George M. Williams

Probabilistic analysis of linear mode vs. Geiger mode APD FPAs for advanced LADAR enabled interceptors

To meet evolving ballistic missile threats, advanced seekers will include a multi-modal imaging capability in which a passive single- or multi-band infrared focal plane array (FPA) shares a common aperture with an active laser radar (LADAR) receiver - likely, a photon-counting LADAR receiver that can resolve photon times of arrival with sub-nanosecond resolution. The overall success of such a system will depend upon its photon detection efficiency and sensitivity to upset by spurious detection events. In the past, to perform photon counting functions, it has generally been necessary to operate near infrared (NIR) avalanche photodiode (APD) FPAs in Geiger Mode. Linear Mode APDs could not provide enough proportional gain with sufficiently low noise to make the photocurrent from a single photon detectible using existing amplifier technology. However, recent improvements in APDs, sub-micron CMOS technology, and concomitant amplifier designs, have made Linear Mode single-photon-counting APDs (SPADs) possible. We analyze the potential benefits of a LADAR receiver based on Linear Mode SPADs, which include: 1) the ability to obtain range information from more than one object in a pixels instantaneous-field-of-view (IFOV), 2) a lower false alarm rate, 3) the ability to detect targets behind debris, 4) an advantage in the endgame, when stronger reflected signals allow dark current rejection via thresholding, and 5) the ability to record signal intensity, which can be used to increase kill efficiency. As expected, multiple laser shots of the same scene improves the target detection probability.

Multi-Gain-Stage InGaAs Avalanche Photodiode With Enhanced Gain and Reduced Excess Noise

We report the design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950-1650 nm wavelength sensing applications. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise and stable linear-mode operation at much higher avalanche gain than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6000 without avalanche breakdown. Excess noise characterized by an effective impact ionization rate ratio below 0.04 were measured at gains over 1000.

Time resolved gain and excess noise properties of InGaAs/InAlAs avalanche photodiodes with cascaded discrete gain layer multiplication regions

To predict pulse detection performance when implemented in high speed photoreceivers, temporally resolved measurements of a 10-stage InAlAs/InGaAs single carrier multiplication (SCM) avalanche photodiode (APD)s avalanche response to short multi-photon laser pulses were explained using instantaneous (time resolved) pulse height statistics of the devices impulse response. Numeric models of the junction carrier populations as a function of the time following injection of a primary photo-electron were used to create the probability density functions (pdfs) of the instances of the avalanche buildup process. The numeric pdfs were used to generate low frequency gain and excess noise models, which were in good agreement with analytic models of multiple discrete low-gain-stage APDs and with measured excess noise data. The numeric models were then used to generate the instantaneous and cumulative instantaneous low order statistics of the instances of the impulse response. It is shown that during the early times o...

High-speed photon counting with linear-mode APD receivers

HgCdTe and InGaAs linear-mode avalanche photodiodes (APDs) were fabricated and tested for properties suitable for high-speed photon counting when integrated with commercially available 2-GHz resistive transimpedance amplifiers (RTIAs). The 2.71-μm, 100-μm-diameter HgCdTe APDs were fabricated in using an n+/p vertical carrier transport architecture designed to reduce carrier drift time and facilitate high-speed operation. At 215 K, a gain of 100 was measured with an excess noise of 2.5. The InGaAs/InAlAs APDs were fabricated using two absorber alloy compositions, one optimized for 950-1300 nm operation and the other for 950-1550 nm operation. Both were fabricated using multiple, cascaded gain regions that allowed for high gain and low avalanche-induced shot noise. Gain exceeding 6000 was observed, and the excess noise factor was measured to be below 20 at a gain of M = 1200 (effective k ~ 0.03). The InGaAs/InAlAs APDs were integrated into receivers consisting of a multi-gain-stage APD coupled to a commercial 2-GHz RTIA and were operated as thresholded photon counters. At a linear gain of M = 1800, a single photon detection efficiency greater than 85% was measured at a maximum count rate of 70 MHz; at a linear gain of M = 1200, single photon detection efficiencies greater than 20% were measured at maximum count rates of 80 MHz. At the temperature tested, 185 K, the receivers dark count rate (DCR) is dominated by electronic amplifier noise from the TIA for low threshold settings, and by dark counts from the APD at high threshold settings.

