Parallel optical coherence tomography system

We present the design and procedures for implementing a parallel optical coherence tomography (POCT) imaging system that can be adapted to an endoscopic format. The POCT system consists of a single mode fiber (SMF) array with multiple reduced diameter (15 microm) SMFs in the sample arm with 15 microm center spacing between fibers. The size of the array determines the size of the transverse imaging field. Electronic scanning eliminates the need for mechanically scanning in the lateral direction. Experimental image data obtained with this system show the capability for parallel axial scan acquisition with lateral resolution comparable to mechanically scanned optical coherence tomography systems.     

Characterization of dentin and enamel by use of optical coherence tomography

Optical coherence tomographic images of human dentin and enamel are obtained by use of polarization-sensitive optical coherence tomography. A birefringence effect in enamel (lambda = 856 nm) and light propagation along dentinal tubules are observed. The group index of refraction for both dentin and enamel was measured at 1.50 +/- 0.02 and 1.62 +/- 0.02, respectively.     

Characterisation of scaffold architecture and tendons using optical coherence tomography and polarisation-sensitive optical coherence tomography

Scaffolds play an important role in the generation of functional tissues using tissue-engineering techniques. To generate highly organised tissue, scaffolds must have specific internal and external architectures. Here, optical coherence tomography (OCT) is exploited to characterise the architectures of various scaffolds, in particular scaffolds which have been fabricated to support the formation of uniaxially orientated collagen bundle for use in tendon tissue engineering. In parallel, a polarisation-sensitive OCT (PSOCT) has been built to assess the collagen fibre organisation in human tendon and monitor the growth of engineering tendon constructs online and non-destructively. The impact of mechanical stimuli on the modulation of tendon tissue formation and organisation was also assessed. It is shown that conventional OCT is capable of characterising scaffold architecture and the pore size, porosity or microchannel dimension can be determined quantitatively and qualitatively. PSOCT generated birefringence images of human tendon and demonstrated that low birefringence images, associated with fewer microstructural variations, correlated to the presence of scar tissue or degenerated tissue; whereas the tissue-engineered tendon exhibited lower degree of birefringence.     

Artificial fingerprint recognition by using optical coherence tomography with autocorrelation analysis

Fingerprint recognition is one of the most widely used methods of biometrics. This method relies on the surface topography of a finger and, thus, is potentially vulnerable for spoofing by artificial dummies with embedded fingerprints. In this study, we applied the optical coherence tomography (OCT) technique to distinguish artificial materials commonly used for spoofing fingerprint scanning systems from the real skin. Several artificial fingerprint dummies made from household cement and liquid silicone rubber were prepared and tested using a commercial fingerprint reader and an OCT system. While the artificial fingerprints easily spoofed the commercial fingerprint reader, OCT images revealed the presence of them at all times. We also demonstrated that an autocorrelation analysis of the OCT images could be potentially used in automatic recognition systems.     

Basic image-processing operations by use of acousto-optics

We describe some basic optical image-processing operations with acousto-optic (AO) Bragg diffraction. Instead of using frequency-plane filters, we place an AO cell behind the object. We then realize experimentally one-dimensional edge enhancement, which utilizes a high-pass filtering effect in the undiffracted order from the AO cell. A numerical simulation compares well with the experimental results. With two AO cells oriented orthogonally to each other, a second-order mixed derivative operation, evident from the four-corner enhancement of a square, is also demonstrated.     

Signal-to-noise ratio analysis of all-fiber common-path optical coherence tomography

We present theoretical analysis and experimental verification of the signal to noise ratio (SNR) of a common-path interferometer-based optical coherence tomography (OCT) system. Based on fully integrated all-fiber implementation of a common-path time-domain OCT system, we derived the SNR of the system including the effect of beat noise, which turns out to be twice as large as the excess noise term. We verified the theoretical SNR through a series of experiments, utilizing both controlled phantom and biological samples such as a rat brain with tumor and a frog retina. The results showed that the source power and the reference reflectivity can be easily controlled to optimize the SNR of OCT imaging. We have also analyzed the effect of the fiber delays and the offset in the fiber autocorrelator of the common-path OCT system on the overall SNR.     

