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 µm air gap. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.