基于圆环达曼光栅的长焦深光学相干层析成像系统

An Extended-Focus Optical Coherence Tomography System Based on Circular Dammann Grating

圆环达曼光栅(CDG)产生的贝塞尔光束照明可以扩展光学相干层析成像(OCT)系统的焦深,使得系统在较大成像深度范围均保持高的横向分辨率。但CDG一级衍射环的分裂效应会导致贝塞尔光束轴向能量分布出现凹陷,削弱了焦深扩展的效果。本研究团队在分析CDG衍射原理的基础上,优化设计了无一级衍射环分裂效应的CDG,并通过仿真证明了该CDG产生的贝塞尔光束不存在轴向能量凹陷,具有更好的焦深扩展效果;然后将优化设计的CDG应用于搭建的OCT系统中,采用该系统对聚苯乙烯微球琼脂凝胶、白胶带等样品进行成像。结果表明,该OCT系统可以在1.8 mm的焦深范围内获得优于3.9μm的横向分辨率,有效扩展了系统焦深。

Objective Optical coherence tomography(OCT)has been widely used in clinical medicine,materials science,tissue engineering,among other fields due to its advantages of non-destructive,non-contact,high speed,and high resolution.In the traditional OCT system,which is based on Gaussian beam illumination,the lateral resolution is determined by the size of the focused spot of the Gaussian beam.However,the smaller the focused spot,the shorter the depth of focus,and the rapid drop of the lateral resolution outside the depth of the focus area.Therefore,improving the depth of the focus,while maintaining a high lateral resolution is one of the important problems to be solved in a high-resolution OCT system.Bessel beam is used to extend the depth of the focus of the OCT system because of its non-diffracting property.Bessel beams can be generated by the optimal solution of the first-order circular Dammann grating(CDG),and the depth of the focus can be extended effectively,while the diffraction efficiency can reach 81%.However,in the existing CDG design,the splitting effect of the first-order diffraction ring will lead to a zero-energy dent in the axial direction of the central spot of the Bessel beam,resulting in a short depth of focus.To solve this problem,this paper proposes a depth of focus extension method,which eliminates the axial energy dent of Bessel beam generated by CDG.Moreover,it can effectively reduce the splitting effect of the first-order diffraction ring,improve the uniformity of axial intensity distribution within the depth of the focus range of Bessel beam,and realize the depth of the focus extension of the OCT system.Methods In this paper,by optimizing the radius of the central circle of the CDG and the ratio of the binary phase in a single period,we design a CDG for generating Bessel beams that eliminate the energy dent,and apply it to the OCT system,which effectively extends the depth of the focus of the system.Based on the principle of the CDG diffraction,the CDG parameters that meet the requirement of eliminating the first-order diffraction ring splitting are derived and the optimal CDG design is obtained.We use the software simulation to prove the feasibility of the design.Based on our simulation,we set up a swept-source OCT system with the CDG.The system is used to imaging the samples of polystyrene microspheres embedded in agarose gel and multilayer white tape to verify the effectiveness of the focal depth extension and the imaging capability of the system.Results and Discussions The energy dent in the depth of focus of the Bessel beam generated by CDG diffraction is eliminated,and the axial intensity uniformity is improved[Fig.8 (c),(d)].Based on the CDG design,we built a swept-source OCT system,the axial resolution in the air is 8.24μm (Fig.11).Imaging results of the polystyrene microspheres measured by the system show that the system can achieve a lateral resolution of 3.9μm over the depth of the focus range of 1.8mm (Fig.12).Additionally,the system is used to image human nails and multilayer white tape samples.In OCT imaging of human nails,the boundary between the cuticle and nail can be clearly distinguished,and the nail extending under the skin can be observed[Fig.13 (a)].Furthermore,the optical thickness of the multilayer white tape sample is about 2mm.The layered structure of the multilayer white tape can be clearly distinguished in the OCT image,and the air layer between the white tape and the upper surface of the glass slide can be observed[Fig.13(b)].Conclusions Based on the diffraction principle of CDG,the energy splitting effect of the first-order diffraction ring of the CDG is reduced by optimizing the central circle radius of the CDG and the ratio of the binary phase in a single period.On the premise of maintaining a higher first-order diffraction efficiency,the axial energy dent of Bessel beam generated by the CDG is eliminated,and the depth of the focus is effectively extended.A swept-source OCT system based on Bessel beam illumination is built by the optimized CDG.By measuring polystyrene microspheres embedded in the agarose gel,it is proved that the system can achieve a lateral resolution better than 3.9μm over the depth of the focus range of 1.8mm,which is consistent with the calculated theoretical result of the depth of the focus and lateral resolution.By using the system,clear tomographic images of human nails,white tape,and other samples were obtained,which verified the imaging ability of the system.