Anna Petoukhova

Path-length distribution and path-length-resolved Doppler measurements of multiply scattered photons by use of low-coherence interferometry.

We report first results of measurements by low-coherence Doppler interferometry of the path-length distribution of photons undergoing multiple scattering in a highly turbid medium. We use a Mach-Zehnder interferometer with multimode graded-index fibers and a superluminescent diode as the light source. The path-length distribution is obtained by recording of the heterodyne fluctuations that arise from the Brownian motion of particles in an Intralipid suspension as a function of the optical path length. The experimental path-length distribution is in good agreement with predictions of Monte Carlo simulations. In the heterodyne spectrum, an increase of the mean Doppler frequency with path length is observed.

Glass-fiber self-mixing intra-arterial laser Doppler velocimetry: signal stability and feedback analysis

We have developed a blood velocimeter based on the principle of self-mixing in a semiconductor laser diode through an optical fiber. The intensity of the light is modulated by feedback from moving scattering particles that contain the Doppler-shift frequency. Upon feedback the characteristics of the laser diode change. The threshold current decreases, and an instable region may become present above the new threshold. The amplitude of the Doppler signal turns out to be related to the difference in intensity between situations with and without feedback. This amplitude is highest just above feedback. The suppression of reflection from the glass-fiber facets is of paramount importance in the obtaining of a higher signal-to-noise ratio. Using an optical stabilization of the feedback, we optimized the performance of the laser-fiber system and the Doppler modulation depth and clarified its behavior with a suitable physical model. We also investigated the effect of the finite coherence length of the laser. We tested the efficiency of the self-mixing velocimeter in vivo with the optical glass fiber inserted in the artery with endoscopic catheters, both in upstream and in downstream blood flow conditions. For the latter we used a special side-reflecting device solution for the fiber facet to allow downstream measurements.

Effects of absorption on coherence domain path length resolved dynamic light scattering in the diffuse regime

A low coherence Mach–Zehnder interferometer is developed for path length resolved dynamic light scattering in highly turbid media. The path length distribution of multiply scatteredphotons in Intralipid is changed by the addition of absorbing dyes. Path length distributions obtained for various absorption levels are shown to be mutually related by Lambert–Beer’s law. Furthermore, the broadening of path length resolved heterodyne light beating spectra of media with identical scattering properties is shown to be independent of the absorption coefficient.

Instrument-independent flux units for laser doppler perfusion monitoring assessed in a multi-device study on the renal cortex

To investigate the feasibility of instrument-independent perfusion units for laser Doppler flowmetry, a comparison was performed of two commercial fiberoptic laser Doppler perfusion monitors measuring the same flux situation for two different types of probes. In vivo measurements were performed on the cortex of pigs kidney, with an ultrasonic arterial flow meter as reference. The flow was mainly varied by internal arterial constriction using a balloon catheter. For each probe, instruments are compared in terms of the ratio of laser Doppler flux and arterial flow. For a given probe, the flux-to-flow ratios of the two instruments show a linear mutual relationship for a wide variety of arterial flows and laser Doppler fluxes. In vitro measurements were performed on an aqueous suspension of polystyrene microspheres. For the probe with interfiber distance 500 microm the ratio of the in vivo fluxes appears to agree within 16% to the value found in vitro, while for the 250-microm probe a difference of 28% was found. For a wide range of fluxes, the in vivo flux values of one instrument can be translated into flux values for the other instrument, in spite of the instrumental differences. This enables the user to render experimental results independent of the specific instrument, thus facilitating multi-center studies.

Recent advances in self-mixing laser-Doppler velocimetry: use as an in-vivo blood flow meter

In the present paper, recent experimental advances obtained with a laser Doppler self-mixing velocimeter are reported. The self-mixing effect in a semiconductor laser is used to realize the velocimeter. The velocity is calculated measuring the frequency peak of the frequency spectrum of the intensity signal generated by the laser diode when modulated by feedback light coming from the moving scattering particles. A special optical fiber version of this velocimeter to be used specifically for intra-arterial blood velocity measurement has been realized and a solution for reducing temperature influence on the semiconductor performances is proposed. The results of the in vivo tests carried out with the proposed sensor are presented.

Suppression of dynamic laser speckle signals in multimode fibers of various lengths.

The effects of fiber coupling and fiber length on photocurrent fluctuations are studied when the light of a laser diode transmitted to and from a dynamic turbid medium by a step-index multimode fiber is studied. When the laser light is coupled asymmetrically, filling only the higher-order modes, the photocurrent fluctuations are suppressed significantly when fiber lengths of as much as 16 m are added between the laser and the medium. Addition of as much as 16 m of detection fiber, or any fiber in the case of symmetric light coupling, leads to much less or no suppression of the photocurrent fluctuations.

Path length distribution of multiple-scattered photons by low-coherence Doppler interferometry

We report results of measurements by low coherence Doppler interferometry of the path length distribution of photons undergoing multiple scattering in a highly turbid medium. We use a Mach-Zehnder interferometer with multimode graded index fibers and a superluminescent diode as light source. The path length distribution is obtained by recording the heterodyne fluctuations arising due to the Brownian motion of particles in an Intralipid suspension as a function of the optical path length. The experimental path length distribution is in good agreement with predictions of Monte Carlo simulations. In the heterodyne spectrum an increase of the mean Doppler frequency with the path length is observed. The path length resolution of the setup was directly evaluated by replacing the turbid medium with randomly moving scatterers by a mirror attached to a harmonically oscillating piezo-element. The maximum (peak-to-peak) mirror displacement was 10% of the optical wavelength. We observed a narrow and strong (signal/noise ratio ~300) interference peak with the full width at the half maximum ~50 microns equal to the coherence length of the superluminescent diode. However, additional weaker satellite peaks are also observed, which may be caused by the intermodal dispersion in our multimode fibers. We demonstrate that our setup allows achieving high path length resolution for biological tissues where the width of the path length distribution is several millimeters.