Holographic thin-beam reconstruction technique for the study of 3-D refractive-index field

A thin-beam holographic reconstruction technique to study the 3-D refractive index field of a phase object is described. The advantages and drawbacks of this method are discussed.     

Dispersion-based optical coherence tomography OCT measurement of mixture concentrations

We present a low-coherence technique for the measurement of concentrations in aqueous solutions. It is based on a second-order dispersion measurement using the phase of the Fourier transform of the interferogram term obtained with an unbalanced Michelson interferometer. We demonstrate this approach by concentration measurements of aqueous mixtures of NaCl and glucose solutions. Results show errors between a few percent for high concentrations, up to 31% for low concentrations. The application of such a technique in OCT might require ultrahigh depth resolution and/or other scattered photon rejection techniques.     

Linear optical coherence tomography system with extended measurement range

Optical coherence tomography (OCT) sensors traditionally use scanning optical delay lines with moving parts and a single detector. OCT systems with a linear detector array (linear OCT or L-OCT) are simple and robust, but a detector with approximately 10,000 pixels is needed for an imaging depth of 2mm, which is necessary for most biomedical applications. We present a new optical setup for L-OCT with an increased measurement range. An additional grating performs a reduction of the spatial frequencies of the fringe pattern on the detector without loss in the signal-to-noise ratio, so the signal can be sampled with a minimal number of pixels. The theory for this approach is addressed and the first measurements are presented.     

Spectral measurement of absorption by spectroscopic frequency-domain optical coherence tomography

A new method of measurement that essentially combines Fourier-domain optical coherence tomography with spectroscopy is introduced. By use of a windowed Fourier transform it is possible to obtain, in addition to the object structure, spectroscopic information such as the absorption properties of materials. The feasibility of this new method for performing depth-resolved spectroscopy is demonstrated with a glass filter plate. The results are compared with theoretically calculated spectra by use of the well-known spectral characteristics of the light source and the filter plate.     

Quantification of a three-dimensional velocity vector using spectral-domain Doppler optical coherence tomography

Multiangle, fiber-based, spectral-domain Doppler optical coherence tomography with a phase-resolved algorithm is presented to measure three components of an arbitrary velocity vector. A beam divider that divides a probe beam to have five independent viewpoints and path length delays was designed. The divider was inserted into the sampling arm of a Doppler optical coherence tomography system between the collimator and the first galvo mirror of a two-axis galvo scanner. The divider produced five independent D k's (the average difference between the wave vectors of incoming and outgoing beams) after passing through the focusing lens while keeping two-axis scanning capability. After calibration, an unknown velocity vector field inside a microtube was quantified by solving a three-dimensional minimization problem.     

The characteristics of three-dimensional skin imaging system by full-colored optical coherence tomography

In the present cosmetic market, the skin image obtained from a hand-held camera is two-dimensional (2-D). Due to insufficient penetration, only the skin surface can be detected, and thus phenomena in the dermis cannot be observed. To take the place of the conventional 2D camera, a new hand-held imaging system is proposed for three-dimensional (3-D) skin imaging. Featuring non-invasiveness, optical coherence tomography (OCT) has become one of the popular medical imaging techniques. The dermal images shown in OCT-related reports were mainly single-colored because of the use of a monotonic light source. With three original-colored beams applied in OCT, a full-colored image can be derived for dermatology. The penetration depth of the system ranges from 0.43 to 0.78 mm, sufficient for imaging of main tissues in the dermis. Colorful and non-invasive perspectives of deep dermal structure help to advance skin science, dermatology and cosmetology.     

Silicon V-shaped groove measurement by spectral domain optical coherence tomography

In this research, we focuses on a new detection method for testing a micro electro mechanical systems V-shaped groove in silicon using spectral domain optical coherence tomography with a thermal light source. By this method, the V-shaped groove depth, width and bottom width were measured. The technique, because of its noncontact operation and high acquisition rate, is suitable for on-line precision measurement of V-shaped grooves in silicon.     

Film thickness measurement and defect inspection using optical coherence tomography

An optical sensing technology based on optical coherence tomography is presented for film thickness measurement and defect inspection. In order to improve the imaging quality, a simple interference spectrum processing procedure is proposed to eliminate the DC and the autocorrelation noise. With the proposed method, we obtain high quality one-dimensional depth and two-dimensional cross-sectional images of the films. Then, the thickness and the defect information of the film can be obtained from the acquired images. The experiment result demonstrates that this nondestructive imaging technique is applicable for measuring film thickness and inspecting defects.     

聚焦法测量梯度折射率透镜的折射率分布

本文针对梯度折射率分布的透镜(以后简称梯析透镜)与光纤在折射率分布上的不同点,对用于光纤及其预制棒测量的聚焦法的原理公式,计算测量方法等进行了重要改进,从而使聚焦法可适用于梯折透镜的测量.本文通过计算机模拟计算,对原理公式及计算方法的准确性和可靠性进行了验证,并同时给出了这一测量方法的精度,最后给出了测量实例及其比较结果.     

