Joseph E. Funk

Crosstalk Analysis of Integrated Geiger-mode Avalanche Photodiode Focal Plane Arrays

Arrays of photon-counting Geiger-mode avalanche photodiodes (APDs) sensitive to 1.06 and 1.55 μm wavelengths and as large as 256 x 64 elements on 50 μm pitch have been fabricated for defense applications. As array size, and element density increase, optical crosstalk becomes an increasingly limiting source of spurious counts. We characterize the crosstalk by measurement of emitted light, and by extracting the spatial and temporal focal plane array (FPA) response to the light from FPA dark count statistics. We discuss the physical and geometrical causes of FPA crosstalk, suggest metrics useful to system designers, then present measured crosstalk metrics for large FPAs as a function of their operating parameters. We then present FPA designs that suppress crosstalk effects and show more than 40 times reduction in crosstalk.

Readout circuitry for continuous high-rate photon detection with arrays of InP Geiger-mode avalanche photodiodes

An asynchronous readout integrated circuit (ROIC) has been developed for hybridization to a 32x32 array of single-photon sensitive avalanche photodiodes (APDs). The asynchronous ROIC is capable of simultaneous detection and readout of photon times of arrival, with no array blind time. Each pixel in the array is independently operated by a finite state machine that actively quenches an APD upon a photon detection event, and re-biases the device into Geiger mode after a programmable hold-off time. While an individual APD is in hold-off mode, other elements in the array are biased and available to detect photons. This approach enables high pixel refresh frequency (PRF), making the device suitable for applications including optical communications and frequency-agile ladar. A built-in electronic shutter that de-biases the whole array allows the detector to operate in a gated mode or allows for detection to be temporarily disabled. On-chip data reduction reduces the high bandwidth requirements of simultaneous detection and readout. Additional features include programmable single-pixel disable, region of interest processing, and programmable output data rates. State-based on-chip clock gating reduces overall power draw. ROIC operation has been demonstrated with hybridized InP APDs sensitive to 1.06-μm and 1.55-μm wavelength, and fully packaged focal plane arrays (FPAs) have been assembled and characterized.

Reliable InP-based Geiger-mode avalanche photodiode arrays

Arrays as large as 256 x 64 of single-photon counting avalanche photodiodes have been developed for defense applications in free-space communication and laser radar. Focal plane arrays (FPAs) sensitive to both 1.06 and 1.55 μm wavelength have been fabricated for these applications. At 240 K and 4 V overbias, the dark count rate (DCR) of 15 μm diameter devices is typically 250 Hz for 1.06 μm sensitive APDs and 1 kHz for 1.55 μm APDs. Photon detection efficiencies (PDE) at 4 V overbias are about 45% for both types of APDs. Accounting for microlens losses, the full FPA has a PDE of 30%. The reset time needed for a pixel to avoid afterpulsing at 240 K is about 3-4 μsec. These devices have been used by system groups at Lincoln Laboratory and other defense contractors for building operational systems. For these fielded systems the device reliability is a strong concern. Individual APDs as well as full arrays have been run for over 1000 hrs of accelerated testing to verify their stability. The reliability of these GM-APDs is shown to be under 10 FITs at operating temperatures of 250 K, which also corresponds to an MTTF of 17,100 yrs.

InP-based single-photon detector arrays with asynchronous readout integrated circuits

We have developed and demonstrated a high-duty-cycle asynchronous InGaAsP-based photon counting detector system with near-ideal Poisson response, room-temperature operation, and nanosecond timing resolution for near-infrared applications. The detector is based on an array of Geiger-mode avalanche photodiodes coupled to a custom integrated circuit that provides for lossless readout via an asynchronous, nongated architecture. We present results showing Poisson response for incident photon flux rates up to 10 million photons per second and multiple photons per 3-ns timing bin.

Design and reliability of mesa-etched InP-based Geiger-mode avalanche photodiodes

Design modifications to InP-based Geiger-mode avalanche photodiodes are described that improve reliability. Geiger-mode aging at multiple conditions can cause significant degradation in some design variants while linear mode (below breakdown) aging does not.

Arrays of 128x32 InP-Based Geiger-Mode Avalanche Photodiodes

Arrays of InP-based avalanche photodiodes operating at 1.06-μm wavelength in the Geiger mode have been fabricated in the 128x32 format. The arrays have been hermetically packaged with precision-aligned lenslet arrays, bump-bonded read-out integrated circuits, and thermoelectric coolers. With the array cooled to -20C and voltage biased so that optical cross-talk is small, the median photon detection efficiency is 23-25% and the median dark count rate is 2 kHz. With slightly higher voltage overbias, optical cross-talk increases but the photon detection efficiency increases to almost 30%. These values of photon detection efficiency include the optical coupling losses of the microlens array and package window.

Reliable large format arrays of Geiger-mode avalanche photodiodes

The fabrication of reliable InP-based Geiger-mode avalanche photodiode arrays is described. Arrays of up to 256 times 64 elements have been produced and mated to silicon read-out circuits forming single-photon infrared focal plane imagers for 1.06 and 1.5 mum applications.

Geiger-mode avalanche photodiodes for laser communications and laser radar

Arrays of photon-counting avalanche photodiodes (APDs) enable laser-communications and laser-radar receivers with unprecedented sensitivity at 1.06-mum wavelength. Near room temperature, the best detectors have: 50% photon detection efficiency, 30-kHz dark count rate, and a 1-mus reset time to avoid after-pulsing. Arrays with 64 elements were fabricated in the InGaAsP/InP materials system and were bump-bonded to a custom CMOS integrated circuit (IC) with a novel nonblocking architecture to continuously report both time-of-arrival for incoming photons as well as their spatial location on the array. Larger arrays with 1024 elements were mated to custom read-out ICs that report time and location data at lower duty cycles (typically 1-5%) and are appropriate for pulsed laser-radar systems

Packaging and qualification of single-photon counting avalanche photodiode focal plane arrays

Avalanche Photodiode (APD) photon counting arrays are finding an increasing role in defense applications in laser radar and optical communications. As these system concepts mature, the need for reliable screening, test, assembly and packaging of these novel devices has become increasingly critical. MIT Lincoln Laboratory has put significant effort into the screening, reliability testing, and packaging of these components. To provide rapid test and measurement of the APD devices under development, several custom parallel measurement and Geiger-mode (Gm) aging systems have been developed. Another challenge is the accurate attachment of the microlens arrays with the APD arrays to maximize the photon detection efficiency. We have developed an active alignment process with single μm precision in all six degrees of freespace alignment. This is suitable for the alignment of arrays with active areas as small as 5 μm. Finally, we will discuss a focal plane array (FPA) packaging qualification effort, to verify that single photon counting FPAs can survive in future airborne systems.

Performance of Reliable Mesa-Etched InP-based Geiger-Mode Avalanche Photodiodes and Arrays

The fabrication of reliable InP-based Geiger- mode avalanche photodiodes are described. Devices passivated with polyimide coated with silicon nitride have not degraded even while aging under more strenuous conditions than those used in fielded systems.