Dual-wavelength time-of-flight interferometric speckle-contrast optical spectroscopy (dual-wavelength TOF-iSCOS) for co-registered blood-flow and hemoglobin sensing

Diffuse optical measurements of blood flow, blood volume, and blood oxygenation often use separate devices and photon populations. Combining these contrasts within a common photon population would enable their direct co-registration while retaining time-of-flight-based depth discrimination. We introduce dual-wavelength time-of-flight interferometric speckle-contrast optical spectroscopy (TOF-iSCOS), built on an interferometric near-infrared spectroscopy (iNIRS) architecture that temporally multiplexes 780 and 852 nm swept lasers through a shared detection channel. It recovers co-registered flow-sensitive field dynamics and wavelength-resolved attenuation from the same complex-field acquisition. Three 50 ps TOF gates sample increasingly late-arriving photons. Within each gate, g_1 was calculated directly as the normalized temporal autocorrelation of the reconstructed complex field. κ2 was reconstructed by finite-time integration of measured g_1 and finite-integration fits yielded decorrelation rates. No camera speckle-variance contrast or Siegert substitution was used. Gate-integrated TPSF intensities yielded optical-density changes δOD, and division by the baseline-TPSF-weighted mean photon path length gave effective δμ_a for two-wavelength hemoglobin inversion. In 12 forearm cuff-occlusion recordings from 11 adults, TOF-iSCOS detected flow suppression during occlusion and reactive hyperemia after release at both wavelengths. At 780 nm, the Late-minus-Early reactive-hyperemia amplitude in the relative blood-flow index (rBFI) was +0.647 (95% CI 0.194 to 1.124), whereas the Early-minus-Late cuff δμ_a contrast was +0.196 cm^{-1} (95% CI 0.123 to 0.280). In the photon-limited 852 nm Late gate, the accepted-frame fraction was 0.994 for κ^2-based fitting versus 0.831 for g_1-based fitting. Exploratory phase-resolved analysis suggested that the initial hemoglobin response was concentrated in early-arriving photons, while later responses differed in magnitude and direction. Wavelength, field dynamics, and photon time-of-flight thus provide complementary physiological information; the largest response need not identify the phase of interest.