Jiancheng Lai

Experimental measurement of the refractive index of biological tissues by total internal reflection

We discuss the refractive-index measurement of biological tissues by total internal reflection. The methodology of the measurement is illuminated comprehensively, and an experimental setup, combined with a data processing program, is developed correspondingly. Refractive indices of typical tissue samples are measured by use of the developed methodology. The agreement of our measurements with the reported results shows the validity of our scheme, which has the potential for being a simple, quick, and low-cost practical means for determining the refractive index of a turbid medium. Moreover, an empirical formula for evaluating the refractive index of Intralipid suspensions with different concentrations is also presented according to experimental measurements.

Complex refractive index measurement of biological tissues by attenuated total reflection ellipsometry

To apply reflection ellipsometry to determine the real and imaginary parts of the refractive index of biological tissues simultaneously, we combine reflection ellipsometry with total internal reflection to warrant minimal influences by the strong scattering and absorption of biological tissues. A K9 glass prism with refractive index 1.51468 at wavelength 632.8 nm and a Glan prism polarizer with an angular sampling interval of 0.1 degrees were used in our experimental setup. Using the setup, the complex refractive indices of some typical mammalian tissues were measured under the wavelength of 632.8 nm. The results show that the indices of porcine muscle, liver, pancreas, and dermis tissues were 1.3713+0.062i, 1.3791+0.0087i, 1.3517+0.0113i, and 1.3818+0.0049i, respectively.

Three-dimensional refractive index reconstruction of red blood cells with one-dimensional moving based on local plane wave approximation

To simplify the reconstruction optical system, an improved method is developed to reconstruct the three-dimensional (3D) quantitative refractive index of live cells based on a local plane wave approximation. In this method, the cell is illuminated by a convergent beam whose cross section is much larger than the cell, so the beam passing through the cell can be treated as a local plane wave. With the one-dimensional moving of cells in the beam cross section, multi-directional phase projections can be obtained using a Mach–Zehnder interferometer and the Hilbert transform phase retrieval algorithm. An inverse Radon–Radon iterative algorithm is used to reconstruct the 3D refractive index distribution which is verified by a simulation reconstruction. The corresponding set-up is applied to obtain the multi-directional phase projections and reconstruct the 3D refractive index of red blood cells (RBCs). The results show that only with a simple device could the method measure the 3D refractive index of cells with high precision. The reconstruction method could also be applied in opto-fluidic microscopy to fabricate a compact on-chip opto-fluidic tomographic microscope.

Phase retrieval method for biological samples with absorption

Interferometric microscopy is an important technique to obtain the phase distribution of cells and tissues. However, in practice, the absorption effect of the biological samples and non-uniformities in the beam could induce errors in solving phase distributions by traditional methods. In this paper, an improved phase retrieval method is introduced to quantitatively calculate the phase distribution of absorbing samples. The proposed method could acquire more accurate phase distributions and optical transmitted sample images physically. Comparing with conventional phase retrieval methods, the proposed method has higher accuracy and calculation efficiency.

Research on refractive index distribution in biological tissues

From the view of refractive index distribution, biological tissue can be regarded as a refractive index random media. Inhomogeneous distribution of refractive index is the physical basis of the high scattering property of biological tissue, whose scale is close to wavelength. Disperse scatter model and statistical distribution function are introdcued to describe the refractive index distribution in biological tissues such as blood tissue. Based on the disperse scatter model and scatter theory, the relations between refractive index fluctuation and scattering coefficient of biological tissue are derived and corresponding formulas are given to calculate the refractive index fluctuation in biological tissue. The formulas are applied to calculate the refractive index distribution of blood tissue in different status, combining the Rayleigh scattering describing the single scattering events. The calculating results are illustrated in the paper.

