Estimation of light detection efficiency for different light guides used in time-resolved near-infrared spectroscopy

Optical Characterization of Homogeneous and Heterogeneous Intralipid-Based Samples

Different scattering processes take place when photons propagate inside turbid media. Many powerful experimental techniques exploiting these processes have been developed and applied over the years in a large variety of situations from fundamental and applied research to industrial applications. In the present paper, we intend to take advantage of Static Light Scattering (SLS), Dynamic Light Scattering (DLS), and Time-Resolved Transmittance (TRT) for investigating all the different scattering regimes by using scattering suspensions in a very large range of scatterer concentrations. The suspensions were prepared using Intralipid 20%, a material largely employed in studies of the optical properties of turbid media, with concentrations from 10−5% to 50%. By the analysis of the angular and temporal dependence of the scattered light, a more reliable description of the scattering process occurring in these samples can be obtained. TRT measurements allowed us to obtain information on the reduced scattering coefficient, an important parameter largely used in the description of the optical properties of turbid media. TRT was also employed for the detection of inclusions embedded in Intralipid suspensions, by using a properly designed data analysis. The present study allowed us to better elucidate the dependence of scattering properties of Intralipid suspensions in a very large concentration range and the occurrence of the different scattering processes involved in the propagation of light in turbid media for the first time to our knowledge. In so doing, the complementary contribution of SLS, DLS, and TRT in the characterization of turbid media from an optical and structural point of view is strongly evidenced.

Liquid-core waveguide TCSPC sensor for high-accuracy fluorescence lifetime analysis

Liquid-core waveguide (LCW) has many advantages such as the elimination of optical artifacts typically exhibited in systems employing lenses and filters. However, due to the effect of temporal dispersion, LCWs are typically employed in steady-state fluorescence detection microsystems rather than in fluorescence lifetime measurement (FLM) systems. In this paper, we present a compact liquid-core waveguide time-correlated single-photon counting (LCW-TCSPC) sensor for FLM. The propagation of excitation within the LCW is analyzed both analytically and in simulations, with results in agreement with experimental characterization. Results reveal an optimal region within the LCW for highly accurate FLM. The proposed prototype achieves excellent excitation rejection and low temporal dispersion as a result of optimization of the propagation length of the excitation within the LCW. The prototype achieves a detection limit of 5 nM for Coumarin 6 in dimethyl sulfoxide with < 3% lifetime error. The techniques proposed for analyzing the LCW for TCSPC based FLM and prototype demonstration pave the way for developing high-performance fluorescence lifetime measurement for microfluidics and point-of-care applications. Graphical abstract A compact liquid-core waveguide time-correlated single-photon counting (LCW-TCSPC) sensor for fluorescence lifetime measurement (FLM) is presented. Results reveal an optimal propagation length region within the LCW for highly accurate FLM. The prototype achieves a detection limit of 5 nM for Coumarin 6 in dimethyl sulfoxide with < 3% lifetime error.

Probe-hosted silicon photomultipliers for time-domain functional near-infrared spectroscopy: phantom and in vivo tests

We report the development of a compact probe for time-domain (TD) functional near-infrared spectroscopy (fNIRS) based on a fast silicon photomultiplier (SiPM) that can be put directly in contact with the sample without the need of optical fibers for light collection. We directly integrated an avalanche signal amplification stage close to the SiPM, thus reducing the size of the detection channel and optimizing the signal immunity to electromagnetic interferences. The whole detection electronics was placed in a plastic screw holder compatible with the electroencephalography standard cap for measurement on brain or with custom probe holders. The SiPM is inserted into a transparent and insulating resin to avoid the direct contact of the scalp with the 100-V bias voltage. The probe was integrated in an instrument for TD fNIRS spectroscopy. The system was characterized on tissue phantoms in terms of temporal resolution, responsivity, linearity, and capability to detect deep absorption changes. Preliminary in vivo tests on adult volunteers were performed to monitor hemodynamic changes in the arm during a cuff occlusion and in the brain cortex during a motor task.

New frontiers in time-domain diffuse optics, a review

The recent developments in time-domain diffuse optics that rely on physical concepts (e.g., time-gating and null distance) and advanced photonic components (e.g., vertical cavity source-emitting laser as light sources, single photon avalanche diode, and silicon photomultipliers as detectors, fast-gating circuits, and time-to-digital converters for acquisition) are focused. This study shows how these tools could lead on one hand to compact and wearable time-domain devices for point-of-care diagnostics down to the consumer level and on the other hand to powerful systems with exceptional depth penetration and sensitivity.