Mr. Emil G. McDowell Profile

Emil G. McDowell

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Proceedings Article | 5 March 2021 Presentation + Paper

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

Jeremiah Wathen, Michael Fitch, Vincent Pagán, Griffin Milsap, Emil McDowell, Lafe Spietz, Zachary Markow, Jason Trobaugh, Edward Richter, Adam Eggebrecht, Joseph Culver, David Blodgett, Scott Hendrickson

Proceedings Volume 11629, 116291P (2021) https://doi.org/10.1117/12.2581482

KEYWORDS: Receivers, Silicon photomultipliers, Transmitters, Transformers, Silicon, Semiconductors, Nanolithography, Modulation, Image resolution, Fiber couplers

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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.

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