Jaro Rička

Dead‐time and afterpulsing correction in multiphoton timing with nonideal detectors

The influence of detector nonidealities on multiphoton timing experiments is investigated. Deviations from ideal detector behavior include dead‐time, afterpulsing, and a smeared detector response. A statistical model of dead‐time effects combined with cumulative afterpulsing is developed which allows the correction of distorted experimental data. The theory is verified with both Monte Carlo simulations and experimentally, whereby particular emphasis is put on fiberoptic optical time‐domain reflectometry measurements. Though the theory is applicable to any kind of detector we discuss the various effects for two examples of commercially available single photon avalanche diode modules. One of the two detectors is the passive quenched EG&G SPCM‐100 while the other module contains an active quenching circuit and is manufactured at the Czech Technical University at Prague.

High-resolution distributed temperature sensing with the multiphoton-timing technique

We report on a multiphoton-timing distributed temperature sensor (DTS) based on the concept of distributed anti-Stokes Raman thermometry. The sensor combines the advantage of very high spatial resolution (40 cm) with moderate measurement times. In 5 min it is possible to determine the temperature of as many as 4000 points along an optical fiber with an accuracy Δ T < 2 °C. The new feature of the DTS system is the combination of a fast single-photon avalanche diode with specially designed real-time signal-processing electronics. We discuss various parameters that affect the operation of analog and photon-timing DTS systems. Particular emphasis is put on the consequences of the nonideal behavior of sensor components and the corresponding correction procedures.

Characterization of optical properties of ZnO nanoparticles for quantitative imaging of transdermal transport

Widespread applications of ZnO nanoparticles (NP) in sun-blocking cosmetic products have raised safety concerns related to their potential transdermal penetration and resultant cytotoxicity. Nonlinear optical microscopy provides means for high-contrast imaging of ZnO NPs lending in vitro and in vivo assessment of the nanoparticle uptake in skin, provided their nonlinear optical properties are characterized. We report on this characterization using ZnO NP commercial product, Zinclear, mean-sized 21 nm. Two-photon action cross-section of this bandgap material (Ebg = 3.37 eV, λbg = 370 nm) measured by two techniques yielded consistent results of ηZnOσZnO(2ph) = 6.2 ± 0.8 μGM at 795 nm, and 32 ± 6 μGM at 770 nm per unit ZnO crystal cell, with the quantum efficiency of ηZnO = (0.9 ± 0.2) %. In order to demonstrate the quantitative imaging, nonlinear optical microscopy images of the excised human skin topically treated with Zinclear were acquired and processed using σZnO(2ph) and ηZnOvalues yielding nanoparticle concentration map in skin. Accumulations of Zinclear ZnO nanoparticles were detected only on the skin surface and in skin folds reaching concentrations of 800 NPs per μm3.

Dynamic light scattering with single-mode receivers: partial heterodyning regime

A frequent source of errors in dynamic light-scattering experiments is partial heterodyning caused by scattering on large particles or imperfections of the sample cell. With a conventional two-pinhole receiver it is impossible to distinguish its effect from the effects of a finite detector area and detector nonlinearity. However, an accurate data analysis is feasible when a single-mode light receiver is employed. We present formulas for single-mode autocorrelation and cross-correlation functions that include a local oscillator and an incoherent background of arbitrary strength and take into account detector nonlinearity (e.g., dead time) up to second order. A simple but accurate method for the determination of the nonlinearity parameters and the effective number of receiver modes is also provided. The success of the data-evaluation procedure is demonstrated by the measurement of the hydrodynamic radius of latex in the presence of deliberately added local-oscillator or incoherent-background contributions.

Design and performance of a new ophthalmic instrument for dynamic light‐scattering measurements in the human eye

A new optical instrument for in vivo dynamic light‐scattering measurements in the human eye is described. The instrument is based on the well‐known dynamic light‐scattering technique. Results can be obtained from the anterior as well as the posterior segment with high spatial resolution and sensitivity. This instrument allows, to our knowledge for the first time, in vivo measurements of dynamic light scattering to be performed in the vitreous. Interesting performance is obtained using single‐mode optical fibers to guide the excitation and the scattered light resulting in a modular, compact system with high‐beam quality and electrical insulation of the patient. The instrument demonstrates good safety characteristics (the optical power impinging the patient’s eye is 36 times below the maximum laser power at the cornea, recommended for intrapupil exposure by ANSI standard). To demonstrate the possible clinical use of this technique, in vivo measurements were made and the results compared with the known eye p...

