Silver halide single-mode microstructured fibers for the middle infrared

Single-mode optical fibers for the mid-IR (λ=3-30μm) are needed for many applications such as IR fiber lasers and spatial filtering for nulling interferometry. In the past, we have already reported the design and fabrication of stepindex single mode fibers for the mid-IR. Index guiding photonic crystal fibers (IG-PCF) offer many advantages over step-index fibers, such as a wide spectral range, large mode area and low bending losses. So far, only limited success has been achieved in the development of such fibers, due to the lack of suitable materials that are transparent in this spectral range. We report here the design, fabrication and optical characterization of single-mode IG-PCFs for the mid-IR. Triangular and octagonal IG-PCFs were fabricated from silver halide polycrystalline materials which transmit well in the spectral range 2-20μm. The photonic crystal fibers were characterized by near-field and far-field measurements and they demonstrated a single-mode behavior with relatively low losses and a large mode area, in agreement with our simulations. As predicted from the simulations, the octagonal arrangement of the rods in the fiber resulted in a single mode fiber with lower losses, a better mode shape and a higher rejection of high order modes, in comparison to the triangular structure.