NTT, in partnership with Fujikura and Hokkaido University, claim they have demonstrated the "world's highest density optical fiber" using a deployable optical fiber diameter of less than 250 micrometers. During the demonstration, NTT used an optical fiber that contained 19 optical paths (cores) that can support six kinds of optical signals (modes), providing 114 spatially multiplexed communication paths (channels) in one optical fiber.
NTT said that a glass diameter of less than 250 micrometers ensures that optical fiber has enough reliability, meaning that it won't break. The ability to accommodate 114 channels in one optical fiber enables NTT to transfer thousands of HDTV videos per second. NTT's work was reported in March as a post-deadline paper at the Optical Fiber Communication Conference and Exposition (OFC 2016). phys.org article