of the Steward Observatory borosilicate honeycomb mirror development program a seventh mirror may also be available, giving the light gathering capability of a 4.8 m aperture but with the resolution of a 25 m aperture, for a facility dedicated to the advancement of interferometric imaging technology and high angular resolution astronomy. By utilizing existing technology and providing weather proofing of the optics on its mount, an expensive telescope enclosure, as well as costly, complex laser metrology and optical delay lines, is avoided in a cost effective installation.
A hardware demonstration of segmented mirror systems for adaptive optics is described. The basis of the phased array mirror extendible large aperture (PAMELATM) concept is that large adaptive mirrors can be fabricated from many small segments by utilizing edge-sensors, which measure the piston error between segments. We have investigated the interaction between the piston and tilt control loops which direct the motion of individual segments. The segment tilt, which is set by a wavefront-sensor-based control loop, directly affects the piston error between segments; therefore, the segment piston control loop must be able to perform corrections much faster than the rate at which the tilt corrections are being performed. In this experiment, we have one fully actuated segment with a wavefront sensor measuring the error in the wavefront gradient. An adjacent segment is driven in piston to produce the piston error signal. We measure and present the bandwidth trade-offs between the two control loops and predict how this will affect the performance of larger systems. This interactive control loop methodology has an advantage over normal adaptive optics systems in that the computationally intensive wavefront reconstruction process can be removed due to the direct measurement of both the tilt and the piston errors.
The large lunar telescope is a proposed moon-based telescope which incorporates a sixteen-meter segmented primary mirror. An error budget is developed for the active control system of the primary mirror. A control methodology for the primary mirror is then described which utilizes piston sensors for measuring the relative piston error between adjacent segments as well as a separate sensor which measures the tilt of each segment with respect to the pointing direction of the telescope. A trade study is conducted in which the following types of tilt sensors are examined to determine their applicability to this program: stellar wavefront sensors, such as a Hartmann-Shack or a shearing interferometer; holographic optical elements; interferometers; scanning systems; and some nonoptical systems which electronically measure the relative tilt between adjacent segments. In addition, two independent methods of quantitatively verifying the performance of the telescope using either a phase retrieval algorithm or an image sharpening technique, both of which are based on the quality of a stellar image, are presented.
Techniques are demonstrated which, at high temporal bandwidths, can control a segmented reflector with many segments. Phased Array Mirror, Extendible Large Aperture (PAMELA) technology is applied, where small hexagonal mirror segments are about equal to the atmospheric coherence length, to allow diffraction-limited visible imaging. Attention is first given to an adaptive optics technique which corrects the wavefront for atmospheric distortion in the isoplanatic patch of the observed object. Wavefront sensing control methods are presented for adjusting the mirror segments. The second technique considered is active optics, whereby a control system employing local figure sensing allows the utilization of the telescope as a traditional telescope. The latter technique is shown to work for objects that are not adequately bright. The incompatibilities of local and global iterative control methods are analyzed, as are hierarchical techniques which cluster the segments. The methods, in conjunction with the reduced stiffness and therefore low mass of a large segmented mirror, present high bandwidth wavefront correction capability in a system which can be used on earth and in space.
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