Compact,broad-bandwidth fiber laser for sub-2-microm axial resolution optical coherence tomography in the 1300-nm wavelength region

A novel, compact, user friendly fiber laser with a broad emission bandwidth (MenloSystems, lambdac = 1375 nm, deltalambda = 470 nm, Pout = 4 mW) was used to achieve unprecedented sub-2-microm axial resolution optical coherence tomography (OCT) in nontransparent biological tissue in the 1300-nm wavelength region. Fresh human skin and arterial biopsies were imaged ex vivo with approximately 1.4-microm axial and approximately 3-microm lateral resolution and 95-dB sensitivity, demonstrating the great potential for clinical OCT applications of this stable, low-cost, and turn-on-key fiber laser.     

In vivo human retinal imaging by ultrahigh-speed spectral domain optical coherence tomography

An ultrahigh-speed spectral domain optical coherence tomography (SD-OCT) system is presented that achieves acquisition rates of 29,300 depth profiles/s. The sensitivity of SD-OCT and time domain OCT (TD-OCT) are experimentally compared, demonstrating a 21.7-dB improvement of SD-OCT over TD-OCT. In vivo images of the human retina are presented, demonstrating the ability to acquire high-quality structural images with an axial resolution of 6 microm at ultrahigh speed and with an ocular exposure level of less than 600 microW.     

Real-time display with large field of view on fourier domain optical coherence tomography at 1310 nm wavelength for dermatology

A Fourier domain optical coherence tomography (OCT) system with 1310 nm light was demonstrated to study inflammatory human skin and the skin coated with a moisturizer in vivo. By using a graphics processing unit (GPU), the display rate could reach 20 frames/s with 1000 A-scans contained in one image. The field of view (FOV) of the cross-sectional image is 7 mm in the lateral direction and the penetration depth is ??1 mm in skin. The result shows that, in inflammatory skin, the epidermis became thicker and had a decreased scattering; furthermore, the region of the severe lesion present an uneven thickness of the epidermis compared with the peripheral area. For the result of a finger tip coated with the moisturizer, the antireflection effect was significant and the stratum corneum became more transparent. In this letter, we demonstrated that real-time display with a large FOV could enable screening of a large tissue area; thereby increasing the dermatologic diagnostic potential of the method by permitting a comparison of the lesion and the normal peripheral region.     

Estimation of maximum measurable distance in optical time domain reflectometry

The dependence of the maximum measurable distance (MMD) in optical time domain reflectormetry (OTDR) upon fiber and receiver parameters, number of averaging times, signal pulse width, and input power launched into the fiber has been investigated for both 1.3- and 1.5-μm wavelengths.

The calculations have been performed for both Ge- and InGaAs/InP-APD receivers. As a result it is found that (1) the (MMD) is larger for the InGaAs/InP-APD receiver and (2) a further increase in the MMD can be achieved by cooling the InGaAs/InP-APD receiver.     

Demonstration of highly efficient waveguiding in a photonic crystal slab at the 1.5-microm wavelength

Highly efficient transmission of 1.5-microm light in a two-dimensional (2D) photonic crystal slab waveguide is experimentally demonstrated. Light waves are shown to be guided along a triple-line defect formed within a 2D crystal and vertically by a strong index-guiding mechanism. At certain wavelength ranges, complete transmission is observed, suggesting lossless guiding along this photonic one-dimensional conduction channel.     

谱域光学相干层析系统中基于解卷积方法的像质优化

谱域光学相干层析（spectral domain optical coherence tomography, 简记为SD-OCT）系统的轴向点扩散函数（point spread function, 简记为PSF）并不具备空不变特性,无法直接应用于解卷积运算.为实现SD-OCT系统成像质量基于解卷积算法的优化,本文采用数值校正后的轴向扫描信号和轴向有效PSF来实施基于Lucy-Richardson算法的解卷积运算,进而实现了SD-OCT系统中图像质量尤其是轴向分辨率的改善.本文理论分析了SD-OCT系统中导致轴向有效PSF随成像深度增大而下降和图像模糊的因素,阐述了利用解卷积算法实现图像质量优化的过程,基于建立的SD-OCT系统实施了不同成像深度位置处PSF的标定,并利用离散轴向位置处PSF的峰值拟合了轴向有效PSF的调制函数.利用调制函数对所有轴向扫描信号进行数值校正,然后根据轴向有效PSF进行解卷积算法.典型样品的解卷积图像重建结果表明,提出的解卷积方法能有效提高系统的轴向分辨率,同时有效抑制系统灵敏度随成像深度增大而下降的趋势. 关键词： 谱域光学相干层析 Lucy-Richardson解卷积 有效点扩散函数     