Silicon avalanche photodiodes as detectors for photon correlation experiments

In view of time correlated photon-counting experiments using wavelengths at the red end of the electromagnetic spectrum, we developed a simple electronic circuit for periodical gated quenching of silicon avalanche photodiodes. We compare the performance of this device with commercially available passive and active quenching modules and a reference photomultiplier. The detection system’s nonlinearities, i.e., dead time and afterpulsing, lead to direct and indirect distortions of photocount correlation functions. We characterize this nonlinear behavior by measuring intensity auto- and cross-correlation functions and supply nonlinearity parameters for each of the four detection systems. In addition, transfer functions are given which allow an estimate for the highest count rates accessible for each detection system.

Determining the optical properties of a gelatin-TiO2 phantom at 780 nm

Tissue phantoms play a central role in validating biomedical imaging techniques. Here we employ a series of methods that aim to fully determine the optical properties, i.e., the refractive index n, absorption coefficient μa, transport mean free path ℓ * , and scattering coefficient μs of a TiO2 in gelatin phantom intended for use in optoacoustic imaging. For the determination of the key parameters μa and ℓ * , we employ a variant of time of flight measurements, where fiber optodes are immersed into the phantom to minimize the influence of boundaries. The robustness of the method was verified with Monte Carlo simulations, where the experimentally obtained values served as input parameters for the simulations. The excellent agreement between simulations and experiments confirmed the reliability of the results. The parameters determined at 780 nm are n = 1.359 ( ± 0.002 ) , μ ′ s = 1 / ℓ * = 0.22 ( ± 0.02 )   mm -1 , μ a = 0.0053(+0.0006-0.0003)  mm -1 , and μ s = 2.86 ( ± 0.04)  mm -1 . The asymmetry parameter g obtained from the parameters ℓ * and μ ′ s is 0.93, which indicates that the scattering entities are not bare TiO2 particles but large sparse clusters. The interaction between the scattering particles and the gelatin matrix should be taken into account when developing such phantoms.

Dynamic light scattering in the vitreous: performance of the single-mode fiber technique

We constructed an apparatus for dynamic light scattering (DLS) measurements in the posterior chamber for the eye. The setup uses single-mode technology and, with regard to future clinical use, it is combined with a commercial ophthalmologic microscope. The scattering volume is illuminated with a Gaussian beam realized by coupling a HeNe laser into a single-mode fiber. The scattered light is measured using a single-mode fiber receiver. The signal is processed with a digital autocorrelator. Our measurements on the vitreous of post mortem porcine eyes show that DLS measurements in the posterior chamber are feasible and that it is possible to keep the excitation intensity below the damage threshold of the retina. In addition, absolute measurements of light scattering in toluene confirmed the validity of DLS theory for single-mode receivers.

Multiple irradiation sensing of the optical effective attenuation coefficient for spectral correction in handheld OA imaging

Spectral optoacoustic (OA) imaging enables spatially-resolved measurement of blood oxygenation levels, based on the distinct optical absorption spectra of oxygenated and de-oxygenated blood. Wavelength-dependent optical attenuation in the bulk tissue, however, distorts the acquired OA spectrum and thus makes quantitative oxygenation measurements challenging. We demonstrate a correction for this spectral distortion without requiring a priori knowledge of the tissue optical properties, using the concept of multiple irradiation sensing: recording the OA signal amplitude of an absorbing structure (e.g. blood vessel), which serves as an intrinsic fluence detector, as function of irradiation position. This permits the reconstruction of the bulk effective optical attenuation coefficient μeff,λ. If performed at various irradiation wavelengths, a correction for the wavelength-dependent fluence attenuation is achieved, revealing accurate spectral information on the absorbing structures. Phantom studies were performed to show the potential of this technique for handheld clinical combined OA and ultrasound imaging.

Simulating light propagation: Towards realistic tissue models

We present a tool for Monte Carlo simulations of polarized light transport in biological tissue samples. This tool can be adapted to various scenarios thanks to a user interface that allows both complex geometrical structures and different illuminating beam profiles to be generated. By combining spheres, cylinders and half-spaces, the user is able to create highly intricate physical models, with each geometrical element able to be assigned its own optical properties: absorption and scattering coefficients, refractive index and scattering law. The scattering law models currently utilized by the tool are the Mie scattering law and the polarized version of the generalized Henyey-Greenstein scattering law: these models can be customized by modifying the appropriate parameters. A tracing method is used to track the propagating photons and handle the reflection/transmission processes (using the Fresnel relations) at interfaces between two different media. Virtual CCDs coupled with polarizers are used to carry out the imaging of the backscattered and transmitted light. The Mueller matrix of the sample can be obtained by measuring the changes that occur in the polarization states (represented by complex Jones vectors) of the propagating photons. The propagating lights temporal and spatial profiles can also be visualized.