Theoretical model of optical coherence tomography for system optimization and characterization

We present a detailed analytical model to describe optical coherence tomography (OCT) systems, which considers the propagation of the optical field within a scattering medium in the framework of the extended Huygens-Fresnel principle. The model includes use of the discrete-particle model and the fractal approach in treating biological tissue as being packed with scattering particles with a power-law distribution. In contrast to previous models, an imaginary lens proximal to the tissue surface is introduced that approximates the real focusing lens in the sample arm of the OCT system. This treatment avoids the consideration of backscattering light as traveling in the free space between the focusing lens and the tissue surface before mixing with the reference beam. Experiments on tissue phantoms were carried out to verify the validity of this model.     

Ultrahigh-resolution full-field optical coherence tomography

We have developed a white-light interference microscope for ultrahigh-resolution full-field optical coherence tomography of biological media. The experimental setup is based on a Linnik-type interferometer illuminated by a tungsten halogen lamp. En face tomographic images are calculated by a combination of interferometric images recorded by a high-speed CCD camera. Spatial resolution of 1.8 microm x 0.9 microm (transverse x axial) is achieved owing to the extremely short coherence length of the source, the compensation of dispersion mismatch in the interferometer arms, and the use of relatively high-numerical-aperture microscope objectives. A shot-noise-limited detection sensitivity of 90 dB is obtained in an acquisition time per image of 4 s. Subcellular-level images of plant, animal, and human tissues are presented.     

Interferometer for optical coherence tomography

We describe a new interferometer setup for optical coherence tomography (OCT). The interferometer is based on a fiber arrangement similar to Young's two-pinhole interference experiment with spatial coherent and temporal incoherent light. Depth gating is achieved detection of the interference signal on a linear CCD array. Therefore no reference optical delay scanning is needed. The interference signal, the modulation of the signal, the axial resolution, and the depth range are derived theoretically and compared with experiments. The dynamic range of the setup is compared with OCT sensors in the time domain. To our knowledge, the first images of porcine brain and heart tissue and human skin are presented.     

Comparative signal-to-noise analysis of fibre-optic based optical coherence tomography systems

Several optical coherence tomography (OCT) systems are proposed using optical-fibre components and based around Fizeau sensing interferometers. The theoretical signal-to-noise ratio (SNR) is calculated for each of the proposed configurations, using a constant set of assumed values for illumination and detection parameters. The SNR values obtained are compared with values calculated for typical existing configurations based around Michelson interferometers. Fizeau-based systems incorporating a secondary processing interferometer offer the advantage over current interferometer configurations of down-lead insensitivity, which prevents signal fading and reduces thermal fringe drift. The most basic form of the Fizeau system makes inefficient use of optical power, and has a low SNR compared with the widely used Michelson configuration. However, the results of the analysis described in this paper show that the SNR for more sophisticated Fizeau configurations, incorporating optical circulators and balanced detection systems, can be as high as the value for the most sensitive existing fibre-based OCT systems. Fizeau configurations therefore offer the combined advantages of optimized SNR and down-lead insensitivity, indicating their suitability for use in relatively poorly controlled environments such as in-vivo measurements.     

Unbalanced versus balanced operation in an optical coherence tomography system

The choice of a balanced optical coherence tomography (OCT) configuration versus an unbalanced OCT configuration with optimized reference-arm attenuation is discussed. The choice depends on the receiver noise, the fiber-end reflection R, and the power to the object. When OCT is used to investigate biological tissue an equivalent R? can be evaluated as the compound reflected light from tissue. In this case an additional parameter has to be considered: the confocal optical sectioning interval of the OCT system.     

线聚焦光学相干层析术的研究

提出了一种高速光学相干层析(OCT)成像技术方案。利用柱面镜的成像特性将传统OCT的点聚焦成像模式改变为线聚焦成像模式,从而降低二维OCT图像的扫描维数,达到提高成像速度的目的。利用ZEMAX光学软件对系统进行光线追迹获得光束经过柱面镜后的聚焦情况。随后采用635nm的激光光源和柱面镜构建了实验系统,实验结果很好地验证了光线追迹仿真结果。     

Low-noise broadband light generation from optical fibers for use in high-resolution optical coherence tomography

Broadband light generation from a single-mode optical fiber was developed for high-resolution optical coherence tomography (OCT). No noise amplification was observed for light broadened by self-phase modulation. The investigation showed that the intensity noise of light broadened by self-phase modulation in a single-mode optical fiber was much lower than that of continuum light from a microstructure fiber (MSF). The spectral width of a femtosecond input laser pulse was successfully broadened by a factor of 11, and a coherence length of 3.7 microm was achieved with this source. The application of light broadened by a single-mode optical fiber and MSF was compared for use in OCT imaging. The results showed that a single-mode fiber with a small core diameter is a useful way to generate low-noise, broadband light for high-resolution OCT imaging.     

