Proposal and Fabrication of Negative-Type Refractive Index Distribution Polymer Optical Fiber

Kenji Tsukada, Yumi Nakagawa, Kouichi Asakura, Eisuke Nihei

In this paper, we propose a new type of graded index polymer optical fiber (GI-POF). Most of reported fibers have a convex-type refractive index distribution, where the refractive index is highest at the central axis and gradually decreases as it approaches the cladding region. The fiber presented in this research has a special refractive index distribution that is rarely found in any of the reported polymer optical fibers. We call its distribution “negative refractive index distribution” where the refractive index is lowest around the central axis and highest at the core-cladding boundary.

In this study, we first determined the transmission bandwidth property of the proposed fiber by ray trace simulation. It became clear that with an optimal refractive index distribution, these fibers could have the same transmission bandwidth as conventional refractive index distribution (convex) type fibers.

Furthermore, we were able to show that the proposed fibers could realize easier connecting/branching than conventional fibers because its light intensity becomes highest near the core-cladding boundary. This is verified by simulation of optical branching. These fibers have potential for practical applications such as in high-sensitivity sensors. In addition, when negative refractive index distribution type polymer optical fiber is uniformly excited at incident surface, it is likely that bending loss will not occur because more of the lower-order modes are excited than the higher-order modes.

In this study, we succeeded in actual fabrication of negative refractive index distribution type polymer optical fibers. This fiber is fabricated by two independent means, namely, multi-step interfacial gel polymerization technique and UV-assisted frontal polymerization technique. We measured its refractive index distribution and how much its transmission pulse widened. When pulsed light with full width at half maximum of 140 ps was transmitted for 4.7 m, the pulse widened to 142 ps. This verifies that its band property is sufficient for practical use in short-distance transmission.