Absolute fast axis determination using non-polarization-maintaining fiber-based polarization-sensitive optical coherence tomography

We report on a new calibration technique that permits the accurate extraction of sample Jones matrix and hence fast-axis orientation by using fiber-based polarization-sensitive optical coherence tomography (PS-OCT) that is completely based on non-polarization-maintaining fiber such as SMF-28. In this technique, two quarter-wave plates (QWPs) are used to completely specify the parameters of the system fibers in the sample arm so that the Jones matrix of the sample can be determined directly. The device was validated on measurements of a QWP and an equine tendon sample by a single-mode fiber-based swept-source PS-OCT system.     

Depth-resolved birefringence and differential optical axis orientation measurements with fiber-based polarization-sensitive optical coherence tomography

Conventional polarization-sensitive optical coherence tomography (PS-OCT) can provide depth-resolved Stokes parameter measurements of light reflected from turbid media. A new algorithm that takes into account changes in the optical axis is introduced to provide depth-resolved birefringence and differential optical axis orientation images by use of fiber-based PS-OCT. Quaternion, a convenient mathematical tool, is used to represent an optical element and simplify the algorithm. Experimental results with beef tendon and rabbit tendon and muscle show that this technique has promising potential for imaging the birefringent structure of multiple-layer samples with varying optical axes.     

Simplified method for polarization-sensitive optical coherence tomography

We report a method for extracting the birefringence properties of biological samples with micrometer-scale resolution in three dimensions, using a new form of polarization-sensitive optical coherence tomography. The method measures net retardance, net fast axis, and total reflectivity as a function of depth into the sample. Polarization sensing is accomplished by illumination of the sample with at least three separate polarization states during consecutive acquisitions of the same pixel, A scan, or B scan. The method can be implemented by use of non-polarization-maintaining fiber and a single detector. In a calibration test of the system, net retardance was measured with an average error of 7.5 degrees (standard deviation 2.2 degrees ) over the retardance range 0 degrees to 180 degrees , and a fast axis with average error of 4.8 degrees over the range 0 degrees to 180 degrees .     

Micromachined array tip for multifocus fiber-based optical coherence tomography

High-resolution optical coherence tomography demands a large detector bandwidth and a high numerical aperture for real-time imaging, which is difficult to achieve over a large imaging depth. To resolve these conflicting requirements we propose a novel multifocus fiber-based optical coherence tomography system with a micromachined array tip. We demonstrate the fabrication of a prototype four-channel tip that maintains a 9-14-microm spot diameter with more than 500 microm of imaging depth. Images of a resolution target and a human tooth were obtained with this tip by use of a four-channel cascaded Michelson fiber-optic interferometer, scanned simultaneously at 8 kHz with geometric power distribution across the four channels.     

全光纤频域光学相干层析成像系统优化研究

针对频域光学相干层析(SD-OCT)系统特有硬件(线阵CCD及分光光栅)参数对成像质量造成的影响,展开了对基于光纤的频域OCT系统中硬件参量的模拟和优化工作,分析了分光计中CCD线列阵像素数及数字化深度、CCD安装偏差等因素对OCT成像质量的影响,并对光谱像素图定位进行了修正.研究表明:纵向分辨率不受CCD线列阵像素数的影响,CCD线列阵像素数的增多将线性地增大最大测量深度;CCD数字化深度小于6 bit将直接导致系统纵向分辨率的锐减;线阵CCD偏离聚焦透镜焦面将导致点扩展函数的强度减弱、系统分辨率降低;在较小角度内转动CCD,将使纵向分辨率得到提高;采用氖灯光谱进行像素图定位校正之后,可以相应地提高系统分辨率.部分模拟结论得到实验验证.采用此模拟优化结果,可根据OCT成像的具体要求对系统硬件参量进行优化选择.     

