  A pulse oximeter is an optical device that monitors tissue oxygenation
levels. Traditionally, these devices estimate the oxygenation level by
measuring the intensity of the transmitted light through the tissue and are
embedded into everyday devices such as smartphones and smartwatches. However,
these sensors require prior information and are susceptible to unwanted changes
in the intensity, including ambient light, skin tone, and motion artefacts.
Previous experiments have shown the potential of Time-of-Flight (ToF)
techniques in measurements of tissue hemodynamics. Our proposed technology uses
histograms of photon flight paths within the tissue to obtain tissue
oxygenation, regardless of the changes in the intensity of the source. Our
device is based on a 45ps time-to-digital converter (TDC) which is implemented
in a Xilinx Zynq UltraScale+ field programmable gate array (FPGA), a CMOS
Single Photon Avalanche Diode (SPAD) detector, and a low-cost compact laser
source. All these components including the SPAD detector are manufactured using
the latest commercially available technology, which leads to increased
linearity, accuracy, and stability for ToF measurements. This proof-of-concept
system is approximately 10cmx8cmx5cm in size, with a high potential for
shrinkage through further system development and component integration. We
demonstrate preliminary results of ToF pulse measurements and report the
engineering details, trade-offs, and challenges of this design. We discuss the
potential for mass adoption of ToF based pulse oximeters in everyday devices
such as smartphones and wearables.
