A 32-channel frequency-domain fNIRS system based on silicon photomultiplier receivers

Frequency-domain (FD) fNIRS is attractive for non-invasive brain imaging because phase-sensitive detection leads to increased resolution and may exhibit improved robustness to motion artifacts. We present an FD-fNIRS system with silicon photomultiplier (SiPM) receivers, where the sensitivity and dynamic range approach those of a first-class continuous-wave (CW-) fNIRS system. This represents a significant step toward fully exploiting the phase degree of freedom provided by FD-fNIRS. The transmitter subsystem includes 32 channels and each supplies 12.5 mW of coherent light at both 690 and 852 nm. A dedicated radio circuit intensity-modulates each laser, and they are independently configured to operate at frequencies up to 400 MHz. The transmitters are on-off-keyed according to a user-specified pattern to mitigate shot noise and maximize dynamic range. The receiver subsystem also includes 32 channels. Each consists of a large-area (2.16-mm diameter), high-NA (0.66) fiber bundle, which carries light to a custom photo-receiver. A three-lens assembly enhances coupling between the fiber-bundle and the SiPM, and the SiPM (ON Semiconductor MICRORB-10020) converts the signal to the electrical domain. The electrical signal is amplified and down-converted to the audio spectrum, and a transformer balances the signal and provides galvanic isolation. Each of the 32 audio waveforms is digitized at 192 kS/s in a bank of commercial audio digitizers. Using a modulation frequency of 211 MHz, swept-power measurements demonstrate that the average noise-equivalent power of the SiPM photo-receivers is 20.5 fW per square root Hz, with about 6 decades of optical dynamic range. This work was funded by a research contract under Facebook’s Sponsored Academic Research Agreement.