Birefringence imaging of human skin by polarization-sensitive spectral interferometric optical coherence tomography

We have developed a spectral interferometric optical coherence tomography (OCT) system with polarization sensitivity that is able to measure a two-dimensional tomographic image by means of one-dimensional mechanical scanning. Our system, which has an axial resolution of 32 mum , calculates the distribution of each element of the Müller matrix of a measured object from 16 OCT images. The OCT system successfully reveals the birefringent nature of human skin tissue.     

Jones Matrix Imaging of Biological Samples Using Parallel-Detecting Polarization-Sensitive Fourier Domain Optical Coherence Tomography

A polarization-sensitive Fourier domain optical coherence tomography (PS-FD-OCT) system has been developed. By using a two-dimensional CCD camera, this PS-FD-OCT simultaneously obtains two spectral interferograms corresponding to orthogonal polarization components of a probing beam. FD-OCT avoids axial mechanical scanning, therefore, two OCT images are obtained by a single lateral scanning. This system requires only two one-dimensional scannings for determining the Müller matrix images of a sample. In the Müller matrix images, the birefringence properties of the inner surface of a porcine esophagus are revealed.     

Polarization-Sensitive Spectral Interferometric Optical Coherence Tomography for Human Skin Imaging

Polarization-sensitive spectral interferometric optical coherence tomography (PS-SI-OCT) is designed for imaging of the inner structure of a biological tissue with polarization properties corresponding to Müller matrix elements. With arbitrary polarization state of incident and reference light, the change of polarization state can be acquired, and the axial structure of the tissue is measured by single detection of the power spectrum, superposition of object and reference light. Using with this OCT system, a cross-sectional Müller matrix images of human skin can be observed.     

复谱频域OCT成像的研究

采用频域技术的OCT系统，深度扫描信息由背向散射光谱的傅立叶反变换获得，简化了轴向扫描过程，从而使快速OCT成像成为可能．为了实现复杂生物组织的OCT快速成像，消除谱频域OCT的“混叠”现象、重建样品的真实层析结构，本文引入了相移干涉技术，构建了一套满量程复谱频域OCT实验系统，并为系统设计了特殊的分束镜和移相器，为最终实现复杂生物组织的OCT快速成像提供了条件．     

Three‐Dimensional,Time‐Resolved Profiling of Ferroelectric Domain Wall Dynamics by Spectral‐Domain Optical Coherence Tomography

We apply here spectral‐domain optical coherence tomography (SD‐OCT) for the precise detection and temporal tracking of ferroelectric domain walls (DWs) in magnesium‐doped periodically poled lithium niobate (Mg:PPLN). We reproducibly map static DWs at an axial (depth) resolution down to ～ 0.6 μm, being located up to 0.5 mm well inside the single crystalline Mg:PPLN sample. We show that a full 3‐dimensional (3D) reconstruction of the DW geometry is possible from the collected data, when applying a special algorithm that accounts for the nonlinear optical dispersion of the material. Our OCT investigation provides valuable reference information on the DWs’ polarization charge distribution, which is known to be the key to the electrical conductivity of ferroelectric DWs in such systems. Hence, we carefully analyze the SD‐OCT signal dependence both when varying the direction of incident polarization, and when applying electrical fields along the polar axis. Surprisingly, the large backreflection intensities recorded under extraordinary polarization are not affected by any electrical field, at least for field strengths below the switching threshold, while no significant signals above noise floor are detected under ordinary polarization. Finally, we employed the high‐speed SD‐OCT setup for the real‐time DW tracking upon ferroelectric domain switching under high external fields.     