Limitations of Geiger-mode arrays for Flash LADAR applications

It is shown through physics-based Monte Carlo simulations of avalanche photodiode (APD) LADAR receivers that under typical operating scenarios, Geiger-mode APD (GmAPD) flash LADAR receivers may often be ineffective. These results are corroborated by analysis of the signal photon detection efficiency and signal-to-noise ratio metrics. Due to their ability to detect only one pulse per laser shot, the target detection efficiency of GmAPD receivers, as measured by target signal events detected compared to those present at the receivers optical aperture, is shown to be highly particular and respond nonlinearly to the specific LADAR conditions including range, laser power, detector efficiency, and target occlusion, which causes the GmAPD target detection capabilities to vary unpredictably over standard mission conditions. In the detection of partially occluded targets, GmAPD LADAR receivers perform optimally within only a narrow operating window of range, detector efficiency, and laser power; outside this window performance degrades sharply. Operating at both short and long standoff ranges, GmAPD receivers most often cannot detect partially occluded targets, and with an increased number of detector dark noise events, e.g. resulting from exposure to ionizing radiation, the probability that a GmAPD device is armed and able to detect target signal returns approaches zero. Even when multiple pulses are accumulated or contrived operational scenarios are employed, and even in weak-signal scenarios, GmAPDs most often perform inefficiently in their detection of target signal events at the aperture. It is concluded that the inability of the GmAPD to detect target signal present at the receivers aperture may lead to a loss of operational capability, may have undesired implications for the equivalent optical aperture, laser power, and/or system complexity, and may incur other costs deleterious to operational efficacy.

Back-illuminated CCD imagers for high-information-content digital photography

The advantages of digital photography are well documented, and digital photography is seeing increased use in demanding photography applications. Of the many implementations of digital cameras, the three-CCD camera provides the optimal resolution, temporal sampling, and color reproduction, which when examined together form the information content of the sensor - a physical measure of the detectors imaging performance. So that uncertainty between the object of interest and the reproduced image is reduced, high quality, precise, color photographic records require sensors with the highest possible information handling capability. Furthermore, high information content images, due to the increases information about the scene, can be compressed to smaller file sizes than can lower fidelity images -thus, allowing reduced transmission data rates. Whereas traditional film is hypersensitive in the blue, conventional CCD imagers have reduced blue response as compared to their red response. A class of CCD called the back-illuminated CCD has uniform spectral response throughout the visible, UV, and NIR spectral regions. By integrating the uniform spectral response of the back-illuminated CCD with ultra-low noise amplifiers, high dynamic range pixels, a high pixel density, a large area detector, and a 3-CCD color prism architecture, a nearly ideal digital camera can be realized. This paper discusses a development effort at PixelVision Inc. to realize a nearly ideal color digital camera. So that a system for evaluating solid state photographic imagers can be established, a methodology of determining the information content of an imager is introduced and the information content of a back-illuminated CCD camera is compared to conventional film and to currently available studio digital cameras.

Linear-mode single-photon APD detectors

In the past, it has been necessary to operate avalanche photodiodes (APDs) in Geiger mode to perform photon counting. The gain and noise performance of available linear-mode APDs was too poor to detect the photocurrent pulse from a single photon using existing amplifier technology. We review the performance thresholds required to achieve linear-mode photon counting, and present measurements from two APD designs that meet the gain and noise requirements. The first design is a previously-reported vertical-junction, electron-avalanche HgCdTe device fabricated from 4.06-μm-cutoff liquid phase epitaxy (LPE)-grown material. These HgCdTe APDs have an excess noise factor of approximately F~1 at a gain of M=150 when measured at 196 K. The second design is a novel InAlAs/InGaAs structure grown by molecular beam epitaxy (MBE) entirely from alloys lattice-matched to InP. The maximum gain found for this new design was as high as M=2000 at 235 K, but the principle of its operation limits the best noise performance of the prototype to gains below M=20, for which it has an excess noise factor of F~2.3 at room temperature (corresponding to k~0.02 when fit to McIntyres model). This design can be scaled to deliver the same noise performance at higher gains.

