Abstract
Interferometric near-infrared spectroscopy (iNIRS) is a time-of-flight- (TOF-) resolved sensing modality for determining optical and dynamical properties of a turbid medium. iNIRS achieves this by measuring the interference spectrum of light traversing the medium with a rapidly tunable, or frequency-swept, light source. Thus, iNIRS system performance critically depends on the source and detection apparatus. Using a current-tuned 855 nm distributed feedback laser as the source, we experimentally characterize iNIRS system parameters, including speed, sensitivity, dynamic range, TOF resolution, and TOF range. We also employ a novel Mach-Zehnder interferometer variant with a multi-pass loop to monitor the laser instantaneous linewidth and TOF range at high tuning speeds. We identify and investigate tradeoffs between parameters, with the goal of optimizing performance. We also demonstrate a technique to combine forward and backward sweeps to double the effective speed. Combining these advances, we present in vivo TPSFs and autocorrelations from the mouse brain with TOF resolutions of 22-60 ps, 36-47 dB peak-sidelobe dynamic range, 4-10 μs autocorrelation lag time resolution, a TOF range of nanoseconds or more, and nearly shot noise limited sensitivity.
Highlights
The field of near-infrared spectroscopy (NIRS) aims to non-invasively probe the physiology of highly scattering biological tissues with near-infrared light [1]
We present in vivo temporal point spread function (TPSF) and autocorrelations from the mouse brain with TOF resolutions of 22-60 ps, 36-47 dB peaksidelobe dynamic range, 4-10 μs autocorrelation lag time resolution, a TOF range of nanoseconds or more, and nearly shot noise limited sensitivity
To assess whether phase nonlinearity is completely removed by interpolation, an “ideal” instrument response function (IRF) can be constructed from the Fourier transform of the resampled and Gaussian shaped fringe envelope, thereby defining the TOF resolution achieved with no residual phase error
Summary
The field of near-infrared spectroscopy (NIRS) aims to non-invasively probe the physiology of highly scattering biological tissues with near-infrared light [1]. We introduce a method for monitoring the instantaneous linewidth and coherence time of temporally coherent and rapidly tuned lasers, based on a novel multi-pass loop variant of a modified Mach-Zehnder interferometer. We show that both forward and backward sweeps nearly achieve the shot noise limit, and combine them to double the temporal sampling of the field autocorrelation. INIRS provides the unique capability to measure TOF-resolved field autocorrelations This capability enables more direct quantification of sample dynamics [12,14] than is possible with intensity autocorrelations [16]. As convolution involves a smoothing along the TOF axis, iNIRS requires a fine TOF resolution to determine optical properties and TOF-resolved medium dynamics
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