Smartphone camera technology is growing in leaps and bounds in the past couple of years and continues to be a major point to differentiate their products. The commercial development of curved image sensors is one of the biggest advance in camera technology in decades, allowing for simpler, flatter lenses with larger apertures as well as dramatically better image quality.
NIKON, Sony and Canon are reported to be in race to develop and market curved sensor camera that operates using lens designs with fewer elements, less weight, less light loss, less internal reflection, less distortion and less aberration, all at lower cost. Now DARPA wants to develop this curved sensor technology for military imaging applications for improved ISR.
The military relies on advanced imaging systems for a number of critical capabilities and applications – from Intelligence, Surveillance, and Reconnaissance (ISR) and situational awareness to weapon sights. These powerful systems enable defense users to capture and analyze visual data, providing key insights both on and off the battlefield.
The Defense Advanced Research Projects Agency (DARPA) launched a new Focal arrays for Curved Infrared Imagers (FOCII) program in August 2019 to expand upon the current commercial trend for visible sensor arrays by extending the capability to both large and medium format midwave (MWIR) and/or longwave (LWIR) infrared detectors. Infrared imaging enables the spotting of targets, intruders and hidden bombs by detecting their heat signatures thereby protecting troops and making the application of force more discriminating.
Today, nearly all imaging systems rely on detector arrays fabricated using planar processes developed for electronic integrated circuits on flat silicon. While significant progress has been made in advancing these technologies for narrow field of view (FOV) systems, optical aberrations can limit the performance at the periphery in wide FOV systems that then require large, costly, and complex optics to correct. The trade-off for correcting optical aberrations by using large, heavy lenses is a reduction in optical signal and a large size penalty, which limits their use for new and emerging capabilities.
“Tremendous progress has been made over the past 20 years towards making multi-megapixel infrared (IR) focal plane arrays (FPA) for imaging systems cost effective and available to the Department of Defense,” said Dr. Whitney Mason, a program manager in DARPA’s Microsystems Technology Office (MTO). “However, limitations to the technology’s performance and size remain. Current advances on the commercial side have shown the viability of small area, curved FPAs (CFPAs) for visible cameras. While these technologies have shown modest benefits, more must be done to achieve the performance and size requirements needed for imaging systems used in emerging defense applications.”
DARPA developed the FOcal arrays for Curved Infrared Imagers (FOCII) program to expand upon the current commercial trend for visible sensor arrays by extending the capability to both large and medium format midwave (MWIR) and/or longwave (LWIR) infrared detectors. The goal of the program is to develop and demonstrate technologies for curving existing state-of-the-art, large-format, high-performance infrared focal plane arrays (FPAs) to a small radius of curvature (ROC) to maximize performance, as well as curve smaller format FPAs to an extreme ROC to enable the smallest form factors possible while maintaining exquisite performance.
FOCII will address this challenge through two approaches to fabricating a curved FPA. The first involves curving existing state-of-the-art FPAs, while keeping the underlying design intact. The focus of the research will be on achieving significant performance improvements over existing, flat FPAs, with a target radius of curvature of 70mm. The fundamental challenge researchers will work to address within this approach is to mitigate the mechanical strain created by curving the FPGA, particularly in silicon, which is very brittle.
The second approach will focus on achieving an extreme ROC of 12.5 mm to enable a transformative reduction in the size and weight compared to current imagers. Unlike the first approach, researchers will explore possible modifications to the underlying design, including physical modifications to the silicon that could relieve or eliminate stress on the material and allow for creating the desired curvature in a smaller sized FPA. This approach will also require new methods to counter the effects of any modifications during image reconstruction in the underlying read-out integrated circuit (ROIC) algorithm.