Monte Carlo simulation of Stokes vectors of polarized light scattering from two-dimensional random rough surfaces

A Monte Carlo model was established to simulate polarized scattering fields of two-dimensional rough surfaces based on the Kirchhoff approximation. Based on this model, numerical studies of the hemispherical distribution of Stokes vectors of scattered light from dielectric and metal rough surfaces were carried out. These surfaces have Gaussian distributions with correlation length of 3.1 µm and standard deviation varying between 0.1 and 0.6 µm. The results reveal that the V component of metal surfaces has peaks antisymmetric with the incident plane, whereas the V component of dielectric surfaces is almost zero. We consider that this property of the V component would provide a new method which could be used to distinguish the target material.

Quantitative phase microscopy of red blood cells with slightly-off-axis interference

Microscopic interferometry is a noncontact technique for quantitative phase imaging of live cells. The method combines the principles of single-shot slightly-off-axis interferometry and confocal microscopy and is characterized by real-time acquisition capabilities and optimized spatial resolution. However, slightly-off-axis interferometry requires less detector bandwidth than traditional off-axis interferometry and fewer phase-shifted steps than on-axis interferometry. Meanwhile, confocal microscopy allows microstructure magnification imaging. To validate the utility of this technique, experimental and theoretical comparisons are given. The potential of the technique for phase microcopy is demonstrated by experiments on red blood cells. This study will set the basis for interferometric phase measurements of dynamic processes with fine spatial details, especially for observing live biological cell dynamics.

Influence of dead-time on detection efficiency and range performance of photon-counting laser radar that uses a Geiger-mode avalanche photodiode

Dead-time has a significant influence on the detection efficiency and range performance of a photon-counting laser radar system with a Geiger-mode avalanche photodiode. In this paper, a rapid universal recursive model of the detection probability of discrete time under various dead-times is proposed, which is verified with controlled parameters. Our model has the advantage of fast computing speed and unifies multi-trigger, single-trigger, and zero-dead-time models. The computing speed is 1 to 2 orders of magnitude faster than Gatts and Zhaos models under a short dead-time condition, with relative errors less than 0.001 and 10-14, respectively. Subsequently, the detection efficiency and range bias and precision with various dead-times are theoretically calculated and Monte Carlo simulated with different parameters. On the one hand, dead-time shorter than the end time of the target achieves better detection efficiency; however, this results in worse range performance. On the other hand, dead-time longer than the end time of the target maintains the detection efficiency at a low level but provides a better range performance. We discover that noise is the key reason for the periodic fluctuation of the detection efficiency and range performance versus different dead-times and the local optimum values of fluctuations occur when the dead-time is a few nanoseconds shorter or longer than 1, 1/2, 1/3, or even 1/4 of the end time of the target; further, this phenomenon becomes more evident when noise increases. Moreover, weaker noise level is crucial to the detection efficiency, and narrow pulse width and nearer target position in the range gate are important factors to improve precision.

Polar decomposition applied to light back-scattering by erythrocyte suspensions

In this paper, polarization property of RBCs was discussed by polar decomposition. Experimental results were compared with a three-dimensional Monte Carlo simulation for the erythrocyte suspensions with the same concentration. And there is a good agreement for both experimental and simulative results. Furthermore, Mueller matrices were measured for erythrocyte suspensions with different concentration under 10%, in this condition light coherent phenomena can be ignored. Using polar decomposition, the conclusion comes out that degree of polarization (DOP) and diattenuator for erythrocyte suspensions decrease with increasing concentration. Because when suspension concentration increases, scattering coefficient will be changed increasingly simultaneously and DOP and diattenuator decreases with added scattering times. These results will be referred as useful information for noninvasive diagnosis of blood.

A computational model for light transporting in biological tissues irradiated by converging laser beam

A semi-analytical method has been developed to calculate the spatially resolved diffuse light in layered biological tissues irradiated by converging laser beam based on the diffusion theory. Monte Carlo method is used to evaluate the correctness of the method, results show that they have good consistency and our method has higher computational efficiency. Numerical calculations disclosure same important features that are uniquely related to the propagation of the converging light in biological tissues. Those features are valuable to optimize the optical diagnosis and therapy.