Matrix approach to quantitative refractive index analysis by Fourier domain optical coherence tomography

The rapid development in the field of optical coherence tomography has demanded increasingly sophisticated numerical models to enable the interpretation of image data and extract quantitative results. We use a matrix formulation of Fresnel's equations for multilayered media to extract layer-dependent thickness and refractive index directly from Fourier domain optical coherence tomography spectrograms. An eigenanalysis spectral decomposition approach is used to constrain the least squares fitting algorithm, avoiding the need for initial estimates of the parameter values. We demonstrate this novel quantitative analysis approach by using a multilayered phantom and show good agreement with the known layer parameter values. This approach introduces a powerful tool for the analysis of layer-dependent optical properties that could have an important role in the differentiation of healthy and diseased tissue.     

Temporal coherence and time-frequency distributions in spectroscopic optical coherence tomography

Traditional analysis of spectroscopic optical coherence tomography (SOCT) signals is limited by an uncertainty relationship between time (depth) and frequency (wavelength). The use of a bilinear time-frequency distribution for analysis, such as those that compose Cohen's class of functions, may provide a way to avoid this limitation. Here we present the relationship between traditional SOCT analysis and the relevant Cohen class functions: the Wigner and Choi-Williams distributions. While cross terms that arise in these bilinear time-frequency distributions have been viewed as an artifact, here we identify these terms with temporal coherence, which contains significant information about the signal through phase relationships. The utility of time-frequency distributions is illustrated through analysis of calculated signals.     

Compound prism design principles, III: linear-in-wavenumber and optical coherence tomography prisms

We extend the work of the first two papers in this series [Appl. Opt. 50, 4998-5011 (2011), Appl. Opt. 50, 5012-5022 (2011)] to design compound prisms for linear-in-wavenumber dispersion, especially for application in spectral domain optical coherence tomography (OCT). These dispersive prism designs are believed to be the first to meet the requirements of high resolution OCT systems in direct-view geometry, where they can be used to shrink system size, to improve light throughput, to reduce stray light, and to reduce errors resulting from interpolating between wavelength- and wavenumber-sampled domains. We show prism designs that can be used for thermal sources or for wideband superluminescent diodes centered around wavelengths 850, 900, 1300, and 1375 nm.     

采用频域快速延迟线的光学相干层析系统的研制

建立了一套单模光纤型光学相干层析(OCT)成像系统。系统采用了不同于国内其他组的新型频域快速扫描延迟线(FD-ODL)和外差平衡接受技术,采取了一系列提高系统信噪比的方法,成功地实现高信噪比(113dB)、高分辨率、大成像深度的层析图像的提取,获得了纵向分辨率9μm、横向分辨率10μm、深度2.88μm的清晰的皮肤OCT图像。该系统比传统的采用时域延迟线的OCT在信噪比上高出了一个量级。     

Speckle reduction by I-divergence regularization in optical coherence tomography

For optical coherence tomography (OCT), ultrasound, synthetic-aperture radar, and other coherent ranging methods, speckle can cause spurious detail that detracts from the utility of the image. It is a problem inherent to imaging densely scattering objects with limited bandwidth. Using a method of regularization by minimizing Csiszar's I-divergence measure, we derive a method of speckle minimization that produces an image that both is consistent with the known data and extrapolates additional detail based on constraints on the magnitude of the image. This method is demonstrated on a test image and on an OCT image of a Xenopus laevis tadpole.     

Speckle statistics in optical coherence tomography

The two previously reported calculations of the amplitude distribution of speckles in optical coherence tomography, each based on a different mathematical formulation, yield different results. We show that a modification of an initial assumption in one of the formulations leads to equivalent results.     

Inverse scattering for optical coherence tomography

Inverse scattering theory for optical coherence tomography (OCT) is developed. The results are used to produce algorithms to resolve three-dimensional object structure, taking into account the finite beam width, diffraction, and defocusing effects. The resolution normally achieved only in the focal plane of the OCT system is shown to be available for all illuminated depths in the object without moving the focal plane. Spatially invariant resolution is verified with numerical simulations and indicates an improvement of the high-resolution cross-sectional imaging capabilities of OCT.