Polarization-Sensitive and Intensity Dual-Channel Optical Coherence Tomographic Method
Objective Polarization-sensitive optical coherence tomography(PSOCT)can noninvasively obtain depth-resolved optical tomographic images of biological tissue samples.PSOCT is based on OCT with the additional function of detecting changes in the polarization states of light backscattered from different depths after polarized light is incident on the sample.It can provide polarization-related information about a sample,such as the Stokes parameter,Jones and Mueller matrix,phase delay,and depolarization,and it can distinguish structural specificities that OCT contrast cannot.PSOCT can be applied to detect changes in the function,structure,and activity of human tissues,which has major application prospects in medical diagnosis.In conventional PSOCT,the interference signals from the interferometer output are measured using horizontal and vertical polarization channels,thus requiring two separate spectrometer cameras.This increases the size of the system,adds additional costs,and requires strict triggering of hardware and software to avoid any time delay between the signal acquisition of the two cameras under the phase-based PSOCT algorithm.It also requires high response consistency of the charge-coupled device(CCD).Accordingly,PSOCT technology that utilizes a single spectrometer is currently under development.Some existing PSOCT systems based on a single camera can achieve only time-sharing or real-time detection.In this work,a PS and intensity dual-channel OCT measurement method is presented that can realize both time-sharing and real-time detection,thus providing a new method for laboratories in the analysis of the polarization information of samples.Methods In this study,a theoretical model of a PSOCT system with dual reference arms for a single camera is first established.Based on the traditional spectral-domain OCT(SDOCT)system,an additional reference arm is introduced,where the function of the two reference arms is to provide a pair of orthogonally polarized lights.A neutral filter is added to the other two reference arms to attenuate the light intensity and to ensure that the intensity of the light returned by the reference arm does not exceed the CCD acquisition threshold.A linear polarizer is added to the light source exit module,and four manual polarization controllers are added to the optical path.The system can realize the conversion between SDOCT and PSOCT imaging.A dual-reference arm detection system based on a single-mode fiber and polarization controller is then constructed.The quarter-wave plate in the existing SDOCT system is replaced with a single-mode fiber and polarization controller.The polarization module function is then expanded,and micron longitudinal high-resolution imaging performance is achieved using a broadband light source.The signals from the horizontal and vertical channels are collected using a single camera in real time.A Jones matrix model of the system is established,and the obtained tomography images are processed by combining the Stokes parameters.Relative reflectance,phase delay,and depolarization information are obtained to reconstruct the intensity,phase delay,and depolarization maps of the in vitro biological tissues,further verifying the feasibility of the proposed PS and intensity dual-channel OCT method as well as the imaging capabilities of the established system.Results and Discussions An OCT system is established to realize the conversion of PSOCT and SDOCT as well as the functions of single-channel time-sharing detection and dual-channel real-time detection(Fig.1).A polarization characteristic model of the system is developed,and the calculation method for the sample polarization parameters is analyzed.Results show that a cross-sectional strength diagram of the sample tissue is obtained by processing the signals detected from the two orthogonal channels.The collected signal strength is then solved along the depth direction,and the defect polarization information is extracted using the Jones matrix and Stokes operation.The birefringence and depolarization parameters of the biological tissue as well as images of the polarization parameters of the biological tissue samples are obtained.The strength,phase delay,and depolarization of the sample beef(Fig.2)and chicken(Fig.4)tendons are reconstructed using time-sharing and real-time detection methods,demonstrating the feasibility of the system scheme and verifying its dual-channel detection capabilities.Conclusions The polarization characteristics of biological tissues can reveal the unique characteristics of biological tissues that cannot be described by isotropic properties,including tissue birefringence and depolarization.Depth-resolved isotropic intensity images and anisotropic polarization parameter images of biological tissue samples can be reconstructed noninvasively using a micron-resolution spectral domain PSOCT system.In this study,a dual-channel PSOCT system is proposed.The system is based on an optical fiber and single camera and has an imaging rate of 104 frame/s.An ultra-wideband light source,large numerical aperture objective lens,and common optical path structure are used to achieve horizontal and vertical resolutions at the micron level,ensuring the detection of tiny structures in tissue samples.The signals of the two channels can be collected at different moments such that horizontally and vertically polarized light can be incident on the same spectrometer at adjacent time to realize time-sharing detection.The interference signals of the two orthogonal channels can be collected simultaneously to achieve real-time detection.The images of the two channels are separated on both sides of the CCD,and the CCD can simultaneously contain the spectral interferograms formed by the horizontal and vertically polarized beam components.The proposed polarization imaging system provides a new method for the practical application of PSOCT and SDOCT in clinical diagnosis.
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