The high-density FAU comprises a support substrate having a grooved front-end section that supports glass end sections of the small diameter low-attenuation optical fibers. A cover is disposed on the front-end section and secured thereto to hold the glass end sections in place. The substrate and the cover can be made of the same glass or glasses having about the same CTE. The glass end sections have a diameter d4 so that the pitch P2 of the fibers at the front end of the FAU can be equal to or greater than d4, wherein d4=2r4, with r4 being the radius of the glass end section as defined by the optical fiber cladding. The glass end section has a radius r4 less than 45 microns, allowing for a high-density FAU and a high-density optical interconnection device.