A potential candidate design for nanosecond-order delay based on high-group index four rows optimized air holes line-defect photonic crystal waveguide

In order to meet the demand of ultra-high optical delay within a limited transmission distance, a high-group index line-defect photonic crystal waveguide (PCW) is designed. By adjusting the parameters of holes adjacent to the defect, a high constant group index about 680 and the corresponding flat bandwidth approximately 0.4 nm are obtained. Group velocity dispersion in the flat-band slow light region is in the order of 10⁹ ps²/km, which is non-negligible. Thus, the pulse broadening caused by dispersion that limits the transmission distance of the pulse in PCW is evaluated through broaden factor (BF). Considering the limitation of BF, a highest time delay up to 70.9 ps for 24 μm delay line is obtained. Assuming return-to-zero modulation format, the above optimized delay line supports a bit rate of the signal up to 12.9 bit/s. The study provides an important approach for realizing large time delay with small size using line-defect PCW.