利用变偏振光谱技术实现显微成像深度选择

在正交偏振光谱(OPS)微循环成像系统的基础上,提出了通过改变信号光的偏振态来实现成像深度的选择。传统的OPS系统利用的是正交线偏振光,只能获得图像在某一断层的二维信息,但作者通过控制系统起、检偏单元椭圆偏振光的椭圆度,可以在不进行机械扫描的情况下将成像光束聚焦在不同的断层,从而获得不同深度处的组织信息。构建的变偏振光谱成像系统将光源发射光谱与红血球吸收光谱相匹配,可以实现微血管的探测,且具有较高的信噪比。对一块含标记物的猪肉脂肪进行实验,并通过图像处理得到了对比度与信号光偏振态间的定量关系。实验结果表明：椭圆度由0°～45°增大,即从线偏振光向圆偏振光转变的过程中,对比度逐渐增大,可探测到的最大深度增大。最后利用该系统,对裸鼠耳廓微血管进行了变偏振光谱测量实验,实验证明了控制偏振态可以实现对血管不同深度的探测,为微血管断层光谱成像提供了一种新的研究手段。     

High-speed fiber based polarization-sensitive optical coherence tomography of in vivo human skin

A high-speed single-mode fiber-based polarization-sensitive optical coherence tomography (PS OCT) system was developed. With a polarization modulator, Stokes parameters of reflected flight for four input polarization states are measured as a function of depth. A phase modulator in the reference arm of a Michelson interferometer permits independent control of the axial scan rate and carrier frequency. In vivo PS OCT images of human skin are presented, showing subsurface structures that are not discernible in conventional OCT images. A phase retardation image in tissue is calculated based on the reflected Stokes parameters of the four input polarization states.     

LCVR和AOTF的光谱偏振测量新策略

现有利用液晶相位可变延迟器(LCVR)和声光可调谐滤光器(AOTF)的偏振测量方法较为繁琐,故提出了一种光谱偏振测量新策略,去除了机械运动,并且相位延迟量的选取从四组减少为两组。采用两个LCVR和一个AOTF,通过两个相同型号探测器分别测量±1级衍射光,实现光谱偏振测量。电脑控制LCVR和AOTF的驱动系统分别实现所需相位延迟量和波长选择,通过扫描射频驱动整个频段得到被测光的光谱信息。叙述了方法的具体原理,分析了AOTF的偏振模型,通过理论计算LCVR和AOTF的Muller矩阵,推导出了相应的斯托克斯(Stokes)矢量中的I,Q,U的测量公式。分析并仿真了相位延迟量微小偏差对整个系统测量误差的影响,结果显示相位延迟量在±π/100范围内相对误差<3%。实验验证了测量系统的可行性和准确性,测量误差总体<6%。为偏振测量提供了一种简单可行且精度较高的新方法,具有重要的应用价值。     

基于OCT技术对古瓷釉气泡特征的初步研究

气泡是古代瓷釉中最常见的一种现象,气泡的大小与分布与胎体和釉层的配方及制作工艺密切相关。气泡表征对研究陶瓷的制作工艺、产地和时代特征等具有重要的意义。为了探讨利用光学相干层析成像技术（OCT）表征古代瓷釉中气泡及其分布特征的可行性,实验采用扫频OCT成像系统对五件不同类型的瓷釉样品进行了测试。根据实验所获得的透明釉层及胎釉结合部位的二维断面图像和三维层析图像,分别进行了瓷釉气泡二维断面和三维切片的特征研究,综合分析了釉层中的气泡特征及其可能产生的原因,认为釉层中的气泡主要是由于胎体中的气体在烧制过程中向釉层溢出所致;同时基于像素点计算了气泡的大小, 并与传统光学显微镜观测到的结果进行了比较;实验还针对不透明釉层中的气泡进行了大小计算、二维断面及三维切片的特征研究。实验结果表明,不同类型的古代瓷釉釉层中的气泡特征差异明显;基于像素点计算的气泡大小与传统光学显微镜测试的结果吻合, 近胎釉结合面的胎体切片能很好的反映瓷釉的气泡特征。本研究提出并验证了利用OCT技术表征瓷釉气泡特征的可行性与有效性,实现了釉层气泡特征的无损检测,特别是对不透明釉层气泡的分析,克服了以往利用传统显微镜技术对釉层气泡分布研究的局限,为古代陶瓷瓷釉的气泡特征分析提供了一种新型的、可靠的分析手段。     