Optically coded nanocrystal taggants and optical frequency IDs

A series of nanocrystal and nanocrystal quantum dot taggant technologies were developed for covertly tagging and tracking objects of interest. Homogeneous and heterogeneous nanocrystal taggant designs were developed and optimized for ultraviolet through infrared emissions, utilizing either Dexter energy transfer or Förster resonant energy transfer (FRET) between specific absorbing and emitting functionalities. The conversion efficiency, target-specific identification, and adhesion properties of the taggants were engineered by means of various surface ligand chemistries. The ability to engineer poly-functional ligands was shown effective in the detection of a biological agent simulant, detected through a NC photoluminescence that is altered in the presence of the agent of interest; the technique has broad potential applicability to chemical, biological, and explosive (CBE) agent detection. The NC photoluminescence can be detected by a remote LIDAR system; the performance of a taggant system has been modeled and subsequently verified in a series of controlled field tests. LIDAR detection of visible-emitting taggants was shown to exceed 2.8 km in calibrated field tests, and from these field data and calibrated laboratory measurements we predict >5 km range in the covert shortwavelength infrared (SWIR) spectral region.

Single-photon-sensitive linear-mode APD ladar receiver developments

New measurements are presented for multi-stage InGaAs avalanche photodiodes (APDs) which have the potential to perform GHz-rate single photon counting in linear mode. No increase in dark current was measured for an 11-device sample of 5-stage APDs following 717 hours of accelerated aging under bias at 50°C, during an initial lifetime study. Impulse response times of 0.45 ns, 0.9 ns, and 1.1 ns were measured directly for 6-, 8-, and 10-stage APDs, respectively, operated at a nominal gain of M=10. To assess the suitability of the technology for a NASA optical communications application, separate samples of 5-stage APDs were irradiated by 1- and 2-MeV protons at the University of Washingtons Center for Experimental Nuclear Physics and Astrophysics (UW CENPA) and by 63.5-MeV protons at the University of California Davis, Crocker Nuclear Laboratory (UCD CNL). Good agreement between calculated non-ionizing energy loss (NIEL) and observed damage was found for the low-energy protons at fluences of 1010 and 1011 cm-2. A NIEL calculation successfully predicted the damage observed following a 5×1010 cm-2 dose of 63.5-MeV protons by extrapolating from 2 MeV data, which suggests that displacement damage is the dominant mechanism.

Discrimination of Photon- and Dark-Initiated Signals in Multiple Gain Stage APD Photoreceivers

We demonstrate the ability of linear mode single carrier multiplication (SCM) avalanche photodiode (APD)-based optical receivers to discriminate single-photon-initiated avalanche events from dark-current-initiated events. Because of their random spatial origin in discrete regions of the depletion region, in the SCM APD the dark-generated carriers multiply differently than the photon-generated carriers. This causes different count distributions and necessitates different statistical descriptions of the signal contributions from photon- and dark-originating impulse responses. To include dark carriers in the performance models of the SCM APD, we considered the influence of the spatial origin of the ionization chains on a receivers noise performance over the times the optical pulse is integrated by the receivers decision circuits. We compare instantaneous (time-resolved) numeric and pseudo-DC analytical models to measured SCM APD data. It is shown that it is necessary to consider both the distribution of spatial origin and the instantaneous properties of the ionization chains to describe statistically an SCM APD receiver. The ability of SCM APD receivers to discriminate single photon events from single dark events is demonstrated, and the effective gain and excess noise contributions of the light- and dark-initiated avalanche events and their influence on receiver sensitivity and signal-to-noise characteristics are shown.