Photonics Spectra article about Gigajot’s QIS Tech

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The March 2022 edition of Photonics Spectra magazine has an interesting article titled "Photon-Counting CMOS Sensors: Extend Frontiers in Scientific Imaging" by Dakota Robledo, Ph.D., senior image sensor scientist at Gigajot Technology.

While CMOS imagers have evolved significantly since the 1960s, photon-counting sensitivity has still required the use of specialized sensors that often come with detrimental drawbacks. This changed recently with the emergence of new quanta image sensor (QIS) technology, which pushes CMOS imaging capabilities to their fundamental limit while also delivering high-resolution, high-speed, and low-power linear photon counting at room temperature. First proposed in 2005 by Eric Fossum, who pioneered the CMOS imaging sensor, the QIS paradigm envisioned a large array of specialized pixels, called jots, that are able to accurately detect single photons at a very fast frame rate . The technology’s unique combination of high resolution, high sensitivity, and high frame rate enables imaging capabilities that were previously impossible to achieve. The concept was also expanded further to include multibit QIS, wherein the jots can reliably enumerate more than a single photon. As a result, quanta image sensors can be used in higher light scenarios, versus other single-photon detectors, without saturating the pixels. The multibit QIS concept has already resulted in new sensor architectures using photon number resolution, with sufficient photon capacity for high-dynamic-range imaging, and the ability to achieve competitive frame rates.

The article uses "bit-error-rate" metric for assessing image sensor quality.

The photon-counting error rate of a detector is often quantified by the bit error rate. The broadening of signals associated with various photo charge numbers causes the peaks and valleys in the overall distribution to become less distinct, and eventually to be indistinguishable. The bit error rate measures the fraction of false positive and false negative photon counts compared to the total photon count in each signal bin. Figure 4 shows the predicted bit error rate of a detector as a function of the read noise, which demonstrates the rapid rate reduction that occurs for very low-noise sensors. 


The article ends with a qualitative comparison between three popular single-photon image sensor technologies.

Interestingly, SPADs are listed as "No Photon Number Resolution" and "Low Manufacturability". It may be worth referring to previous blog posts for different perspectives on this issue. [1] [2] [3]

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