Ultra Compact and Broadband Fiber Facet Graphene Oxide Microlens Array Fabricated on a Beam-Expanding Fiber Facet Using Femtosecond Laser Direct Writing Technique.

We demonstrate an ultra-compact fiber facet graphene oxide (GO) microlenses array fabricated through femtosecond laser direct writing (FLDW). Taking advantage of the broadband transparency, mechanical, and laser-tunable optical properties of GO, the device realizes phase modulation in a 400 nm film, enabling multi-focus beam splitting and focusing. A coreless fiber segment is fusion-spliced to a standard single-mode fiber to expand the mode field, and a high-density array of 19 microlenses is integrated on the beam-expanding fiber facet; the device achieved focal lengths ranging from 8.0 to 9.6 μ m and numerical apertures (NA) from 0.60 to 0.74. Experimental results at 850, 1310, and 1550 nm demonstrate broadband compatibility. This single-step fabricated, ultrathin device offers a compact scalable solution for fiber-probe imaging, multi-point sensing, micro-optical manipulation and integrated nanophotonic systems.