Digital-Pixel Focal Plane Arrays

Over the last decade, Lincoln Laboratory has pioneered the development of a new generation of mid- and long-wave infrared (IR) focal plane arrays based on all-digital readout integrated circuits—the digital-pixel focal plane array (DFPA).

Typical modern IR focal plane arrays (FPAs) are hybrid devices: an array of detectors is mated with a silicon readout integrated circuit (ROIC).  Following hybridization of the detector array and ROIC, each unit cell makes an electrical connection to a corresponding detector in the detector array, as shown in Figure 1.  

Focal plane arrayFigure 1. An infrared detector array is hybridized to a readout integrated circuit to form a focal plane array.

The array of detectors converts photons into electrical signals measured by the silicon CMOS ROIC and read out the image from the array to a receiver. 

Lincoln Laboratory's digital readout integrated circuits (DROIC) contain a complete Nyquist-rate analog-to-digital converter (ADC) in every unit cell. The ADC is very simple and low power for implementation into large-format two-dimensional imaging arrays. Digitization at the pixel enables the imaging device to achieve low-noise performance and wide dynamic range. In addition, high-speed digital data transfer can be used to read data off the array quickly to support fast framing applications.

Unlike per-pixel ADC devices developed elsewhere, Lincoln Laboratory's DROICs also provide the capability to perform simple data processing on the image data prior to read out from the array. In fact, our DROIC actually contains a simple computer in every pixel. Additions and subtractions can be performed at the pixel level. Also, digital image data can be communicated to neighboring pixels within the focal plane array prior to readout of image data. This capability is in contrast to other approaches, which are fundamentally based on spatial sampling multiplexers, either analog or digital (or both), to measure pixel content for image readout. In-pixel computation and on-FPA data communication allow Lincoln Laboratory DROICs to process imagery and extract or enhance the information content prior to transmitting data to a display or computer (process then read). Almost all other FPAs simply collect image data that must be passed to another subsystem for data processing and/or viewing (read then process).

The DFPA architecture was developed from a system perspective to provide high performance, deliver new capability, and enable system design flexibility. Some of the powerful capabilities that the Laboratory's DROICs can provide for mid- and long-wave applications and which far exceed the industry state-of-the-art include high dynamic range, large well depth, low noise-equivalent-delta temperature, and fast frame rate. Other benefits that take advantage of the DFPA’s unique processing capability include on-FPA electronic image stabilization, on-FPA spatial filtering, in-pixel coherent signal detection, on-FPA background subtraction, and on-FPA velocity filtering.

 

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