High-resolution ultrasound-modulated optical tomography in biological tissues

We present a novel implementation of high-resolution ultrasound-modulated optical tomography that, based on optical contrast, can image several millimeters deep into soft biological tissues. A long-cavity confocal Fabry-Perot interferometer, which provides a large etendue and a short response time, was used to detect the ultrasound-modulated coherent light that traversed the scattering biological tissue. Using 15-MHz ultrasound, we imaged with high-contrast light-absorbing structures placed >3 mm below the surface of chicken breast tissue. The resolution along the axial and the lateral directions with respect to the ultrasound propagation direction was better than 70 and 120 microm, respectively. The resolution can be scaled down further by use of higher ultrasound frequencies. This technology is complementary to other imaging technologies, such as confocal microscopy and optical-coherence tomography, and has the potential for broad biomedical applications.     

Energy transport in silicon–aluminum composite thin film during laser short-pulse irradiation

Energy transport in silicon–aluminum composite thin films due to short-pulse laser irradiation is examined. Frequency dependent phonon transport in the silicon film is considered to formulate equivalent equilibrium temperature while modified two-equation model is used in the aluminum film to obtain electron and phonon temperatures. Thermal boundary resistance across the films is incorporated in the analysis. Transmittance, reflectance, and absorption of the incident laser beam are determined using the transfer matrix method. Equivalent equilibrium temperatures resulted from frequency dependent and frequency independent solutions are compared. It is found that phonon temperature increase at the aluminum interface is suppressed by phonon transport to the silicon film, which is more pronounced at low laser pulse intensities. The influence of the ballistic phonons on equivalent equilibrium temperature in the silicon film is found to be significant.     

Dual femtosecond laser multiheterodyne optical coherence tomography

A time domain optical coherence tomography (OCT) system without moving parts is described, which is based on multiheterodyning utilizing two mode-locked femtosecond lasers. By synchronizing the two lasers to slightly different repetition rates and coupling to an interferometric OCT setup, we obtain amplitude-modulated beat signals representing the structure of the specimen under investigation. Our system is suitable for biological imaging as well as technical applications. We demonstrate high axial imaging depths of 150 mm with up to 5000 axial scans per second, achieving equivalent path scanning velocities of 750 m/s.     

Polarimetry of short-pulse gamma rays produced through inverse Compton scattering of circularly polarized laser beams

We have developed a polarimetry of ultrashort pulse gamma rays based on the fact that gamma rays penetrating in the forward direction through a magnetized iron carry information on the helicity of the original gamma rays. Polarized, short-pulse gamma rays of (1.1+/-0.2)x10(6)/bunch with a time duration of 31 ps and a maximum energy of 55.9 MeV were produced via Compton scattering of a circularly polarized laser beam of 532 nm off an electron beam of 1.28 GeV. The first demonstration of asymmetry measurements of short-pulse gamma rays was conducted using longitudinally magnetized iron of 15 cm length. It is found that the gamma-ray intensity is in good agreement with the simulated value of 1.0x10(6). Varying the degree of laser polarization, the asymmetry for 100% laser polarization was derived to be (1.29+/-0.12)%, which is also consistent with the expected value of 1.3%.     

Nonstationary two-flux model of radiation transport for optical tomography of scattering media

A nonstationary two-flux model is formulated for the transport of radiation in an inhomogeneous scattering medium and is applied to the situation where such a medium is irradiated by the narrow beam of a pulsed laser. It is shown that when the time distribution of the transmitted photons is measured it is possible simultaneously to reconstruct the two spatial functions (the coefficients of absorption coefficient and of scattering of the radiation by the medium) by means of an inverse Radon transformation and the solution of a system of nonlinear differential equations on the projection lines. An analytic solution is obtained in quadratures for these differential equations. The results constitute a method of solving problems in optical tomography in an inhomogeneous scattering medium Zh. Tekh. Fiz. 67, 61–65 (May 1997)     

基于δ声波场的生物组织光学断层成像研究

利用δ声波场和近红外光漫射理论实现生物组织的断层成像是一种新颖医学 成像方法. 通过构建一个δ声波场并作用到生物组织中，仅改变组织内作用点上的光学特性参数，并影响近红外光在组织中的传播特性而改变组织表面的光强分布. 利用组织表面光强改变量与作用点上组织光学参数改变量之间的内在关系，可以通过对δ声波场作用点上断层的扫描实现组织内部的断层成像. 该方法具有广泛的临床医学应用前景，为乳腺癌的早期检测提供一种有效方法. 关键词： 光学断层成像 δ声波场 漫射方程 生物组织     

High performance wavelength-swept laser with mode-locking technique for optical coherence tomography

We demonstrate a Fourier-domain mode-locked wavelength-swept laser that uses a polygon-based narrowband optical scanning filter and a high-efficiency semiconductor optical amplifier. Peak and average output powers of 98 mW and 71 mW have been achieved, respectively, without an external amplifier, while the wavelength was swept continuously from 1247 nm to 1360 nm. A unidirectional wavelength sweeping rate of 7452 nm/ms (65.95 kHz repetition rate) was achieved by using a 72 facet polygon scanner with a rotation rate of 916 revolutions per second. The instantaneous linewidth of this laser is 0.09 nm, which corresponds to a coherence length of 16 mm. This laser is most suitable for optical coherence tomography applications.     

