Alexei A. Kamshilin

Continuous reconstruction of holographic interferograms through anisotropic diffraction in photorefractive crystals

Abstract A new technique is presented for a continuous reconstruction of volume holograms in cubic photorefractive crystals of the sillenite family using the effect of anisotropic diffraction. In the scheme suggested, only two waves (the reference and object beams) are needed for simultaneous recording and reconstruction of holograms. The major advantage of the technique is an automatic self-adjustment of the Bragg diffraction at the volume hologram. This simple and efficient scheme can be used for producing holographic interferograms of objects of arbitrary shapes in real time. The experimental results are presented both for Bi12SiO20 and Bi12TiO20 crystals.

Photoplethysmographic imaging of high spatial resolution

We present a new method of formation photoplethysmographic images with high spatial resolution from video recordings of a living body in the reflection geometry. The method (patent pending) is based on lock-in amplification of every pixel of the recorded video frames. A reference function required for synchronous detection of cardiovascular pulse waves is formed from the same frames. The method is featured by ability to visualize dynamic changes in cardiovascular pulse wave during the cardiac (or respiratory) cycle. We demonstrate that the system is capable to detect the minimal irritations of the body such as gentle scratching of the skin by own finger.

Adaptive interferometry with photorefractive crystals

This work presents a review of progress and development in the field of adaptive laser interferometry. This method enables highly precise and reliable measurement of various physical parameters under unstable environmental conditions, which makes it very attractive for numerous industrial applications.

A new look at the essence of the imaging photoplethysmography

Photoplethysmography (PPG) is a noninvasive optical method accepted in the clinical use for measurements of arterial oxygen saturation. It is widely believed that the light intensity after interaction with the biological tissue in vivo is modulated at the heartbeat frequency mainly due to pulsatile variations of the light absorption caused by arterial blood-volume pulsations. Here we report experimental observations, which are not consistent with this model and demonstrate the importance of elastic deformations of the capillary bed in the formation of the PPG waveform. These results provide new insight on light interaction with live tissue. To explain the observations we propose a new model of PPG in which pulse oscillations of the arterial transmural pressure deform the connective-tissue components of the dermis resulting in periodical changes of both the light scattering and absorption. These local changes of the light-interaction parameters are detected as variations of the light intensity returned to a photosensitive camera. Therefore, arterial pulsations can be indirectly monitored even by using the light, which slightly penetrates into the biological tissue.

Photorefractive crystals for the stabilization of the holographic setup.

A photorefractive crystal of the sillenite family in a two-wave mixing experiment is used as a fundamental component in a negative feedback system for stabilizing a holographic setup. The behavior of such a stabilization system is theoretically analyzed, and the advantages and limitations of using a real-time nonpermanent recording material (photorefractive crystal) are discussed. We present experimental results using a Bi12TiO20 crystal, compare actual and predicted performances, and discuss the optimization of relevant parameters for better performance of the whole stabilizing system. Theoretical analysis and experimental results show this system to have interesting perspectives for further development.

Adaptive interferometer based on wave mixing in a photorefractive crystal under alternating electric field

Coupling of two waves with different polarization states in a photorefractive crystal under an ac electric field allows us to achieve the linear transformation of a small transient phase shift into the intensity modulation without any output polarization filtering. This can lead to design of an adaptive interferometer with sensitivity that reaches the classical homodyne detection limit.

Fast adaptive interferometer on dynamic reflection hologram in CdTe:V.

We present an adaptive interferometer based on the reflection dynamic hologram recorded in photorefractive CdTe:V crystal with no external electric field. Linear phase-to-intensity transformation is achieved by vectorial mixing of two waves with different polarization states (linear and elliptical) in the anisotropic diffraction geometry. Comparison of reflection and transmission geometries considering both sensitivity and adaptability is carried out. It is shown that the reflection geometry is characterized by better combination of these parameters provided that the crystal possesses high enough concentration of photorefractive centers.

Variability of Microcirculation Detected by Blood Pulsation Imaging

The non-invasive assessment of blood flow is invaluable for the diagnostic and monitoring treatment of numerous vascular and neurological diseases. We developed a non-invasive and non-contact method of blood pulsation imaging capable of visualizing and monitoring of the two-dimensional distribution of two key parameters of peripheral blood flow: the blood pulsation amplitude and blood pulsation phase. The method is based on the photoplethysmographic imaging in the reflection mode. In contrast with previous imaging systems we use new algorithm for data processing which allows two dimensional mapping of blood pulsations in large objects areas after every cardiac cycle. In our study we carried out the occlusion test of the arm and found (i) the extensive variability of 2D-distribution of blood pulsation amplitude from one cardiac cycle to another, and (ii) existence of the adjacent spots to which the blood is asynchronously supplied. These observations show that the method can be used for studying of the multicomponent regulation of peripheral blood circulation. The proposed technique is technologically simple and cost-effective, which makes it applicable for monitoring the peripheral microcirculation in clinical settings for example, in diagnostics or testing the efficiency of new medicines.

Growth of single-crystal photorefractive fibers of Bi12SiO20 and Bi12TiO20 by the laser-heated pedestal growth method

Abstract Photorefractive single-crystal fibers of Bi 12 SiO 20 up to 90 mm in length and Bi 12 TiO 20 up to 50 mm in length with diameters of 650–1200 μm were grown by the laser-heated pedestal growth (LHPG) method. Both Bi 12 SiO 20 and Bi 12 TiO 20 fibers were pulled from stoichiometric (6:1) melts at pulling rates of 0.3 and 0.2 mm/min, respectively, the feeding rate was 0.1 mm/min. Growth conditions, morphology and chemical composition of the grown fibers were studied. The possible mechanisms of crystallization of incongruently melting Bi 12 TiO 20 fiber from the stoichiometric melt were discussed. The energy-exchange effect was observed in the BSO fibers at the wavelength λ = 0.5145 μm .

Intensity redistribution in a thin photorefractive crystal caused by strong fanning effect and internal reflections

Propagation of the light beam in a thin photorefractive crystal with the diffusion-type mechanism of nonlinearity has been studied both experimentally and theoretically. Total internal reflections of fanned beams from the crystal’s side surfaces and their coupling with the pump beam result in the light-intensity redistribution and in the generation of the photorefractive surface wave. We propose a theoretical model of the intensity redistribution that considers the light-energy flow from the pump beam to the fanning beam and backward after the fanning beam reflection. A numerical simulation shows that the total internal reflection off the surface, toward which the flow of the light energy is directed, is responsible for the light-energy self-concentration.