Jones matrix analysis for a polarization-sensitive optical coherence tomography system using fiber-optic components

We present an analysis for polarization-sensitive optical coherence tomography that facilitates the unrestricted use of fiber and fiber-optic components throughout an interferometer and yields sample birefringence, diattenuation, and relative optic axis orientation. We use a novel Jones matrix approach that compares the polarization states of light reflected from the sample surface with those reflected from within a biological sample for pairs of depth scans. The incident polarization alternated between two states that are perpendicular in a Poincaré sphere representation to ensure proper detection of tissue birefringence regardless of optical fiber contributions. The method was validated by comparing the calculated diattenuation of a polarizing sheet, chicken tendon, and muscle with that obtained by independent measurement. The relative importance of diattenuation versus birefringence to angular displacement of Stokes vectors on a Poincaré sphere was quantified.     

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.     

In vitro birefringence imaging with spectral domain polarization-sensitive optical coherence tomography

Spectral domain polarization-sensitive optical coherence tomography （SDPS-OCT） is a depth-resolved polarization-sensitive interferometry which integrates polarization optics into spectral domain optical co- herence tomography （SD-OCT）. This configuration can obtain birefringence information of samples and improve the imaging speed. In this paper, horizontally polarized light is used to replace natural light of the source. Then, right-rotated circularly polarized light is the incident sample light. To obtain two orthogonal components of the polarized interferogram, the reflected light of the reference arm is set to be 45° linearly polarized light. These two components are acquired by two spectrometers synchronously. The system was employed to achieve 12.8μm axial resolution and 4.36μm transverse resolution. We have imaged in vitro chicken tendon and muscle tissues with these svstem.     

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.     

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.     

Simulation of polarization-sensitive optical coherence tomography images by a Monte Carlo method

We introduce a new Monte Carlo (MC) method for simulating optical coherence tomography (OCT) images of complex multilayered turbid scattering media. We demonstrate, for the first time of our knowledge, the use of a MC technique to imitate two-dimensional polarization-sensitive OCT images with nonplanar boundaries of layers in the medium like a human skin. The simulation of polarized low-coherent optical radiation is based on the vector approach generalized from the iterative procedure of the solution of Bethe-Saltpeter equation. The performances of the developed method are demonstrated both for conventional and polarization-sensitive OCT modalities.     

Two-dimensional depth-resolved Mueller matrix of biological tissue measured with double-beam polarization-sensitive optical coherence tomography

A double-beam polarization-sensitive system based on optical coherence tomography was built to measure the Mueller matrix of scattering biological tissue with high spatial resolution. The Jones matrix of a sample can be determined with a single scan and subsequently converted into an equivalent nondepolarizing Mueller matrix. As a result, the system can be used to measure the Mueller matrix of an unstable sample, such as soft tissue. The polarization parameters of a porcine tendon, including magnitude and orientation of birefringence and diattenuation, were extracted by decomposition of the measured Mueller matrix.     

Three-dimensional optic axis determination using variable-incidence-angle polarization-optical coherence tomography

Polarization optical coherence tomography (PSOCT) is a powerful technique to nondestructively map the retardance and fast-axis orientation of birefringent biological tissues. Previous studies have concentrated on the case where the optic axis lies on the plane of the surface. We describe a method to determine the polar angle of the optic axis of a uniaxial birefringent tissue by making PSOCT measurements with a number of incident illumination directions. The method is validated on equine flexor tendon, yielding a variability of 4% for the true birefringence and 3% for the polar angle. We use the method to map the polar angle of fibers in the transitional region of equine cartilage.     

A fiber-based single-unit dual-mode optical Imaging system: Swept source optical coherence tomography and fluorescence spectroscopy

We propose a fiber optic single-unit but dual-mode optical imaging system that can provide fast cross-sectional imaging capabilities of swept-source optical coherence tomography (SS-OCT) and functional capabilities of fluorescence spectroscopy (FS). By adopting a fiber optic FS system into a fiber-based SS-OCT system, a compact and effective multimodal single-unit SSOCT-FS system is achieved. Here, the key element of the proposed multimodal imaging system is a specially designed fiber coupler based on double-clad fiber (DCF), which has only cladding-mode coupling capability. The DCF couplers are fabricated with home-drawn DCF by several fabrication methods; a twisting method, a side-polishing method and a fused biconical tapered (FBT) method. Experimentally, the FBT method provides rather flat cladding mode coupling efficiency over 40% in a wide wavelength range. With this specially designed DCF coupler, the OCT signal and the fluorescence signal is measured independently but with a single-unit system. The performance of the SSOCT-FS system is confirmed by measuring the cross-sectional image and the fluorescence signal of a photosensitizer chlorin e6 injected in-vivo rat tumor model.     