Optical Coherence Tomography Implemented by Photonic Crystal Fiber

We report a photonic crystal fiber (PCF)-based optical coherence tomography (OCT) system, in which conventional single-mode fiber for the transmission line and the fiber coupler for the beam splitter/combiner are replaced with PCF and PCF coupler. The PCF coupler fabricated using the fused biconical tapered (FBT) method showed a nearly flat coupling ratio over a broad spectral bandwidth of 400 nm, which provided an axial resolution of 3-μm for OCT imaging. With a white-light source, the 8-μm thick air gap between two stacked cover glasses was measured, and with a conventional superluminecent diode (SLD) source, the in vitro images of rat eye and Misgurnus mizolepis skin were successfully obtained. The PCF and PCF coupler might enable a white-light as the source for the cost effective and high-resolution OCT system.     

Fourier domain optical coherence tomography using optical demultiplexers imaging at 60,000,000 lines/s

We describe high-speed Fourier domain optical coherence tomography (OCT) using optical demultiplexers (ODs) for spectral dispersion. The OD enables separation of a narrow spectral band of 14 GHz (0.11 nm) from a broadband incident light at 256 different frequencies in 25.0 GHz intervals centered at 192.2 THz (1559.8 nm). OCT imaging of 60,000,000 axial scans per second was achieved through parallel signal acquisition using 256 balanced photoreceivers to simultaneously detect all the output signals from the ODs in a Fourier domain OCT system. OCT imaging at a 16 kHz frame rate, 1100 A-lines per frame, 3 mm depth range, and 23 microm resolution was demonstrated using a resonant scanner for lateral scanning.     

Use of a scanner to modulate spatial interferograms for in vivo full-range Fourier-domain optical coherence tomography

We report a new yet simple method to achieve full-range complex Fourier-domain optical coherence tomography (OCT) for in vivo imaging. The method utilizes a scanner that is dedicated for lateral scanning in the system to introduce a constant carrier frequency into the OCT spectral interferograms during the scanning. This is achieved by simply offsetting the sampling beam spot away from the pivot point of the scanning mirror. We demonstrate the method experimentally for in vivo full-range imaging of the anterior segment of a human eye. The method is free from complex conjugate mirror image and self-cross-correlation image artifacts.     

Ultra-high resolution spectral domain optical coherence tomography using supercontinuum light source

An ultra-high resolution spectral domain optical coherence tomography (SD-OCT) was developed using a cost-effective supercontinuum laser. A spectral filter consists of a dispersive prism, a cylindrical lens and a right-angle prism was built to transmit the wavelengths in range 680–940 nm to the OCT system. The SD-OCT has achieved 1.9 μm axial resolution and the sensitivity was estimated to be 91.5 dB. A zero-crossing fringes matching method which maps the wavelengths to the pixel indices of the spectrometer was proposed for the OCT spectral calibration. A double sided foam tape as a static sample and the tip of a middle finger as a biological sample were measured by the OCT. The adhesive and the internal structure of the foam of the tape were successfully visualized in three dimensions. Sweat ducts was clearly observed in the OCT images at very high resolution. To the best of our knowledge, this is the first demonstration of ultra-high resolution visualization of sweat duct by OCT.     

基于受激辐射信号的谱域光学相干层析分子成像方法

鉴于不同生理病理状态下组织复折射率实部的变化不大,传统光学相干层析(OCT)成像技术在分子特异性识别上存在先天不足.为此,本文提出了基于受激辐射信号的OCT成像方法,可在实现传统散射成像的同时,实现基于受激辐射信号的分子成像.在超高分辨率谱域OCT系统的基础上,通过增设光谱分光与调制抽运光支路,建立了基于单宽谱光源的抽运探测谱域OCT系统,详细推导了调制抽运下受激辐射信号的获取与成像公式.利用搭建的抽运探测谱域OCT系统,实现了瞬态受激辐射信号的相干探测.基于同时获取的受激辐射OCT信号和传统OCT信号,成功重构了氮化物粉末构建样品的基于受激辐射信号的分子对比OCT图像.     