谱域光学相干层析系统中基于解卷积方法的像质优化

谱域光学相干层析（spectral domain optical coherence tomography, 简记为SD-OCT）系统的轴向点扩散函数（point spread function, 简记为PSF）并不具备空不变特性,无法直接应用于解卷积运算.为实现SD-OCT系统成像质量基于解卷积算法的优化,本文采用数值校正后的轴向扫描信号和轴向有效PSF来实施基于Lucy-Richardson算法的解卷积运算,进而实现了SD-OCT系统中图像质量尤其是轴向分辨率的改善.本文理论分析了SD-OCT系统中导致轴向有效PSF随成像深度增大而下降和图像模糊的因素,阐述了利用解卷积算法实现图像质量优化的过程,基于建立的SD-OCT系统实施了不同成像深度位置处PSF的标定,并利用离散轴向位置处PSF的峰值拟合了轴向有效PSF的调制函数.利用调制函数对所有轴向扫描信号进行数值校正,然后根据轴向有效PSF进行解卷积算法.典型样品的解卷积图像重建结果表明,提出的解卷积方法能有效提高系统的轴向分辨率,同时有效抑制系统灵敏度随成像深度增大而下降的趋势.     

相位敏感型光时域反射传感系统光学背景噪声的产生机理及其抑制方法

相位敏感型光时域反射(Φ-OTDR)传感系统具有响应速度快、灵敏度高等优点,能够实现对微弱扰动的分布式检测,在重大设施的入侵警戒、大型工程结构的健康监测等领域具有广阔应用前景.然而,与传统的OTDR传感系统不同,Φ-OTDR系统中存在着激光器中心频率漂移、偏振相关的噪声、光纤应变与干涉强度非线性对应关系引起的测量失真等光学背景噪声,对有效信号的提取形成了不可忽视的干扰,从而限制了Φ-OTDR传感系统在实际应用环境下的传感性能.本文对这些光学背景噪声的产生机理进行了深入分析,并提出了相应的噪声抑制方法.实验结果表明,本文提出的方法可以有效抑制Φ-OTDR传感系统中的光学背景噪声,并显著提高传感系统性能.     

Fourier domain mode locking at 1050 nm for ultra-high-speed optical coherence tomography of the human retina at 236,000 axial scans per second

A Fourier domain mode-locked (FDML) laser at 1050 nm for ultra-high-speed optical coherence tomography (OCT) imaging of the human retina is demonstrated. Achievable performance, physical limitations, design rules, and scaling principles for FDML operation and component choice in this wavelength range are discussed. The fiber-based FDML laser operates at a sweep rate of 236 kHz over a 63 nm tuning range, with 7 mW average output power. Ultra-high-speed retinal imaging is demonstrated at 236,000 axial scans per second. This represents a speed improvement of approximately10x over typical high-speed OCT systems, paving the way for densely sampled volumetric data sets and new imaging protocols.     

Performance limit of a multi-frequency probe based coherent optical time domain reflectometry caused by nonlinear effects

The nonlinear effects that limit the performance of the multi-frequency probe(MFP)based coherent optical time domain reflectometry(C-OTDR)are investigated.Based on theoretical analysis and experimental results,compared with conventional C-OTDR,when the probe pulse has power gradient within the pulse width,self-phase modulation(SPM)and cross-phase modulation(XPM)are strengthened in the new C-OTDR scheme.The generation of four-wave mixing(FWM)is dependent on SPM and XPM,and with modulation frequency of phase modulator higher than 40 MHz,the stimulated Brillouin scattering(SBS) threshold can be enhanced by more than 5 dB,which benefits the maximum dynamic range of the MFP C-OTDR.     

Substance identification by depth resolved spectroscopic pattern reconstruction in frequency domain optical coherence tomography

We analyze a method to extract additional depth resolved spectroscopic information from frequency domain optical coherence tomography (FDOCT) data. The reconstruction of depth resolved spectra is obtained by a Fourier transform of the individual peaks in the complex FDOCT depth profiles. We demonstrate a validation of this concept with theoretical simulations and with accurate experimental studies on a multilayer sample with four different characteristic absorbers. The spatially resolved spectroscopic patterns of all individual sample layers are calculated from the depth resolved reconstructed spectra. With an additional pattern recognition algorithm, these reconstructed patterns are compared automatically to the spectral characteristics of the expected substances. This provides an allocation of the reconstructed spectra to the substances with high reliability. Thus, we present an automated substance identification directly from conventional FDOCT data, which increases significantly the information content of the image.     

