Few modes in infrared photonic crystal fibers

Photonic crystal fibers guide light by trapping it in a periodic array of elements in the cladding area. We fabricated photonic crystal fibers by multiple extrusions of silver halide (AgCl _xBr _1-x) crystals which are highly transparent in the middle infrared. The core of such a fiber consisted of pure silver bromide AgBr (n=2.16 at 10.6 μm), and the cladding area consisted of concentric rings of fiber-optic elements made of pure silver chloride AgCl (n=1.98 at 10.6 μm), which lowered the refractive index of the clad. Two types of photonic crystal fibers were fabricated, one with two concentric rings and one with five concentric rings of fiber-optic elements around the core. The characterization of the fibers, such as the power distribution, the attenuation, and the numerical aperture were measured. Both fibers behaved like regular core-clad structures. Simulations on these structures showed that each of these fibers guided a small number of modes and that adding rings to the structure lowered the number of bound modes in the core. This would pave the way for the fabrication of single-mode fibers. Photonic crystal fibers offer many advantages compared to conventional fibers, and they will be extremely useful for many applications in the middle and far infrared.