Procedures of optical control dedicated for laser welding process of biological tissues

In this contribution, an optical method of controlling the state of soft biological tissues in real time exposed to laser radiation is discussed. The method is based on the assumption that the change dynamics of the amplitude of the scattered diagnostic radiation (λ = 635 nm) is compatible with the change dynamics of the tissue inner structure exposed to the Nd:YAG laser radiation (λ = 1064 nm). In this method, the measurement of the tissue temperature is omitted. Exemplary results of the laboratory research on this method and an interpretation of the results are presented.     

激光加载下金属锡材料微喷颗粒与低密度泡沫混合实验研究

金属材料的微喷是冲击加载下金属表面发生的一种动态破碎现象,微喷研究在很多领域都具有重要意义,包括惯性约束聚变(ICF)和烟火制造等.由于激光实验特有的优势,近几年国内外开展了很多利用强激光驱动冲击加载研究材料微喷过程的实验.利用泡沫材料对微喷颗粒进行静态软回收虽然可以获得颗粒的形态分布、颗粒尺寸及颗粒质量等定量结果,但并不能反演微喷颗粒从进入泡沫到停滞过程中的动态混合过程.为此,在神光Ⅱ升级装置上利用皮秒脉冲激光照射金丝产生高能X射线,实现了对锡微喷颗粒与低密度泡沫混合过程的高时间分辨和高空间分辨背光照相.背光图像面密度结果证实微喷颗粒在泡沫中并没有发生二次破碎.静态回收结果表明,在锡材料与泡沫紧贴放置的情况下,微喷颗粒在泡沫中的穿透深度随着加载压强升高呈现先增大后减小的规律,与非紧贴放置的实验结果有明显的差别.     

Continuous wavelength-tunable optical short-pulse generation by use of two Fabry-Perot laser diodes in a mutual injection-seeding scheme

A simple system for producing continuous and widely wavelength-tunable optical short pulses in a mutual injection-seeding scheme is presented. The system exhibits a good side-mode suppression ratio of greater than 32.7 dB over a large wavelength tuning range of 33.8 nm. The system is easy to operate and convenient for continuous wavelength tuning.     

An overview on recent radiation transport algorithm development for optical tomography imaging

Optical tomography belongs to the promising set of non-invasive methods for probing applications of semi-transparent media. This covers a wide range of fields. Nowadays, it is mainly driven by medical imaging in search of new less aggressive and affordable diagnostic means. This paper aims at presenting the most recent research accomplished in the authors’ laboratories as well as that of collaborative institutions concerning the development of imaging algorithms. The light transport modelling is not a difficult question as it used to be. Research is now focused on data treatment and reconstruction. Since the turn of the century, the rapid expansion of low cost computing has permitted the development of enhanced imaging algorithms with great potential. Some of these developments are already on the verge of clinical applications. This paper presents these developments and also provides some insights on still unresolved challenges. Intrinsic difficulties are identified and promising directions for solutions are discussed.     

Rate predictions for two photon spectroscopy of highly charged ions through laser induced recombination

Laser nitriding is used for the fast and easy production of nitride coatings on iron and alloys. Here, first results of the laser nitriding process applied to stainless steel are reported. The laser treatment led to the appearance of additional lines in the Mössbauer spectra, which are attributed to γ-Fe(N) produced by the laser nitriding process. The Mössbauer results are discussed in connection with the results obtained from X-ray diffraction and resonant nuclear reaction analysis. Furthermore, the results of isochronical annealing treatments of laser nitrided iron are reported.     

Functional optical coherence tomography for detecting neural activity through scattering changes

We have demonstrated functional optical coherence tomography (fOCT) for neural imaging by detecting scattering changes during the propagation of action potentials through neural tissue. OCT images of nerve fibers from the abdominal ganglion of the sea slug Aplysia californica were taken before, during, and after electrical stimulation. Images acquired during stimulation showed localized reversible increases in scattering compared with those acquired before stimulation. Motion-mode OCT images of nerve fibers showed transient scattering changes from spontaneous action potentials. These results demonstrate that OCT is sensitive to the optical changes in electrically active nerve fibers.     