Double-Doppler wavelength conversion of infrared optical pulses by moving grating of refractive index in semiconductors

Optical and Quantum Electronics - A time-domain model of an excited semiconductor medium as a moving Bragg grating was used to investigate a double-Doppler wavelength conversion of an infrared...     

Real-time measurement of in vitro flow by Fourier-domain color Doppler optical coherence tomography

The possibility of measuring a full Doppler flow depth profile in parallel by use of frequency-domain optical coherence tomography is demonstrated. The method is based on a local phase analysis of the backscattered signal and allows for imaging of bidirectional Doppler flow. The Doppler frequency limit is 5 kHz for the presented measurements and is set by half of the frame rate of the CCD detector array. We measured the flow of 0.3-microm microspheres suspended in distilled water at controlled flow rates and in vitro human blood flow through a 200-microm capillary with a real-time color-encoded Doppler tomogram rate of 2-3/s.     

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.     

High-speed, high-resolution Fourier-domain optical coherence tomography system for retinal imaging in the 1060 nm wavelength region

A high-speed (47,000 A-scans/s), ultrahigh axial resolution Fourier domain optical coherence tomography (OCT) system for retinal imaging at approximately 1060 nm, based on a 1024 pixel linear array, 47 kHz readout rate InGaAs camera is presented. When interfaced with a custom superluminescent diode (lambda(c) = 1020 nm, Deltalambda = 108 nm, Pout = 9 mW), the system provides 3.3 microm axial OCT resolution at the surface of biological tissue, approximately 4.5 microm in vivo in rat retina, approximately 5.7 microm in vivo in human retina, and 110 dB sensitivity for 870 microW incident power and 21 mus integration time. Retinal tomograms acquired in vivo from a human volunteer and a rat animal model show clear visualization of all intraretinal layer and increased penetration into the choroid.     

Power-efficient Fourier domain optical coherence tomography setup for selection in the optical path difference sign using Talbot bands

A power-efficient Fourier domain optical coherence tomography fiber system is presented free of mirror terms. The object and reference beams from the interferometer are spatially separated to illuminate different parts of the diffraction grating in the spectrometer according to a method inspired from the use of Talbot bands. In this way, the system is made sensitive to only one sign of the optical path difference in the interferometer.     

Quantification of color alteration in human teeth with optical coherence tomography

It is necessary to develop a laboratory model to evaluate tooth discoloration, because there are several limitations to assessment methods at present stage. Therefore, in this letter, we report the results from a pilot study on using optical coherence tomography imaging method to quantify color alteration in the human teeth treatment with 35% hydrogen peroxide bleaching in vitro. Quantitative comparison of chromogens reduction in dental tissue showed that near infrared attenuation coefficient (μ) increased for enamel with the bleaching passage time and diminution for dentine. Therefore, the precise detection of the change in attenuation coefficient is can accurate quantitative chromogens alteration in tooth. OCT has a potential to become a useful tool for the assessment color alteration in human teeth.     

Speckle noise attenuation in optical coherence tomography by compounding images acquired at different positions of the sample

This study demonstrates a simple method for attenuating the speckle noise generated by coherent multiple-scattered photons in optical-coherence tomography images. The method could be included among the space-diversity techniques used for speckle reduction. It relies on displacing the sample along a weakly focused beam in the sample arm of the interferometer, acquiring a coherent image for each sample position and adding the individual images to form a compounded image. It is proven that the compounded image displays a reduction in the speckle noise generated by multiple scattered photons and an enhancement in the intensity signal caused by single-backscattered photons. To evaluate its potential biomedical applications, the method is used to investigate in vitro a caries lesion affecting the enamel layer of a wisdom tooth. Because of the uncorrelated nature of the speckle noise the compounded image provides a better mapping of the lesion compared to a single (coherent) image.     

Quantification of magnetic domain disorder and correlations in antiferromagnetically coupled multilayers by neutron reflectometry

The in-plane correlation lengths and angular dispersion of magnetic domains in a transition metal multilayer have been studied using off-specular neutron reflectometry techniques. A theoretical framework considering both structural and magnetic disorder has been developed, quantitatively connecting the observed scattering to the in-plane correlation length and the dispersion of the local magnetization vector about the mean macroscopic direction. The antiferromagnetic domain structure is highly vertically correlated throughout the multilayer. We are easily able to relate the neutron determined magnetic domain dispersion to magnetization and magnetoresistance experiments.