Bruce Hancock

Dual-photoelastic-modulator-based polarimetric imaging concept for aerosol remote sensing

A dual-photoelastic-modulator- (PEM-) based spectropolarimetric camera concept is presented as an approach for global aerosol monitoring from space. The most challenging performance objective is to measure degree of linear polarization (DOLP) with an uncertainty of less than 0.5% in multiple spectral bands, at moderately high spatial resolution, over a wide field of view, and for the duration of a multiyear mission. To achieve this, the tandem PEMs are operated as an electro-optic circular retardance modulator within a high-performance reflective imaging system. Operating the PEMs at slightly different resonant frequencies generates a beat signal that modulates the polarized component of the incident light at a much lower heterodyne frequency. The Stokes parameter ratio q = Q/I is obtained from measurements acquired from each pixel during a single frame, providing insensitivity to pixel responsivity drift and minimizing polarization artifacts that conventionally arise when this quantity is derived from differences in the signals from separate detectors. Similarly, u = U/I is obtained from a different pixel; q and u are then combined to form the DOLP. A detailed accuracy and tolerance analysis for this polarimeter is presented.

First results from a dual photoelastic-modulator-based polarimetric camera

We report on the construction and calibration of a dual photoelastic-modulator (PEM)-based polarimetric camera operating at 660?nm. This camera is our first prototype for a multispectral system being developed for airborne and spaceborne remote sensing of atmospheric aerosols. The camera includes a dual-PEM assembly integrated into a three-element, low-polarization reflective telescope and provides both intensity and polarization imaging. A miniaturized focal-plane assembly consisting of spectral filters and patterned wire-grid polarizers provides wavelength and polarimetric selection. A custom push-broom detector array with specialized signal acquisition, readout, and processing electronics captures the radiometric and polarimetric information. Focal-plane polarizers at orientations of 0 degrees and -45 degrees yield the normalized Stokes parameters q=Q/I and u=U/I respectively, which are then coregistered to obtain degree of linear polarization (DOLP) and angle of linear polarization. Laboratory test data, calibration results, and outdoor imagery acquired with the camera are presented. The results show that, over a wide range of DOLP, our challenging objective of uncertainty within +/-0.005 has been achieved.

An enhanced-performance CMOS imager with a flushed-reset photodiode pixel

A new front-end for photodiode-based CMOS imagers is presented. Degradation in imaging performance due to conventional hard- and soft-reset of pixels is analyzed. To overcome these limitations, the design and operation of a flushed-reset pixel is described. The flushed-reset pixel combines the best of hard- and soft-reset to simultaneously provide excellent radiometric accuracy, high linearity, no image lag, high saturation level, and reduced read-noise. The new front-end is implemented by changes to the column-circuitry only, leaving the pixel unchanged, preventing degradation of any unrelated imaging performance. It is compatible with large format imager implementation, has minimal impact on the frame-rate, and does not introduce any additional hot-carrier stress in the pixel. Data from a large format (512/sup 2/) imager demonstrates the efficacy of the flushed-reset pixel approach.

Reset noise suppression in two-dimensional CMOS photodiode pixels through column-based feedback-reset

We present a new CMOS photodiode imager pixel with ultralow read noise through on-chip suppression of reset noise via column-based feedback circuitry. In a 0.5 /spl mu/m CMOS process, the pixel occupies only 10/spl times/10 /spl mu/m/sup 2/ area. Data from a 256/sup 2/ CMOS imager indicates imager operation with read noise as low as 6 electrons without employing on- or off-chip correlated double sampling. The noise reduction is achieved without introducing any image lag, and with insignificant reduction in quantum efficiency and full-well.

Single Photon Counting UV Solar-Blind Detectors Using Silicon and III-Nitride Materials

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness.

A single-chip programmable digital CMOS imager with enhanced low-light detection capability

The advent of high performance imaging in CMOS technology using active pixel sensors has enabled ultra-low power, miniature, single-chip, digital camera systems. We report at fully digital, programmable, 5-wire, large format (512/spl times/512) camera-on-a-chip that integrates the imager array, control logic, ADC, and bias generation on the same chip. The chip runs off a single (3.3 V) power supply, consumes only 10 mW, is capable of electronic panning, and produces high quality images with a low noise of <40 e/sup -/, and excellent response linearity down to read noise levels.

