Broad-Bandwidth Frequency-Domain Near-Infrared Spectroscopy System on a Chip

The design and evaluation of an integrated circuit (IC) that provides gain and phase measurements for a broad modulation bandwidth frequency-domain near-infrared spectroscopy (fd-NIRS) system is described. This circuit includes a voltage-controlled oscillator (VCO) that provides modulating frequencies from 50 MHz up to 1 GHz, as well as analog measurements of amplitude and phase. The phase detector is based on an N-path filter and allows for accurate phase measurement. The IC works with a laser diode and an avalanche photodiode (APD) to implement a complete fd-NIRS spectroscopy system. The IC-based system was used to characterize tissue-simulating phantoms with varying absorption and reduced scattering properties with modulation frequencies of 105-405 MHz with the upper frequency limited by the APD used in the system. Mean optical property accuracy was within 0.00064 and ${0.054}~\text {mm}^{-{1}}$ for absorption and reduced scattering, respectively, compared to a reference system. These results show a promising path toward scalable and wearable optical sensing systems based on fd-NIRS.