Chen Xianpei, Reichenbach Kristen Lantz, Xu Chris
School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA.
Opt Express. 2008 Dec 22;16(26):21598-607. doi: 10.1364/oe.16.021598.
Flexible endoscopes commonly use coherent fiber bundles with high core density to facilitate in vivo imaging. Small, closely spaced cores are desired for achieving a high number of resolvable pixels in a small diameter fiber bundle. On the other hand, closely spaced cores potentially lead to strong core-to-core coupling. Based on numerical simulations, it was previously explained that image fiber bundles can successfully transmit images because of nonuniformities in the core size that reduce coupling. In this paper, we show numerically and experimentally that, due to the randomness of the structural nonuniformity, significant core-to-core coupling still exists in fiber bundles that are routinely used for imaging. The coupling is highly dependent on the illumination wavelength and polarization state. We further show that the resolution achievable by a fiber bundle depends not only on the core density, but also on the inter-core coupling strength. Finally, we propose that increasing the core-cladding index contrast is a promising approach to achieve a fiber bundle with low core coupling, high core density, and effectively single moded propagation in individual cores.
柔性内窥镜通常使用具有高纤芯密度的相干光纤束以利于体内成像。为了在小直径光纤束中实现大量可分辨像素，需要小的、间距紧密的纤芯。另一方面，间距紧密的纤芯可能导致强烈的芯间耦合。基于数值模拟，之前曾解释图像光纤束能够成功传输图像是因为纤芯尺寸的不均匀性减少了耦合。在本文中，我们通过数值和实验表明，由于结构不均匀性的随机性，在常规用于成像的光纤束中仍然存在显著的芯间耦合。这种耦合高度依赖于照明波长和偏振态。我们进一步表明，光纤束可实现的分辨率不仅取决于纤芯密度，还取决于芯间耦合强度。最后，我们提出增加纤芯 - 包层折射率对比度是实现具有低芯间耦合、高纤芯密度且在各个纤芯中有效单模传播的光纤束的一种有前景的方法。