Simultaneous intensity, birefringence, and flow measurements with high-speed fiber-based optical coherence tomography

We demonstrate that tissue structure, birefringence, and blood flow can be imaged simultaneously by use of techniques of polarization-sensitive optical coherence tomography and phase-resolved optical Doppler tomography. An efficient data-acquisition procedure is implemented that optimizes the concurrent processing and display of all three image types. Images of in vivo human skin acquired with a high-speed fiber-based system are presented.     

Invivo depth-resolved birefringence measurements of the human retinal nerve fiber layer by polarization-sensitive optical coherence tomography

To our knowledge, this is the first demonstration of in vivo depth-resolved birefringence measurements of the human retinal nerve fiber layer (RNFL) by use of polarization-sensitive optical coherence tomography (PS-OCT). Because glaucoma causes nerve fiber layer damage, which may cause loss of retinal birefringence, PS-OCT is a potentially useful technique for the early detection of glaucoma. We built a fiber-based PS-OCT setup that produces quasi-real-time images of the human retina in vivo . Preliminary measurements of a healthy volunteer showed that the double-pass phase retardation per unit depth of the RNFL near the optic nerve head is 39+/-6( degrees )/100microm .     

Velocity-estimation accuracy and frame-rate limitations in color Doppler optical coherence tomography

Color Doppler optical coherence tomography (CDOCT) is a recent innovation that allows spatially localized flow-velocity mapping simultaneously with microstructural imaging. We present a theoretical model for velocity-image formation in CDOCT. The proportionality between the heterodyne detector current Doppler power spectrum in CDOCT and the optical source power spectrum is established. We show that stochastic modifications of the Doppler spectrum by fluctuating scatterer distributions in the flow field give rise to unavoidable velocity-estimation inaccuracies as well as to a fundamental trade-off between image-acquisition rate and velocity precision. Novel algorithms that permit high-fidelity depth-resolved measurements of velocities in turbid media are also reported.     

Forward-imaging instruments for optical coherence tomography

We discuss the design and implementation of forward-imaging instruments for optical coherence tomography (OCT), which require the delivery, scanning, and collection of single-spatial-mode optical radiation. A hand-held surgical probe for use in open surgery can provide cross-sectional images of subsurface tissue before surgical incisions are made. A rigid laparoscope for minimally invasive surgical OCT imaging provides a simultaneous enface view of the area being imaged. OCT imaging is demonstrated on in vitro human specimens.     

Imaging needle for optical coherence tomography

We describe a miniature optical coherence tomography (OCT) imaging needle that can be inserted into solid tissues and organs to permit interstitial imaging of their internal microstructures with micrometer scale resolution and minimal trauma. A novel rotational coupler with a glass capillary tube is also presented that couples light from a rotating single-mode fiber to a stationary one. A prototype needle with a 27-gauge (~410-mum) outer diameter has been developed and is demonstrated for in vivo imaging. The OCT needle can be integrated with standard excisional biopsy devices and used for OCT-guided biopsy.     

Spectroscopic optical coherence tomography

Spectroscopic optical coherence tomography (OCT), an extension of conventional OCT, is demonstrated for performing cross-sectional tomographic and spectroscopic imaging. Information on the spectral content of backscattered light is obtained by detection and processing of the interferometric OCT signal. This method allows the spectrum of backscattered light to be measured over the entire available optical bandwidth simultaneously in a single measurement. Specific spectral features can be extracted by use of digital signal processing without changing the measurement apparatus. An ultrabroadband femtosecond Ti:Al(2)O(3) laser was used to achieve spectroscopic imaging over the wavelength range from 650 to 1000 nm in a simple model as well as in vivo in the Xenopus laevis (African frog) tadpole. Multidimensional spectroscopic data are displayed by use of a novel hue-saturation false-color mapping.