Active pixel sensors for mass spectrometry

Abstract Active pixel sensors (APS) are micro-fabricated CMOS amplifier arrays that are rapidly replacing CCD devices in many electronic imaging applications. Unlike the pixels of a CCD device, the sensing elements of the APS will respond to locally situated electrostatic charge, owing to the amplifier present in each pixel. We have built two small test arrays with microscopic aluminum electrodes integrated onto standard APS readout circuitry for the purpose of detecting low-energy gas-phase ions in mass spectrometers and other analytical instruments. The devices exhibit a near-linear dynamic range greater than four orders of magnitude, and a noise level of less than 100 electrons at room temperature. Data are presented for the response of the APS detectors to small ions in a miniature magnetic sector mass spectrometer and in an atmospheric pressure jet of helium. Data for individual highly-charged electrospray droplets are presented as well. Anticipated improvements suggest that in the near future APS ion detectors will posses noise levels approaching 10 electrons and will have a useful dynamic range over six orders of magnitude.

Mapping electrical crosstalk in pixelated sensor arrays

Electronic coupling effects such as Inter-Pixel Capacitance (IPC) affect the quantitative interpretation of image data from CMOS, hybrid visible and infrared imagers alike. Existing methods of characterizing IPC do not provide a map of the spatial variation of IPC over all pixels. We demonstrate a deterministic method that provides a direct quantitative map of the crosstalk across an imager. The approach requires only the ability to reset single pixels to an arbitrary voltage, different from the rest of the imager. No illumination source is required. Mapping IPC independently for each pixel is also made practical by the greater S/N ratio achievable for an electrical stimulus than for an optical stimulus, which is subject to both Poisson statistics and diffusion effects of photo-generated charge. The data we present illustrates a more complex picture of IPC in Teledyne HgCdTe and HyViSi focal plane arrays than is presently understood, including the presence of a newly discovered, long range IPC in the HyViSi FPA that extends tens of pixels in distance, likely stemming from extended field effects in the fully depleted substrate. The sensitivity of the measurement approach has been shown to be good enough to distinguish spatial structure in IPC of the order of 0.1%.

A Hybrid Readout Solution for GaN-Based Detectors Using CMOS Technology

Gallium nitride (GaN) and its alloys are becoming preferred materials for ultraviolet (UV) detectors due to their wide bandgap and tailorable out-of-band cutoff from 3.4 eV to 6.2 eV. GaN based avalanche photodiodes (APDs) are particularly suitable for their high photon sensitivity and quantum efficiency in the UV region and for their inherent insensitivity to visible wavelengths. Challenges exist however for practical utilization. With growing interests in such photodetectors, hybrid readout solutions are becoming prevalent with CMOS technology being adopted for its maturity, scalability, and reliability. In this paper, we describe our approach to combine GaN APDs with a CMOS readout circuit, comprising of a linear array of 1 × 8 capacitive transimpedance amplifiers (CTIAs), implemented in a 0.35 µm high voltage CMOS technology. Further, we present a simple, yet sustainable circuit technique to allow operation of APDs under high reverse biases, up to ≈80 V with verified measurement results. The readout offers a conversion gain of 0.43 µV/e−, obtaining avalanche gains up to 103. Several parameters of the CTIA are discussed followed by a perspective on possible hybridization, exploiting the advantages of a 3D-stacked technology.

CMOS digital imager design from a system-on-a-chip perspective

Due to substantial mixed analog-digital circuit integration in one chip, CMOS digital imager cannot be considered only as a photoelectric transducer. In this paper, we have identified timing and circuit layout considerations that are critical for implementing a digital CMOS camera-on-a-chip. An optimized binary-scaled tree-topology power routing has been shown to be critical for minimizing chip area and providing low spatial pattern noise. Imaging artifacts due to timing asymmetry have been quantified, and methods for elimination of the artifacts have been demonstrated. The impact of on-chip bias-generation and drive circuits on the on-chip ADC performance has been shown. New timing and circuit layout techniques have been presented for enabling random noise limited performance of a CMOS imager.