CMOS Demodulation Image Sensor for Nanosecond Optical Waveform Analysis

An image sensor with 256 $,\times,$ 256 pixels and a pitch of 6.3 $\mu{\rm m}$ , suitable for resolving ultrashort optical phenomena, is developed. It is based on a standard CMOS process with pinned photodiode option. The pixel comprises three transfer gates to allow versatile sensor operation whereas repetitive exposure and integration are used to increase the lowest signal level that can be detected. The image sensor is fully functional and demonstrates the ability to demodulate signals in the time-domain with contrast higher than 92% up to 100 MHz. Algorithms for fluorescence lifetime imaging microscopy and three-dimensional time-of-flight imaging are proposed. They allow measurements to be insensitive to background light, subsurface leakage, dark current leakage, and subthreshold leakage of the transfer gates. Lifetimes of free quantum dots are resolved using time-domain demodulation. Range imaging is also shown to be possible with frequency-domain demodulation although background light cannot be suppressed efficiently in the present implementation of the image sensor due to the limited full well capacity.