一维和二维傅里叶谱相位测量

我们利用正交偏振方法将傅里叶谱的实部和虚部分开,从而测定了一维和二维傅里叶谱的相位。文中报道了位移狭缝和位移方孔傅里叶谱相位的测量结果,它们与用位移量计算出的相位值符合很好,因此证实了该方法的可靠性。在此基础上我们还考察了其它二维二进制图形傅里叶谱的实部和虚部。     

Spectral Filtering in OCT System Implemented by Mechanically Induced Long-Period Fiber Grating

A spectral filtering method that can enhance the axial resolution and suppress sidelobes of the point spread function in Optical Coherence Tomography (OCT) system is demonstrated. A mechanically induced long-period fiber grating (MLPG) is used to tailor the spectral shape of a homemade Er-doped fiber amplified spontaneous emission (ASE) source. By using a MLPG, the high and narrow characteristic peak, centered at 1533 nm, of the ASE source was fairly well removed, which enabled us to use a broader spectral bandwidth of the source. The experimental results show that the proposed spectral filtering method can suppress the sidelobes in the axial point spread function up to 5.4 dB and thus enhance the axial resolution of an OCT system by four times. The OCT images of an extracted human premolar taken with and without the spectral filtering are presented and compared with each other.     

Faraday mirror-typed optical current transformers and its theoretical analysis

A Faraday mirror-typed optical current transformer (OCT) design with turn-back optical paths is proposed to overcome the harmful effects of the linear birefringence inside bulk glass current sensing head, which can double the sensitivity and improve the state of polarization of the system output. The theoretical analysis of the working principle, the simulation of the polarization state of the output light beam and the comparison between it and the OCT with polarization preserving total reflection (PPTR) layers are presented. The results show that this design has a good suppressing effect on the degeneration of the polarization state of the output light, which has certain reference significance on the performance improvement of the present bulk glass OCT, and the development of practical OCTs.     

Paired-angle-rotation scanning optical coherence tomography forward-imaging probe

We report a novel forward-imaging optical coherence tomography (OCT), needle-probe paired-angle-rotation scanning OCT (PARS-OCT) probe. The probe uses two rotating angled gradient-index lenses to scan the output OCT probe beam over a wide angular arc (approximately 19 degrees half-angle) of the region forward of the probe. Among other advantages, this probe design is readily amenable to miniaturization and is capable of a variety of scan modes, including volumetric scans. To demonstrate the advantages of the probe design, we have constructed a prototype probe with an outer diameter of 1.65 mm and employed it to acquire four OCT images, with a 45 degrees angle between adjacent images, of the gill structure of a Xenopus laevis tadpole. The system sensitivity was measured to be 93 dB by using the prototype probe with an illumination power of 450 microW on the sample. Moreover, the axial and the lateral resolutions of the probe are 9.3 and 10.3-12.5 microm, respectively.     

Elastic scattering susceptibility of the high temperature superconductor Bi2Sr2CaCu2O(8+delta): a comparison between real and momentum space photoemission spectroscopies

The joint density of states of Bi2Sr2CaCu2O(8+delta) is calculated by evaluating the autocorrelation of the single particle spectral function A(k, omega) measured from angle resolved photoemission spectroscopy (ARPES). These results are compared with Fourier transformed (FT) conductance modulations measured by scanning tunneling microscopy (STM). Good agreement between the two experimental probes is found for two different doping values examined. In addition, by comparing the FT-STM results to the autocorrelated ARPES spectra with different photon polarization, new insight on the form of the STM matrix elements is obtained. This shines new light on unsolved mysteries in the tunneling data.     

Differential phase contrast in optical coherence tomography

We report on a modification of optical coherence tomography (OCT) that allows one to measure small phase differences between beams traversing adjacent areas of a specimen. The sample beam of a polarization-sensitive low-coherence interferometer is split by a Wollaston prism into two components that traverse the object along closely spaced paths. After reflection at the various sample surfaces, the beams are recombined at the Wollaston prism. Any phase difference encountered between the two beams is converted into a change of polarization state of the recombined beam. This change is measured, and the resulting signals are converted to differential phase-contrast OCT images. The first images obtained from simple test objects allowed us to determine path-difference gradients with a resolution of the order of 5 x 10(-5) .