基于单相机的微米谱域偏振光学相干层析成像方法

Micron Polarization-sensitive Spectral-domain Optical Coherence Tomography Based on Single Camera

提出一种基于单相机的偏振谱域光学相干层析方法。该方法基于全单模光纤光路,使用宽带光源实现微米纵向高分辨率成像性能。通过硬件和软件两方面对系统色散进行矫正补偿,提高系统的实际分辨率。为实现样本偏振态的测量,入射至样品表面及参考臂的光偏振态由偏振片和四个偏振控制器进行调制。此外,利用偏振片在不同时间的通道切换实现两正交通道光谱信号的分时探测。根据得到的正交光谱信息,计算重构样品强度图和相位延迟图。该系统成功实现了单相机的偏振OCT测量,通过获取离体生物组织的强度图和相位延迟图验证了系统的成像能力。该方法为实现小型化在体高分辨偏振成像打下基础。

Optical Coherence Tomography(OCT)is an interferometric imaging method,and it is mostly used in the field of imaging for its non-invasive,high-resolution and high-speed properties.This technology can also be applied to detect defects in materials.Although OCT can provide images of morphological structure,it can not distinguish tissues with similar light intensity properties in pathologies.Polarization-sensitive Optical Coherence Tomography(PS-OCT)is an imaging system extended from conventional OCT,enabling functional imaging.It can use Stokes parameters,Jones and Muller matrices to calculate the polarization properties of the samples,like the birefringence phase retardation,the optic axis orientation and depolarization.PS-OCT has been used in a number of medical applications,such as burn depth determination and tumor yield assessment.And it also can be applied to examination of stress-induced birefringence of materials.In previous systems,it is required to detect the two orthogonally polarized components by using dual cameras based spectrometers.But there are problems with this system arrangement.For example,it has high cost and requires complex hardware and software designs.In addition,it is hard to achieve the uniform detection for two cameras,the mismatch between the two channels can lead to polarization distortions and failure to calculate the true information and additional algorithms are necessary to tackle it.So a series of single cameras based methods have been proposed.Single cameras based systems can achieve time-sharing detection or real-time detection of two orthogonal channels with relatively lower cost and simpler system setup.Wollaston prism,optical switch,grating and multi-camera are often used to achieve single camera detection.Improving the axial resolution of the system can enable it to have more potential applications.The OCT system with micron axial resolution can achieve cellular and subcellular level imaging and detect subsurface defects in ceramics or other materials.In order to achieve such a high axial resolution,super-continuum light source is generally used to increase the bandwidth of imaging.Because the optical path of the reference arm and the sample arm are not completely symmetrical,as the spectral bandwidth increases,the system can introduce serious dispersion and affect axial resolution.In this paper,we demonstrate a polarization-sensitive spectral domain optical coherence tomography imaging system using a single camera with micron axial resolution.It is an all single-mode fiber-based system,and a broad bandwidth light source is used to achieve micron axial resolution.In order to increase actual axial resolution of the system,the system dispersion effects are compensated by using both hardware and software methods.After compensating the first order and second order dispersion,the measured axial resolution of the system is about 1.61μm for the sample with an approximated refractive index of 1.4.In order to realize the measurement of the polarization state of the light reflected from the sample,the polarization state of light incident on the sample surface and the reference arm is modulated by the polarizer and four polarization controllers.The horizontal and vertical polarization interference signals are separately measured via channel switching of a polarizer and they can achieve time-sharing detection by using only one camera.Intensity and phase retardation information of the samples can be calculated by the signals obtained from the two channels at different times.To verify the capability of our system to measure the polarization information,we succeed in polarization imaging with a single camera and obtaining the images of intensity and polarization parameter contrast of the biological tissue in vitro by using Stokes vector.From the retardation image of bovine tendon at different position,it can be obviously observed that the phase retardation varies periodically with the increase of the depth in the tissue.This method is characterized by its simple system arrangement and lay the basis for miniaturizing in vivo high resolution polarization parameter imaging.