Appl Opt. 2021 Mar 10;60(8):2393-2399. doi: 10.1364/AO.420591.
The diagnostic capability of high-resolution optical coherence tomography (OCT) may be enhanced by using extended depth-of-field (EDOF) imaging that retains high transverse resolution over long depths. A recently developed mirror-tunnel optical probe design (single-mode fiber to multimode fiber to lens structure) that generates coaxially focused modes has been previously shown to enable EDOF for endoscopic OCT applications. Here, we present ray-tracing optical modeling of this optical configuration, which has the potential to guide performance improvement through optimization. The Huygens wave propagation of the field was traced through probe components with initial lengths. The irradiance along the - plane was analyzed, yielding an average full width at half-maximum (FWHM) of 9 µm over a 640 µm DOF (defined as the axial range over which the beam's transverse FWHM is maintained). A custom merit function was then defined, based on the focal region illumination intensity profile that yielded the maximum possible depth having a constrained FWHM size. An orthogonal gradient descent optimization algorithm was then applied using this merit function, using the multimode fiber, spacer, and lens lengths as variables. Optimization resulted in a modeled mean 6 µm FWHM spot diameter over an EDOF of 1 mm. Following optimization, a probe was fabricated, and was validated using a custom-built near-field scanning pinhole beam profiler. The experimental results (6 µm mean FWHM over 800 µm EDOF) showed reasonable correspondence to the simulated predictions, demonstrating the potential utility of optical modeling and optimization for improving EDOF performance in mirror-tunnel endoscopic OCT probes.
高分辨率光学相干断层扫描（OCT）的诊断能力可以通过使用扩展景深（EDOF）成像来增强，这种成像在长深度范围内保持高横向分辨率。最近开发的镜筒光学探头设计（单模光纤到多模光纤到透镜结构），能够产生共轴聚焦模式，以前已经证明可用于内窥镜 OCT 应用的 EDOF。在这里，我们提出了这种光学配置的光线追踪光学建模，它有可能通过优化来指导性能的提高。通过初始长度的探头组件跟踪场的惠更斯波传播。分析了 - 平面上的辐照度，在 640 µm 的景深（定义为光束横向 FWHM 保持的轴向范围）内获得了 9 µm 的平均半最大值全宽（FWHM）。然后，基于聚焦区域照明强度分布定义了一个自定义优点函数，该函数产生具有约束 FWHM 尺寸的最大可能深度。然后使用此优点函数应用正交梯度下降优化算法，将多模光纤、间隔物和透镜长度作为变量。优化结果是在 1mm 的 EDOF 上模拟出的平均 6 µm FWHM 光斑直径。优化后，制造了一个探头，并使用定制的近场扫描针孔光束轮廓仪对其进行了验证。实验结果（800 µm EDOF 上的 6 µm 平均 FWHM）与模拟预测具有很好的对应关系，证明了光学建模和优化在提高镜筒内窥镜 OCT 探头 EDOF 性能方面的潜在效用。
