Analysis of bandpass filtering behaviour of singlemode depressed-core-index photonic-bandgap fibre

The modulus of the transfer function of a depressed-core-index photonic-bandgap (DCI-PBG) fiber is studied both theoretically and experimentally. It is shown that the theoretical 3 dB bandwidth of 40 cm of cladding fiber with seven high index layers, designed for transmission at 1060 nm, is approximately 150 nm. Experimental measurements with a 40 cm piece of manufactured DCI-PBG fiber exhibit a 3 dB bandwidth of 80 nm around 1080 nm. The slope of the transfer function reaches 1 dB/nm out of the transmission bandwidth.