Miniaturized Probe for Time-Domain Near-Infrared Spectroscopy

This paper concerns demonstrating the feasibility of designing a miniature optical probe for imaging applications based on time-domain near infrared spectroscopy (TD-NIRS). In a first step towards miniaturization, a compact optical probe was designed by housing Vertical Cavity Surface Emitting Laser (VCSEL) diode, Single Photon Avalanche Diode (SPAD) and control circuit in one single QFN package of dimensions 12mm x 12mm, often referred to as System-in-Package approach. The integrated optical module can provide short optical pulses at 100 MHz repetition rate, demonstrating its suitability for time-domain measurements. The experimental characterization of the time gated detector features the ability to reject undesired strong light pulses by sharp temporal filtering of impinging photons that would otherwise mask the signal of interest. Furthermore, the size of the probe is reduced to 12 mm by 2.5D integration of all the heterogenous chips including the ancillary components on a silicon interposer platform.