Research on temperature field characteristics of optical films under 1064 nm short-pulse laser radiation

The 3-D temperature field distribution (TFD) equation of optical films (OFs) was analyzed, based on the film temperature field distribution (FTFD) model irradiated by Gaussian short-pulse laser, and short-pulse laser-induced temperature field distribution of TiO₂/SiO₂ composite membranes on K9 glasses was obtained by means of numerical simulation. The results show that the thermal effect of optical films is very obvious under the Gaussian short-pulse laser. Temperature at the center of the spot rises faster than in other areas, and it decreases along the radius rapidly. It provides theoretical basis for progress analysis of short-pulse laser acting on optical films.     

Continuous optical coherence tomography monitoring of nanoparticles accumulation in biological tissues

In this study, dynamics of nanoparticles penetrating and accumulating in biotissue (healthy skin) was investigated in vivo by the noninvasive method of optical coherence tomography (OCT). Gold nanoshells and titanium dioxide nanoparticles were studied. The processes of the nanoparticles penetration and accumulation in biotissue are accompanied by the changes in optical properties of skin which affect the OCT images. The continuous OCT monitoring of the process of the nanoparticles penetration into skin showed that these changes appeared in 30 min after application of nanoparticles on the surface; the time of accumulation of maximal nanoparticles concentration in skin was observed in period of 1.5–3 h after application. Numerical processing of the OCT signal exhibited the increase in contrast between upper and lower parts of dermis and contrast decay of the hair follicle border during 60–150 min. The transmission electron microscopy technique confirmed accumulation of the both types of nanoparticles in biotissue. The novelty of this study is presentation of OCT ability to in vivo monitor dynamics of nanoparticles penetration and their re-distribution within living tissues.     

短脉冲激光对硅基探测器的热作用模型选择

从傅里叶模型和非傅里叶模型的基本方程出发,通过有限差分方法对方程进行数值求解。分别分析了10,1．0,0．1,0．01 ns这4种脉宽的脉冲激光作用于硅材料时两种传热模型温度曲线的相对变化;讨论了热弛豫时间对非傅里叶模型数值结果的影响。结果表明:脉宽小于或等于100 ps的激光作用于硅材料时,表层温度上升缓慢,会发生载流子效应,非傅里叶模型可以合理地反映这种现象;对于一般材料,载流子效应发生的条件是脉宽小于或等于材料热弛豫时间,此时应当用非傅里叶模型描述加热过程。     

Measurement of the laser beam intensity distribution using optical tomography

Optical tomography is proposed as a method for studying the intensity distribution in the laser beam cross section. The tomographic projection of the beam cross section is obtained with the help of an optical fiber possessing a high radiation resistance, which makes it possible to employ this technique for studying high-power laser radiation. The results of tomographic reconstruction of the bean cross section from experimental projections are compared with the results of reconstruction from model projections.     

Generation of ultra-intense ion beams by a short-pulse laser

This paper summarizes briefly the main experimental and numerical results of the IPPLM team studies on the generation of ultra-intense ion beams by a short (≤1?ps) laser pulse. Basic laser-driven ion acceleration schemes capable of generating such ion beams are described including the target normal sheath acceleration (TNSA) scheme, the skin-layer ponderomotive acceleration (SLPA) scheme and the laser-induced cavity pressure acceleration (LICPA) scheme. It is shown that an efficient way for achieving high ion beam intensities and fluencies lies in using a short-wavelength laser driver of circular light polarization. In such a case, SLPA clearly dominates over TNSA, and dense and compact ion bunch is generated with high energetic efficiency. The LICPA scheme operating in the photon (radiation) pressure regime can be even more efficient than SLPA. As it is demonstrated by particle-in-cell simulations, the LICPA accelerator with a picosecond, circularly polarized laser driver of intensity ～ 10²¹_?W/cm² can produce sub-picosecond light ion beams of intensity ～ 10²²?W/cm² and fluence?>?1?GJ/cm² with the energetic efficiency of tens of percent. Laser-driven ion beams of such extreme parameters could open up new research areas in high-energy-density science, inertial fusion or nuclear physics.     

Heating of thin foils with a relativistic-intensity short-pulse laser

K-shell x-ray spectroscopy of sub-100 nm Al foils irradiated by high contrast, spatially uniform, 150 fs, Ilambda (2)=2 x 10(18) W microm(2)/cm(2), laser pulses is obtained with 500 fs time resolution. Two distinct phases occur: At /=500 fs the resonance transitions dominate. Initial satellites arise from a large area, high density, low temperature (approximately 100 eV) plasma created by fast electrons. Thus, contrary to predictions, a short, high intensity laser incident on a thin foil does not create a uniform, hot dense plasma.