Connection loss reduction in variable optical delay line by using graded-index type photonic crystal fiber

A low-loss butt-joint connection method between photonic crystal fibers (PCFs) by using a graded-index type photonic crystal fiber (GI-PCF) is proposed. The GI-PCF, in which the diameter of the air holes is chosen in a parabolic distribution in the effective refractive index, is analyzed numerically by a beam propagation method. It is found that the mode extent in the GI-PCF varies periodically along the propagation direction when the LP01 mode of the PCF is launched into the GI-PCF. This characteristic is successfully applied to reduce the butt-joint connection loss between the PCFs by 1.6 dB with a 20.6-mm GI-PCF under 200 mu m air gap. (c) 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.