Spectral domain optical coherence tomography with sub-micrometer sensitivity for measurement of central corneal thickness

We demonstrated a method for measurement of central corneal thickness(CCT) with a sub-micrometer sensitivity using a spectral domain optical coherence tomography system without needing a super broad bandwidth light source. By combining the frequency and phase components of Fourier transform, the method is capable of measurement of a large dynamic range with a high sensitivity. Absolute phases are retrieved by comparing the correlations between the detected and simulated interference fringes. The phase unwrapping ability of the present method was quantitatively tested by measuring the displacement of a piezo linear stage. The human CCTs of six volunteers were measured to verify its clinical application. It provides a potential tool for clinical diagnosis and research applications in ophthalmology.     

扫频光学相干层析成像系统的波数校正与相位测量研究

相位敏感光学相干层析成像(OCT)系统可以用于高灵敏度的相位探测,在细胞分析、材料检测等方面具有重要应用,但扫频光源的不稳定性会影响扫频OCT系统的相位测量精度.本文提出了一种基于马赫-曾德尔干涉仪(MZI)时域相位信息的波数校正方法.利用MZI时域包裹相位的互相关运算确定各采集波数序列的相对偏移量,鉴于时域包裹相位的非严格周期特征,可确定偏移量的大小不受限制.依据相对偏移量对各序列信号进行时域同步,并基于同步后的MZI时域解包裹相位实施待测干涉信号在位相域的等间隔重采样.基于所提出的波数校正方法,实施了各扫频序列波数偏移量的校正,开展了基于位相信息的光程重复性测量实验.结果表明,即使在不稳定扫频光源的前提下,也能获得高精度的相位测量结果.     

Sampled fiber Bragg grating for simultaneous refractive-index and temperature measurement

We present a novel scheme for simultaneous measurement of temperature and refractive index by use of a single sampled fiber Bragg grating (SFBG). The intrinsic nature of a SFBG that possesses both fiber Bragg grating and long-period grating spectral responses permits the temperature effect to the measured solely from the former, whereas the refractive-index information is extracted from the latter. Using such a dual-parameter sensor, we successfully demonstrated measurement of the dependence on temperature of the refractive index of an aqueous solution of sucrose.     

In vivo measurement of retinal physiology with high-speed ultrahigh-resolution optical coherence tomography

Noninvasive in vivo functional optical imaging of the intact retina is demonstrated by using high-speed, ultrahigh-resolution optical coherence tomography (OCT). Imaging was performed with 2.8 microm resolution at a rate of 24,000 axial scans per second. A white-light stimulus was applied to the dark-adapted rat retina, and the average reflectivities from different intraretinal layers were monitored as a function of time. A 10%-15% increase in the average amplitude reflectance of the photoreceptor outer segments was observed in response to the stimulus. The spatial distribution of the change in the OCT signal is consistent with an increase in backscatter from the photoreceptor outer segments. To our knowledge, this is the first in vivo demonstration of OCT functional imaging in the intact retina.     

Variable coherence tomography

By adjusting the spatial coherence of a quasi-monochromatic beam, one can control the distance between two volumes of coherence. We propose to use such a beam in a typical scattering experiment and present a method for determining the degree of spatial correlation of a quasi-homogeneous medium by recording the scattered intensity in only one direction.     

Optical coherence tomography

Optical Coherence Tomography (OCT) is a new technology for performing high-resolution cross sectional imaging. OCT is analogous to ultrasound imaging, except that it uses light instead of sound. OCT can provide cross sectional images of tissue structure on the micron scale in situ and in real time. OCT functions as a type of optical biopsy and is a powerful imaging technology for medical diagnostics because unlike conventional histopathology which requires removal of a tissue specimen and processing for microscopic examination, OCT can provide images of tissue in situ and in real time. OCT can be used where standard excisional biopsy is hazardous or impossible, to reduce sampling errors associated with excisional biopsy, and to guide interventional procedures.     

Holographic measurement of high viscosities

We describe a holographic method for measuring very high viscosities, up to 10¹⁵ poise, or higher. The method consists of detecting small displacements, due to the sample deformation, by the holographic double exposure technique. Measurements performed on amorphous selenium are reported and compared with those obtained with the usual technique, showing that our method gives the correct results. The advantages of the proposed technique are briefly discussed.     

Holographic measurement of reverberation time

A system employing the radio frequency holographic method for the measurement of the reverberation time of an acoustic chamber is proposed. This system has the advantage of supplying visible information on the diminishing intensity of the acoustic wave. A simple mathematical model has been taken to discuss the basic principles involved in the system. A possible experimental set-up for the measurement of the reverberation time is described in detail.     

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.     

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.