Improving the time-of-flight filter in highly parallel interferometric diffusing wave spectroscopy (iDWS)

Blood flow index (BFI) has proven to be a quantitative and specific signal to assess the human brain with near-infrared light. Interferometric diffuse optics has enabled the fusion of two features crucial for measuring BFI with high brain specificity: highly parallel detection and time-of-flight (TOF) discrimination. This fusion has been demonstrated earlier with sinusoidal laser optical frequency tuning to reduce effective coherence, generating a Bessel function squared TOF filter. However, this TOF filter has oscillating sidelobes, reducing brain specificity particularly at shorter source-collector (S-C) separations. In this work, we demonstrate a significantly improved TOF filter with a custom-designed tuning waveform, reducing the amplitude of the first sidelobe in the TOF filter to 0.15% of the maximum value (compared to 16.2% for the previous Bessel-squared filter). A reduction in scalp sensitivity compared to continuous wave operation is demonstrated at 1 cm S-C separation with the new custom TOF filter.