Loss reduction in few-mode photonic crystal fiber by reducing inner surface imperfections in air holes.

We studied both theoretically and experimentally the additional loss in photonic crystal fiber (PCF) that results from inner surface imperfections such as contamination and the surface roughness of air holes. We estimated the modal loss dependence of these imperfections using a model with a "defective layer" for the first time. The theoretical studies suggest that higher order modes have a larger loss due to imperfections in the air holes. By minimizing the inner surface imperfections of the six innermost air holes, we can theoretically expect any additional loss to be reduced to a negligible level. Moreover, we examined our theoretical prediction experimentally. We fabricated few-mode PCFs by employing a suitable inner surface treatment for just the six innermost holes. As expected theoretically, the transmission loss was greatly reduced by employing these processes. The lowest transmission losses in the 1550 nm band were 0.31 dB/km for the LP01 mode and 0.43 dB/km for the LP11 mode. Our theoretical model will be useful with a view to realizing few-mode PCF with a loss comparable to that of conventional fibers.