To perform time-resolved fluorescence imaging in the lung at video rates it is vital for our optical sensors to incorporate fast on-chip signal processing.
Two of the most important areas where we have achieved world-record data throughput rates are in time-to-digital conversion and on-chip histogramming.
Time-to-Digital Conversion (TDC)
Time to digital conversion is the process of measuring time intervals by means of electronic circuitry and representing time as a digital number or code which can be processed or stored. The conversion is performed by a category of circuits known as time to digital converters (TDCs) which are intricate systems capable of measuring time intervals typically with tens of picoseconds of resolution.
Such circuits require two main trigger signals for them to operate, a start signal indicating the start of the time interval to be measured, and a stop signal which halts the measurement resulting in a representative digital output.
An equivalent analogy to these circuits is the stopwatch which is used to measure fine time intervals between the start of a race and the time an athlete crosses the finish line. To put this in context consider the example above, where a laser is placed a certain distance away from a photo-detector with both of them connected to a TDC circuit. The start signal is a laser trigger indicating that a laser pulse has been generated and the athlete is the photon that is racing towards the finish line or the photo-detector. Once the photo-detector or SPAD detects the photon, it generates the stop signal causing the TDC or the stopwatch to record the time of arrival with respect to the laser start.