TECHNOLOGY AREA(S): Sensors
OBJECTIVE: Design, develop, demonstrate, and produce a prototype 4 x 4 avalanche photodiode focal plane array with dynamic biasing and low multiplication noise.
DESCRIPTION: Low photon flux detectors are a major focus of military electro-optical systems for sensing, communications, and quantum computing. Historically, these detectors have been implemented on active systems (e.g. LADAR) in the form of linear or Geiger mode avalanche photodiode (APD) focal plane arrays (FPAs). The two APD FPA forms can be differentiated by the timing circuitry of the read-out, and the dynamic range of the signal. A Geiger mode FPA will momentarily bias the APD beyond breakdown, creating a strong response to the presence of any electron present in the multiplication region. These electrons can be generated optically (signal) or thermally (noise), but the output level is the same. Meanwhile, a linear mode FPA will maintain a time invariant gain and can produce varying output levels for any number of photons, but cannot detect single photons as easily as Geiger mode APDs. The limiting factors for linear mode detectors are the multiplication noise and dark current. Given these constraints, electro-optical system designers are forced to trade signal dynamic range and overall receiver sensitivity, even when they have knowledge of their photon flux and pulse timing. An opportunity exists to create APD arrays that have higher sensitivity for known pulse trains, which would enhance signal, reduce noise, and maintain dynamic range across 100s of signal photons.
Dynamically biasing an APD below its breakdown field is a potential method to increase low-noise gain and improve detectivity. The goal of this program is (a) to explore dynamic bias APD FPA designs in Phase I, (b) to produce a photodiode array with dynamic bias in Phase II, and (c) to demonstrate an imaging array with enhanced detectivity in Phase III. The basic requirements for meeting these goals are: the detector should operate at or above 200 K; the readout should be capable of adapting to changes in the pulse train to maximize SNR at frequencies greater than 10 MHz; and the APD spectral cutoff should be 1.6 microns or greater. Preference will be given to designs that offer better noise equivalent photon values and higher signal dynamic range. No government materials, equipment, data, or facilities will be provided.
PHASE I: Develop a generic model for dynamic biasing of a given APD design. Optimize performance concurrently in the APD and readout for several different pulse train examples. Demonstrate lower excess noise using dynamic bias. Provide simulation code for testing/verification.
PHASE II: Demonstrate statically biased APD operation on single element devices (Gain x EQE > 100%). Demonstrate spectral cut-off wavelengths of 1.6 microns or greater. Demonstrate APD operation with dynamic bias and various pulse trains. Model and design a dynamically biased readout FPA (50 micron pitch or smaller). A proof of concept FPA is desirable, but not required.
PHASE III: Demonstrate a 4 x 4 APD FPA, or larger, using dynamic biasing and adapting to the pulse train for maximum SNR.
REFERENCES: 1. Hayat, M. M. and Ramirez, D. A., Multiplication theory for dynamically biased avalanche photodiodes: new limits for gain bandwidth product. Optics Express, 2012. 20(7): p. 8024.; 2. Hayat, M. M., et al, Breaking the buildup-time limit of sensitivity in avalanche photodiodes by dynamic biasing. Optics Express, 2015. 23(18): p. 24035.; 3. Namekata, N., 1.5 GHz single-photon detection at telecommunication wavelengths using sinusoidally gated InGaAs/InP avalanche photodiode. Optics Express, 2009. 17(8): p. 6275.; 4. US Patent US9354113 B1. “Impact ionization devices under dynamic electric fields” The United States has certain rights, including a license to have practiced on behalf of the United States the invention. Please see USPTO Reel/Frame number 034697/0153KEYWORDS: APD, Avalanche Photodiode, Infrared Detector, SWIR, III/V, Compound Semiconductor, ROIC, Readout Integrated Circuit, Dynamic Bias, Gain Modulation, LADAR, LIDAR
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