Assess practicality of coupling a small-area APD to an optical fiber for TD‑fNIRS

Ascertain the practical difficulty and feasible methods for coupling a small-area avalanche photodiode chip (approximately 0.04 mm^2 active area) to an optical fiber in the described time-domain fNIRS detector assembly, to enable efficient light collection without excessive loss or alignment complexity.

Background

In comparing detector technologies, the authors highlight that while an APD may offer lower dark count rates, its very small active area could pose significant coupling challenges when interfacing with optical fibers typical of fNIRS systems.

This mechanical/optical integration uncertainty affects the viability of APDs in a wearable, multi-channel TD‑fNIRS device and must be resolved to make an informed detector choice.