Depolarization characteristics of spatial modes in imaging probe using short multimode fiber

Optical coherence tomography (OCT) is one of the standard imaging modalities at present, that is widely used in the medical and biological fields, to obtain three-dimensional (3D) images with high spatial resolution. However, the depth up to which the 3-D images can be directly obtained is limited to within 3 mm. Therefore, the suitability of many kinds of catheters and needles has been considered for minimally invasive imaging. We have examined the utility of a short multimode fiber (SMMF) using graded index optical fibers for minimal invasive imaging of deeper areas, up to 6–8 mm. The diameter and length of the SMMF are 125 μm and 6–8 mm, respectively. In the core of the SMMF, scattering and multi-refraction occur due to small variations in the refractive index to generate deforms and depolarization of images. In order to investigate the depolarization characteristics, the images reflected at the facet of the SMMF were measured by changing the angle of the polarizer, using an LED as the light source. The reflection image almost corresponds to that obtained with combined linearly polarized (LP) modes with the ratio of LP01 : LP11 : LP21 equal to 1 : 0.2 : 0.7. Comparing the measured results with simulations in the simple model, the depolarization ratio was estimated at 0.7 in the core. The degrees of polarization were measured to be 0.15 around the center and increased to 0.90 at the periphery.