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

Jeremiah J. Wathen, Michael J. Fitch, Vincent R. Pagán, Griffin W. Milsap, Emil G. McDowell, Lafe Spietz, Zachary E. Markow, Jason W. Trobaugh, Edward J. Richter, Adam Eggebrecht, Joseph P. Culver, David W. Blodgett, Scott M. Hendrickson

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

4 Scopus citations


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.

Original languageEnglish
Title of host publicationOptical Techniques in Neurosurgery, Neurophotonics, and Optogenetics
EditorsV. X. D. Yang, Q. M. Luo, S. K. Mohanty, J. Ding, A. W. Roe, J. M. Kainerstorfer, L. Fu, S. Shoham
ISBN (Electronic)9781510640931
StatePublished - 2021
EventOptical Techniques in Neurosurgery, Neurophotonics, and Optogenetics 2021 - Virtual, Online, United States
Duration: Mar 6 2021Mar 11 2021

Publication series

NameProgress in Biomedical Optics and Imaging - Proceedings of SPIE
ISSN (Print)1605-7422


ConferenceOptical Techniques in Neurosurgery, Neurophotonics, and Optogenetics 2021
Country/TerritoryUnited States
CityVirtual, Online


  • Diffuse optical tomography
  • Frequency domain
  • Functional imaging
  • Functional near-infrared imaging
  • Functional neuroimaging
  • Near infrared spectroscopy


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