Interferometric near-infrared spectroscopy directly quantifies optical field dynamics in turbid media

Abstract

Sensing and imaging methods based on the dynamic scattering of coherent light (including laser speckle, laser Doppler, diffuse correlation spectroscopy, dynamic light scattering, and diffusing wave spectroscopy) quantify scatterer motion using light intensity fluctuations. The underlying optical field autocorrelation, rather than being measured directly, is typically inferred from the intensity autocorrelation through the Siegert relationship, assuming that the scattered field obeys Gaussian statistics. Here, we demonstrate interferometric near-infrared spectroscopy for measuring the time-of-flight (TOF) resolved field and intensity autocorrelations in turbid media. We find that the Siegert relationship breaks down for short TOFs due to static paths whose optical field does not decorrelate over experimental time scales. We also show that eliminating such paths by polarization gating restores the validity of the Siegert relationship. The unique capability of measuring optical field autocorrelations, as demonstrated here, enables the study of non-Gaussian and non-ergodic light scattering processes. Moreover, direct measurements of field autocorrelations are more efficient than indirect measurements based on intensity autocorrelations. Thus, optical field measurements may improve the quantiffcation of scatterer dynamics with coherent light.

Interferometric near-infrared spectroscopy (iNIRS): performance tradeoffs and optimization

“…Similarly, TOF-resolved, un-normalized iNIRS field autocorrelations, G 1 iNIRS (τ s , τ d ), are functions of both TOF (τ s ) and lag time (τ d ) [12,13]:…”

“…As a rule of thumb, we assume that, due to decorrelation during the sweep and sampling considerations, accurate measurement of the decay rate is possible up to TOF values where ξ(τ s ) ~1/δτ d , where δτ d is the autocorrelation lag time (ALT) resolution. In our previously reported results [12][13][14], the tuning speed was limited to F S = 50 kHz, and we employed only forward sweeps, with δτ d = 1/F S = 20 μs. In order for iNIRS to measure autocorrelations at longer TOFs (corresponding to larger penetration depths), both high sensitivity and lag time resolution (speed) are desired.…”

“…Interferometric near-infrared spectroscopy (iNIRS) is a new time-of-flight (TOF-) resolved optical modality that can assess optical properties and dynamics with a single instrument. The theoretical iNIRS framework [12,13] and in vivo application [14] have been described previously. In this work, for the first time, we characterize and optimize the iNIRS setup with respect to speed, sensitivity, dynamic range, TOF resolution (related to laser tuning range), and TOF range (related to laser linewidth, sampling, and detection bandwidth).…”

Reflectance-mode interferometric near-infrared spectroscopy quantifies brain absorption, scattering, and blood flow index in vivo

“…A two-dimensional measurement of reemitted light provides TOF-resolved autocorrelations, from which optical and dynamical properties can be derived [10]. The iNIRS method recently was shown to also provide TOF-resolved field autocorrelations, more directly related to scatterer dynamics than TOF-integrated intensity autocorrelations provided by DCS [13]. In this Letter, we validate iNIRS in reflectance mode using calibrated phantoms, and in the brain of a nude mouse, showing that iNIRS can simultaneously and non invasively quantify optical properties and BFI in vivo .…”

“…The static contribution is understood to be nonergodic over a certain time scale [9]. Hence, the autocorrelation measured in iNIRS becomes [13]…”