Optical sensors, based on the time correlated single photon counting (TCSPC) technique, are found in a range of applications from medical to consumer. This chapter provides an overview of the challenges in TCSPC sensor design for mobile applications. We describe the design of a proof-of-concept TCSPC optical sensor with 10 GS/s conversion rate folded flash time to digital converter (TDC) and on-chip histogram generation, designed to minimize time-domain distortion and have high power efficiency. The proof of concept IC is fabricated in STMicroelectronics 130 nm SPAD foundry process. The system consumes 178.1 pJ per photon at 899 M photon/s, and the TDC achieves state of the art 0.48 pJ/S energy efficiency.