Enlargement of omnidirectional bandgap in a hollow core Bragg fiber with linearly-chirped multilayered cladding for mid-infrared multicomponent trace-gas detection

For the enlargement of omnidirectional photonic bandgap (OPBG) in the mid-infrared (mid-IR) range of 3–5 μm, a hollow core Bragg fiber (HC-BF) with the linearly-chirped cladding (LCC) is proposed. We numerically investigate its OPBG structure and modal loss characteristics using the transfer matrix and ray-optics methods. Compared with a conventional periodic cladding, the quasi-periodically distributed cladding is capable of enlarging the OPBG by increasing the linear increment of cladding period or the layer number, despite different reasons for these two approaches. Moreover, the influence of increasing refractive index contrast on the enlargement of OPBG is also analyzed. The transmission loss is still lower than 1 dB/m in the OPBG range for both HE11 and TE01 modes even with inclusion of material absorption. The enlargement of OPBG in our proposed HC-BF would help to build a miniaturized and compact system for mid-IR multicomponent trace-gas detection.